Category: Climate change and palaeoclimatology

While I write this issue of EPN it is supposed to be early spring outside, and that is clearly what the ducks reckon as well – they are beginning to, er um, frolic.  But there has been two weeks of snow and frost.  Britain and the rest of Europe owe the frigid snap to cold air spilling westwards from northern Asia; the influence of the Siberian winter high-pressure area.  Although somewhat lost in the recent kerfuffles about whether or not global warming is a fact or a misreading of data, the inevitable build up of mid-continental cold dense air in winter might have interesting consequences, should climate warm.  Normally, areas far from the oceans remain dry as well as getting very cold through radiative heat loss in winter.  When spring comes, such snow as there is soon disappears and the extremes of cold are replaced by surprisingly high summer temperatures, as anyone who has visited Siberia or Northern Canada will know.  Should moist air find its way into such areas during winter, vastly more snow would fall.  Its melting would take longer, and more solar radiation would be reflected back to space in spring.  Such an albedo feedback could induce generalised cooling.  Now evidence has emerged that the earliest known growth of land ice in North America was linked to warming of the ocean from which winds blew over it (Haug G.H. et al. 2005.  North Pacific seasonality and the glaciation of North America 2.7 million years ago.  Nature, v. 433, p. 821-825).  In fact it is axiomatic that growth of continental ice sheets requires a supply of moisture and snow that exceeds the rate of summer melting and ablation, as well as cold winters.

Most theorising about the onset of Northern Hemisphere glaciation has centred on changes in North Atlantic circulation due to closures of the straits where the Isthmus of  Panama now links North and South America, and the start of southward deep-water circulation from the latitude of Iceland.  In fact both are known to have preceded the last Ice Age by a good 2 Ma.  The actual start around 2.7 Ma coincided with an increase in obliquity of the Earth’s orbit that would have led to periods with cold northern summers.  Without abundant mid-continent snowfall, that in itself would not have set ice sheets forming in earnest.  The multinational team of oceanographers studied sea-floor sediment cores from the sub-Arctic Pacific.  To their surprise, sea-surface temperatures provided by evidence from planktonic organisms show evidence at 2.7 Ma for on the one hand cooling of the sea surface (from foraminifer oxygen isotopes) yet considerable warming on the other (from organic chemicals secreted by coccolithophores).  Resolving this paradox requires a careful assessment of the ecological behaviour of the two groups of organisms.  The authors’ explanation involves the onset of density stratification in the North Pacific, so that the surface warmed quickly in summer, retaining warmth during autumn, and warmed slowly in spring from its minimum temperature.  Both result from the high thermal inertia of water.  The productivity of silica-secreting diatoms plummeted to a fifth of its earlier levels at 2.7 Ma as well, explained by ocean stratification reducing the supply of nutrients from deep water upwellings.  Intuitively, a warm sea upwind of the North American continental interior should have generated high snowfall in late autumn and winter.  Haug and colleagues modelled the contrasting effects of an ocean with water overturn and mixing with one that tends to become stratified, to simulate snowfall over the North American Arctic.  From a situation in the Pliocene with snowfall over Greenland and the Arctic islands, the scenario shifts to heavy snow over the whole Arctic in the earliest Pleistocene.  It seems that the trigger for the Great Ice Age was a hemisphere away from the “usual culprit”, the North Atlantic, although its vagaries, once glacial cycles were underway, probably controlled the details thereafter.

Readers will be familiar with the to-ing and fro-ing that surrounds the idea of Neoproterozoic Snowball Earth episodes from earlier issues of EPN.  The leading proponent and sturdy defender of the hypothesis, Paul Hoffman of Harvard University, re-enters the fray as co-author of a paper that builds on the idea that following global glaciation the climate became not only very warm but also violent (Allen, P.A. & Hoffman, P.F. 2005.  Extreme winds and waves in the aftermath of a Neoproterozoic glaciation.  Nature, v. 433, p. 123-127).  They document evidence from “cap carbonates” in northern Canada and Spitzbergen that succeed diamictites of “Marinoan” (~635 Ma) age, in the form of large-scale sedimentary structures.  Many of these are submarine ripples with amplitudes up to 40 cm, and forms that suggest they were produced by sea-bed motion due to surface waves, down to 200-400 m, far deeper than modern storm-wave base.  Central to their argument is hydrodynamic modelling of wind speeds that might have produced such large ripples, and their specific shapes – steep sided.  Being based on experiment and observation of modern sea-bed processes, the theory seems quite rigorous.  It retrodicts wave periods that are somewhat longer than those commonly seen in modern ocean storms.  From that they derive sustained wind speeds that exceed 70 km per hour across open oceans, extraordinary by modern ocean wind standards.

Despite its latitude (above the Arctic Circle) the sedimentary depocentre of northern Alaska is becoming famous for its Cretaceous terrestrial flora and fauna.  Plant remains indicate luxuriant vegetation cover, and high excitement greeted the discovery of 8 species of dinosaurs (4 herbivores and 4 theropod predators (Fiorillo, A.R. 2004.  The dinosaurs of Arctic Alaska.  Scientific American, v. 291(6), p. 60-67).  How dinosaurs were able to survive the darkness of the Arctic winter is a bit of a mystery, unless the migrated as do modern caribou – Fiorillo cites evidence for small juveniles that would have been unlikely to have migrated far, because compared with adults they were much smaller than young caribou.  There would have been sufficient winter biomass for survival during the Cretaceous, but seeing and being active as cold-blooded reptiles pose problems.  At least one of the species had unusually large eyes, so one of the conditions for dinosaur’s remaining year-round seems established.  New data regarding climatic conditions in the far north have turned up after an most unusual and intrepid programme of drilling through a drifting island of pack ice over the Arctic Ocean’s Alpha Ridge, not far short of the geographic North Pole.  An extraordinary feature of the programme is that it took place between 1963-74, the core having only been examined in detail in the last year (Jenkyns, H.C. et al. 2004.  High temperatures in the Late Cretaceous Arctic Ocean.  Nature, v. 432, p. 888-892).  The Late Cretaceous part of the cores is black mud rich in terrestrial vegetation remains and marine diatoms, and totally lacking in evidence for dropstones and other debris from floating ice shelves.  Unfortunately, the Arctic sediments lack carbonate-shelled plankton remains,  so the now standard method of sea-surface temperature measurement is not possible.  However, Jenkyns et al. were able to use a method based on the fatty acids that survive in plankton membranes, results from which match oxygen-isotope palaeo-temperature measurements in Cretaceous cores from lower latitudes.  Astonishingly, even at polar latitudes, the Cretaceous Arctic Ocean seems to have been as warm as 15°C.  Climate modelling based on lower latitude data and estimates of CO₂ concentration in the Late Cretaceous atmosphere falls around 10° short of these levels.  The conventional modelling requires 3 to 6 times more “greenhouse” warming than generally accepted, to account for Arctic sea temperatures in which we could swim in moderate comfort.  Possibly the modelling is awry.  One of the most important features of Late Cretaceous palaeogeography was a major seaway across North America that connected the Arctic with tropical latitudes.  It existed because global sea level was far higher than now, probably due to the oceans’ volume having been substantially reduced by huge magmatic outpourings on the floor of the West Pacific basin (the Ontong-Java Plateau), earlier in Cretaceous times, together with higher rates of sea-floor spreading.  The seaway would have been shallow, and thereby easily warmed.  Had poleward currents been possible in it, their flow would have acted very like the modern Gulf Stream to warm high latitudes.  Despite palaeoclimatologists reliance on models of heat circulation, it needs to be remembered that they are based on grossly simplified geographic features.  If they get it very wrong indeed for the well-studied Cretaceous, that casts doubts on climate modelling’s predictive powers for the course of current climate evolution.

See also: Poulsen, C.J. 2004.  A balmy Arctic.  Nature, v. 432, p. 814-815

Two recent papers add weight to the “against” view expressed in For and against “Snowball Earth in EPN of October 2004”  One gives age of 709±5 Ma for tuff immediately beneath a supposed Sturtian diamictite from the western USA (Fanning, C.M & Link, P.K. 2004.  U-Pb SHRIMP ages of Neoproterozoic (Sturtian) glaciogenic Pocatello Formation, southeastern Idaho.  Geology, v. 32, p. 881-884), which does not tally with the radiometric age (685 Ma) of similar rocks not far away.  The other (Calver, C.R. et al. 2004.  U-Pb zircon age constraints on late Neoproterozoic glaciation in Tasmania. Geology, v. 32, p. 893-896), gives a 575±3 Ma age for sills intruding a “Marinoan” diamictite in Tasmana, and 582±4 Ma for a rhyodacite immediately beneath it.  This suggests that these antipodean glaciogenic rocks are correlative with those in Newfoundland and Norway, that are supposedly representatives of the Varangerian glacial epoch.  Yet the authors are pains to state that the Marinoan and the Varangerian are one and the same.  Read these papers if you are still confused!

Geoscientists have become used to masses of climate data, often with better than 50 years resolution, from cores through ice sheets and sea-floor sediments.  But all of it is from some kind of proxy; oxygen isotopes for air temperature and land-ice volume, methane for humidity, dust for windiness, and so forth.  One aspect of both climate and the British obsession with weather is raininess, for which there is scant evidence.  How many rainy days occur in a British summer is interesting, but for studies of past climate evidence for the onset or disappearance of seasonality, and the annual intensity and duration of rainfall would be invaluable, if it could be had.  A piece of ingenious research shows that it is possible (Kano, A. et al. 2004.  High-resolution records of rainfall events from clay bands in tufa.  Geology, v. 32, p. 793-796).  Akihiro Kano and Japanese colleagues studied the well-known layering of tufa – carbonate veneers laid down in freshwater that has high dissolved bicarbonate and calcium ions.  In “hard-water” areas tufa can be deposited very quickly, at rates above a few millimetres per year, and it tends to be preserved, being quite tough.  So tufas have the potential for preserving annual records of various fluctuations.  Kano and colleagues saw that colour laminations represented clays deposited in the tufa when the water was turbid after prolonged rainfall.  To record the variations they simply measured fluorescent X-rays emitted by silicon when slices of tufa were examined in an electron microprobe – silicon is present in clays and silt, but not in carbonate minerals.  Because they used tufa deposited in recent times (1988-2002) they were able to correlate variations in clay content with detailed weather records from the site, thereby calibrating their method.  The match was very good and followed rainfall closely at the level of a few days.  Of 112 high rainfall days in the abnormally wet year of 1993, 100 showed up in the clay record.  So, tufas are potentially more revealing than even the annual growth rings in wood, and some tufa deposits preserve long records.

Details of the last interglacial climate

Worries about how anthropogenic warming will affect the course of the Holocene interglacial in which we live might be tempered or exacerbated by knowing what went on during the previous, Eemian interglacial that ended about 120 ka ago.  Data from cores through the Greenland and Antarctic ice sheets have been both ambiguous and plagued by resolution that does not show enough detail, but a core from a new position in Greenland seems to resolve both problems (North Greenland Ice Core Project members 2004.  High-resolution record of Northern Hemisphere climate extending into the last interglacial period.  Nature, v. 431, p. 147-151).  Uniquely, the NGRIP ice still preserves the annual snow layering as far back as 123 ka.  This is because the site shows little sign of the deformation at deep levels that characterised previous Greenland cores.  That is probably because the site lies above a zone of high heat flow through the underlying crust, so that the base of the ice has melted.  Melting helps prevent internal deformation, but that in itself is a surprise because the site was chosen because it is colder and drier at the surface than other sites.  The drilling objective was to penetrate older ice than the Eemian to give a fuller record than from earlier cores, yet anticipated poor time resolution.  The presence of resolvable annual records from depth was both a surprise and a bonus, although the melting had removed ice from the earliest part of the last interglacial.  Despite that, preliminary oxygen-isotope results from the NGRIP core suggest that the Eemian had a remarkably stable climate and one that was warmer than that of the Holocene by about 5ºC; maybe it is an analogue for climate evolution during a future, artificially warmed world.  That possibility stems from the observation that around 115 ka, North Atlantic climate suddenly warmed  Thereafter, interglacial conditions did not suddenly change to glacial, as happened several times during the course of the last glacial epoch, but took around five millennia after the sudden warming.  The authors make no claims that their preliminary data help resolve current fears of warming collapsing to glacial conditions in a matter of years to decades.  That grim scenario has been widely trumpeted both by the media and some climate scientists.  There is more to the Eemian than the period after 123 ka, and who knows what the eventual annual resolution will show up?  The data presented in the paper are from a coarse sampling of 55 cm that represents about 40 year intervals.

See also: Kuffey, K.M. 2004.  Into an ice age.  Nature, v. 431, p. 133-134

For and against “Snowball Earth”

Reputedly glaciogenic sediments in the Neoproterozoic are reckoned to represent at least three separate cold episodes, the Sturtian (~720 Ma), Marinoan (~600 Ma) and Varangerian (~580 Ma).  Sadly, the diamictites that characterise these episodes are not easily dated.  Only two have well-defined radiometric ages, the Gubrah Member in the Oman (713 Ma), said to be Sturtian, and the Gaskiers Formation of Newfoundland (580 Ma), a possible example of the Varangerian that is better exposed in northern Norway.  The truly whopping Sturtian and Marinoan diamictites of Australia are fitted to a global stratigraphy on the basis of carbon isotope variations, as are those of Namibia on which Paul Hoffman and colleagues stake their claims to “Snowball Earth” events. Another Hoffman, native to Namibia, and geochemists at MIT, have finally given a believable age to one of the Namibian diamictites (Hoffman, K.-H. et al 2004.  U-Pb zircon dates from the Neoproterozoic Ghaub Formation, Namibia: constraints on Marinoan glaciation.  Geology, v. 32, p. 817-820).  Their zircons come from a thin volcanic ash within isolated Neoproterozoic diamictites in central Namibia, and yield an age of 636±1 Ma.  Correlating the studied diamictites with the Namibian sequences elsewhere in the country relies on the presence of a supposed cap carbonate rather than lateral continuity.  The authors link them with the younger of the two Namibian diamictites, the Ghaub Formation, rather than the Chuos Formation that lies at depth, despite the fact that both well-studied units are sometimes overlain by carbonate sediments.  The conclusion is that the Ghaub is Marinoan, previously thought to be somewhere between 600 and 660 Ma.  Interestingly, the new occurrence of diamictites is divided vertically by two thick sequences of volcanic lavas, neither of which have been dated by the authors.

