Category: Climate change and palaeoclimatology

It is well-established that the first large ice sheets that presaged descent into the oscillating climate of the Neogene formed about 34 Ma ago (the Eocene-Oligocene boundary) on Antarctica. Some 21 Ma before, at the Palaeocene-Eocene boundary, global temperatures had leaped following what many believe was a massive blurt of methane previously held in cold storage in ocean-floor sediments as gas hydrate. A monstrous ‘greenhouse’ climatic system must sometime in the interim have reverted to the cooling trend begun at the outset of the Cenozoic. Defining that transformation relies on assembling and interpreting newly available, high-resolution records of climatic proxies through the Eocene and Early Oligocene (Tripati, A. et al. 2005. Eocene bipolar glaciation associated with global carbon cycle changes. Nature, v. 436, p. 341-346). Hitherto, the Eocene part of the ocean-floor sedimentary column had been poorly sampled, so that only broad trends showed.

As you might expect, the change was not a simple transition. At about 42 Ma the record of the Pacific Ocean calcite compensation depth (CCD – the depth at which carbonate remains are dissolved in the deep oceans) shows a remarkable perturbation long before the CCD dipped decisively from about 3.5 km to around 5 km at the start of the Oligocene. A close look at the oxygen isotope record of that age in a highly detailed marine sediment core shows an increase in d ¹⁸O that corresponds to either some 6° of cooling or a 120 m fall in sea-level due to build-up somewhere of ice on land. Coinciding with this perturbation are shifts in the carbon-isotope record in carbonates. The authors suggest that the mid-Eocene cooling and continental glaciation that produced falling sea level triggered the weathering of shallow-water carbonates, which together with river transport increased the oceans’ alkalinity. That would have increased deep-water carbonate formation enormously and accelerated the effective ‘burial’ of carbon from the atmosphere

High latitudes in the North Atlantic, especially on its eastern side, are warmed today by the Gulf Stream.  That current, which defies the Coriolis effect, is pulled northwards by the sinking of cold dense sea water between Greenland, Iceland and Scandinavia to form North Atlantic Deep Water (NADW).  The thermohaline circulation here is driven by both cooling of salty surface water in the Gulf Stream and further salinisation as sea ice forms in this area each winter.  The Younger Dryas cold period between 13 and 11.5 ka is regarded by most oceanographers and climatologists to have resulted from sudden freshening of the North Atlantic at these critical high latitudes, so that surface water density became too low to sink.  Such a process had occurred several times during the last glacial period, each of which has been correlated with release of massive amounts of glacial ice as icebergs.  There melting caused the freshening. The Younger Dryas is a different kind of event, because it occurred well into the period of global warming that brought the Ice Age to an end.  A seemingly plausible explanation was suggested by Wallace Broecker in 1989, who looked to explosive release of meltwater trapped in glacial lakes roughly along the Canadian-US border along the present St Lawrence River Valley, effectively flooding the source of NADW with a surface layer of low-density, low-salinity water.

The problem with Broecker’s mechanism is that sea-level records through the Younger Dryas show no sudden rise, whereas at about 14 ka a meltwater pulse had resulted in a 20 m rise over about 500 years, with no sign of a climatic response to a shutdown of the Gulf Stream by the freshening that it caused.  A similar event occurred shortly after the waning of the Younger Dryas.  There is no doubt that throughout high northern latitudes the great ice sheets were melting since about 18 ka. A new approach to the Younger Dryas concentrates on where the meltwater formed in northern North America probably escaped to the sea (Tarasov, L & Peltier, W.R. 2005.  Arctic freshwater forcing of the Youner Dryas cold reversal.  Nature, v. 435, p. 62-665).  Through their analysis of the drainage chronology of the Canadian Shield Tarasov and Pelter conclude that at the time of the onset of the Younger Dryas most flow was roughly along the present MacKenzie River valley to the Arctic Ocean.  Freshening of the Arctic Ocean would escape through the narrow Fram Straits directly to the source region for NADW. It would not necessarily have been through currents, for escape of increased amounts of pack ice would have much the same effect.  Central to their hypothesis are new data that relate to extraordinarily thick continental ice in the Keewatin glacial dome, that formed just to the east of modern Great Slave Lake.

