Author: zooks777

Former Senior Lecturer at British Open University, research in remote sensing of arid lands, groundwater exploration, Precambrian tectonics and geochemistry

Phanerozoic CO2 levels

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Because climate depends partly on the retention of solar heat by carbon dioxide in the atmosphere, a record of past CO2 fluctuations is important in linking evidence for shifting climate and environments to models.  Conversely, models that seek to mimic climates of the past depend heavily on the assumption that the “greenhouse” effect and the carbon cycle underpin global temperature and precipitation.  Current theorists consider that shifts in CO2 content of the atmosphere reflect a balance between its release through volcanism (itself a reflection of the rate of plate tectonics) and  its removal by weathering of silicate minerals and burial of dead biomass. 

The GEOCARB III model predicts rising atmospheric CO2 following the ice-house condition of the late-Precambrian, when rapid sea-floor spreading broke up and began to reassemble supercontinents during the Lower Palaeozoic.  In the early Cambrian CO2 levels come out at 25 times the modern amount.  Colonization of the land by plants through the Upper Palaeozoic, and the burial of a proportion of the increased amount of carbon fixed by them, allows the model to predict a massive fall in CO2.  That tallies very well with the long period of glaciation in southern Pangaea during the Carboniferous and Permian.  GEOCARB III suggests a recovery in levels through the Mesozoic, punctuated by extraordinary releases from plume activity, such as that implicated in the formation of ocean plateaux beneath the Pacific about 120 Ma ago.

From GEOCARB modelling stem predictions of the overall forcing of global temperatures.  However, only the last 100 Ma can be assessed as regards temperatures, by using accurate proxies provided by oxygen isotopes and the Ca:Mg ratio of marine carbonates.  Two of the leading climatic theorists, Thomas Crowley and Robert Berner of Texas A&M and Yale universities usefully summarise the range of other proxies that help validate their kind of modelling (Crowley, T.J. and Berner, R.A. 2001.  CO2 and climate change.  Science, v. 292, p. 870-872).  These include estimates from fossil soils, carbon isotopes in sediments, the pores in plant leaves (see Plant respiration and climate below) and how much boron is taken up in the shells of fossil animals.  There are considerable discrepancies with modelling, albeit encompassed by the high uncertainties in the calculations.  Crowley and Berner acknowledge the complexity of other factors that affect the global redistribution of heat, such as continental configurations in terms of area, geographic position, their effects on ocean circulation and even on the pace of the carbon cycle.  They see the need to expand climate models, taking other factors on board, in an attempt to quantify the discrepancies.

Methane and escape from Snowball Earth

Palaeomagnetic pole positions determined from areas characterized by thick glacigenic deposits around 750 Ma old leave little doubt that large volumes of ice covered the Earth to tropical latitudes.  Such evidence suggests an ice-bound world from which escape would have been very difficult because much of the Sun’s energy would have been reflected back to space.  Extreme and prolonged frigidity, from which Earth’s climate did escape is seen by a growing number of palaeobiologists as the most profound influence over later evolution and diversification of life.  The first fossil metazoans appear in the record shortly after a “Snowball Earth” event at 650 Ma, and the Cambrian explosion of animals with hard parts followed close on the heels of the last.  Carbon isotope studies from marine carbonates suggest that each global glaciation witnessed massive extinctions of single-celled organisms, and surviving life was presented with a virtual tabula rasa of niches to fill.  Such survivors, possessing characters that had ensured their survival – at which we can only guess – exploited them to the full.  It is reasonable to speculate that without such climatic upheavals life would not be as it is now, and that our eventual appearance depended on them.

That Earth’s climate broke out of runaway ice-house conditions is obvious, the question being how was that possible.  Volcanic emissions of carbon dioxide, which neither the Neoproterozoic biosphere nor silicate weathering were able to draw down into ocean water and sediments, would have accumulated in the atmosphere, to create “greenhouse” conditions.  That simple scenario, envisaging a spectacular shift from frigid to hot conditions, has its problems.  In order for climate to stabilize, without rushing into runaway heating along the path followed by Venus, demands implausibly high rates of silicate weathering to draw down CO2 in the period following the end of each “Snowball” event, and strontium isotopes that record the rate of continental weathering shwo no sign of anything so dramatic.  It also poses the question of how global ice cover could remain while CO2 slowly built up.  The key seems to lie in carbonates that everywhere cap the glacigenic deposits of this age.  The cap carbonates record rapid falls in the 13C proportion of the carbon in carbonate.  13C shows a rise in the glacial epochs that signifies massive burial of dead organic matter (enriched in lighter 12C), probably through mass extinction.  In a review of the geochemical basis for changes in oceanic carbon isotopes, and high-resolution data from cap carbonates, scientists from the University of California and the Lamont-Doherty Earth Observatory, suggest that the isotopic excursions could reflect massive release of methane from gas-hydrate layers in sediments that were frigid during the Snowball event (Kennedy, M.J. et al. 2001.  Are Proterozoic cap carbonates and isotopic excursions a record of gas hydrate destabilization following Earth’s coldest intervals?  Geology, v. 29, p. 443-446).  Backing up this hypothesis are examples of structures in cap carbonates that are identical to those formed in modern sediments affected by break down of gas hydrates and release of methane from the sea floor.