One of the leading experts on what actually constitutes incontrovertible evidence for glacial sedimentation is Nicholas Eyles of the University of Toronto.  He has become increasingly disenchanted with notions of Snowball conditions, on the basis of ambiguity in the very evidence said to signify them; diamictites with drop stones.  He and Nicole Januszczac have assembled a monumental paper that counsels caution, and perhaps more (Eyles, N. & Januszczac, N. 2004.  “Zipper-rift”: a tectonic model for Neoproterozoic glaciations during breakup of Rodinia after 750 Ma.  Earth-Science Reviews, v. 65, p. 1-73).  Part of their argument rests on the very lack of robust ages for Neoproterozoic diamictites that prevents believable correlations from continent to continent.  It is the globally synchronous nature assumed for these glaciations that gave rise to the “Snowball Earth” notion.  The palaeomagnetic latitudes are often used to support this, but they are error prone both palaeogeographically and geochronologically.  Accepting evidence for glaciation at low latitudes is no guarantee of support for even cold extremes, let alone an icebound world.  Solar heating in the Neoproterozoic was lower than now, and so, therefore, would be the elevations at which glaciers might form at different latitudes.  But the main problem is reconciling the features of many supposed glaciogenic diamictites with modern ideas of what truly constitutes evidence for glacial transport and deposition.  Few of the units on which the “Snowball Earth” hypothesis is based stand up to modern scrutiny.  Most of the diamictite packages occur in tectonically controlled basins, that were subject to episodic rifting.  Each can be considered to form the base of a “tectonostratigraphic” cycle, and many show abundant evidence of having formed as mass flows from a shelf into the basin.  They include olistostromes with huge rafts of carbonates likely to represent failure of carbonate platforms and huge submarine landslides, similar to those being discovered off many large islands today.  The 750 to 580 Ma period was one of the most dramatic episodes of continental break-up in Earth’s history as the Rodinia supercontinent was disassembled.  Continental uplift, resulting either from mantle plume activity or rebound of rift shoulders, could have resulted in large areas rising above the ice limit, even at low latitudes in those cooler times.  Those diamictites that are undoubtedly glaciogenic could easily have formed haphazardly in time.  The carbon isotope record of immense shifts in d¹³C during the Neoproterozoic, linked by some to repeated collapses and resurrections of life, might just as easily have occurred through efficient organic burial in active extensional basins and repeated major volcanism from plumes.  Only evidence of timing will tell, and three good dates for “Snowball Earth” events are simply not enough.

See also: Fanning, C.M & Link, P.K. 2004.  U-Pb SHRIMP ages of Neoproterozoic (Sturtian) glaciogenic Pocatello Formation, southeastern Idaho.  Geology, v. 32, p. 881-884. Gives age of 709±5 Ma for tuff immediately beneath a supposed Sturtian diamictite. Also:  Calver, C.R. et al. 2004.  U-Pb zircon age constraints on late Neoproterozoic glaciation in Tasmania. Geology, v. 32, p. 893-896.   Gives 575±3 Ma age for sills intruding a “Marinoan” diamictite, and 582±4 Ma for a rhyodacite immediately beneath it, similar to Gaskiers age above – worth a read later

Ice extracted from ice sheets by core drilling has provided the most detailed historical information on climate variation at high latitudes and about the varying gas and dust content of the atmosphere.  It provides the best time-resolution currently available, sometimes of the order of 50 years. Cores from the Greenland ice sheet revolutionised ideas about the controls over short-term climate shifts in the northern hemisphere – the millennial-scale Heinrich and Dansgaard-Oeschger events.  It is from those revelations that fears have arisen about the consequences of deep-ocean circulation shut-downs that might arise from current global warming.  The Greenland ice goes back only to cover the last glaciation and part of the interglacial period the preceded it.  Until recently, the Vostok ice core from Antarctica gave the greatest penetration into past climatic events, to around 430 ka that covers the last four glacial epochs.  Again, Vostok revolutionised our understanding of past climate change, principally the differences between climate behaviour in interglacials, and those between the records from northern and southern hemispheres.  North and south have not been in exact harmony, at least as far as high latitudes are concerned.  Ocean-floor sediment cores and those from mid-latitude glaciers do give hints of a global harmonisation of events though.  Since we live in an interglacial period, for the last three of which the previous ice-core records suggest a span around 10 ka, it has seemed likely that ours wouldn’t have lasted much longer than it already has under purely “natural” conditions.  Modelling the possible effects of anthropogenic warming on climate that may be about to change anyway within this millennium, has left climatologists undecided about the future.  That blurring is as much to do with the unknown direction that an unstable climate might take and the limitations of modelling, as with knowledge of past events.  So, the more information on past interglacials, the better the chance of getting a “handle” on the climatic frying pan out of which humanity seems to be on the point of jumping.  The European Project for Ice Coring in Antarctica (EPICA), which involves 57 scientists from 10 European countries, has dramatically expanded the scope for comparison with the past by a 3 kilometre core from one of the deepest parts of the Antarctic ice (EPICA, 2004.  Eight glacial cycles from an Antarctic ice core.  Nature, v. 429, p. 623-628).  The potential information that eventually will flow from the core will dwarf that from any previous climatic research project.  It covers the period when climate settled into a roughly 100 ka rhythm, probably linked with the weakest of the astronomical controls of solar heating, that of orbital eccentricity, and thereby a bit of a mystery even if it twangs the harmonics of purely terrestrial climatic processes.

The first focus, naturally enough, is on the fourth interglacial epoch before the present one, which ended about 400 ka ago.  In terms of overall astronomical forcing, that is the time when insolation patterns were most similar to those during the Holocene.  Vostok only covered the latter stages, but now its entire span is covered.  All the preliminary time-series for it indicate that it was considerably longer than the last three interglacials, around 25 ka rather than 10.  Its initiation following the waning of the preceding full glacial period follows a similar patter to the early Holocene; the warming was interrupted by a sudden, one-off cooling, somewhat like the Younger Dryas around 12 ka ago.  Although the first EPICA report contains preliminary ideas on several important topics, the one that has caused a stir is that duration of the 5^th interglacial.  Maybe out own warm times will be naturally prolonged for several more millennia, in which case fears of instability and a plunge to full glaciation soon could be set aside with some relief.  However, the abstract to the article, concludes ny saying, “…our results may imply that without human intervention, a climate similar to the present one would extend well into the future” [my italics].  But we do intervene, and nobody knows the outcome of that on a climatic pace of change that follows the almost infinitesimally small orbital-obliquity forcing of probable oceanic process that really call the tune.

Smoking gun for end-Palaeocene global warming: an igneous connection

The sudden warming of the Earth at the start of the Eocene 55 Ma ago has been a topic touched on several times in EPN.  It is widely regarded as a consequence of rapid release of methane from sea-floor gas hydrate, a risk that modern anthropogenic warming presents if deep-water temperatures rise much above their present near-freezing temperatures.  However, no evidence gives a direct connection to the “clathrate gun”.  The disturbance in carbon isotopes of marine sediments at the P-E boundary is most easily linked to a massive methane release at the time, but precisely where it began has been unknown.  Many shallow marine basins, such as the North Sea, have a pockmarked modern floor attributed to minor gas release in much more recent times.  The phenomenon can destabilise the sea bed, so more recent releases have been carefully documented where oil-production platforms are situated.  A clue to the much larger release at 55 Ma stems from detailed seismic exploration of western Norway that involved over 150 thousand kilometres of profiling (Svenson, H. et al. 2004.  Release of methane from a volcanic basin as a mechanism for initial Eocene global warming.  Nature, v. 429, p. 542-545).  The surveys revealed that beds immediately beneath the base of Eocene sediments are riddled with hydrothermal vents complexes, which take the form of mounds, craters and eye-shaped structures.  Some are huge, extending to 5 km across. The profiles also show that beneath the vents are pipes of disrupted strata which extend to the depth of a complex of igneous sills of the North Atlantic large igneous complex, itself emplaced at about 55 Ma.  The sills underlie about 80 thousand square kilometres and most of the vents occur within this area.  Biostratigraphic dating of the youngest sediments disrupted by the vents gives ages between 55.0 and 55.8 Ma.  Intrusion of magma into a deep sedimentary sequence unsurprisingly would set hydrothermal circulation going.  If, as they did, the hot fluids reached the sea bed, they would pass through a zone of gas hydrate, destabilise it and release massive amounts of methane to the atmosphere.  In the case of the Norwegian shelf, the intrusions were into deeply buried organic rich rocks, further encouraging methane formation; probably a great deal more than from gas hydrate.  An estimate of 10¹² tonnes of methane generated thermally off Norway is enough to result in a change in carbon isotopes as large as that known from the P-E boundary.  In fact, similar sediments throughout the end-Palaeocene North Atlantic large igneous province are likely to have been “over matured” in this way, and no other explanation for the increase in “greenhouse” gases seems necessary.  The clear connection with large scale magmatism in thick sedimentary basins may help focus ideas about similar methane-related episodes of global warming, such as the C-isotope excursions at the Permian-Triassic and Triassic-Jurassic boundaries, and within Jurassic and Cretaceous sequences.

Earth’s early climate and methane

At the time the Earth accreted, some 4.6 billion years ago, the Sun was less bright than it is now, so that its warming effect was 30% less.  Without some means of retaining in the ancient atmosphere what heat was available, the Earth would have been frigid.  This “faint, young Sun” problem would have persisted into the time when the geological record begins, around 4 billion years ago, slowly increasing in its energy output to its modern level.  Even in the oldest rocks, there is abundant evidence for the dominance of liquid water at the surface in the form of oceans and river transport across continents.  Low solar warming would have made that impossible, and pole-to-pole ice would have made the Earth a highly reflective planet that could never escape glacial condition.  That is, unless the atmosphere contained sufficient “greenhouse” gases to retain far more solar energy than now.  The favoured gas, until recently, has been the same one that dominates fears of global warming today – carbon dioxide – that volcanoes probably emitted throughout Earth’s history.  However, estimates of how much would have been needed to keep the surface free of sea ice and land glaciers, for which there is no evidence until about 2.3 billion years, are extremely high (hundreds of time greater than now).  Levels greater than 8 times present levels encourage the precipitation of iron carbonates in soils, yet soils from the late Archaean and Palaeoproterozoic contain none.  At those times, CO₂ concentrations less than 8 times present ones would not have prevented runaway “ice-house” conditions, so some other gas had to be involved in atmospheric warming.  James Kasting of the University of Michigan, who has been involved in studies of ancient atmosphere and climate for 25 years, summarises the case for methane being the means of keeping Earth free of ice while the sun was fainter in a recent article (Kasting, J.F. 2004.  When methane made climate.  Scientific American, v. 291(1), p. 52-59).  Only about 1000 parts per million of atmospheric methane would have been needed to keep the early Earth ice-free, because its “greenhouse” effect is extremely efficient.  After oxygen rose to become a major atmospheric gas (since 2.2 billion years), heating induced by methane releases has been tempered by its rapid oxidation to CO₂.  At several times in the past, when there were massive methane releases from sea-floor sediments, such as the end of the Palaeocene, that oxidation prevented the opposite problem, a runaway “greenhouse”.  That is “another story”, involving the rise of photosynthesising organisms.  Kasting’s main theme is the role of methane-generating Archaea (once known as archaebacteria) soon after the origin of life.  In the absence of oxygen, rising methane from thriving methanogen communities could itself have produced irreversible heating, were it not for methane’s ability to polymerise to heavier hydrocarbons through photochemical reactions.  That would have produced a “smog” that not only would have acted as a reflector for solar radiation, but would have added chemical “feedstock” to early life.  Kasting gives a fascinating, all-sided summary, but misses what seems to be an obvious point.  Without atmospheric methane, any water on Earth would have frozen soon after it appeared, however that happened, perhaps by outgassing, perhaps delivered by comets.  Without liquid water, life processes cannot develop.  That opens the possibility for a much earlier origin of life, of the methane generating variety, than anyone has dared to speculate on.  Many methanogens metabolise hydrogen and CO₂.  Volcanoes emit small amounts of hydrogen gas, but an even larger source is from sea-floor hydration of ultramafic lavas, common in early times.  Almost certainly the very earliest times would have provided a suitable environment for methanogens to emerge.