Acidification of the oceans

When gases such as CO₂ and H₂S permeate through ocean water they dissolve to form weak acids: carbonic and sulfurous acid respectively. So many organisms, plants as well as animals, incorporate carbonates into their hard parts that changes in acidity constitute an important kind of stress.  The acidity of water combines with increasing pressure as water deepens to create a zone (the lysocline) in which water is undersaturated in calcium carbonate.  Below the lysocline carbonate shells begin to dissolve.  Deeper still is a level (the carbonate compensation depth, or CCD) below which there is no free CaCO₃ in the water column.  Falling shelly material dissolves completely, so that deep-ocean sediments contain few if any shells other than those of silica-secreting organisms.  At present the CCD is around 4 km deep.  Any shift in the pH of the oceans causes the CCD either to rise or fall.  The signatures of such shifts lie in the composition of ocean-floor sediments.  In the deepest parts, where silica and clays dominate, layers in which carbonate shells are preserved signify a decrease in acidity (increased pH) and descent of the CCD to below the elevation of the ocean floor.  On the other hand, the appearance of pure clay-silica oozes in otherwise shelly muds, where the sea floor has been well above the CCD for long periods, show that acidity increased (a drop in pH) over a period.  Such anomalous sediment layers are often easy to see in cores because their colour is different from the common sediments.

In cores from ocean depths between 2 and 4 km, the second kind of anomaly appears consistently at the level of the Palaeocene-Eocene boundary: it signifies a massive increase in acidity (Zachos, J.C. et al. 2005.  Rapid acidification of the ocean during the Paleocene-Eocene thermal maximum.  Science, v. 308, p. 1611-1614). Carbon-isotope measurements from the same cores also show a marked shift.  The sediments are depleted in ¹³C, which has generally been taken to indicate a huge release of methane from storage as gas hydrate in sea-floor sediment at the time of the Palaeocene-Eocene boundary.  Most palaeoclimatologists consider the C-isotope “spike” to be a proxy for sudden, intense warming that resulted from methane – a more efficient ‘greenhouse’ gas than CO₂ – and the carbon dioxide produced as it was oxidized.  The range of water depths where the carbonate-free layers occur enables marine geochemists to estimate the rate of acidification.  In around only 10 ka the CCD rose 1.3 to 2.0 km above its current level.  From the degree of acidification needed it seems that considerably more than 2 x 10¹² t of carbon was released in the form of methane that eventually oxidized to CO₂, and returned to the ocean.  The carbonate content of the ocean sediments rose gradually over the next 100 ka, by the end of which the former balance was restored. This information in turn gives a picture of the rate at which sudden ‘greenhouse’ events subside once their cause has stopped being produced, almost certainly by the drawdown of atmospheric CO₂ by weathering of silicate minerals exposed on the continental surface.

At the end of the Palaeocene, the effect on organisms was mainly restricted to benthic foraminifera that live in moderately deep water, which show a selective extinction.  The eventual release by human activity of carbon contained in accessible fossil fuel reserves, will give a mass of carbon in ‘greenhouse’ gases of about twice that released at the Palaeocene-Eocene boundary over perhaps 300 years.  Such rapid release may result in acidity that is incompatible with carbonate-secreting organisms anywhere in the oceans: the CCD will effectively be at the sea surface

Now and again in the geological record, evidence turns up that suggests that the deep oceans were devoid of oxygen.  Ocean anoxia encourages burial of dead organic remains that gives rise to carbon-isotope “excursions”: signals of the anoxia itself.  A likely mechanism that starves the deep oceans of oxygen is the shut down of that part of the ocean “conveyor” driven by sinking of cold, dense brines, as happens today in the North Atlantic and around Antarctica.  Gases dissolve more efficiently in cold water than in warm.  Quite probably most oceanic anoxia events are related to global warming and increases in the “greenhouse” effect due to CO₂ rises in the atmosphere.  A group of US and British geoscientists have examined one such anoxia event in the Lower Jurassic (~183 Ma) of Denmark using both carbon isotopes and the density of pores (stomata) on fossil leaves (McElwain, J.C. et al. 2005.  Changes in carbon dioxide during an oceanic anoxia event linked to intrusion into Gondwana coals.  Nature, v. 435, p. 479-482).  Stomatal density is inversely related to the amount of CO₂ in the atmosphere, so is very useful in seeking evidence for an anoxia-climate link.

This particular anoxia event has been linked either to release of methane, which quickly causes warming and then oxidises to CO₂, from gas hydrate or to massive release of carbon dioxide itself.  McElwain et al. neatly show that the event first experienced drawdown of ”greenhouse” gas and cooling of around 2.5 °C, then sudden quadrupling of CO₂ and warming of around 6.5°C.  Such an odd pattern cannot be ascribed to methane release, but coincides with the formation of the Karroo-Ferrar continental flood-basalt igneous activity in southern Africa and Antarctica.  That involved massive intrusion into coal-bearing strata, whose thermal metamorphism would have released huge amounts of “greenhouse” gases.  Calculations of the amount of carbon mobilised to cause the shifts in CO₂ suggest between 2.5 and 4.4 trillion metric tons, vastly more than the probable amount of methane hydrate beneath the Jurassic sea floor.