Plant respiration and climate

Leaf surfaces are pockmarked by pores (stomata), through which cell metabolism draws in the carbon dioxide involved in photosynthesis and transpires its products, including oxygen.  When CO2 levels are low, more pores are needed, and vice versa.  Surprisingly, museum specimens of leaves collected since the start of the Industrial Revolution do show a decrease in the density of such pores that matches the documented rise in atmospheric CO2 levels.  Were it possible to find fossils of the same plant species, pore density would be an excellent proxy for the “greenhouse” effect.  That is not possible, because of evolution.  However, plants related to the Ginkgo have a pedigree that goes back about 300 Ma.  Morphologically, the four genera of Ginkgo-like leaves are very similar, so using them potentially gives an independent record of the “greenhouse” effect.

Gregory Retallack of the University of Oregon has measured the stomatal index of sufficient Ginkgo and related leaves to assess CO2 levels in a broad-brush sense for the period since the early Permian (Retallack, G.J. 2001.  A 300-million-year record of atmospheric carbon dioxide from fossil plant cuticles.  Nature, v. 411, p, 287-290).  His results tally broadly with oxygen-isotope and other proxies for palaeotemperature variations, and to some extent with CO2 modelling (see Phanerozoic CO2 levels above).  However, the stomatal record shows changes up to 10 Ma in advance of shifts in temperature.  That might be due to coarse resolution in Retallack’s data, but could signify other forces at work other than the “greenhouse” effect.  The most significant advance provided by leaf studies is that they help account for mismatches between evidence for cooling and predictions of highCO2 by modelling, for the Jurassic and Cretaceous, that have been a thorn in the side of the modellers.  Given fossil leaves more closely spaced in time, and using other plant groups, Retallack’s method potentially could revolutionize climate analyses and extend them back as far as 400 Ma ago.

See also:  Kürschner, W.M.  2001.  Leaf sensor for CO2 in deep time.  Nature, v. 411, p. 247-248.

Loss to geology

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Robert Shackleton FRS died aged 91 on 3 May 2001. Shackleton’s long career began as a survey geologist in Africa.  After a period at Liverpool University he took up a chair at Leeds, and became an Honorary Senior Research Fellow at the Open University.  He was not a retiring man, and was of the school of which it was said, “The best geologist is the one who sees the most rocks”.   His peregrinations were legendary.  Shackleton’s forte was structural geology and tectonics, and he was a central figure in driving forward our understanding of Africa’s evolution.  Sadly, he did not live to witness the publication of his Geology of Africa project.  His touch and his flair were felt by many throughout the world, and they will be missed.

Java girl

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As if the jumble in cladistics of African hominins was not enough, the skull SM3, dubbed by some as “Java Girl”, adds to the bag of spanners that disrupts attempts to rationalise the human evolutionary bush  (See Earth Pages Apr 2001, Skulduggery, migration and confusion).  Java, of course is where the whole thing began, with Eugene Dubois’ (See Review of Pat Shipman’s biography of Dubois in Nature v. 410, p. 869) discovery of what seemed to him as Darwin’s “missing link”, in the form of Pithecanthropus (now Homo) erectus in 1892.  Miss palaeo-Java, is odd by comparison, largely because her brow ridges did not meet and her forehead was “nobly” high.  Morphologically, her skull shows features that could be transitional between H. erectus and H, sapiens.  New Scientist ran an article (Soares, C.. Talking heads.  New Sientist, 14 April 2001 issue, p. 26-29) that charts how her skull, found recently in a New York antique shop – she was smuggled out of Indonesia two decades ago, has been grist to the mill for the multiregionalists, already gleeful at the DNA sequence of Australia’s “Mungo Man” (See Earth Pages Feb 2001, Out of Africa hypothesis confounded?).  Thoughtfully, Christine Soares also mentions the growing doubts that shapes of skulls and even whole skeletal anatomies can contribute a great deal resolving the multiregional vs out-of-Africa debate.  This arises from Todd Disotell’s studies of  modern monkeys, where he found that genetically distant species had almost identical morphologies, whereas much more closely related species were the most different from each other cranially.

Impacts and human evolution

Few Earth scientists disagree with the notion that our planet’s evolution and that of its life has been repeatedly punctuated by catastrophic impacts with comets and asteroids.  The Moon’s surface is an excellent record of that bombardment in near-Earth space since about 4.45 Ga ago, when it formed in orbit around the Earth.  Both dating of impact glasses from the Apollo programme and assessment of the relative ages of lunar craters provide continually refined statistics of the distribution of impact events of different magnitudes through time.

Dr Benny Peiser, a social anthropologist at Liverpool John Moores University and Michael Paine, an impact researcher from the Planetary Society in Australia, applied these statistics to the roughly 5 Ma time span of human and hominin evolution.  Their suggestions were presented by Peiser at the Charterhouse Conference 2001 “Celebrating Britain’s Achievements in Space” in London (see the Cambridge Conference Network  archives at http://abob.libs.uga.edu/bobk/cccmenu.html .