Ray Bradbury wrote a seminal political fiction in the 1950s, called Fahrenheit 451.  It is about a repressive regime that tries to snuff out dissent by burning books, the title referring to the temperature (233ºC) at which paper spontaneously bursts into flame in the modern atmosphere.  With no reference to book burning by some future oligarchy, geoscientists have speculated on the possibility of higher atmospheric oxygen contents being able to induce massive conflagration of green vegetation after lightning strikes or meteorite impacts.  One often cited case is at the K/T boundary, where the thin layer that signifies the mass extinction event contains a high proportion of sooty particles.  Late Cretaceous air probably had significantly higher oxygen content than now, generated by pole-to-pole luxuriant vegetation, and the idea of a global wildfire gained much support when first mooted.  During the Carboniferous, there is very good evidence that oxygen levels were as high as 35% compared with 21% today.  It was a time of giant flying insects, whose size is limited by the availability of oxygen. Carboniferous and Permian strata contain much charcoal, which suggests that indeed fires then were a great deal fiercer and more capable of spreading.  They might have destroyed vegetation, despite evidence that the tree-sized plants of the period had developed fire-resistant structures.  Experiments to simulate the effects up to now have used strips of paper in different oxygen levels, and showed a strong correlation between the minimum energy for ignition and oxygen concentration.  US geologists, foresters and engineers have repeated the experiments using a range of natural plant materials as well as paper (Wildman, R.A. et al. 2004.  Burning of forest materials under late Paleozoic high atmospheric oxygen levels.  Geology, v. 32, p. 457-460).  Their results approximately confirm Bradbury’s fictional paper-combustion temperature, but monkey-puzzle (Araucaria) leaves are more easily set alight.  However, the temperature for ignition does not change as oxygen levels increase, although burning is faster.  How natural materials burn depends on their relative proportions of cellulose and lignin, the higher the latter, the greater the temperature for complete combustion.  They behave very differently from paper.  Another finding was that the rate at which burning spreads did not rise as dramatically as expected for Carboniferous conditions.  The limiting factor is moisture content, although that for no-burn does increase with oxygen levels.  This is particularly important for the firing of dead vegetation lying on the surface, which is essential for catastrophic wildfires.  Natural fires are started by lightning, and that occurs during heavy rainfall, when surface debris is thoroughly saturated.  Fires in the canopy would have occurred at higher frequencies and with greater intensities, but the authors consider they would not have seriously threatened plant life.

Six years ago vast areas of Indonesia caught fire after an unusually dry phase in the El Niño – Southern Oscillation (ENSO).  Burning forest and peat deposits swathed a vast area in smoke, but another alarming aspect was the greatest addition of carbon dioxide to the atmosphere in half a century.  Such a wildfire on a global scale is thought to have marked the end of the Mesozoic, perhaps triggered by the K-T impact event and encouraged by higher oxygen content in the atmosphere.  Present oxygen levels seem to be at a balance that staves off spontaneous combustion of green vegetation, but only a few percent more would render vegetation much more prone to bursting into flame.  The end of the Palaeocene involved a sudden global warming that coincides with a decrease in the proportion of ¹³C in marine carbonates.  Since photoynthesis, at the base of the trophic pyramid, favours light ¹²C, such a negative d¹³C “spike” is generally ascribed to an unusually high release of organic carbon to the environment.  The end-Palaeocene warming may have resulted from a massive release of methane from gas-hydrate buried in shallow seafloor sediments (See Methane hydrate – more evidence for the ‘greenhouse’ time bomb and Plankton and the end of the Palaeocene-Eocene global warming August and October 2000 issues of EPN).  However, massive burning of living biomass could also produce the carbon-isotope signal.   Telling the two mechanisms apart requires information from other organic-related cycles.  One key is comparing the carbon- and sulphur-isotopic records that enables the place in which carbon had been stored geologically.  For marine burial, the effect of aerobic bacteria that completely oxidises hydrocarbons back to carbon dioxide and water needs to have been suppressed.  Periods of massive marine carbon burial coincide with oceanic anoxia episodes, when anaerobic bacteria beneath the seafloor reduce dissolved sulphate ions to sulphides, thereby depositing lots of iron sulphide (pyrite) in black organic mudrocks.  This sequesters sulphur that is depleted in ³²S into marine sediments, so that the marine carbon- and sulphur-isotope records fluctuate in a clearly related way.  During the Palaeocene this relationship is absent, while overall the carbon isotopes do signify progressive burial of organic carbon.  The decoupling of the two cycles points to carbon burial on the continents, forming peat and eventually coal deposits.

Playing games on Snowball Earth

For as long as anyone can remember there has been a parade of geoscientific bandwagons in town.  Three of the floats today carry banners saying, “Snowball Earth”, “Climate models” and “continental erosion and CO₂ drawdown”.  Of course there is serious science aboard each, but they are getting overcrowded, especially as separate bands try to jump from one to another.  When it sometimes seems, as now, that the “next Big Thing” is some way off, we get the unseemly spectacle of some bands trying to straddle two or even several of the wagons.  Three is quite a feat, yet the 18 March 2004 issue of Nature contains perhaps not a vast human pyramid, but at least a tetrahedron of the genre (Donnadieu, Y. et al. 2004.  A “snowball Earth” climate triggered by continental break-up through change in runoff.  Nature, v. 428, p. 303-306).  From about 1100 to 750 Ma ago, the bulk of continental lithosphere was gathered in a supercontinent known as Rodinia (from the Russian for “Mother Earth”).  By analogy with modern Eurasia, and the stratigraphic record from the Phanerozoic Pangaea supercontinent, the centre of Rodinia would almost certainly have been dry, being so far from the ocean.  Break-up of that continental mass would also probably have allowed moist maritime air to penetrate over a larger proportion of the fragments.  The hypothesis that Donnadieu and colleagues try to test using linked geochemical and climate models is that such a tectonic change would increase continental weathering and reduce the “greenhouse” effect.  The weak acid formed by solution of carbon dioxide in rain water can provide hydrogen ions to break down silicate minerals.  The reactions contribute bicarbonate and soluble metal ions to surface and subsurface water.  Ultimately, both reach the oceans and contribute to its chemistry.  If conditions are suitable, calcium ions in particular combine with bicarbonate to precipitate calcium carbonate on the ocean floor, either through the action of organisms or inorganically.  The two chemical equilibria involved result in a net burial of one carbon atom out of the two involved in the weathering, thereby drawing down carbon dioxide from the atmosphere.  The climate model used in their cyber-experiment resolves the Neoproterozoic Earth into cells that are 10 x 10 degrees (about 100 thousand km²) and considers Rodinia at 800 Ma and the result of its break-up at 750 Ma, the time of the first good evidence for extensive low-latitude glaciation.  The results, after some tinkering, suggest that increased continental weathering could have reduced CO₂ levels to 250 parts per million.  Taking account of a 6% less energetic Sun at the time, this would have produced sufficient cooling for ice caps to exist to sea level at the equator.  So, taken at face value, the hypothesis seems plausible.  However, there are major snags.  First, in a mere 50 million years their model sees continental dispersion on a scale that has not yet happened to Pangaea in about 200 Ma of Phanerozoic time.  Second, since continental area remains constant, the proportion of rainfall, and therefore weathering and runoff, involving continental crust also stays fixed.  Third, continental weathering refers to the crystalline part of its crust, in which there are unstable minerals, such as feldspars, that can do the chemical trick.  We have little idea how much of the continents at that time was veneered by sediments that are the products of earlier chemical weathering, and contribute nothing to the process.  Exposing such deep crust depends to a large extent on mountain building, which continental extension does not encourage.  Fourth, carbon dioxide is not the only source of hydrogen ions that are involved in weathering, especially as much of it goes on in groundwater – bacterial action and oxidation of iron sulphides create much more acid conditions that rainwater.  Fifth, and most important, where is the complementary geochemical evidence?  Feldspars of the continental crust, on which the hypothesis mainly rests, have high contents of rubidium compared with their oceanic counterparts, and they are old.  Much of Rodinia was underpinned by crust formed as far back as 4 billion years ago.  Prolonged decay of ⁸⁷Rb to radiogenic ⁸⁷Sr makes the strontium isotopes of continental material very different from those of the ocean floor – it has a much higher ⁸⁷Sr/⁸⁶Sr ratio.  Since soluble strontium would be released to runoff by continental weathering, that signature makes its way to the ocean and should pop up in marine carbonates.  Although the ocean strontium isotopes in the Neoproterozoic did rise a little, it did not peak until the very end.  In fact, the details show that the periods around supposed “snowball” conditions involved downturns in radiogenic strontium supply to the oceans.  Whatever the model suggests, all that it amounts to is the equivalent of a table-top train set

The vast reserves of peculiar methane-water ice deposits (gas hydrate or clathrate) in sea-floor sediments are the most likely source of methane releases that could generate sudden warming events, such as that at the end of the Palaeocene, and left traces in polar ice cores during the last few glacial-interglacial episodes.  Methane probably leaks from the sea floor all the time, but is soon oxidised to the lesser “greenhouse” gas CO₂ in the atmosphere, so muting its potential effects to a low background level.  For methane to have a sizeable effect on global warming, lots of it has to blurt out suddenly.  Possibly the only mechanism that can trigger such explosive releases are failures of sea-floor sediments, either by those beneath a steep surface slope collapsing under gravity, or as a result of seismicity.  Geoscientists from University College London and the British Geological Survey have tried to correlate known peaks in atmospheric methane from the recent past (shown by ice cores) with episodes of mass flow on the seabed (Maslin, M. et al. 2004.  Linking continental-slope failures and climate change: Testing the clathrate gun hypothesis.  Geology, v. 32, p. 53-56).  They found that the periods of greatest disturbance of continental-slope sediments over the last 45 ka took place at the tail-end of the last glaciation, between 13 and 15 ka and 8 to 11 ka.  Each correlates with methane highs in the Greenlandic ice cores and with bouts of rapidly rising sea level (the Bølling-Ållerød and Preboreal warming periods).  So they conclude that there is support for a “clathrate gun” model for sudden warming associated with glacial to interglacial transitions.  However, seafloor collapses also correlate with Heinrich events (ice-sheet surges that launched iceberg “armadas” to low latitudes) that punctuated glacial times.  These marked brief periods, repeating every 1000 years or so, which mark cooling when sea-levels were low.  None are associated with upsurges in atmospheric methane., although the following interstadial warmings are.  This lack of correlation rules out a “clathrate gun” influence on millennial-scale climate fluctuations during glaciations.

Super-eruptions and climate

The biggest known, young volcanic crater is that of Toba on Sumatra, which is a caldera complex measuring 30 x 100 km.  Around 74 ka Toba emitted an eruption that dwarfed any in more recent times, and spread a dust cloud around the world – it is present in ice cores from Greenland, and has been linked with a cooling step during the onset of the last glaciation.  It happened around the time that fully modern humans had begun to spread across Asia after migrating from NE Africa – an Acheulean hand-axe has been found in the Toba Tuff – and may have deeply affected those pioneering bands.  There are older ash levels that can also be attributed to Toba eruptions, one found 2500 km away in the sediments of the South China Sea (Lee, M-Y. et al. 2004.  First Toba supereruption revival.  Geology, v. 32, p. 61-64) and at other sites up to 3000 km from Toba.  This gives an age around 800 ka.  Lee and colleagues from Academica Sinica (Taiwan), the National Taiwan University and the University of Rhode Island estimate that almost 1000 km³ of ash was expelled by the eruption.  Unlike the 74 ka ash, this layer falls in the transition from a glaciation to an interglacial period; instead of a possible cooling influence through dust blocking solar heating, there is a warming trend.  Although not quite as big as the 74 ka eruption of Toba, that of 800 ka is still vastly bigger than any other explosive volcanism during the Pleistocene.  So, it suggests that super-eruptions are not significant climate triggers after all.

Since the resurrection of Chamberlin’s idea that the rate of chemical weathering of continental crust helps regulate atmospheric CO₂ by Maureen Raymo, the hypothesis has not yet been supported by convincing geochemical evidence.  There is such a lag between changes in ocean chemistry and evidence for global climate change, that correlations are flimsy.  The need is for a proxy for weathering of the land surface that resides in seawater for a geologically very short period.  Such an element is osmium (Os), which passes from river water through the oceans to sea-floor sediments in about 25 thousand years, so changes in its abundance in sediments ought to match the pace of any climatic shifts.  In principle, there are two main sources for elements in seawater, from sea-floor hydrothermal alteration of oceanic crust, and from continental weathering.  The first can be considered to be more or less constant, except on time scales of tens of million years.  Continental weathering is a response to climate change, and keeps pace with it.  Researchers at the UK Open University and the University of Köln in Germany analysed samples for osmium and carbon isotopes through a sequence of Jurassic mudstones on the NE coast of England (Cohen, A.S. et al. 2004.  Osmium isotope evidence for the regulation of atmospheric CO₂ by continental weathering.  Geology, v. 32, p. 157-160).  The carbon isotopes show a sudden drop in d¹³C within a very hydrocarbon-rich unit famous for it contribution of jet (oil-rich lignite) to Victorian funereal jewellery.  This negative excursion is recognisable world-wide at around 180 Ma.  The most likely explanation is a monstrous blurt of methane from destabilised gas hydrate on the Jurassic sea floor (see Methane hydrate – more evidence for the ‘greenhouse’ time bomb, August 2000 issue of EPN).  The Jet Rock of the Whitby coast therefore preserves a nice example of sudden climatic change, and by the end of its deposition carbon isotopes returned to Jurassic background values.  Methane, a powerful “greenhouse” gas, is rapidly oxidised to CO₂ in the atmosphere, so reducing its initial warming effect, but climate would have been hotter for some time afterwards until the excess CO₂ was drawn down somehow.  Interestingly, the Jet Rock also shows a sudden leap in the abundance of ¹⁸⁷Os, reflected in the ¹⁸⁷Os/¹⁸⁶Os ratio of the samples, and an upward step in the value of the ⁸⁷Sr/⁸⁶Sr ratio – one of the fastest rises known.  The latter is generally assigned to an increase in continental weathering, since continental crust contains more radiogenic ⁸⁷Sr than does oceanic crust.  The implication of the osmium-isotopic shift is odd; it requires an increase in the rate of continental weathering by 4 to 8 times that in the preceding period.  That is a vast change, even if it only lasted for a short period, but it tallies with what is known about the temperature dependence of the dissolved loads of rivers in more recent times.  If the osmium isotope excursion truly reflects massive continental weathering, then it is possible to calculate the drawdown of the excess CO₂ in the atmosphere from a commensurate flux of calcium and magnesium ions from the continents, that would eventually form marine carbonates.  The authors estimate a mere 37-123 ka to get rid of it.  Yet continent-derived radiogenic ⁸⁷Sr remained high for much longer, and the authors’ arguments become tricky.  One interesting aside is that, unlike today, more groundwater found its way to the oceans than surface run-off during the Jurassic, perhaps 6 times more.  It is easy to look on weathering as what happens at the interface between rocks and the weather; the land surface.  Not so.  A great deal of chemistry that releases soluble ions goes on in the subsurface, above and below the water table.  It is by no means as simple as reactions between carbonic acid in rainwater and silicate minerals.  Weathering is the product of hydrogen ions’ (whatever their source) effects on silicates.  Bacteria are extremely important actors in modifying pH below the surface, for example the sulphate-sulphide reducers, and the oxidative dissolution of sulphides produces sulphuric acid.  Even more interesting for the chemistry of groundwater is the curious role of iron hydroxide.  Under oxidising conditions it adsorbs many elements from solution, including platinum-group elements, such as osmium.  Should conditions become reducing, dissolution of goethite skins on sedimentary grains releases the accumulated elements.  A warming trend almost inevitably results in increased precipitation, and rising water tables.  It also should boost biological productivity on land and an increase in the amount of buried organic matter, which create reducing conditions in groundwater.