enveloping glaciations during the Neoproterozoic Eon, that notion of “Snowball” conditions has received many severe knocks, charted by numerous items in EPN.  Geochemists and geologists from the Universities of Vienna and Witwatersrand realised that a good test of the hypothesis would be to concentrate on a rather obvious property of an ice-bound planet (Bodiselitsch, B. et al. 2005.  Estimating duration and intensity of Neoproterozoic Snowball glaciations from Ir anomalies.  Science, v. 308. P. 239-242).  Whatever falls on an ice sheet, whether it is cosmic dust from outside the Earth or ash from volcanoes, becomes trapped in the annual layers of ice.  When the ice melts, that accumulated content is transferred to the oceans very quickly.  With weathering in suspended animation during the glacial epoch, transport of many elements would have slowed to very low levels.  So, marine sediments deposited immediately after the diamictites that are allegedly glaciogenic ought to contain anomalously high levels of several elements.  The most important of these would be those which show very different abundance patterns in meteorites form those in terrestrial rocks.

Bodiselitsch et al. hit what seems to be “paydirt” in carbonates above a prominent diamictite in central Africa.  Their samples are impeccable, being from diamond-drill cores produced during evaluation of sediment-hosted mineralization in the famous Neoproterozoic Copper Belt of Zambia and Congo.  The core contains a prominent iridium anomaly at the very base of the carbonates, with a “signature” relative to other anomalous elements that points to a cosmic origin.  Normally such an anomaly would be ascribed to a meteorite impact, but in this case the coincidence would be too good to be true.  Instead, the authors use the magnitude of the anomaly to estimate how long cosmic dust had to accumulate to build up such a high level if it was released by rapid deglaciation.  Deep-ocean sediments from the last 80 Ma are a guide to the long-term accumulation rate of cosmic material.  If that rate is applied to the cap-carbonate anomaly, it gives a total time for accumulation in the hypothesised global ice cover of around 12 Ma.  Presumably this would have been from ice immediately overlying the area being studied.  An ice age that long defies any idea of more “normal”, astronomically forced glaciation, which would be expected to have cyclically formed and receded many times, thereby releasing the dust particles much more gradually.  Any anomalies would be expected in the diamictites themselves, yet there are none.  Although sample spacing is rather patchy through the entire succession, they are most dense around the anomaly itself.  Moreover, another suspected glaciogenic “package” higher in the sequence shows exactly the same iridium “spike”. 

Arguing against such support for the “Snowball Earth” hypothesis will be difficult, but other sequences require similar tests, most importantly those of Namibia, where Hoffman and colleagues developed their ideas, and the much more extensive deposits of Australia.  This diamictite sequence is reckoned to represent both postulated deep-freeze events of the Neoproterozoic, around 710 Ma (Sturtian) and 635 Ma (Marinoan). There is one nagging problem.  Data from one area are likely to record ice-retained cosmic dust only from ice in its immediate vicinity, and therefore do not represent the entire planet.  Much of the controversy is between supporters of a whole-Earth ice cover, and those who favour patchy glaciation (the “Slushball” model).  Unfortunately, Neoproterozoic stratigraphic correlation and radiometric age calibration is not sufficiently good to detect the same intervals elsewhere and look for anomalies there.  In fact, the stratigraphy is generally correlated from place to place by matching the diamictites themselves.  There is plenty of evidence that they may all coincide in time.

Tracking ocean circulation during the last glacial period

The use of various ocean-floor sediment proxies for climate change, such as the ups and downs of heavy ¹⁸O that chart waxing and waning continental ice cover, has progressively revealed the complexity of shifts during glacial and interglacial periods. Yet more emerged from finer-resolution time-series contained with Greenland and Antarctic ice cores.  The diversity of information that proxy for many different, climate-related processes has in the last decade enabled palaeoclimatologists to begin piecing together possible causative mechanisms, beyond the initial discovery of an astronomical signal in early oxygen-isotope records.  One of enormous significance is the possibility that sudden millennial-scale cooling and warming link to changes in ocean circulation, especially that performed by the Gulf Stream driven by thermohaline processes at high northern latitudes.  Shutting down that poleward transfer of heat, probably because freshwater made high-latitude surface water less dense, has been implicated in sudden cooling or “stadials”, and its restart linked to warming or “or interstadials”.  The last such sudden climate event, the Younger Dryas between about 12 and 11 thousand years ago, is widely believed to have resulted from a collapse of the Gulf Stream.  That has raised fears that current anthropogenic warming might achieve the same thing, thereby plunging Western Europe into a counterintuitive frigid period through loss of its maritime warming.