They calculate that 552 impacts that formed craters between 5 and more than 20 km across occurred on land during human evolution, with an additional 6 ocean impacts that could be expected to produce moderate to severe global climate disruption. So far, 32 impact craters have been discovered that are younger than 5 million years.  Earth’s active erosion and sedimentation are like to have obscured more craters, even in such a brief period.

No-one would seriously dispute Peiser and Pain’s calculations, but where they proceed from them is a different matter.  They assign an impact origin to the genetic bottlenecks, which seem to be implicated in speciation and which show up in modern human gene sequences (see More molecular evidence for Cro-Magnon migration into Europe – Earth Pages Jan 2001 – and Eve never met Adam – Earth Pages Nov 2000).  No doubt the aftermath of sizeable impacts would place terrestrial life under considerable stress, but to jump from impact statistics to a hypothesis of external causes for hominin speciation is not likely to find much support.  It does not use evidence at all, but probabilities, as often quoted that each of us is as likely to perish from extraterrestrial impact as from a firework accident or murder.

The record of human evolution is blurred to a large degree by:

1.  the tiny number of fossils

2.  the dates assigned to those fossils

3.  the significance assigned to their morphology by different palaeoanthropologists – there are “lumpers” and “splitters”

4.  the total lack of knowledge about the interplay between physiology, culture and social interaction, as regards what constituted “fitness” in natural selection.

Aside from the bottlenecks implied by modern human genetic diversity, or rather lack of it, we do not have a clue when Orrorin, “Lucy” , H. erectus, “Bonzo” the chimp or fully modern humans appeared as species.  And there is another matter; the post-Miocene period has been punctuated by climatic shifts of dreadful magnitude that came thick and fast through Milankovich pacing.  To suggest any other trigger for speciation, without a “smoking crater” and a precise date coinciding with the first individual of a species, is neither sensible nor necessary – as if…  This is grandstanding, and the press have had a field day.

New human evolution web site

Science magazine’s NetWatch (10 April 2001) includes news of the Becoming Human web site developed by the Institute of Human Origins at Arizona State University.  http://www.becominghuman.org is multimedia, including a 30 minute “webcast” by Donald Johanson, the director, who found “Lucy” in 1974.  That can be skipped, and the meat found in various Exhibits, a glossary, references, links and news.  The site plans to launch a teachers’ resource centre in May.

The origin of microcontinental terranes

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Slivers of ancient crust make up part of the collages of accreted terranes found in many ancient orogens.  How they form is not well-known.   Clues might lie in modern microcontinents that still remain surrounded by oceanic lithosphere, such as Jan Mayen, the Seychelles and the East Tasman Plateau.  Geologists from the Universities of Sydney, and Aarhus and the Geological Survey of Canada believe that such fragments of continental crust form early in the evolution of passive margins, as a result of plume activity followed by asymmetric sea-floor spreading (Müller, D.M. 2001.  A recipe for microcontinent formation.  Geology, v.  29, p. 203-206).

One suspected microcontinent in the southern Indian Ocean is the Kerguelen Plateau – its shape is odd.  In the few places where it breaches surface in the Kerguelen Archipelago, there are rare occurrences of silicic plutonic rocks.  However, evidence from dredged samples seems to show that most of the Plateau formed by plume-related basaltic volcanism that began at the same time as the formation of the Rajmahal Traps in Bangladesh (about 117 Ma ago).  ODP drilling now reveals fluviatile sediment layers that contain high-grade gneisses, whose ages range back to the Proterozoic (Nicolaysen, K. and many others 2001.  Provenance of Proterozoic garnet-biotite gneiss recovered from Elan Bank, Kerguelen Plateau, southern Indian Ocean.  Geology, v.  29, p. 235-238).  The authors do not see this as directly supporting a Kerguelen microcontinent, but the formation of the plateau close to eastern India around 110 Ma ago, from where abundant Precambrian crustal debris would have been shed.  However, the presence of continental geochemical signatures in Kerguelen Plateau basalts, otherwise having plume affinities, might indicate a fragment of former Gondwanan lithosphere at the core of the Plateau, akin to the now exposed Danakil block in the nascent Red-Sea – Afar rift in NE Africa, that spalled off during the break-up of the Mesozoic supercontinent.

The start of North Atlantic Deep Water formation

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The most favoured means whereby the weak fluctuation in solar radiation due to the Milankovich-Croll Effect become amplified to affect climate’s ups and downs is the switching on and off of thermohaline circulation in the North Atlantic Ocean.  The key to such ocean circulation is formation today of dense, cold brine through sea-ice formation around Iceland.  To set circulation in motion, however, depends on these brines being able to move southwards, which they do now in a sea-floor channel between Shetland and the Faeroe Islands.  When the North Atlantic began to open, this route was blocked by a ridge between Greenland and Shetland, buoyed up by residual warmth in the lithosphere from volcanic activity at the Iceland plume.

