Author: zooks777

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

Empirical geochemistry points to continents’ role in mantle dynamics

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Major-element chemistry of basalts provides proxies for key parameters involved in magmatism.  Sodium content, normalized to an MgO content of 8%, relates to the degree of mantle melting, and similarly normalized iron content helps assess the depth of melt production.  Such proxies help establish potential mantle temperatures – the temperature of magma that would erupt after rising adiabatically from different mantle depths.  Low Na8.0 suggests high potential temperature in a magma’s source.

Vast repositories of basalt chemistry relate to every conceivable setting of magmatism, so Na8.0 and Fe8.0 numbers are useful in testing various hypotheses.  One of these is that slabs of continental lithosphere affect mantle convection, by forming insulating “lids” that control surface heat flow.  Eric Humler and Jean Besse, of the Université Denis Diderot in Paris, focus on the relationship between mantle potential temperature beneath ocean-ridge systems and their distance to passive continental margins (Humler, E. & Besse, J. 2002.  A correlation between mid-ocean ridge basalt chemistry and distance to continents.  Nature, v. 419, p, 607-609).  Leaving out the complicating factors of continental margins that involve subduction and ridges affected by hot spots, they found that recent ridge basalts show higher potential temperatures when the ridge is close to continental lithosphere than for more distant ridges.  This suggests that the mantle cools away from continents by between 0.05 to 0.1°C per kilometre.  This matches the well-known increase in depth to ridges as they become further from continents.  Rather than being inert passengers on modern plates, continents do play a role in the mantle’s thermal structure.

The scope for synopsis of geochemical data is boosted by wider availability of existing data.  How tedious it used to be, trawling paper journals for tables of analyses with which to compare ones own.  It is still quite a task, but there is light on the horizon, because geochemists at the University of Mainz in Germany have made their compilations for ocean-island volcanic rocks and those from large igneous provinces (flood basalts) available on the web as the initial input to the GEOROC (Geochemistry of Rocks of the Oceans and Continents) database (http://georoc.mpch-mainz.gwdg.de ).  A similar database for ocean-floor basalts is PETDB at Columbia University in the USA (http://petdb.ldeo.columbia.edu/petdb/).  Between them, the two web sites amass over 200 thousand analyses of major- and trace-elements, and isotopes, enough for even the most ardent user of  MS Excel!

Detrital platinum-group grains and “plum pudding” mantle heterogeneity

Evidence for the degree and longevity of geochemical heterogeneities in the mantle has largely stemmed from studies of basalts derived by mantle melting.  The great diversity of melting and fractionation processes involved in their genesis obviously complicates assessment of whether or not the mantle is a mixture of several chemical domains, even though it is suspected.  Indeed it is only to be expected as a result of 4.5 billion years of mantle melting events and recycling of surface materials that find their way into subduction zones, unless, that is, long-term convection is an efficient means of mixing.  A novel approach by a team from Stanford University, the University of Copenhagen and the US Geological Survey uses a combination of the rhenium-osmium radioactive decay scheme and the tendency for Re to enter melts, while Os is highly compatible to address this long-standing conundrum (Meibom, A. et al. 2002.  Re-Os isotopic evidence for long-lived heterogeneity and equilibration processes in the Earth’s upper mantle.  Nature, v. 419, p. 705-708).  The novelty lies in their use of detrital grains of platinoids in alluvium derived from the many ultramafic masses in the western USA, rather than individual basalts or peridotites themselves.

Measurements of 187Os/188Os in the grains span a wide range from extremely unradiogenic values to those signifying a high component of radiogenic 187Os.  The data occupy a bell-shaped (Gaussian) frequency distribution.  While that probably reflects equilibration of old, unradiogenic material with radiogenic Os in melts derived from the mantle ultramafic rocks, and the destruction of any age information, it does point to mantle dotted with patches with different origins.

Reviews of climate and the hydrological cycle

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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 CO2­ in a difficult to verify process of lithospheric flatus.