Whether or not you believe that the Earth was totally encased in ice up to four times during the Neoproterozoic Era, there is convincing evidence that ice sheets did extend to the tropics during such “Snowball” episodes.  How such extremely cold episodes came to prevail for several million years has been the subject of debate for 5 years, since Harland’s notion of global glaciations was resurrected by palaeomagnetic evidence for the low latitudes of Neoproterozoic glaciogenic rocks.  Ice extending almost to the Equator, even if just on the continents, would have driven down global temperatures simply because it would have reflected away solar radiation.  Increased albedo helps explain why frigid conditions lingered, but some other cooling mechanism must first have encouraged the widespread formation of ice sheets.  Essentially, the supply of the “greenhouse” gas CO₂ by volcanic activity must have been outstripped by burial or solution of carbon in some form.  The two usually identified candidates are increased deposition of carbonate sediments and the accumulation of unoxidised organic carbon in sea-floor muds.  It is the first of these that dominates climate control today, by the accumulation of carbonate shells of marine plankton, and that has probably prevailed since foraminifera and coccolithophores began to proliferate in the Mesozoic.  No shelled organisms existed during the Precambrian, so a major factor in damping down climate fluctuations was missing before the start of the Phanerozoic.  This crucial difference between the modern and Precambrian world focussed the attention of Andy Ridgwell, Martin Kennedy (University of California) and Ken Caldeira (Lawrence Livermore National Laboratory) in seeking an explanation for “Snowball” events (Ridgwell, A.J. et al. 2003.  Carbonate deposition, climate stability and Neoproterozoic Ice Ages.  Science, v. 302, p. 859-862).

Carbonate sediments are plentiful in the Precambrian record.  Some formed as a result of organic action (stromatolitic limestones) and others show evidence for direct, inorganic precipitation of carbonates from sea water.  The latter indicate sea water in which calcium and carbonate/bicarbonate ions exceeded the solubility of calcite and the ability of organic activity to remove calcite from solution.  Evidence for such extreme oversaturation is rare, but the cap carbonates that overlie Neoproterozoic glaciogenic rocks are important examples.  The key area of carbonate deposition has always been on shallow continental shelves, the main secreters of carbonates during the Precambrian having been blue-green bacteria that can photosynthesise only in shallow water.  Falls in sea-level or a reduction in the area of shelves during the Phanerozoic reduced this sink for CO₂ in the build-ups of coral and shelly limestones, but plankton of the open oceans continued to accumulate on the deep sea floor.  Because calcite can be dissolved at depth, the deepest sea floor does not contain much carbonate.  However, a fall in sea level,  increases the area suitable for deep-water burial of shelly material, because the carbonate compensation depth or lysocline also falls.  In the absence of shelly plankton, this modern balancing mechanism for ocean chemistry did not exist during the Precambrian.  Superficially, it might seem that a reduction in the area of shelf deposition of carbonates, brought on by a sea-level fall, would allow CO₂ to build up in the atmosphere, driving towards warmer conditions.  However the way in which atmospheric carbon dioxide is related to dissolved carbonate (CO₃^2-) and bicarbonate (HCO₃^–) ions tells a very different story.  This is the equilibrium: CO₂ + CO₃^2- +H₂O = 2HCO₃^–.  Less carbonate accumulation on reduced continental shelves would drive up the carbonate-ion concentration of sea water, and also its pH.  So, according to Le Chatelier’s Principle, the equilibrium proceeds to the right and adds to the more soluble bicarbonate ions in sea water.  This consumes CO­₂, and drives down the “greenhouse” effect.  Ridgwell and colleagues developed a model around this equilibrium, and applied it to conditions of falling sea level when carbonates were only deposited on continental shelves.  Their results show that decreased shelf-carbonate burial during a period of sea-level fall would rapidly drive down the warming effect of atmospheric carbon dioxide.  Combined with the lower solar energy output during the Neoproterozoic, that would be sufficient to create protracted periods of frigidity.  Alkalinity of the oceans would increase through periods of glaciation, so that once sea-level rose, massive carbonate precipitation would form cap carbonates on the newly inundated shelves, thereby reducing the oceanic drawdown of CO₂.

Ridgwell et al’s model is not easy to grasp, and relies on its initiation by falling sea-level.  Either that resulted from build up of continental glaciers because of some other climatic mechanism, or internal processes increased the volume of the ocean basins.  An example of the last is a decrease in sea-floor spreading, when cooling of the lithosphere increases it density so that it sags down.  Periods of accelerated creation of oceanic lithosphere displace sea water upwards, and perhaps that might explain an increase in shelf areas, which would allow warming according to the new model.  The model also needs special pleading to account for the 1 billion-year absence of glaciation before the period of Snowball events.  The authors suggest that it could have been prevented by much wider shelves during earlier times, but without quoting evidence.

Continental erosion and climate

Maureen Raymo suggested in 1988 that long term climate change was modulated by the rise of mountain chains and their erosion and weathering.  This is because chemical weathering of silicate minerals is a net consumer of atmospheric carbon dioxide.  Raymo’s hypothesis, based on T.C. Chamberlin’s theory of glaciation, has set climatically concerned geochemists to analysing the trace element content of river water in many mountainous regions, because those such as strontium are proxies for the amount of weathering going on today.  Others have looked at the flux of elements into seawater through the Phanerozoic in particular, by analysing marine carbonates, to see if the ups and down’s of water composition through time match the record of climate change.  These time series do suggest some matching, but not precise enough for all to agree with the hypothesis.  Measurements of river-water composition have also met set-backs.  Much of the weathering flux from mountains seems to stem from dissolution of carbonate rocks, and that does not lead to long-term loss of CO₂ from the atmosphere.  In a bid to resolve the contributions of carbonates and silicates, Andrew Jacobson and Joel Blum of the University of Michigan have studied the flux from part of the Alps of New Zealand’s South Island (Jacobson, A.D. & Blum, J.D. 2003.  relationship between mechanical erosion and atmospheric consumption in the New Zealand Southern Alps.  Geology, v. 31, p. 865-868).  Their area is a good choice because the New Zealand Alps are actively rising, precipitous and drenched with continual heavy rain and snowfall. Moreover, they offer something that the Andes and Himalaya do not; the rocks are pretty uniform.  What they find will not please Raymo’s followers.  As in many mountain ranges, mechanical erosion favours carbonate weathering over that of the CO₂ sequestering alteration of silicates.  With a low ratio of  silicate:carbonate chemical weathering, mountain building in New Zealand does draw down carbon dioxide, but only by a factor of about 2.  They conclude that more stable areas with lower relief are more likely to affect climate.  Although chemical weathering in them is lower than in mountains, that of silicates is far higher than for carbonates.  Moreover, active mountain ranges are minuscule compared with the extent of more subdued land.  It seems likely from Jacobson and Blum’s findings that the major control of weathering over climate depends to a large degree on where continents are located relative to warm, humid climatic zones.  For much of the early Cenozoic, the dominantly crystalline Precambrian shields of India, Africa, Australia and South America straddled the Equator, and witnessed intense weathering.  Maybe that relationship helped draw down carbon dioxide, and gradually cooled the planet from the hot and humid climate of the late Mesozoic.

Ten years ago the records of climate proxies from the Greenland ice sheet set new benchmarks for understanding how climate has varied over the last 100 thousand years – annual ice layers allowed division of that data to as fine as decades.  Variations in the ice cores helped explain many of the variations found in more blurred data from sea-floor sediment cores in the Northern Hemisphere.  Variations could be correlated with changes in the formation of North Atlantic deep water at high latitudes and the destabilisation of North American and Scandinavian glaciers.  The whole hemisphere behaved in concert, through long-distance connections in climatic processes, but high-latitude processes seemed to dominate.  Development of ²³⁴U/²³⁰Th dating extended high precision to carbonates that have been precipitated from groundwater to form stalagmites or speleothem.  The latest results from speleothem, collected on the Indian Ocean island of Socotra, cover 14 thousand years between 56 and 42 ka, and resolve down to only 8 year intervals (Burns, S.J.  et al. 2003.  Indian Ocean climate and an absolute chronology over Dansgaard/Oeschger events 9 to 13.  Science, v. 301, p. 1365-1367).  They show variations in rainfall on the island, though the d¹⁸O proxy, and thus changes in the strength of the Indian Ocean monsoon.  In terms of shape, the stalagmite record closely resembles d¹⁸O changes in the Greenland ice cores, although the two have opposite senses, because the Greenland proxy is for air temperature above the ice cap.  During the frigid Heinrich events that saw massive southward waves of icebergs, rainfall over Socotra was low.  It became higher as high-latitude conditions warmed in Dansgaard-Oeschger events.  The fine speleothem resolution shows a dramatic change-over that took only 25 years or so.  The explanation is that warmer conditions increased equatorial evaporation from the oceans.  But water vapour is the dominant “greenhouse” gas, and a wetter atmosphere would become warmer.  So the question of whether low- or high latitudes drove the changes is still an open one.  If North Atlantic events were the driver, then the tropical processes would greatly amplify their effects.  One big problem emerges from the joint research by US, Swiss and Yemeni scientists.  The highly reliable U/Th dating gives ages for each event that are about 3000 years older than those interpreted from the ice cores.  The authors are convinced that the ice-core ages need revision, yet there are discrepancies with the event-ages from other similarly dated speleothems.  Commenting on the paper, Frank Sirocko of Johannes Gutenberg University of Mainz in Germany (Sirocko, F. 2003.  What drove past teleconnections.  Science, v. 301, p. 1336-1337) makes the point that maybe the quality and age of ice core records lie behind the widely accepted view that high-latitude process drive climate.  He presents an excellent global image of modern sea-surface temperatures that show the main oceanic shifts of energy – the leakage of cold circum-Antarctic waters northwards, the westward movement of equatorial warm waters to which the El Niño – Southern Oscillation (ENSO) is due, and the unique movement of warm water to Arctic regions in the North Atlantic that is connected to deep water formation.  To that he adds the major effect of continental winter snow cover in central Eurasia, that affects albedo and the size of the winter high-pressure zone there.  Is there a teleconnection between that and events in the North Atlantic?  Nobody knows, because there are no data to compare, yet.  Another uncharted but likely linkage is between the ENSO and processes in the circum-Antarctic current.  Using currently accepted dating of ice cores, records from those in the Antarctic show air temperature changes that precede those from Greenland by several thousand years.  In that respect, the Socotra record possibly has a link with the South Polar climate.  Until the issue of dating is sorted out, it will always be difficult to make concrete statements about global climate change.

Interestingly, in the same issue of Science, sea-floor data (between 9 and 16 ka) from the Cariaco Basin off Venezuela, at about the same latitude as Socotra, mimic the Greenland records to within 30 to 90 years (Lea, D.W. et al, 2003.  Synchroneity of tropical and high-latitude Atlantic temperatures over the last glacial termination.  Science, v.  301, p. 1361-1364).

“Greenhouse” controls challenged

There’s data gathering and there’s theorising.  In palaeoclimate studies the two come into conflict.  Theory suggests that CO₂ is likely to be the principal driver for climatic ups and downs, probably on all time scales.  Atmospheric CO­₂ estimates from the past are based on proxies of different kind, and the various models that they support do not tally vary well.  Worst of all they do not fit climate records through the Phanerozoic at all well, except in the crudest possible way.  Only the long-lived Carboniferous to Permian “icehouse” and Tertiary cooling tally, and then only in Berner’s GeocarbIII model.  One of the best records of major climate shifts, aside from continental tillites, are marine sediments that contain ice-rafted debris, in particular the palaeolatitudes to which they extend.  They record four major cooling episodes: Late Ordovician; Devonian to Late Permian; Late Jurassic to Mid Cretaceous; and those since about 35 Ma ago.  The oxygen isotope record from Phanerozoic fossils, partly correlated with ocean temperatures also suggest 4 global coolings in the last 545 Ma.  Either the CO₂ modelling needs more detail, or the whole issue of the “greenhouse” effect is under question.  That is the conclusion of a study by Nir Shaviv of the Hebrew University of Jerusalem, and Ján Veiser of the Ruhr University and The University of Ottawa (Shaviv, N.J. & Veiser, J.  2003.  Celestial driver of Phanerozoic climate?  GSA Today, Huly 2003, p. 4-10).  Veiser has been analysing the chemistry of carbonates, especially their oxygen isotopes, for his 30 year career, and has amassed more data than any other geochemist on carbonate-related issues.  The two have worked together because their interests fit together extremely well.  Shaviv has reconstructed the variation of cosmic ray flux from studies of the exposure of iron meteorites to them, blended with analysis of how the Solar System moves through the spiral arms of our galaxy.  Cosmic rays are known to affect the Earth’s cloudiness and therefore albedo.  Greater cosmic ray flux should increase the amount of solar energy reflected away by the Earth, thereby causing global cooling.  The degree of fit between the cosmic ray flux and palaeoclimatic records is so good that up to 2/3 of climate variation may be connected with the Earth’s celestial position.  That is, as it passes through the star-rich spiral arms cosmic rays intensities go up.  This happens every 140 Ma or so, which fits very well with the 4 icehouse periods during the Phanerozoic.  They even suggest that the climate-CO₂ relationship may be the opposite of that generally agreed; climate might drive carbon dioxide levels.  A secondary role for “greenhouse” gases wreaks havoc on attempts at modelling climate change feared to result from increasing anthropogenic releases.  Shaviv and Veiser’s work comes at a particularly awkward time for climate modellers, who have just initiated a programme for  running huge simulations by corralling the combined computing power of millions of home PC users, similar to the approach pioneered by the SETI Institute (Allen, M.R.  Possible or probable.  Nature, v. 425, p. 242).  Perhaps the view of Phillip Stott, that climate modelling is a complete waste of time (Stott, P. 2003.  You can’t control the climate.  New Scientist, 20 September 2003, p. 25) might sink in as a result of the possible link between cosmic ray flux and climates of the past.  Stott believes that acting on the output of such models might perhaps even be dangerous, since we clearly do not understand short-term climate change well enough.