It is important to assess when the Shetland-Faeroe “gateway” formed, so that the effects of thermohaline circulation on pre-glacial climate can be assessed.  Petroleum exploration using high-resolution seismic reflection profiles and drilling has resolved this particular issue.  Geologists and geophysicists from Exxon and Cardiff University have found signs that sediment drift dragged by such a deep flow began in the early Oligocene (about 35 Ma ago) (Davies, R.  et al. 2001.  Early Oligocene initiation of North Atlantic Deep Water formation.  Nature, v. 410, p. 917-920).  The evidence takes the form of multiple, moat-like erosion surfaces down to the base of sediment fill between the Faeroes and Shetland, shown superbly by the seismic data.  Drilling shows that these signs of deep-water flow stop abruptly in Early Oligocene sediments.

Astrology and ice

The early Oligocene marked the onset of serious ice cover on Antarctica, and it shows as a dramatic increase in d18O values in the ocean-floor record of benthic forms – lighter 16O had been trapped in land ice.  That may or may not be a coincidence with the finding about the start of  North Atlantic thermohaline flow in the previous item.  A lesser, but still dramatic increase marks the Oligocene-Miocene boundary, suggesting further growth of the Antarctic ice sheet, which is not so readily matched empirically.  Detailed study of the isotopic  “blip” at this time by a team from the Universities of California, Cambridge and South Florida (Zachos, J.C. et al. 2001.  Climate response to orbital forcing across the Oligocene-Miocene boundary.  Science, V.  292, p. 274-278) suggests that it related to a remarkable coincidence in the astronomical record of solar heating.

Round 23 Ma ago, the orbital eccentricity dropped almost to zero – Earth’s orbit would have been circular – at the same time as its axial tilt became very stable, the one reinforcing the climatic effect of the other.  The isotopic “blip” coincides exactly with the coincidence.  The detailed record also shows very clearly that minor fluctuations in climate at that time were in step with the 400 and 100 ka periods in the eccentricity variations, and with those of 41 ka that relate to changes in axial tilt.  If nothing else, these results confirm that it is unnecessary to turn to extraterrestrial influences over climate other than those which are predictable from Milankovich’s theory (see Impacts and human evolution, above).

Additional source:  Kerr, R.A. 2001.  An orbital confluence leaves its mark.  Science, v. 292, p. 191.

Start of Pleistocene environmental change in tropical Africa

Pollen records from an ODP core drilled off the Congo estuary provide a record of the fluctuation in the monsoon of western tropical Africa (Dupont, L.M. et al.  2001.  Mid-Pleistocene environmental change in tropical Africa began as early as 1.05 Ma.  Geology, v.  29, p. 1195-198).  Before 1.05 Ma there is little sign of a glacial-interglacial pulse in the fluctuation of vegetation in the Congo Basin.  Thereafter, ups and downs in pollen from various vegetation groups correlate well with the benthic foram oxygen-isotope time series.  However there are a few surprises.

Conventional wisdom is that Africa experienced drying during glacial epochs, rain forest expanding during interglacials.  In the Congo basin, grasses and savannah trees increased during interglacials while mountain trees fell in their influence, up to 600 ka.  This suggests the opposite trend of  warm, dry interglacials and cool, humid conditions during glacial periods, similar to the record for tropical South America.  In the later Pleistocene, the fluctuation switched to that indicated by fluctuating lake levels throughout the continent.  The pollen variations are backed up by variations in dinoflagellate cysts, which show that discharge from the Congo dropped during interglacials.  The other surprise is that the onset of astronomically paced environmental change in west Africa predated the change to a 100 ka domination of global climate, and the increase in amplitude of changes in land-ice volume at 900 ka by a hundred thousand years.  Dupont et al. suggest that the changes in albedo in tropical West Africa in response to vegetation changes could have had an influence on global climate when the fluctuations began.

As well as being interesting in terms of climate change, the new data throw doubt on the hypothesized link between climate in Africa and pulses of migration of early human species, such as H. ergaster and H. erectus.  There were fluctuations in humidity in the earlier Pleistocene, but they show no link to global climate change.  So, it seems unwise simply to look to the Milankovich forcing as a pacemaker in early human affairs.

A Late-Jurassic methane “gun”

Massive releases of methane from gas hydrate layers beneath the ocean floor, and its subsequent oxidation to carbon dioxide have been implicated in major climatic and oceanographic changes in the mid-Jurassic, Cretaceous and Palaeocene.  They can be detected by drops in the 13C content of marine carbonates, caused by the “light” carbon trapped in biogenic methane.  All those known also correlate with evidence for climatic warming.

The Swiss Jura mountains are a repository of great thicknesses of Jurassic carbonates, whose ammonite faunas allow fine stratigraphic division.  Between 157 and 156 Ma (late Middle Oxfordian) there is a major negative excursion in d13C whose duration was as short as 180 ka (Padden, M. et al. 2001.  Evidence for Late Jurassic release of methane from gas hydrate.  Geology, v.  29, p. 223-226).  The Swiss-French geochemists who discovered the anomaly believe that the release may have linked to opening of the ocean gateway that connected currents between Tethys and the easter Pacific oceans through what is now the Atlantic.

Surviving in salt?