Ocean circulation has lacked a proxy that might help resolve such worrying scenarios, but it seems that one has arrived, because of improvements in mass spectrometry (Piotrowski, A.M. et al. 2005.  Temporal relationships of carbon cycling and ocean circulation at glacial boundaries.  Science, v. 307, p. 1933-1938).  Different bodies of ocean-surface water have subtly different chemical compositions, due to the varied geochemistry of surrounding landmasses.  Weathering of exposed rocks results in some elements entering solution in river water, and that mixes with surface water in the nearby ocean.  Among the most useful elements are those with an isotope to which radioactive decay of unstable isotopes of another element contributes.  A good example is ⁸⁷Sr that is formed when ⁸⁷Rb decays.  Where continents expose  large expanses of very ancient rocks they contribute more ⁸⁷Sr to seawater than do continents veneered with younger rocks.  Strontium isotopes have been used successfully for charting very-long term changes in the overall erosion of continental crust, in relation to climate shifts, but being related to calcium are taken up quickly by carbonate secreting organisms, such as foraminifera, at many different levels in the ocean as it circulates.  So they are not very useful for short-term studies.  A more useful isotopic system involving an daughter of slow radioactive decay is that of neodymium, because it does not get taken up in this way.  It does however enter the manganese minerals that slowly precipitate on the deep ocean floor.  Moreover, its isotopic composition varies greatly in different ocean-water masses.  Piotrowski et al. used neodymium isotopes from deep ocean cores to see if changes in this circulation proxy coincided with known climate proxies.  For interstadial, warming events there is a match, so a Gulf-stream control over millennial-scale climate shifts is indeed supported.  But for the start and end of the full glacial period control by ocean circulation did not happen.  Instead, changes in the neodymium record lag behind the climate proxies, suggesting climatic control of circulation, which then “kicked in” to boost changes that were well underway.

See also: Kerr, R.A. 2005.  Ocean flow amplified, not triggered, climate change.  Science, v. 307, p. 1854.

The competing periodicities of  the three astronomical “drivers” of climate – orbital eccentricity (~100 ka), axial obliquity (~40 ka) and axial precession (~20 ka) – lie behind several models for the climate changes of the last 0.7 Ma.  Taking in the theories that sway towards the influence of variables in the Earth system itself, around 30 models have some currency at present.  Since climate forecasters have to take account of which factors drive climate in the absence of human emissions, as well as piece together their own particular models, it is easy to see how critics of global warming get a wide hearing: compared with creationists, they have it easy!  Is there any way of resolving what is quite bluntly a theoretical mess?  It is a mess simply because the available data are so complex, and in the case of both main sources, ocean-floor sediments and ice cores, not only are their devils in the detail, but there are whopping contradictions, such as the mismatches in timing between the Greenland and Antarctic ice cores.  Add all the other sources, such as stalactites, tree rings etcetera, together with caveats like the difficulty in time calibration using ¹⁴C dating,  and the volume of diverse records become bewildering.  It is tempting that a reversion to a statistical approach, that includes more bells and whistles than hitherto (see Evolutionary rhythms below), can resolve matters.  Peter Huybers and Carl Wunch, of Woods Hole Oceanographic Institution and MIT, have tried that for pacing of the last 0.7 Ma of climate cycles (Huybers, P. * Wunsche, C. 2005.  Obliquity pacing of the late Pleistocene glacial terminations.  Nature, v. 434, p. 491-494).  Generally accepted “wisdom” holds that the last 7 glacial-interglacial cycles are paced by ~100 ka eccentricity forcing, even though it has the weakest effect on solar heating, by a very long way.  But there are smidgens of evidence for some interaction between that and the much stronger influence of changes in the Earth’s axial tilt or obliquity.  Huybers and Wunsch go for the Popperian rigor of first defining a null hypothesis, that obliquity has no effect, and then designing a test.   It isn’t easy to decide how the contrary hypothesis that it does can be evaluated though.  The clearest features in all climate records are the ends of glacial epochs or termination: they are sudden, sharp and generally look the same.  Most other features have some kind of pattern, but little consistent comparability.  Using the most advanced statistical techniques, which employ many iterations to test for stability in statistical models, they can show that the null hypothesis fails.  The positive result is that the time between terminations that are repeatedly modelled  falls into two envelopes, around 120 and 80 ka, which simple arithmetic shows are divisible by 40 ka.  But how can axial obliquity only have an effect every two of three of its cycles, while a single cycle does not appear in the time-series; is it nature skipping beats somehow.  One means that the authors suggest is that the underlying pace of eccentricity can effect the temperature at the base of ice sheets, depending on their thickness.  If they are thin, then the heating is insufficient to trigger ice-sheet collapse because the base is very cold, whereas if ice is thick the effects of thermal conductivity and heat flow makes the ice base warmer and more subject to perturbation beyond its failure limit.  It was at this point that I gave up, but wish the authors good luck in promoting their possibly unifying hypothesis for what finishes off glacial epochs…..

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