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

Microgravity and diamonds

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Prospecting for diamonds relies either on lucky finds in sediments or locating the odd kimberlite pipes that brought diamonds from depths greater than 100 km in the mantle, where they form.  Such has been the centuries-old frenzy for diamonds that most deposits of the trip-over kind have been found.  One of the last major diamond fields turned up in Arctic Canada, after prospectors panned their way upstream of glaciers that had dropped the odd diamond in Canadian Shield tills.  It is simply too costly to keep repeating this painstaking exercise to satisfy the enduring demand for diamonds of all qualities.  New sources probably exist in huge, unexplored regions of Canada, Australia, Africa and north Asia, yet kimberlites, often having broken down to clays and forming little by way of topographic features, are not easy to find.  Great efforts have been made to harness conventional remote sensing that uses reflected and emitted electromagnetic radiation, but with little success.  Aside from the innocuous nature of kimberlites, most prime ground is either flat, vegetated steppe in areas once affected by glacial conditions, the featureless soil covered tracts of interior Australia or tropical rain forest, where remote sensing simply does not work well enough.

Kimberlite pipes have round traces at the surface and the rock has a different density from common rocks of the upper crust, so one means of locating them is by looking for circular patterns on gravity maps.  But they are small relative to the resolution of regional gravity maps, which are generally constructed by careful measurement of gravitational field potential at points on the surface.  It is not that gravimeters are incapable of detecting differences due to rocks with anomalous density, but that sample spacing is too coarse (>1km) because of the high cost of field surveys.  Maps of the Earth’s magnetic field and emissions of gamma-rays by radioactive isotopes are routinely created at suitable resolution by aerial surveys, but kimberlites show only subtle features on them.  Airborne gravity surveys have been a grail for explorationists for many physical resources, but insufficient economic interest has blunted the search for a way of overcoming the effects of turbulent accelerations during flight, which spoil measurements of the actual gravity force field.  Mining company Broken Hill Proprietary – Billiton’s venture into diamonds after their acquisition of the Ekati deposit in northern Canada has encouraged them to seek a cunning approach to the problem.  Whereas measuring gravitational potential from the air is a tough nut to crack, the US navy had developed an instrument to measure changes in the gradient of the gravitational field that can overcome varying accelerations, to help nuclear submarines navigate without recourse to giveaway sonar “pings”.  BHP-Billiton is into this technology in a big way, now that it has been declassified.  While gravity gradiometry offers one way of revolutionizing the precision of gravity surveys, other methods are possible, and it is rumoured that geophysicists who try to measure even tinier shifts in the gravitational field to monitor the rise and fall of magma in volcanoes are onto a cheaper and less convoluted method………

Source:  Nowack, R. 2002.  Pulling power.  New Scientist, 21 September 2002,p. 42-45.

Kennewick Man freed for research

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Some years back, a near complete skeleton emerged from a terrace on the Columbia River, in the north-western USA, near to Kennewick.  Preliminary examination suggested that the skull had distinct European features, and some thought that these were the remains of some early pioneer.  Kennewick Man attracted considerable attention when the terrace was dated at 9300 years, because the individual would then have been among the earliest known colonizers of the Americas.  Five local tribes of  Native Americans laid claim to the bones under the Native American Graves Protection and Repatriation Act, considering him to be an ancestor.  The bones were taken into custody, thereby halting further research.  Several academics saw this in a malevolent light, since if it was proven that the skeleton was indeed of European origin instead of Asian that would undermine a major plank in Native Americans’ claims for primary occupation of land; the central issue in a vast raft of legislation over ownership of mineral reserves.  Pressure for release of the bones for research has built over the last two years, finally to overcome concerted opposition that wished to re-bury the bones with due resepct.  The magistrate who judged the case found the original decision for sequestration “arbitrary and capricious”, and so investigations can resume.  Quite possibly DNA will be preserved, and that could set the cat among the pigeons in Native American circles.  However, some experts who had a quick look at the skull suggested that it might well be of an Ainu, one of the earliest inhabitants of the Japanese islands, who bear passing resemblance to Caucasian people..