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In the manner of Count Dracula’s dogged refusal to shed his mortal coil by hiding from sunlight, is it conceivable for primitive organisms to be immortal by being protected from UV radiation?  That it might be possible emerged from the revival of bacteria trapped in fluid inclusions in Permian rock salt by Russell Vreeland and William Rozenzweig of West Chester University in Pennsylvania (see Earth Pages, November 2000, The undead).  Despite taking stringent precautions to avoid any contamination of their samples by modern bacteria, Vreeland and Rozenzeig’s claim has been fiercely challenged.  It is possible that the dormant bacteria could have entered the salt in much younger solutions permeating the deposit (incidentally one of the most stable tectonically and hydrogeologically – it is the prospective site for burial of US radioactive wastes).

Vreeland’s team  found 4 bacterial strains – all salt-tolerant halobacteria – but have genetically fingerprinted only one so far.  It is related to a modern genus living in the Dead Sea that forms spores.  The minute fluid inclusions from which samples came have insufficient energy and nutrients to have sustained cell growth and division.  The inactivity involved in spore formation, combined with the slowing down of biological processes by dense brines in the inclusions, might just allow immensely long survival for 250 Ma without breakdown of the DNA essential for revivable dormancy.  Hydrogen diffusing into the salt and biological materials could have played a role in maintaining DNA’s integrity.  One snag is that the DNA sequence of the revived bacteria is 99% identical to that of its closest modern relative.  Using the theory of molecular clocks, they should have been different by 5 to 10%.  Yet, says Vreeland, salt deposits continually add to the surface environment, being soluble.  Any dormant bacteria within them would replenish fully living stocks in similar environments to those which formed the salt originally.  Such continual addition might preserve ancient genetics, that would otherwise evolve steadily.

Aside from giving comfort to proponents of life spreading throughout the universe as spores adrift on dust driven in the manner of a solar sail, the results encourage probing of older salt deposits, which go back in almost undisturbed form to the Mesoproterozoic.

(Source:  Knight, J.  The Immortals.  New Scientist, 28 April 2001, p. 36-39).

How the Earth works: “mega-blobs” in the mantle

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Seismic waves generated by large earthquakes arrive at different times at seismographs arranged in a world-wide network.  When they arrive depends on the relative positions of epicentres and receivers, but most importantly on variations in physical properties within the Earth that affect the speed at which they travel.  Given enough high-quality seismic records and powerful computing, such data allow geophysicists to map how wave speeds change with depth in the mantle and produce 3-D models.  In other words, seismic energy can produce geophysical homologues of medical CAT scans.  The second important means of visualizing the unseeable comes from the geochemistry of basaltic lavas formed by partial melting of the mantle in different tectonic settings.  Results from such studies reveal that the composition of the mantle is not homogeneous.  Combining information from both sources, in the light of motions of the lithosphere, provides a powerful input to modelling how the Earth behaves as a whole  (see Earth Pages, July 2000, Geodynamics).

Seismic tomography’s most important derivative stems from the manner in which wave speed depends on variations in the mechanical properties of the mantle.  For P-waves, speed varies with the mantle’s differing resistance to compression, and S-wave speed is directly proportional to the rigidity of the mantle.  Unusually high mantle temperatures cause decreases in compression resistance and rigidity, and therefore drops in the speeds of both kinds of body wave.  The cooler the temperature, the higher both speeds.  So, velocity variations in seismic tomographs are proxies for changing mantle temperature, and in turn for regions of different density – the hotter a material is, the lower is its density.  The implications are quite simple; high-speed anomalies signify cool, potentially sinking regions in the mantle, whereas low speeds suggest that matter is able to rise.  In practice, modelling the fundamental dynamics of the Earth’s mantle using seismic tomography is computationally difficult, often ambiguous and blurred because of the lack of suitable data.

Seismic tomography gave the first clues to the idea that subducted slabs penetrate all the way down to the core mantle boundary, and that at least some of the plumes suspected to underpin hot spots have their source at such depths.  Together, these findings support whole-mantle convection.  As well as improving the amount of high-quality seismic data and the software to analyse them, combining physical parameters with sketchy knowledge of variations in mantle chemistry and mineralogy is the next step in “sharpening” the focus of mantle models.  That seems to have been taken by Alessandro Forte and Jerry Mitrovica of the Universities of Western Ontario and Toronto (Forte, A.M. and Mitrovica, J.X. 2001.  Deep-mantle high-viscosity flow and thermochemical structure inferred from seismic and geodynamic data.  Nature, v. 410, p. 1049-1056).  Their work confirms the concept of whole-mantle convection resulting from thermal anomalies, but has an added bite.  They show evidence for vary large variations in deep-mantle composition – to megaplumes they have added “mega-blobs”.  Although the results of their analyses are limited by data availability and reliability, and by simplifying assumptions, they imply that such blobs can respond to temperature changes by rising and sinking periodically.  That is, the mantle may move as vast domes and downwellings as well as in the more tightly constrained plumes and sinking slabs.  One intriguing possibility is that such blobs may be primitive and retain high concentrations of elements that evolution of other parts of the mantle has transferred to the continental crust.  Such primitive signatures are passed on to the geochemistry of basalts forming from plumes beneath ocean islands.  However, there is a long way to go before a blob-plume-ocean island connection can be made.  If it proves to be plausible, then such ancient blobs would have to be very viscous to have resisted mixing over time with more evolved mantle.  Another possibility is that the blobs are themselves highly evolved, through the progressive accumulation of subducted slab material.