Precambrian CO₂ levels

Whether or not fluctuations in the “greenhouse” effect drive climate change, the fact remains that CO₂, methane and water vapour all act to retain solar heat in the Earth system.  Were it nor for their presence in the atmosphere, the Earth would be about 33 degrees colder than it is.  It would be covered by ice.  Theoretical modelling of how stars evolve suggests that the Sun had progressive less energy output going back in Earth’s history.  Only gaseous heat retention could have prevented a sterile, frigid planet.  Yet periods of cooling sufficient to hold large amounts of water in surface ice have occurred only a few times, 4 in the Phanerozoic, a flurry of so-called “Snowball” epochs in the Neoproterozoic and the earliest known glaciation around 2200 Ma ago.  The earliest coincided with the first evidence for free oxygen in the atmosphere, and may have been caused by that.  Methane, a more powerful “greenhouse” gas than water or carbon dioxide and abundantly produced by anaerobic decay, is easily oxidised.  In later time, it has been ephemeral in the atmosphere, unless continuously released, for instance by destabilisation of gas hydrate in sea-floor sediments.  Warming by CO₂ has undoubtedly kept total frigidity at bay since then.  The problem is charting just how much was in the air, because most estimates have been based on studies of palaeosols that give odd and very imprecise results for the early Palaeozoic (see Shaviv and Veiser, 2003; previous item).

Photosynthetic organisms derived their carbon from CO₂, either in the air or dissolved in water through equilibration with the atmosphere.  The extraction favours lighter ¹²C, so biological activity results in their products being depleted in the heavier ¹³C by about 25 parts per thousand (‰) relative to carbon in air and water.  If organic carbon becomes buried, the remaining carbon in the surface environment gets richer in ¹³C, and that signature becomes fixed in contemporaneous carbonates, both organic and inorganic.  It is therefore possible to use the two carbon-isotope signatures to estimate the reservoir of CO₂; its proportion in contemporary air. However, the degree of fractionation depends on the specific carbon metabolism of different organisms, yet most organic carbon in sediments is a mixed product of widely differing life styles.  That severely blurs estimates of atmospheric carbon dioxide content.  What is needed are data from a single source with known metabolism.  Acritarchs are fossil remains of single-celled marine eukaryotes that were, and still are, marine photosynthesisers.  They are made of degraded hydrocarbons.  Advanced ion-microprobe resolution is now sufficient to produce carbon-isotope measurements of individual fossils (about 200 micrometres across).  Sediments from northern China, roughly 1400 Ma old, contain abundant little-altered acritarchs and carbon isotope data from them give good estimates of atmospheric CO₂ levels, that are independent of other methods (Kauffman, A.J. & Xiao, S. 2003.  High CO₂ levels in the Proterozoic atmosphere estimated from analyses of individual microfossils.  Nature, v.  425, p. 279-282).  The estimates suggest between 10 to 200 times higher contents than today, but just about sufficient to keep the Earth above the limit of glacial temperatures when solar luminosity was about 88% of the present.  Acritarchs are present throughout the Neoproterozoic, and it should prove possible to examine the critical periods of “Snowball” conditions using this method.

The main driver for biological activity in the oceans far from land is the availability of iron, and this helps control the burial of organic carbon and hence aspects of global climate.  At low Fe concentrations, as they have been since the oxygenation of the surface environment from 2 billion years ago, iron is cycled in the marine environment in a matter of a few hundred years.  So, ocean water responds very quickly, in geological terms, to changes in the source of any dissolved iron.  There are two main sources, discharge of hydrothermal fluids from the oceanic lithosphere and delivery of river water and dust derived from the continents.  Of the last, riverine sources probably end up in near-shore sediments and only dust contributes significantly to deep ocean water.  The slowly growing nodules and crusts, composed mainly of iron and manganese compounds, on the ocean floor can chart variations in the relative proportions of these sources, because their growth produces zonation.  Measurements of d⁵⁶Fe in various materials show that the two sources are different in isotopic composition (Beard, B.L. et al. 2003.  Iron isotope constrains on Fe cycling and mass balance in oxygenated Earth oceans. Geology, v. 31, p. 629-632).  While continent derived materials exude iron that is essentially the same as that in terrestrial volcanic rocks (d⁵⁶Fe ~0.0‰), ocean-floor hydrothermal activity is significantly depleted in ⁵⁶Fe (‰⁵⁶Fe ~ -0.38‰).  From 6 Ma to 1.7 Ma iron-manganese crusts record iron with a dominant hydrothermal origin, but during the glaciation-dominated period since 1.7 Ma the contribution of continent-derived dusts becomes overwhelming – cooling forces drying on a global scale.  Because hydrothermal contributions probably stay much the same over very long periods, because of the sluggishness of plate tectonics, iron isotopes in deep marine sediments, such as Fe-Mn crusts,  may be important tracers for glacial events in the distant past, such as the glaciations during the Neoproterozoic and Palaeozoic. Interestingly, the largest iron-rich deposits on the planet, the BIFs that peaked during Archaean and Palaeoproterozoic times, record far larger excursions in iron isotopes than any other.  The very low d⁵⁶Fe values of some BIFs (down to – 2.4‰) probably signify the dominance of sea-floor sources, although a non-oxidising atmosphere would have mobilised dissolved iron from the continents too, which explains the range in BIFs up to +1.0‰.

The gas-hydrate “gun”

As fears of anthropogenic climate warming have risen, so more geoscientists have looked in detail at the stratigraphic record for signs of past warming, and funds have become more targeted towards palaeoclimatology.  One of the most important discoveries was that the end of the Palaeocene, about 55 Ma ago, was a time of sudden global warming during the overall cooling that has characterised the Cenozoic.  The first sign that something strange had happened then came from using the oxygen isotope geothermometer on plankton tests from marine drill core that passed through the boundary.  There seemed to have been a 7º C jump in surface seawater temperature.  An explanation for the thermal spike arose after carbon isotopes revealed a coincident spike in the lighter ¹²C.  Periods of low primary biological production can impose such anomalies, because photosynthesis selectively binds light carbon in carbohydrate.  However, some of that light carbon ends up buried in sea-floor sediments, so another explanation for a negative excursion in d¹³C is that organic carbon has somehow been released from sedimentary storage to the atmosphere.  So, either there was a sterile ocean or a massive release of organic carbon at the Palaeocene/Eocene boundary.  Some kind of erosion to achieve the second possibility could not have led to such a speedy shift in carbon isotopes.  The accepted explanation, suggested in 1995, stemmed from organic carbon that had been metabolised by methanogen bacteria in anaerobic sea-floor sediments to form methane.  Given low enough sea-bottom temperatures and sufficient pressure, methane can crystallise with water to form an icy substance, known as gas-hydrate or clathrate, in sea-floor sediments.  Being an unstable compound, gas hydrate can break down rapidly if seafloor temperature rises or sea-level falls.  And, of course, the methane can rush to the surface as bubbles.  Being 4 times more efficient than carbon dioxide at trapping thermal radiation emitted by the Earth’s surface, methane releases are excellent explanations for sudden warmings in the stratigraphic record.  And there is a great deal of methane locked as gas hydrate beneath the sea floor, about 2 teratonnes (2 x 10¹² t).  Quirin Schiermeier reviews the basic concept (Scheiermeier, Q. 2003.  Nature, v. 423, p. 681-682), but poses the question of how methane-induced warming is reversed.  Methane is quickly oxidised to CO₂ in the atmosphere, so lessening its warming effect.  So a “spike” that lasts thousands of years has to be fed by continual releases.  Since warming drives gas hydrate breakdown, something must intervene to stop the releases before the warming becomes a “runaway greenhouse”.  One view, and probably the correct one, is that warmth and more CO₂ drives up biological activity so that the increased atmospheric carbon is “pumped” down by living processes, back to sedimentary burial.  If sufficient nutrients are available, there is no way of stopping this negative feedback until a balance is restored.  Schiermeier reports that new ocean drilling plans to test the hypothesis that the Palaeocene/Eocene warming accelerated continental erosion, which was able to wash the crucial nutrients phosphorus and iron into the oceans.  Experiments have shown that increased iron in ocean-surface water far from land – now pretty sterile because it is iron-deficient  – sparks up photosynthetic plankton.  That is one possible way of artificially drawing down anthropogenic CO₂.  The problem is, if such a process was involved in cooling the Eocene Earth, it took about 100 thousand years.

Red Sea record links to northern hemisphere climate

In his forthcoming book, Out of Eden: the Peopling of the World (Constable and Robinson, July 2003), Stephen Oppenheimer offers the novel suggestion that fully modern humans left Africa by island hopping on log rafts across the Straits of Bab el Mandab, which connects the Red Sea to the Indian Ocean.  The rationale to his suggestion is that sea-level falls during major glaciations would have partially exposed the shelf that lies beneath the Straits, presenting a route to SW Arabia across only 18 km of island-dotted sea. As today, it would have been impossible to trek across the deserts of the Middle East after a northward African migration along the Nile, without chains of wells.  His thesis then sees humans migrating along coasts eventually to reach east Asia at about 70 ka.  Precisely when the Straits of Bab el Mandab became shallow enough would have been determined by global climatic conditions, for only glacial maxima result in sufficient sea-level falls for such island hopping to be possible. 

The shallowing of the shelf across the southern outlet of the Red Sea would have had a profound impact on seawater circulation.  Already having restricted connection to the world’s oceans, Red Sea water has elevated ¹⁸O levels, because evaporation from it favours loss of lighter ¹⁶O.  With more restricted circulation, evaporation would have driven this up further.  Geoscientists from the Universities of Southampton, Tuebingen and Göttingen, and the Geological Survey of Israel have analysed the variation in oxygen isotopes of foraminifera from a Red Sea core to quantify ups and downs in  sea level in more detail than possible from open-ocean cores, which have uncertainties of about ±30m) (Siddall, M. and 6 others 2003.  Sea-level fluctuations during the last glacial cycle.  Nature, v. 423, p. 853-858).  The method that they used models the effects on Red Sea oxygen isotopes of evaporation and changed circulation to estimate how the depth of the Straits of Bab el Mandab changed.  They claim a precision of ±12m.  Through the period from 70 to 20 ka, leading up to the last glacial maximum, their sea-level record tallies nicely with climate records from both Antarctic and Greenland ice cores, including shifts linked to the short-lived Heinrich and Dansgaard-Oeschger cycles. During the last glacial maximum(18-20 ka), sea-level fell by almost 120 m, so that the Straits of Bab el Mandab were on average only 15 m deep.  The first human Exodus out of Africa to populate Eurasia would have been between 120 to 130 ka, as suggested by Oppenheimer, when sea level probably fell a little further.  However, at about 65 ka, sea level dropped to about 100 m below modern levels, perhaps presenting another window of opportunity.

Broecker reviews climate triggers

Wallace Broecker, of the Lamont-Doherty Earth Observatory at Columbia University, was the first to quantify in 1975 the 19^th century prediction of Svante Arrhenius that increasing atmospheric carbon dioxide would drive up global temperatures.  Broecker’s early work lies at the centre of concern about global warming, and his subsequent contributions are enmeshed with the entire study of past climate change.  A review by him of current ideas on palaeoclimates of the recent past is therefore compulsory reading, for all geoscientists (Broecker, W.S. 2003.  Does the trigger for abrupt climate change reside in the ocean or in the atmosphere?  Science, v. 300, p. 1519-1522.  As well as the astronomically connected cyclicity that is apparent in all kinds of climate record through the Pleistocene, those records are punctuated by sudden, short-lived phenomena, whose magnitudes and pace are sufficiently dramatic to focus attention on processes that are probably entirely terrestrial.  Foremost among these during the last glacial interglacial cycle are the astonishing coolings of Heinrich’s iceberg armada events and the possibly catastrophic (in a human as well as an ecological sense) Younger Dryas, which reversed warming from the last Glacial Maximum, and the equally sudden warmings associated with Dansgaard-Oeschger events.  Broecker’s review focuses on the two mechanisms that have been suggested to underlie these overturns.  One links such changes to shifts in whole-ocean water circulation, especially the ons and offs of deep-water circulation beneath the North Atlantic, the other to perturbations of the way in which atmosphere and ocean interact in the tropics.