Sea level fluctuations and large igneous provinces

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On a global scale, shifts in sea level recorded by stratigraphers and on seismic profiles stem from one of two main processes: changes in land-ice volume and the volume of the ocean basins.  The latter most often results from changing rates of sea-floor spreading, so that when it is rapid a greater volume of the lithosphere near spreading centres retains sufficient buoyancy to displace the oceans onto continental margins.  During slow spreading, cooling of the lithosphere and an increase in its density enlarges the deep abyssal plains, so that the oceans withdraw to low levels.  The mid-Cretaceous saw vast outpourings of plume-related lavas onto the floor of the West Pacific.  So large, that they reduced the volume of the Pacific basin enough to result in continental flooding that was unprecedented in the Phanerozoic Eon.

On a local scale, changes in sea level recorded by the stratigraphic record include those due to local processes, generally ascribed to tectonic events at continental margins, which involved rising continental lithosphere.  However, one of the greatest forces for local change in the continental freeboard is changing density of the lithosphere due to thermal effects.  Anywhere once affected by major igneous events should record relative falls in sea level during the acme of magmatism, and rises when activity waned.  The British Tertiary Igneous Province, a precursor to the eventual rifting of the North Atlantic under the influence of the Iceland plume is a good candidate for charting magma-sea level connections.  The central volcanic complexes of the Hebrides, and their enveloping flood basalt piles formed at the start of the Palaeocene (~60 Ma).  Around that time, much of the British Isles underwent several kilometres of vertical uplift and exhumation, whose effects remain today.  In the surrounding marine basins, this event is recorded by Palaeogene sandstone bodies, presumable derived by erosion of the uplifted crust.  Yet local Palaeogene sediments also record episodes of rising sea level.  John Maclennan and Brian Lovell of the French Institut de Physique du Globe and Cambridge University have modelled the likely effect on sea levels around the British Isles by crustal underplating of magmas formed during the BTIP magmatism (Maclennan, J. & Lovell, B. 2002.  Control of regional sea level by surface uplift and subsidence caused by magmatic underplating of the Earth’s crust.  Geology, v. 30, p. 675-678).

Up to 8 km of mafic igneous rocks seem to have ponded at the base of the British Isles’ crust while the BTIP was active.  This estimate stems from the fact that the lavas of the province evidence high-pressure fractional crystallization.  Calculations of the percentage of cumulates needed to generate the bulk chemistry of the BTIP lavas suggest that their volume far outweighs that of the volcanic part of the province.  Given estimates of the volume of underplated cumulates, modelling boils down to examining the consequences for lithospheric density of initial heating and its subsequent relaxation.  The Palaeogene sedimentary record provides good support for the model, with massive uplift from 60-56 Ma (the period when the BTIP was forming).  Sudden sea-level rise at the end of this period never reached the level prior to magmatism; in fact it amounts to one half the estimated uplift.  That is precisely in line with the underplating model.

Prediction of earthquake periodicity founders

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In an number of well-studied areas of chronic seismicity it appears from historical records that earthquakes recur with regularity.  If that was so, it might be possible at least to prepare to throw many methods of detecting imminent movements at such areas, when they are “due” to go off.  The theory behind time-predictability is that earthquakes relieve tectonic stresses along faults, and that if the forces are maintained, stress builds up again, to be released after a roughly fixed time (the same might apply to volcanism where magma production stays constant).  A corollary is that high-magnitude events have longer periodicities than those lower on the Richter scale.  One of the best cases thought to support this view is a 25-km stretch of the San Andreas Fault near Parkfield in California.  The area has had  5 or 6 earthquakes greater than magnitude 6 since 1857, roughly every 22 years, the last being in 1966.  There ought to have been one in 1988, but the poor statistics give an uncertainty either way of 10 years.  By now there should have been a magnitude-6 event in the area, but it hasn’t happened.  Jessica Murray and Paul Segall of Stanford University have analyzed the physics of the last event, and of the period that followed it.  (Murray, J. & Segall, P 2002.  Testing time-predictable earthquake recurrence by direct measurement of strain accumulation and release.  Nature, v.  419, p. 287-291).