(See also:  Manga, M.  2001.  Shaken, not stirred.  Nature, v. 410, p. 1041-1042)

Atmospheric oxygen: yet more

Following last month’s Earth Pages briefing (Mantle overturn and oxygenation of the atmosphere)  Nature (19 April 2001) ran a news feature on the competing theories for when oxygen began to accumulate in Earth’s atmosphere (Copley, J.  2001.  The story of O.  Nature, v. 410, p. 862-864).  The paradox between evidence for oxygen production by photosynthetic cyanobacteria since 3.5 Ga and that supporting the first major influence of oxygen in redbeds at 2.2 Ga may be resolved by the ideas of Hiroshi Ohmoto of Pennsylvania State University.

Redbeds – terrestrial sediments containing abundant ferric hydroxides – form when iron enters its Fe-3 state, and are insoluble.  That results in weathering processes being unable to leach soils of their iron content, unless the waters involved have been rendered reducing by bacterial activity.  The most dramatic expression of this is laterite that blankets ancient erosion surfaces of most of the Gondwanan continents, much of which formed in Palaeocene times.  Palaeosols older than 2.2 Ga do not show the characteristic laterite ferricrete cap, implying that iron existed consistently in its soluble Fe-2 form and could be leached away.  Most geochemists regard that as evidence for a reducing atmosphere, lacking oxygen except as a trace.  Ohmoto suggests that organic acids formed by terrestrial cyanobacteria might also create the reducing conditions necessary for iron leaching..  He sees such “blue-greens” as having had a dual role, fixing iron in soils through oxidation and then releasing it to solution by formation of organic acids.  Ohmoto and Antonio Lasaga are developing a geochemical model for the iron, oxygen, carbon and sulphur cycles during the Archaean.  Early runs suggest that only 30 Ma after the appearance of cyanobacteria at 3.5 Ga their release of oxygen would have built up high levels in the early atmosphere.

That bucks the evidence for low oxygen provided by detrital sulphides and uranium oxide grains in Archaean high-energy sediments, such as the conglomerates of the Witwatersrand basin in South Africa – in the presence of oxygen, both should break down quickly in water.  Archaean banded iron formations, thought to form by reaction between Fe-2 ions in ocean water and oxygen produced locally by shallow-water cyanobacteria, have a dual significance – abundant oceanic Fe-2 suggests global lack of oxygen, and BIF deposition of ferric oxide would have formed a sink for any oxygen in the environment.  Ohmoto cites the re-appearance of BIFs at several times in the Proterozoic Eon as a sign that BIF formation was possible when atmospheric oxygen was abundant.

The debate seems destined to run, for two reasons.  Studies of sulphur isotopes – Ohmoto’s speciality – give evidence for fractionation through the influence of ultraviolet radiation.  Once oxygen rose in the air, its formation of ozone gas would have blocked UV and ended this kind of selective take-up of sulphur isotopes.  James Farquhar of the University of California in San Diego has found its effects common in Archaean rocks, but no sign in later rocks.  That favours an oxygen-poor early atmosphere.  Ohmoto counters with abundant evidence in the Archaean for the activity of bacteria that reduce sulphate ions to sulphide – in an oxygen-poor world, sulphate formation would have been suppressed.

Oxygen build-up demands complementary burial of organic matter formed by photosynthesis before it oxidized.  The influence of organic carbon burial  is to take with it 12C that biological processes favour over heavier 13C, so that carbon-rich rocks show higher 12C than carbonates precipitated from the seawater that was left.  Such enrichment in 12C shows up most clearly after 2.7 Ga ago, when carbon burial must have been stoked up somehow.  That points to a late build-up of oxygen in the air.  But why?  James Kasting, also of Pennsylvania State University, suggests a change in the Earth’s mantle from reducing to oxidizing conditions.  Before that time volcanic gases would have been dominated by reduced gases that could mop up any free oxygen.  Afterwards, oxidized volcanic gases could have co-existed with free oxygen.

Mantle overturn and oxygenation of the atmosphere

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The presence of abundant oxygen in Earth’s atmosphere defies Le Chatelier’s Principle – it should react rapidly with the rest of the environment through oxidation.  That it does not is sufficient evidence for an alien observer to conclude that our planet is dominated by photosynthetic life at its surface and the burial of carbohydrate by geological processes.  So, Le Chatelier is not defied on the long term, because the CO2 + H2O = carbohydrate + oxygen equilibrium does not reach a balance because of continual removal of organic material from the right-hand side!  That Mars has no atmospheric oxygen bears witness to its lifelessness in that respect, as concluded decades back by James Lovelock.