An entirely plausible scenario for climate-driving changes in North Atlantic water circulation is flushes of freshwater from the surrounding continents, so that formation of sea ice leaves residual water that is not saline or dense enough to sink and drag in water from lower latitudes.  The problem is that the complete thermohaline cycle, which impacts on global atmospheric circulation, has a period longer than the changes that might be induced by its perturbation in the North Atlantic.  Tropical atmosphere-ocean dynamics are the largest elements in global climate, in terms of the energy and mass that are shifted, so they are a natural candidate for a driving mechanism.  Tropical climate shifts abruptly today in well-known ways, most important being the El Niño-La Niña cycle.  There is no ponderous underlying dynamic that would damp down connections between cause and global effect, and prevent sudden climate change.  Yet, some kind of “flywheel” is essential to keep long-term cyclicity going and lock sudden changes into century to millennium-long climate “states”, which should rapidly decay if effect rebounded on cause, as it does in the case of El Niño-La Niña.  Broecker covers all the critical evidence that has borne on both hypotheses up to now.  His conclusion is interesting.  Both hypotheses are very much model led, and in need of as much empirical support as can be had.  Yet, and here is the nub, the crucial data are those bearing on correlating times of events that are recognised all over the place.  Time resolution is of the greatest importance, since climate transitions are fast; faster in fact than we can presently resolve before historical times.  It is entirely likely that suitable resolution of times past may be absolutely impossible.  Both hypotheses have a lot of empirical and theoretical support.  So, what is the problem of combining them in a cunning way?  Partly, that may be because reductionism (controlling a few variables and looking for developments in another simple set) still plagues science.  That is odd in climatology, where all motions and energy changes palpably relate to one another, with no control of a rational kind.  Reductionism demands ever more staggering computing power and speed, to “keep all the eggs in the air”.  There is always the feeling, as Jimmy “Shnozzle” Durante observed in his musical monologue, The Man Who Found The Lost Chord, that if you find a hitherto overlooked connection, then everything goes well; if you can remember it!  Broecker suggests that the missing connection must “transmit” from deep ocean water to tropical atmosphere.

Tropical rainforest in Africa and South America is the most diverse biome on the planet, both as regards plants and animals.  One view of how such luxuriance arose is that the forests have blanketed the humid tropics for as long as 50 or 60 million years, and the fact that they encompass a huge variety of environments created by different levels in the dominant and diverse vegetation.  Thousands of niches and the interactions between organisms that exploit them during lengthy stasis inevitably drives rapid evolution towards all kinds of specialisation.  The other view is that rainforests are by no means static over millions of years, but climate shifts have caused them to retreat and advance, perhaps hundreds of times during the Cenozoic.  Amazonia in particular shows surprising variation in diversity, some patches being far more biologically rich than others, and having regionally distinct assemblages of plants and animals.  This theory suggests that climatic stress, probably drying associated with globally cool episodes, resulted in rainforest shrinking to “refugia”.  In them, populations of plants and animals shrank, thereby reducing the gene pool and giving greater chance for evolution by natural selection; different in different refuge areas.

Tropical soils are continually reworked and their highly oxidising nature destroys organic remains.  So no record of its development exists in rainforest.  However, wind and rivers transport spores, pollen and other biomarkers to seafloor sediments, where a complete record of fluctuations in biomass and diversity becomes preserved.  A test of the popular refugia hypothesis is therefore to analyse organic matter in continuous cores taken from offshore sediment.  Known fluctuations in global climate, from the oxygen isotope record should be matched by changes in the record of terrestrial biomarkers carried to the sea.  Cores from the deep-sea sediment fan off the mouth of the Amazon potentially provide such a test (Kastner, T.P. & Goñi, M.A. 2003.  Constancy in the vegetation of the Amazon Basin during the late Pleistocene: Evidence from the organic matter composition of Amazon deep sea fan sediments.  Geology, v. 31, p. 291-294). Kastner and Goñi, from the University of South Carolina, examined phenols and organic acids in the cores, which can discriminate between grassy plants and trees that would have dominated savannah and rainforest, whose relative cover of the Amazon basin should have changed, according to the refugia hypothesis, as climate shifted from globally cool-dry to warm-humid..  Although their record only spans the last glacial cycle since 70 ka, they detected no significant change in the proportion of grasses and trees in the Amazon catchment.  Moreover, the biomarkers remained similar to those carried by the Amazon today, right through the last glacial maximum, when drying of the tropics would have been most likely to have driven a shrinkage of rainforest area.  It seems unlikely that forest refugia developed during one of the most extreme climate shifts in the last 55 Ma.  Global climate fluctuations were considerably less before 1 million years ago, when the current round of 100 ka cycles began.  So there is little reason to doubt that the Amazon rainforest has had a more or less constant area for much of the Cenozoic.  The same cannot be said for those in Africa and SE Asia, partly because there are no useful data from offshore sediments, but also because those regions have experienced changing topography due to major tectonic activity, whereas eastern South America has remained stable.  To conclude, as the authors do, that the data signify no great fluctuation in rainfall is not so certain.

There is a large body of opinion, supported by plenty of circumstantial evidence, that the end of the last glacial maximum around 20 ka was controlled by processes that operated in the North Atlantic and its seaboard.  A favoured mechanism is the re-establishment of thermohaline circulation involving North Atlantic deep water that dragged surface water northwards from the tropics, to set up the Gulf Stream.  Temporary shut-down of thermohaline flux, probably by massive release of freshwater to the North Atlantic from melting of ice sheets, is widely understood to have triggered the sudden reversal to frigid conditions in the Younger Dryas around 11.5 ka.  The largest warming pulse in the northern hemisphere, between 14.6 to 14.0 ka, is recorded by a sudden increase in d¹⁸O of ice in the Greenland cores, and is known as the Bølling-Allerød warm interval.  Around that time, sea level rose by 20 m in a few hundred years, and that involved production of fresh glacial meltwater at a rate equivalent to the continual flow of five rivers the size of the Amazon.  Such rapid sea-level rise drowned coastlines and in some areas killed coral reefs.  On such drowned reef in the Caribbean gave a date of 14.2 ka, which since 1989 has been the only indicator of precise timing for the massive influx of meltwater to the oceans.  The date is within the Bølling-Allerød, hence the link between warming and events around the North Atlantic.  That central hypothesis is now under threat, following the dating of drowned coral reefs on the Sunda Shelf at 14.7 ka, and a re-evaluation of the Caribbean data. (Weaver, A.J. et al. 2003.  Meltwater pulse 1A from Antarctica as a trigger of the Bølling-Allerød warm interval.  Science, v. 299, p. 1709-1713).

Using the revised ages and climate modelling, Andrew Weaver and colleagues from the Universities of Victoria and Toronto, Canada and Oregon State University see the massive ice-melting as the precursor to the Bølling-Allerød warm interval and deglaciation of lands around the North Atlantic.  A more plausible source of freshwater influx is a major melting event in Antarctica, so warming in the south may well have driven that of the northern hemisphere.

See also: Kerr, R.A. 2003.  Who pushed whom out of the last ice age.  Science, v. 299, p. 1645.

For about a decade it has been suggested that the Tibetan Plateau, which rises to more than 5000 metres, has a profound effect on climate.  This may be partly due to the way such a high and enormous area deflects regional wind patterns, but largely to its profound interconnection with the South Asian monsoon.  When such a circulation barrier arose is critical to understanding how it relates to climate evolution in the latter part of the Cenozoic.  There are various suggestions, based on aspects of its structural and magmatic evolution.  Theory suggests that the southern part came into being in Eocene times, possibly because a segment of the lithosphere beneath broke off to subside into the mantle – there are volcanic rocks whose chemistry does suggest such a mechanism.  About 8 Ma ago the southern Plateau began to spread laterally, producing a series of N-S extensional basins, which suggests that by then sufficient gravitational potential had accumulated to make the thickened crust unstable.  About that time various signatures arose in foraminifera of the Indian Ocean and sediments derived by erosion, which suggest that the monsoon increased in intensity.

When the Plateau attained sufficient elevation above sea level to start spreading sideways and affect atmospheric circulation largely rests on these theoretical judgements.  For the ideas to firm up needs some means of estimating topographic elevation, which is not easy to do.  One way is to use plant remains that can give clues, either because the species involved are sensitive to elevation today, or the morphology of their leaves shows signs of physiological adaptation to elevation.  The first is ruled out in old sediments, simply because the species present are now extinct..  Plants metabolism is dependent on diffusion of water and CO₂ into their leaves during photosynthesis, and features, such as stomata density, give clues to the conditions for such diffusion.  Luckily, sediments from southern Tibet do contain well-preserved plants, and a multinational group led by Bob Spicer of the British Open University have attempted to assess palaeo-elevation for the time at which they were deposited (Spicer, R.A. and 7 others 2003.  Constant elevation of southern Tibet over the last 15 million years.  Nature, v. 421, p. 622-624).  Their method relies on linking leaf morphology to a property of the atmosphere, known as moist static energy (MSE), through estimates of atmospheric enthalpy from the leaves.  That is not the end of the estimation, because MSE needs to be related to elevation and the only way is to use climatic modelling for the past.  Whatever, Spicer and colleagues reckon that 15 Ma ago their sampling site was more or less at the same elevation as today, around 4.5 km above sea level.  If true, they have established that the south part of the Plateau was already in existence during the Middle Miocene.  Being so convoluted, despite its apparent precision, the leaf analysis method does need independent confirmation.  There is a much easier and arguably more reliable method, based on the change in the size of bubbles formed by gas escaping from lavas, according to atmospheric pressure (see Cunning means of estimating uplift in November 2002 issue of Earth Pages News).  There are lavas in southern Tibet that date from Cretaceous times, including some about a million years younger than the plant remains.

Precambrian warmth and methane

Methane is a more efficient “greenhouse” gas than CO₂, but it soon oxidises in the presence of oxygen.  During the Phanerozoic there have been several massive releases of methane, probably from gas hydrates in deep-ocean sediments, which produced warming spikes that decayed away quickly in geological terms.  Before there was much, if any, oxygen in the atmosphere, methane could linger and add to the retention of heat by carbon dioxide and water in the atmosphere.  One of the longest running disputes in environmental geochemistry concerns when oxygen levels became significant in the Precambrian, and what they were compared with later times.  Whether the Earth was warm or cold has a bearing on this.  Cosmological theory suggest that stars similar to the Sun progressively grow more energetic with time.  Without some kind of greenhouse effect, the Earth would have been condemned to frigidity from its outset.  Even today, with a more radiant Sun, only atmospheric retention of solar heat keeps overall temperature from being well below freezing.  The further back in time, the greater the “greenhouse” effect would have to have been to stave off complete ice cover and a runaway “icehouse”.  Methane almost certainly played a part in this once methane generating organisms evolved, up to about 2200 Ma, when there are signs (continental redbeds and soils rich in iron oxides) that atmospheric oxygen was appreciable.  However, warmth prevailed for about 1.5 billion years thereafter, until the plunges into frigid conditions of the so-called “Snowball Earth” period from about 700 to 550 Ma.  Somehow, the greenhouse effect lingered.

Alexander Pavlov of the University of Colorado, and colleagues from Pennsylvania State University have addressed the implications of this continued warmth in terms of maximum oxygen levels needed to avoid complete oxidation of methane releases (Pavlov, A.A. et al. 2003.  Methane-rich Proterozoic atmosphere?  Geology, v. 31, p. 87-90).  Today, more than 90% of all methane production beneath the ocean floor is consumed by bacteria, depending on the amount of dissolved oxygen and sulphate ions (for aerobic and anaerobic methanotrophs).  There is plenty of evidence that deep Precambrian ocean water was anoxic, so a great deal more methane would have emerged from them.  That it was also poor in sulphate ions is shown by their low levels in solid solution with carbonates and Proterozoic sulphur isotopes in marine sediments.  The authors argue that this signifies low atmospheric oxygen levels, around 5 to 18 percent of modern concentrations.  The scene may have been set for an excess of methane production over its oxidation, thereby keeping the “greenhouse” warming above the levels when glaciation would have been widespread..  If so, something completely upset this balancing act in the Neoproterozoic, to drive down temperatures several times – the “Snowball Earth” events.  The trigger may have been a boost in oxygen production and retention in the atmosphere.

El Niño in the Eocene

The oceanographic-climatic phenomenon in the equatorial Pacific, known as the El Niño-Southern Oscillation (ENSO), now seems to be major force in driving climate shifts far afield, such as the current drought in the Horn of Africa.  Its cyclicity relieves the suffering brought by El Niño events, yet the processes may well be highly unstable.  Some believe that it is only a matter of time before ENSO reverts to a permanent El Niño condition, with disastrous consequences.  Such a stabilisation in the past may have resulted in warming at high latitudes that permitted lush vegetation in near-polar regions, during the Cretaceous and the Eocene.  The Eocene was much warmer than now, as a result of a massive release of methane from seafloor sediments around 55 Ma.  So it makes sense to look at its climate record to check for a permanent El Niño.  Matthew Huber and Rodrigo Caballero of the University of Copenhagen have compared climate records from annually layered lake sediments from the Eocene of Germany and Wyoming in the western USA with climate models to test the hypothesis (Huber, M. & Caballero, R. 2003.  Eocene El Niño: Evidence for robust tropical dynamics in the “hothouse”.  Science, v. 299, p. 877-881).  The climate data from the lake sediments (thickness variations in annual layers) show clear signs of a roughly 5-year cycle of climate change, attributed to an Eocene ENSO.  This tallies nicely with simulations for the Eocene continent-ocean set-up.  Although the authors claim that their findings refute the hypothesis that global warming tends to shut down ENSO, which is a comforting thought, Eocene ocean and air circulation was not the same as now by any means.  There have been interglacial periods during the Pliocene to present climate system in which temperatures exceeded those of the Holocene.  Surely, annually layered sediments from those times will provide a better test.