Their work involved using precise geodetic measurements obtained over the last four decades to assess the 1966 Parkfield earthquake’s size, which combines the movement then along the San Andreas Fault, the area involved in the slip and how “stiff” the crust is locally.  Comparing this with geodetic data since then suggests strongly that the strain released in 1966 must have recovered between 1973 and 1987.  They have shown that another Parkfield earthquake is long overdue.  Their method rigorously allows for the effects of movements along other nearby fault, and inherent unpredictability seems inescapable.  While other tests of the time-predictability principle, theoretically the most plausible approach, will continue, most devastating earthquakes continue to occur without forewarning.  That reflects the fact that there are only enough seismologists with fancy equipment to cover threatened areas in a few extremely rich countries.  Most people who live along active fault zones know whether or not high-magnitude earthquakes occur in their vicinity, yet will not have the privilege of scientists and equipment to provide warnings of this kind for a very long time, for simple economic reasons.  Perhaps some effort and funds should be diverted to providing warnings within days of a serious event, using less “robust” methods.

See also:  Stein, R.S. 2002.  Parkfield’s unfulfilled promise. Nature, v.  419, p. 257-258.

British Geological Survey sued over arsenic

The world’s largest ever class action has been launched in London against the British Geological Survey, over claims that it failed to spot arsenic contamination during a 1992water survey in Bangladesh. As many as 40 million Bengalis risk arsenic poisoning, following a major groundwater development programme in the 1970s and 80s.  Arsenic poisoning at non-fatal doses often shows first as water blisters on hands and shins. Long-term exposure via drinking-water causes cancer of the skin, lungs, urinary bladder, and kidney.

Aid agencies, led by UNICEF sank four million wells deep into alluvium, in the hope that groundwater use would alleviate the chronic problem of heavily polluted surface water in Bangladesh.  The arsenic is of natural origin, and stems from leaching of the toxic element from sulphide minerals by deep, reducing waters.  The case hinges on BGS’ failing to test for arsenic, which is easily detected using low-cost semi-quantitative methods, only 3 years after they had completed a comprehensive evaluation of groundwater quality in Britain that did include arsenic measurements.  Accusations of double standards have been flying.  However, UNICEF also failed to test for arsenic during the original drilling, because they did not expect to find it in the water.  World Health Organization guidelines are very clear that arsenic does pose a threat in groundwater, but most cases in the past have been associated with former mining areas.

Considerable work on measures to clean up well water has been conducted since the Bengal arsenic crisis surfaced.  Under oxidizing conditions, arsenic is adsorbed by ferric hydroxide, and a simple remedy is passing the water through iron wool or over ground-up rust or natural ochres.

Conodonts and late Devonian mass extinction

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The Late Devonian saw sufficient extinctions (around 55 % of all genera) for it to rank among the Big Five, but most genera that disappeared were shallow-water marine, particularly rugose and tabulate corals.  Although the Woodleigh impact structure, just north of Perth in Western Australia, has been suggested as a possible culprit, its age is not reliable.  Another possible cause is climatic cooling at low-latitudes, because the extinction was followed by the spread to tropical localities of high-latitude faunas.  The key to supporting a climatic influence is temperature data from areas most affected by the extinctions.  Unusually, a recent study selected phosphatic conodonts (tooth-like microfossils) for oxygen-isotope investigations – carbonate-shelled creatures are the usual choice.  Michael Joachimski and Werner Buggisch, of the University of Erlangen in Germany, found prominent oxygen isotope excursions close the Frasnian-famenian boundary (Joachimski, M.M & Buggisch, W. 2002.  Conodont apatite d18O signatures indicate climatic cooling as a trigger of the Late Devonian mass extinction.  Geology, v. 30, p. 711-714).  Their data are well controlled stratigraphically, because the rapid evolution of conodonts in the Devonian allows fine biostratigraphic division.