Before 2.5 Ga ago, in the Archaean, atmospheric oxygen was a trace gas.  Preservation of detrital grains of sulphides and uranium oxides in Archaean clastic sequences, that would have broken down in an oxidizing environment, is the main evidence for that.  The other side of the coin is that oxygen-producing photosynthesizers – the cyanobacteria – were abundant throughout the Archaean, leaving their trace as common stromatolitic carbonates and signs of the crucial enzyme rubisco in kerogens and the carbon-isotope record.

If cyanobacteria generated oxygen, then why did it not build up in the atmosphere throughout the Archaean, instead of from about 2.2 Ga ago?  The most likely explanation is that Archaean magmatism released vast amounts of Fe-II or ferrous iron to sea water, which then reacted with available oxygen to form the ferric oxide of banded iron formations (BIFs), with the biproduct of hydrogen gas that further drove Archaean environmental chemistry into a reducing condition.  Seawater circulating through Archaean ocean crust would also have enriched basalts in ferric iron by the same oxidizing reaction.  Such a chemical model still leaves unexplained the shift to an oxygenated atmosphere after the Archaean.

Norman Sleep of Stanford University, reviews an article by Kump et al. in  Geochemistry, Geophysics, Geosystems (2001) that deals with this dilemma (Sleep, N.H.  2001.  Oxygenating the atmosphere.  Nature, v. 410, p. 317-319).  Kump and his co-workers suggest that, rather than relating to a change in palaeoecology, the shift arose from subduction of dense ferric oxide-rich lithosphere to settle at the core-mantle boundary.  By the end of the Archaean oxidized material filled the lower mantle.  Heating reduced its density so that it became buoyant.  If that deep oxidized layer rose to displace more primitive, reducing mantle, later magmatism would have released less Fe-II, thereby allowing biologically generated oxygen to build up.  The converse effect would have been to bring down levels of reducing atmospheric gases, such as hydrogen, methane and carbon monoxide, to trace levels.

Except to its primitive producer – cyanobacteria – oxygen would have been anathema to the dominant anaerobic Bacteria and Archaea that constituted Archaean life.  An end-Archaean mantle overturn, implicated by the tectonic pandemonium from 2.7 Ga, could well have triggered accelerated extinction and evolution that encouraged the rise of the eukaryote cell that requires oxygen for its basic metabolism.  Nonetheless, such an upheaval would have been directly connected with earlier living processes.  That is something which will delight followers of the Gaia hypothesis.

Taming Lake Nyos, Cameroon

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On 21 August 1986 a huge cloud of carbon dioxide gas released from Lake Nyos in the Highlands of Yaounde District of Cameroon, killed 1,700 local people by suffocation

Lake Nyos is one of several maars produced by one-off explosive events in the recent past.  Isotopic analyses of gas remaining dissolved in the lake show that the CO2 is of volcanic origin.  The lakes are fed by springs on their beds, which is where the CO2 enters, so that CO2-rich water builds up at the bottom.  A thermal overturn of Lake Nyos may have caused dissolved gas to come out of solution as pressure decreased. 

Since 1986, gas levels have built up, so Lake Nyos once again threatens the local people and their livestock.  An international team, headed by George Kling a geologist at Michigan University, USA, has devised a means of venting the gas harmlessly.  This involves polyethylene pipes that descend to the lake bed.  Once primed by pumping, gas bubbles form as pressure drops.  Their rise up the pipe drags more water upwards, as in a soda siphon.  Fifteen years after the disaster, the first such siphon began operating with spectacular effects (Jones, N.  2001.  The monster in the lake.  New Scientist, 24 March 2001, p 36-40).  This only keeps pace with addition of CO2 and a full solution requires several siphons.

Some scientists worry that siphoning itself may disturb a precarious balance in the lake, so the French engineers who built it have included sensors and shut-off valves.  Not everyone agrees that the 1986 disaster resulted from processes within the deep lake itself.  That should have led to a regular succession of gas releases, for which there is little evidence.  Landslips or a gaseous eruption might have been the trigger.  Reducing dissolved CO2 levels in Lake Nyos and nearby Lake Monoun would seem to lessen risks of a future disaster, but could also lull locals into a false sense of security.

Bacterial sulphides from the Archaean

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Most of the sulphide mineralization involved in base-metal ore bodies formed by reaction between metal ions and those of sulphur released by bacteria that reduce sulphate ions in water.  They do that while oxidizing organic matter or hydrogen in their metabolism, under completely anaerobic conditions.  Like other biological processes, sulphide production at the cell level fractionates the isotopes of sulphur so that it becomes possible to chart sulphate-reducing bacteria through time.  Depletion of 34S in sedimentary sulphides relative to that in co-existing sulphates (such as baryte) was previously known with certainty back to 2.7 Ga.  Danish and Australian bio-geochemists have now pushed this particular bacterial metabolism back by 750 Ma (Shen, Y.  et al. 2001.  Isotopic evidence for microbial sulphate reduction in the early Archaean era.  Nature, v. 410, p. 77-81).