Records of seawater oxygen isotopes and its Ca/Mg ratio shows that a substantial permanent ice sheet first formed in Antarctica in the Oligocene Epoch, about 34 Ma ago.  The favoured explanation, until this month, was that the South polar continent became thermally isolated from the rest of the planet when circumpolar currents were able to flow around it, once South America and Australia had separated from Antarctica and opened the “gateways” of the Drake and Tasmanian Passages.  But what if atmospheric CO₂ played a role?  A drop in the “greenhouse” effect and global cooling could have driven polar temperatures low enough for ice formation without an oceanographic influence.  Once established, the albedo effect of a large ice sheet would seal Antarctica into permanent freeze-up.  Factoring all the likely components in a general circulation model leads to a surprise (DeConto, R.M. & Pollard, D. 2003.  Rapid Cenozoic glaciation of Antarctica by declining atmospheric CO₂, Nature, v. 421, p. 245-249).  The opening of the Drake and Tasmanian Passages was not accompanied by a sufficient depth of water to support massive current reorganisation until several million years after the ice cap left its clear imprint on the marine record.  DeConto and Pollard’s model shows that even with closed Passages an ice cap would have formed, if CO₂ levels had fallen below three times those that prevailed in the Holocene, before industrial emissions began.  Global cooling had begun somewhat earlier than Antarctic freeze-up, following the high around the Palaeocene/Eocene boundary (~55 Ma), falling to a plateau about 40 Ma ago.  Undoubtedly CO₂ concentrations had fallen globally for this to have happened.  Of course, there is no Oligocene ice, from which glaciologists might extract trapped bubbles and samples of ancient air with which to refute or confirm the model.  However, a decrease in carbon dioxide would also cause the acidity of rainfall to decrease as well as the amount of rainfall globally, and that might show up in changed weathering processes, especially in the tropics of the time. 

How patterned ground forms

Visiting flat areas of permanently frozen ground brings you face to face with truly bizarre patterns at the ground surface.  Some are perfect hexagons of stones around finer soils, others doughnut-like circles and then a perplexing range of other features that look for all the world as though they were built by humans.  Undoubtedly, they result from the forces at work when the top soil layer freezes and thaws annually, together with soil creep down extremely shallow slopes, repeated over millennia.  However, exactly how the patterned ground develops has eluded geomorphologists for more than a century.  Rejecting the reductionist approach that any landform’s evolution can be deduced from basic principles of physics seems to be the key (Kessler, M.A. & Werner, B.T. 2003.  Self-organization of sorted patterned ground. Science, v. 299, p. 380-383).  Kessler and Werner of the University of California modelled the two likely processes of ice lensing that sorts stones and finer soil, and the transport of individual stones along the lines of accumulated stones as freezing fines expand, building in elements of spatial and time scales plus other parameters such as surface slope.  Their model is self-organising, and proceeds to mimic many of the intricacies of patterned ground, even the most labyrinthine.  It might seem a little heavy handed to crunch numbers to help explain what are really quite minor features.  But having demonstrated the power of non-linear modelling here, the authors open up a novel approach to landscape evolution of every scale and antiquity.

The curious mix of water ice and methane, known as gas hydrate or clathrate, which is stable at ocean depths greater than 300 m, is one of the largest potential components of the active carbon cycle (~10¹³ t).  Its methane content stems from bacterial breakdown of organic matter buried in anaerobic sea-floor sediments.  As well as being pressure sensitive, gas hydrate also has a narrow stability “window” as regards temperature.  Geothermal heat therefore limits the depth of gas-hydrate accumulations to a few tens to hundreds of metres below the seabed.  Its vast methane content is clearly something on which energy transnationals have an eye.  However, methane is almost four times more powerful as a “greenhouse gas” than CO₂ emissions.  Carbon-isotope studies from sedimentary rocks show signs that several times in the distant past methane was released catastrophically to the atmosphere, the timing coinciding with signs of rapid global warming.  The last major event of this kind was around 55 Ma ago, when the end of the Palaeocene Epoch witnessed an 8°C global temperature rise in a matter of a few thousand years (Thomas, D. et al. 2002. Warming the fuel for the fire: Evidence for the thermal dissociation of methane hydrate during the Paleocene-Eocene thermal maximum.  Geology, v. 30, p.1067-1070).  The warming “spike” eases because methane is quickly oxidised to water and CO₂ in the atmosphere, but that still allows abnormally warm conditions to linger.

Sonar surveys of the seabed, including that of the North Sea, reveal pits and funnels that probably mark sites of past methane releases from destabilised gas hydrates.  In theory, two general processes lead to their instability: falling global sea level that reduces the pressure on gas hydrates formed at shallow water depths; a rise in the temperature of ocean-bottom water.  The second could produce more widespread methane release than the first.  Refining these crude prognoses needs detail about the structure of gas-hydrate zones beneath the seabed.  Conventional seismic surveys conducted at the sea surface show the clathrate-rich zones just beneath the sea floor, but no detail.  Towing sources and receivers just above the seabed reveals intricate structures (Wood, W.T. et al. 2002.  Decreased stability of methane hydrates in marine sediments owing to phase-boundary roughness.  Nature, v. 420, p. 656-660).  Wood and co-workers from the US Naval Research Laboratory, the University of Victoria and the Pacific Geoscience Centre in British Columbia, Canada surveyed the Pacific floor off Vancouver Island.  Their most striking observation is of many vertical, chimney-like structures that puncture the gas-hydrate zone in the upper sediment layer.  They reckon that these structures are where methane and warm fluids find their way to the seabed; they are probably the expression in cross section of the surface pitting formed by past degassing.  They also may supply gas to the zone where it becomes locked in metastable water ice.  The sheer number of the “chimneys” indicates that the surface area of gas-hydrate stability is many times larger than previously supposed, as a result of their “roughening” effect.  Since the base of the gas-hydrate stability zone is most prone to the effect of warming of sea-bottom water, which shifts the geotherm slightly, an increase in its surface area, together with its closer approach to the seabed around the “chimneys”,  could further increase its sensitivity to small changes.  Up to now, many specialists have suggested that major methane releases resulted from sudden collapses of sea-floor sediments in tectonically unstable areas, such as the Storegga Slide off western Norway.  They may instead have been due to more widespread instability resulting from environmental change.  Since the largest pressure decreases due to sea-level falls accompanied glacial epochs, some clues to whether the “chimney” effect has had an influence may come from a fresh look at methane contents of trapped air bubbles in Antarctic and Greenlandic ice cores.  The extent to which methane releases might effect climate depends on how much is oxidised to CO₂ in sea water, before it can enter the atmosphere to enhance the “greenhouse” effect.  Little is know about such processes.

See also:  Pecher, I.A. 2002.  Gas hydrates on the brink.  Nature, v. 420, p, 622-623.

Palaeomagnetic data from localities famed for their Neoproterozoic glaciogenic rocks point persuasively to several epochs between 750 and 550 Ma when widespread continental glaciation took place at low latitudes.  It is this evidence, along with theoretical consideration of drastic changes in the Earth’s albedo that would result from tropical land ice, that encouraged the idea of pole to pole ice cover.  Only a build-up of volcanogenic CO₂ in the atmosphere could prevent such a “Snowball Earth” lasting indefinitely, and even with such relief it would have endured for millions of years.  Much of the geological evidence cited by those who support and promote this neo-catastrophic idea comes from excellent, but geographically quite limited occurrences of tillites or glaciomarine sediments, such as those of Namibia.  Some occurrences have never been seriously analysed, except as examples that superficially support the hypothesis.  One such sequence is that of Arabia, easily accessed in northern Oman and described by a British-Swiss team (Leather, J. et al. 2002.  Neoproterozoic snowball Earth under scrutiny: Evidence from the Fiq glaciation of Oman.  Geology, v. 30, p. 891-894).

Isotopic studies of carbonates from glaciogenic sediments (see Meltdown for Snowball Earth? in Earth Pages News for February 2002) seriously undermined several arguments by “Snowball Earth” supporters, but are open to various interpretations.  Hard geological evidence is less easy to rationalize.  A growing number of  Neoproterozoic glaciogenic sequences, such as the Port Askaig Tillite of the Scottish Dalradian Supergroup and others from the Congo and Kalahari cratons, and Laurentia, show dropstone-rich diamictites interbedded with sediments that show little if any sign of a glacial influence (Condon, D.J. et al. 2002.  Neoproterozoic glacial-rainout intervals: Observations and implications.  Geology, v. 30, p. 35-38).  Such evidence can be explained by climatic change and a fully functioning hydrological cycle.  The report on the Omani example by Leather and colleagues highlights splendid examples of sediments that mark cycles of glacial advance and retreat, reminiscent of those of the Pleistocene glacial epoch and more or less the same as in many Neoproterozoic occurrences.  It can only be a matter of time before Australian geologists enter the fray decisively, for glaciogenic sediments comprise up to 30% of the many-kilometres thick Umberatana Group in the Neoproterozoic of the Flinders Range in South Australia, and there are several other stratigraphically distinct diamictite sequences.

It seems likely that the “Snowball Earth” hypothesis is waning; an embarrassment for those geologists who have promoted it so assiduously over the last several years.  However, the enigma of low-latitude glaciation on a vast scale is likely to remain, unless, that is, all the diamictites can be shown to have non-glacial origins, which is not as unlikely as it might seem.  The Fiq sequence of Oman, like the Dalradian example in Scotland, formed in an actively extending basin.  Repeated seismicity on rift-bounding faults could have launched debris flows to deposit diamictites (a purely descriptive term for sediments containing a wide variety of clast sizes).  The most spectacular diamictite in the Dalradian Supergroup, and perhaps anywhere, is the Great Breccia of the Garvellachs.  Recent work suggests strongly that it is not glaciogenic, but the product of such a debris flow (Arnaud, E. & Eyles, C.H. 2002.  Catastrophic mass failure of a Neoproterozoic glacially influenced continental margin, the Great Breccia, Port Askaig Formation, Scotland.  Sedimentary Geology, v. 151, p. 313-333).  The supposedly clinching evidence for diamictites’ origin from iceberg armadas is the way in which some clasts (“dropstones”) puncture underlying stratification.  All that is required is a means of puncturing, and sediment compaction around large, resistant clasts in a water saturated matrix is quite capable of doing that.  Even the long-held belief that glaciation is uniquely signified by polished and striated surfaces beneath diamictites containing similarly scratched clasts is coming into question.  Sites of large impacts, such as the Ries crater in Germany, include exactly similar features caused by ejecta blasted from the crater, cited by Vern Oberbeck, formerly of NASA, in a little-cited paper that proposed an impact origin for diamictites (Oberbeck, V.R. et al. 1993.  Impacts, tillites and the breakup of Gondwanaland.  Journal of Geology, v. 101, p. 1-19).

Post-apocalypse weathering in the Early Triassic

Environmental crises do not come bigger than that at the end of the Permian, when marine ecosystems virtually collapsed, and similar extinctions of terrestrial flora and fauna are becoming clear.  Whereas the Siberian Traps may indeed have been a triggering mechanism, there are carbon-isotope indicators that vast amounts of methane entered the atmosphere shortly afterwards, rapidly being oxidised to CO₂.  The density of respiratory openings (stomata) in fossil leaves from the lowest Triassic is unusually low, indicating an abundance of CO₂ in the atmosphere and probably enhanced “greenhouse” conditions.  Hot and humid conditions encourage weathering of the continental surface, and there are many Early Triassic palaeosols, some which mimic those in the tropics being found at unusually high palaeolatitudes.  Such soils harbour crucial evidence for surface conditions, and the high-latitude ones present a surprise (Sheldon, N.D. & Retallack, G.J. 2002.  Low oxygen levels in earliest Triassic soils. Geology, v. 30, p. 919-922).  Unlike tropical laterites, which are rich in kaolinite, high-latitude soils are dominated by illitic clays that signify incomplete breakdown of silicates.  The surprise comes in the form of an unusual mineral, berthierine; a green, serpentine-like mineral that is easily confused with chlorites in hand specimen.  It can form by reaction between clays and ferric oxy-hydroxides, but only under highly reducing conditions.  Because most soils since about 2000 Ma ago have formed in contact with an increasingly oxygen-rich atmosphere, achieving suitably reducing conditions demands input of a reductant to the soil “atmosphere”.  The most likely candidate is methane, whose oxidation would consume oxygen.  However, methane’s residence time in the air is around 10 years, because it is quickly oxidised to CO₂, so methane release following the P-Tr boundary event seems as if it was sufficiently prolonged to influence considerably longer term soil formation.

Melting of low-latitude glaciers in Africa is so rapid that, unless they are cored soon, their content of long-term climate data may soon be gone forever.  So the first detailed isotopic record from Africa’s highest glacier on Kilimanjaro is cause for some relief.  Intrepid glaciologist Lonnie Thompson welded a large team together for this important task (Thompson, L. 2002. Kilimanjaro ice core records: evidence of Holocene climate change in tropical Africa.  Science, v. 298, p. 589-593).  The annually layered ice goes back only about 12 ka, but nonetheless gives a precious account of climate change at the heart of the continent, far more detailed than sparse lake-bed cores from various places.

The core confirms a broad pattern of warm, wet conditions from 11 to 4 ka, before the long-term cooling and drying of historical times.  These reflect likely weakening of monsoonal conditions in the late Holocene.  However, assigning precise ages to depth in the cores is not as easy as in those from high-latitude ice sheets, because of a lack of good layering (presumably) and dateable carbon.  At about 5200 years ago, the record shows an abrupt fall in d¹⁸O, a sign of drying and cooling that took place over perhaps a matter of decades.  This correlates with disruption of early civilisations in India, Egypt and the Middle East, and probably stemmed from cooling in the North Atlantic.  However, an equally rapid deterioration occurred around 6300 years bp, although not so extreme, to presage a millennium of arid conditions at the heart of Africa.  Important as these data are, the team’s estimates of current retreat rates of the Kilimanjaro glaciers are alarming.  Quite probably, the white cap of Africa’s highest mountain will have disappeared within the next 20 years.