The extinction event is bracketed by two espisodes of sea-surface cooling, estimated to involve a drop of 6°C from an otherwise constant ambient temperature of around 32°C.  They coincide with significant positive shifts in d13C of seawater, interpreted by the authors as evidence of the burial of much organic carbon debris.  Therein lies a possible cause for the cooling.  Carbon burial would have drawn down atmospheric CO2 levels.  The extinction does seem to have been a response to temperature stress, tallying with the colonization of low-latitude seas by high-latitude faunas.  However, that still begs the question of why carbon burial underwent two spurts.  Was there an increase in sediment supply to the oceans that might augment burial rates, or are the positive carbon-isotope excursions reflections of the extinctions themselves?  The second still leaves open the possibility that the undoubted cooling events may have had other causes, such as an increase in stratospheric aerosols, resulting either from major explosive volcanism or perhaps impacts that are yet to be found.

Landsat to be privatised, once more?

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Remote sensing, once the domain of researchers seeking hitherto undiscovered potato fields, lost cities and the intricacies of drainage patterns, entered the commercial domain in a big way about a decade ago.  As well as giving lugubrious views of factories reputed to be manufacturing weapons of mass destruction, the aftermath of their bombing and that of villages alleged to harbour agents of the “axis of evil”, remote sensing helps find physical resources, spots farmers who fraudulently claim subsidies for non-existent crops and is used to site cell-phone transmitter networks.  There are now several orbiting systems launched by commercial outfits that offer pin sharp and spectrally revealing information, at a cost.  The workhorse of remote sensing since 1972 has been the US Landsat series.  Following the addition of the Thematic Mapper in 1984, pressure grew for Landsat’s privatization in 1988.  Prices jumped tenfold, to the horror of researchers, and the venture became uneconomic because of insufficient private-sector interest.  Landsat 7, which carries an Enhanced Thematic Mapper, made orbit in 1999, and is administered by the US Geological Survey.  Landsat-7 ETM data sell at $600 per scene, which is a bargain.  Such has been the demand for data that US authorities are once more trying to shed responsibility for data provision to private hands, by asking for bids to develop, launch and market the next Landsat.  Prices will once again leap to profitable levels.  The joint US-Japan ASTER system aboard the ostensibly research-oriented Terra satellite rivals Landsat ETM in quality, and many scientists have been trying out the data.  Again, to their disquiet, pressure reputedly from the Japanese partners has resulted in once free data being assigned a price of $55 per scene

Protocol wars

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Finding a new species of fossil organism is not usually a big deal.  There are lots out there, and palaeontological journals publish formal descriptions regularly.  The finder moves on, and as often as not allows other scientists in the field free access to the original specimens.  Free exchange of published data, allowing colleagues to add to knowledge of materials by direct study, and, in most branches of science, verification by inter-laboratory analysis of material is part and parcel of research.  The priceless Apollo lunar samples and many meteorites move freely because of these informal protocols.  Things are different when the materials are “hot news”, none more so than remains from the human bush of evolution (Gibbons, A. 2002.  Glasnost for hominids: seeking access to fossils.  Science, v. 297, p.1464-1468).

Protocols for hominid specimens often allow access only to the finders, their colleagues and trusted friends, until they have performed the most minute investigation and written detailed monographs.  The rules are sometimes laid down legally at governmental level.  This can extend even to casts and CT-scan facsimiles. There are often delays of a decade between first publication of a new species and basic information, and the fossils’ entering the public domain.  Unsurprisingly, this frustrates palaeoanthropologists who do not have the luck to make a major discovery – useful hominid material is exceptionally rare, despite the fanfares which greet its first publication.  Consequently, eager students of human origins try various ploys to get in on the act, such as detailed photography of specimens in museums, and furtive digs for new material at the original sites.  Sometimes they are thwarted, sometimes not (See April Earth Pages News, Homo erectus unification?).  Berhane Asfaw, of the Middle Awash Research Team that has done so much to advance knowledge of our early ancestors, commented, “You don’t know how we suffered in the field to get these fossils”, when putting a halt to such a disingenuous attempt to snaffle pictures.

Long-range forecast: a prolonged interglacial

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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.