The data from the Pilbara Craton of Western Australia helps calibrate the evolutionary bush of the prokaryotes, which is based on comparisons between RNA in different living organisms.  The trouble is, sulphate-reducing species with very primitive genetics and similar lifestyles (hyperthermophilic) occur among both the Bacteria and Archaea.  Shen et al. go for the Bacteria Thermodesulfobacterium as the most likely organism responsible.  Their argument is that the mineralization replaces originally sedimentary gypsum, formed at low temperatures, and probably represents hydrothermal processes in which thermophilic organisms could have thrived.  Bacteria that reduce sulphate ions at low temperatures – gram-positive and purple bacteria – are genetically more advanced than their candidate.

See also:  Slime to the rescue Earth Pages December 2000

“Piltdown” bird

Fragmentary remains of vertebrates in particular are notoriously prone to misguided reconstruction – Gideon Mantell placed the Iguanodon’s thumb on its nose, thereby obscuring evidence for the first hitchhiking dinosaur for many decades.  The forger of fossils has two possible motives – spite in the case of Piltdown Man, or profit.  The skilled forgeries of Silurian trilobites by quarrymen from Dudley in Britain’s West Midlands are now more valuable because they were made for rapacious Victorian antiquaries, than bona fide Calymene specimens.  Missing links sought by professional palaeontologists and archaeo-biologists are in a field of their own.  It has long been suspected that birds evolved from small carnivorous dinosaurs, and the early Cretaceous of China has provided spectacular transitional fossils.  Archaeoraptor was announced as the final missing link in 1999.  Within a year it was denounced as a forgery that combines very skilfully the bones of a primitive bird with those of a non-flying dromeosaurid dinosaur.  How it was assembled has finally been revealed using X-ray tomography, which shows that as many as 5 different specimens were “cut and pasted” together (Rowe, T. et al. 2001.  The Archaeoraptor forgery.  Nature, v. 410, p. 539-40).

Cretaceous water lilies

Readers of Earth Pages will be delighted to learn that fossil flowers of Nymphaeales (water lilies) have been found in the Lower Cretaceous of Portugal.  (Friis, E.M. et al. 2001.  Fossil evidence of water lilies (Nympaeales) in the Early Cretaceous.  Nature, v. 410, p. 357-360).

When modern corals emerged

Fossil corals  fall into three taxonomic groups or Orders: tabulate, rugose and scleractinian.  Only the last group is alive today.  Scleractinian corals have been central to the “carbonate factories” that have drawn down CO2 from the atmosphere throughout the Mesozoic and Cainozoic Eras to form reef limestones.  They are major regulators of long-term climate fluctuation.  However, there is something very odd about their appearance in the fossil record, as discussed recently by George Stanley and Daphne Fautin (Stanley, G.D. and Fautin, D.G. 2001.  The origins of modern corals.  Science, v. 291, p. 1913-1914).

The rugose and tabulate corals were exclusively Palaeozoic colonial, carbonate-secreting organism.  Their record ends abruptly with the end-Permian mass extinction.  No examples of scleractinian corals have been found in rocks older than Triassic.  The oddity is a 14 Ma gap in known coral fossils in the earliest Triassic.  Scleractinians secrete calcium carbonate as aragonite, whereas rugose corals formed from calcite; an important difference in processes at the cellular level.  It is hard to avoid the conclusion that the ancestors of scleractinians did not secrete carbonate and were entirely soft-bodied taxa during the Palaeozoic Era.  If Permian Rugosa and Tabulata happily secreted carbonate, while proto-Scleractinia did not, there ought to be a biochemical or geochemical explanation for the last taking on a reef building role in Mesozoic times.

Molecular evidence suggests that scleractinian ancestry goes back to the Late Carboniferous, and that there is a complex “lawn” (as opposed to tree or bush) of genetic relationships between modern hard corals and soft-bodied organisms that are closely related.  The puzzle can potentially be resolved if modern corals and their ancestral lines lost and regained skeleton building several times in the Mesozoic and Cainozoic.  Exploring that requires more understanding of how carbonate is secreted at the cell level, and the geochemical conditions in seawater that underpin the need for secretion.

Following the greatest ever mass extinction at the end of the Permian, early Triassic oceans were almost sterile and anoxic.  Global CO2 levels were high, yet little carbonate was deposited in the marine environment.  That would have increased the amount of calcium and bicarbonate ions in sea water.  Many corals harbour algal symbionts that are involved in calcification.  As calcium carbonate saturation drops so too does carbonate secretion, and vice versa.  Calcium is a two-edged sword in cell metabolism.  On the one hand it is vital in “information” transfer, yet above a threshold it combines with CO2 to form crystalline carbonate within the cell wall, that spells cell death.  In Palaeozoic oceans rugose and tabulate corals, as well as a host of other carbonate secreting animals, would have buffered calcium concentrations below levels tolerable by other, soft-bodied animals.  Their sudden demise 251 Ma ago, along with most everything else, would have left calcium to build up in the early Triassic “Strangelove” ocean.  Survivors of the holocaust would have had a fierce task coping with potential calcium toxicity, and the scleractinians may well have adopted calcification as a survival mechanism.  Thereafter, oceans restocked with reef building organisms would have had tolerable calcium concentrations for most organisms, those now able to secrete carbonate having the benefit of armour against predation and a solid substrate for colony building.