Lonnie Thompson is obviously both keyed- and clued up about extracting climatic data from ice at high elevations.  So much so, that Science has printed a lengthy account of his exploits, mainly on low-latitude glaciers (Krajick, K. 2002.  Ice man: Lonnie Thompson scales the peaks for science.  Science, v. 298, p. 518-522

Earth Pages News  has commented several times on developments in the connection between ocean currents and climate, over the last 3 years.  The subject has many aspects, and these have been bundled and brought up to date in one of a series of review articles on the relationship between climate and the hydrological cycle in Nature’s occasional Insight series (Rahmstorf, S. 2002.  Ocean circulation and climate during the last 120,000 years.  Nature, v.  419, p. 207-214).  Stefan Rahmsdorf covers the evidence to date that implicates changes in deep circulation in rapid and dramatic climate shifts, such as changed air temperatures over the Greenland ice cap and iceberg armadas in the North Atlantic.  Another review outlines the longer-term perspective of links between atmosphere, oceans, ice sheets, solid-Earth processes and astronomical forcing in shifts of climate and sea level over the last 3 Ma.  Central to this linked system is the transfer of tens of millions of cubic kilometres of water from tropics to poles, and from ice sheets to sea levels (Lambeck, K. et al. 2002.  Links between climate and sea levels for the past three million years. Nature, v.  419, p. 199-206).

Alaskan source proposed for end-Palaeocene warming

Between 58 and 52 Ma, around the Palaeocene-Eocene boundary, Earth’s climate bucked the long-term cooling trend during the Cenozoic, by warming considerably.  Since the warming lasted for so long, it seems likely to have been caused by an enhanced atmospheric “greenhouse” gases rather than by either astronomical or oceanic causes.  Carbon isotope data around the P-E boundary can be interpreted in terms of massive releases of biogenic methane, perhaps from gas hydrates on the sea floor.  However, such releases are likely to have been sudden, and a more continual release of “greenhouse” gases fits the record better; but that begs the questions where and how?  Catastrophic methane release has been invoked for the dramatic rise in deep-ocean and high-latitude temperatures within 10 thousand years exactly at the P-E boundary.

Lengthy climatic warming can stem from increased volcanism and sea-floor spreading, but there is scanty evidence for either during this period.  Another possibility is production of gases as a result of tectonic activity, either by involvement of carbonate sediments in metamorphism, which releases CO₂, or “stewing” organic matter in thick sedimentary sequences.  Candidates for the last are the thick accretionary prisms at Pacific destructive margins, an especially appropriate example being that of the Gulf of Alaska which grew rapidly during this period (Hudson, T.I. & Magoon, I.B. 2002. Tectonic controls on greenhouse gas flux to the Paleogene atmosphere from the Gulf of Alaska accretionary prism.  Geology, v. 30, p. 547-550).  Oceanic and continental margin sediments scraped off descending oceanic lithosphere contain buried organic matter.  Increased heat flow, perhaps associated with rising magmas, can cause organic debris to break down to hydrocarbons.  Over-maturation results in the formation of methane, potentially in vast volumes, that can leak continually to the atmosphere.  Methane rapidly oxidizes to CO₂, decreasing the warming effect, but able to linger for considerable periods.  Hudson and Magoo calculate such enormous releases, that even disputes over the amount of accreted sediment in the Gulf of Alaska do little to rule out its being a major source for climatically implicated gases.  This first suggestion of a role for accretionary prisms in climate change may spur studies of such processes elsewhere, in an attempt to remove much of the load from the BLAG hypothesis that involves metamorphic release of CO_2­ in a difficult to verify process of lithospheric flatus.

See also:  Clift, P. & Bice, K. 2002.  Baked Alaska.  Science, v.  419, p.129-130

Provided the Milankovich theory of astronomical influences on insolation is indeed behind the pacing of glacial-interglacial episodes of the near past, it should be easier to forecast future change in overall climate than that of weather.  It turns out that the fluctuation of Earth’s orbital eccentricity (behind the roughly 100 ka periodicity of climate change for the past 1 Ma) is entering an historic low, due to the 400 ka period of one of its two cycles.  Modelling future insolation at high northern latitudes results in a damping of its fluctuations over the next 100 ka (Berger, A. and Loutre, M.F. 2002.  An exceptionally long interglacial ahead?  Science, v. 297, p. 1287-1288).  Left to climates own devices, the small changes in insolation may prolong the Holocene interglacial for as much as another 50 ka, instead of being now on the cusp of a descent into more frigid conditions.  Until recently, many climatologists looked to the last, Eemian interglacial as the model for the current one, and that lasted only 10 ka.

Of course, climate is no longer at the whim of astronomical forces and the Earth’s own circulation of energy, principally by the flow of energy in North Atlantic water, driven by deep water formed by sea-ice around Iceland.  Atmospheric CO₂ stands about 30% higher than during previous interglacials, because of anthropogenic emissions.  Berger and Loutre factor in the “greenhouse” influence of the additional CO₂, to find an ominous possibility that the Greenland ice sheet might well melt, with the climate entering an irreversible warming.  The climate, however, is not a model, and there is really no inkling of what surprises are in store from counter-intuitive behaviour of the many forces at work in it, under conditions that have no analogue during the whole of human evolutionary history.

Analogue of Archaean carbon cycle in Black Sea reefs

The Archaean world almost certainly had an atmosphere and oceans that were more or less free of oxygen.  Under such conditions the fate of dead organisms in the ocean, perhaps the remains of photosynthesizing cyanobacteria, would have been bacterial fermentation and the production of massive amounts of methane.  Along with volcanic emissions of carbon dioxide, methane in the atmosphere would have helped warm the planet at a time when the Sun emitted considerably less energy than it does now.  Methane is more strongly depleted in ¹³C than any organic or inorganic carbon compound.  So large falls in the d¹³C composition of organic carbon in Archaean rocks, around 2700 Ma have been taken by some palaeobiologists to signify methane metabolism.  Most methane-consuming bacteria today produce oxygen as a biproduct, so the negative excursions might indicate an early build up of more than a trace of oxygen in the Archaean atmosphere.  Discovery of bacterial communities on the floor of the Black Sea, which consume methane without oxygen production (Michaelis, W. and 16 others 2002.  Microbial reefs in the Black Sea fuelled by anaerobic oxidation of methane.  Science, v. 297, p. 1013-1015), suggest strongly that there may be little reason to suppose that Archaean conditions did involve free oxygen.

Off the coast of Crimea there are numerous sea-bed methane seeps in shallow water.  Surprisingly they are well-colonized by primitive bacteria, which produce thick mats held together by carbonate precipitates in completely anoxic conditions.  Laboratory cultures of the communities reveal that the consist of archaea and bacteria that respectively consume methane and reduce sulphate ions to sulphide.  The net result is that methane is oxidized by sulphate to produce calcium and magnesium carbonates, and lots of hydrogen sulphide (methane donates electrons for sulphate reduction, thereby becoming a source of carbon for cell metabolism).  Since much of the methane’s carbon ends up in stable carbonate – perhaps ten times more than in organic matter, such a process in the Archaean would have helped stabilize the “greenhouse effect” then.

One of the fundamental discoveries about climate change during the Plio-Pleistocene ice ages is how many climate fluctuations with periods too short to be ascribed to astronomical forcing link to shifts in deep-ocean circulation.  In the case of the North Atlantic Ocean, if high-latitude seas become diluted by fresh water cold dense brines are less able to form.  It is their sinking as a residue from the formation of sea ice that helps drive the “ocean conveyor” and draws warmer water into the Arctic from the tropics.  If they do not form, then the conveyor shuts down and high-latitudes cool.  The most spectacular of these ocean-driven events was the Younger Dryas cooling from about 12.9 to 11.6 ka, and it may well have occurred because of the sudden drainage of a giant lake of glacial meltwater down the St Lawrence Seaway to dilute the North Atlantic.  The waning of every major ice sheet covering North America would have generated vast amounts of freshwater, and because repeated glaciation created basins by erosion and sagging of the low-relief surface, drainage of such lakes would have been characteristic of every transition to interglacial warmth.  Steven Colman of the US Geological Survey reviews recent attempts to model how flooding may have escaped from the ice-sheet margins (Colman, S.M. 2002.  A fresh look at glacial floods.  Science, v. 296, p. 1251-1252).

The Hadean was cool

James Hutton’s observation that the geological history of Scotland had “no vestige of a beginning” applies everywhere, for no-one has dated rocks that are older than about 4.0 billion years (Ga) old, despite a great deal of effort.  It seems that continental crust only became capable of remaining at the surface in large volumes almost 600 Ma after the Earth formed from the Solar nebula.  Indirect isotopic evidence and dating of meteorites do indicate that the Earth accreted from dust and planetesimals about 4.56 Ga ago.  There are terrestrial materials that break the 4 Ga barrier, but they are so few and so tiny that they could be lost with one powerful sneeze.  These are crystals of the highly resistant mineral zircon, found as detrital grains in mid-Archaean sandstones in Western Australia.  The oldest of these is a single grain dated at 4.404 Ga.  All of them formed in igneous rocks produced by partial melting of the mantle, which concentrates zirconium in magma.  Following their liberation to sedimentary processes by weathering, the zircons have probably been through several sedimentary cycles since the formed.  So the pre-Archaean history of our world has left relics, but they are minuscule.  Because of the absence of pre-4Ga crust, that period was probably turbulent, partly through rapid convective turnover of the mantle and higher degrees of melting because of higher heat production, and partly due to far more large impacts that the lunar surface shows during those times.  Dating of lunar cratering and impact glasses suggests that bombardment reached a crescendo around 4.0 to 3.9 Ga.  It is now fairly certain that the Moon formed from incandescent material ejected from the Earth when it collided with a Mars-sized planet around 4.45 Ga.  Earth and its companion would, in that likely scenario, have begun their geological evolution completely molten in the case of the Moon and with a deep magma ocean on Earth.  “Hellish” is a barely adequate adjective for such conditions, and the period before 4 Ga has been termed the Hadean.  A vital question concerns when such extreme conditions waned to become potentially supportive of biochemistry and the origin of life.

Minute as they are, the pre-4.0 Ga zircons provide useful oxygen-isotope data, and their d¹⁸O is no different from that of more common zircons throughout the Archaean Aeon.  The explanation for this is that the mantle and the magmas produced from it contained an H₂O phase.  Either the mantle has always had a water content – no surprise as it still does – or the magmas from which the zircons crystallized encountered near-surface water vapour, possibly as a result of hydrothermal exchange with a hydrosphere.  Reviewing these data, John Valley and colleagues from the University of Wisconsin USA and Curtin University Australia pursue the second conjecture (Valley, J.W. et al. 2002.  A cool early Earth.  Geology, v. 30, p. 351-354), and argue for a surface temperature below the boiling point of water since 4.4 Ga, only 50 Ma years after geochemical “year zero”.  The crux of their argument is that the high d¹⁸O values of four Hadean zircons indicate their equilibration with water vapour at temperatures below water’s critical point (374°C).  If crystallization at depth was below that temperature, then the Earth would have had surface oceans.  But is this such a surprising conclusion?  Loss of heat by radiation being proportional to the fourth power of absolute temperature, an incandescent Earth’s surface at the time of Moon formation would have cooled below 100°C well within 50 Ma, unless it was blanketed by an opaque atmosphere.  Impacts of the size of those which produced the lunar maria around 4.0-3.9 Ga could have boiled away any surface water from time to time, only for the surface to cool quickly once again.  Conditions for bio-geochemistry could well have been present throughout the Hadean.  The significance of that for the origin of life is hard to judge, because large impacts and ocean boiling would have extinguished any progress, so that the process may have had to restart again and again.

Hothouse conditions were forced by massive emission of CO₂ during the mid-Cretaceous superplume event that created huge submarine basalt plateaux and began the development of many island chains that litter the floor of the central Pacific.  It was at this time that dinosaur-infested forests cloaked high latitudes, almost to both poles.  Terrestrial evidence suggests that conditions cooled somewhat in the later Cretaceous, and sequence stratigraphy indicates cyclic sea-level fluctuations, ascribed by some to the development of Antarctic ice sheets.  Resolving later Cretaceous global mean temperatures, and the ice-sheet question relies on oxygen isotopes from sea-floor sediments.  These are now available with sufficient precision and resolution to show that hothouse conditions lasted a great deal longer than suspected (Huber, B.T. et al. 2002.  Deep-sea paleotemperature record of extreme warmth during the Cretaceous.  Geology, v. 30, p. 123-126).

The Pacific superplume’s maximum activity was over a period of 15 Ma from 125 to 110 Ma (Barremian and Aptian), although it lasted until the early Campanian (80 Ma).  Contrary to the supposed magnitude of CO₂ release by volcanism, heating reached a maximum from 94 to 80 Ma.  Even at high southern latitudes, deep-ocean water remained at 14 to 19°C for these 14 Ma.  Until the end of the Cretaceous it rarely fell below 10°C.  The data rule out any circulation of cold, dense brines into the deep ocean basins from the formation of boreal sea ice, and consequently any influence by polar ice sheets.  Sea level reached its highest during this period, almost certainly because the volume of the ocean basins shrank, being floored by young, warm, low-density crust formed by the superplume.  Mid to late-Cretaceous flooding of the continental margins created uniquely favourable conditions for an explosive development of carbonate-secreting organisms of many kinds.  Despite the burial of vast carbonate platforms, as well as thick boreal coal seams, these limestone “factories” seem incapable of having kept pace with greenhouse warming.  Was CO₂ the only means then of global warming?