Author: zooks777

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

End-Permian devastation of land plants

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The mass extinction that marks the boundary between the Palaeozoic and Mesozoic Eras snuffed out more than 90% of marine animal species and about 70% of terrestrial vertebrates.  The most complete record of the Permian-Triassic boundary is in marine sediments atop an obducted ophiolite in Japan (Isozaki, Y., 1997.  Permo-Triassic boundary superanoxia and stratified superocean: records from lost deep sea.  Science, v. 276, p. 235-238).  These record a 20 million-year period when deep ocean water was lacking in oxygen, and the anoxia reached extreme conditions for about 4 million years across the boundary.  All the palaeontological signs are that shallow marine faunas dwindled slowly in the 10 million years before the P-T event.  Carbon isotopes from hydrocarbon-rich boundary strata in Canada suggest that over a period of  only 1000 years the oceans were almost devoid of life.  The open oceans had become dead from top to bottom; a scenario graphically expressed by Ken Hsu as a “Strangelove” ocean.  Whatever the pace of preceding extinctions the boundary event was a catastrophe, and the Japanese and Canadian sections suggest that maybe a half-million years passed before surviving organisms began to recover and diverge.

The much-studied K-T boundary’s association with abundant evidence for an associated giant impact, prompted geologists to look for a similar story for the near end of Earth’s life 190 million years earlier.  Supporting evidence has yet to emerge, although the boundary includes the period when huge volumes of continental flood basalts poured over what is now Siberia.

Terrestrial records are far less easy to divide into fine time divisions, partly because they record both deposition and erosion, and partly because fossils are less well-preserved than in marine sediments.  Continental sediments spanning the P-T boundary are particularly frustrating, because of the wide extent of arid to semi-arid conditions then.  The Karoo Basin of South Africa does record wonderfully the fate of vertebrates (only 6 out of 44 genera survived the  boundary event), but less so that of plants.  Abrupt changes in plant-life are equally as important as those of animals, simply because they are at the base of the terrestrial food chain.  One way of addressing vegetation shifts of the most general kind is to look for evidence of how river systems changed their patterns of deposition, and this is what a team from the University of Washington (Seattle) and the South African Museum have done in the Karoo Basin (Ward et al., 2000.  Altered river morphology in South Africa related to the Permian-Triassic extinction.  Science, v. 289 8 September 2000, p. 1740-1743).

Peter Ward, David Montgomery and Roger Smith examined sedimentary structures produced by river channels in the sandstone members of the Karoo sedimentary pile.  Permian rivers seem to have flowed in distinct, meandering channels, whereas those of Triassic age laid down sands that show consistent evidence for intricately braided  channel systems.  The shift from one to the other type falls right at the P-T boundary.  Meanders of large river channels typify land surfaces with abundant vegetation that binds alluvium.  Where vegetation cover is sparse, there is little to constrain river flow and alluvial erosion, and wide braided river courses develop.  The authors conclusion is that vegetation suffered a catastrophic die off at the P-T boundary, leaving formerly lush plains as sandy wastes.  Such a loss of plants that would previously have contributed to balancing the atmosphere’s CO2 levels and the proportions of light and heavy carbon isotopes in the global environment would have helped produce the “Strangelove” signal in the ocean sediments.  The land was seared, and evidence from similar sediments in Australia and Antarctica suggests a global loss of plant life.  Incidentally, the boundary in many places shows a leap in the abundance of fungal spores, so the Mesozoic began with decay on a grand scale.

See also: Kerr, R.A., 2000.  Biggest extinction his land and sea.  Science, v. 289 8 September 2000, p. 1666-1667.

Carbon isotopes of individual microfossils

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Organisms at the base of the food chain, autotrophs that synthesise biological compounds directly from carbon dioxide, water and other fundamental materials in their environment, favour incorporating the lighter of the two common isotopes of carbon, 12C, as opposed to 13C.  Consequently, one of the prime signatures of life in the carbon found in rock is a depletion in 13C, usually expressed as d13C with a negative value.  It is this signature that has allowed the origin of life to be pushed back almost to the age of the oldest rocks on Earth (around 3.9 billion years ago) from carbon isotope studies of carbonaceous compounds (kerogen) in ancient sediments.

Different organisms alive today, particularly among the ecologically diverse bacteria, use different biochemical reactions in synthesising living material.  Each of these have different effects on d13C.  Potentially these differences could be used to identify roughly the kinds of bacteria that lived in the distant past.  Up to now, however, isotopic studies of organic carbon have only been possible for bulk extracts from rock.  That enables some bold conclusions, such as the current suggestion that oxygen-producing blue-green bacteria were around 3.5 billion years ago, but whole-rock results are ambiguous because of mixing of carbon originating from different metabolic pathways. 

Being able to analyse carbon isotopes from individual fossil cells is a major breakthrough, and a team of palaeobiologists from the universities of California and Regensburg, Germany has done just that (House, C.H. et al., 2000.  Carbon isotope composition of individual Precambrian microfossils.  Geology, v. 28, p. 707-710).  They used an ion microprobe that allowed the discovery of biological carbon encapsulated in resistant materials from 3.8 billion-year old metamorphosed iron formations from West Greenland.  That involved probably mixed carbon of biological origin.  In the new work, the isotopic analyses are from individual bacterial cells preserved in 850 and 2100 Ma banded iron formations, and suspected to be blue-green bacteria.  The results clearly distinguish one metabolic pathway – the Calvin cycle used by blue-greens – from other possibilities.

Tangible bacterial fossils go back, albeit rarely, to more than 3 billion years ago.  It is the older life forms that are most intriguing, because by 2100 Ma ago the Earth’s atmosphere had become oxygen bearing, thereby allowing the rise of the Eucarya from which we stem.  Older material might give clues to the more primitive Bacteria and Archaea that were the exclusive rulers of the biosphere before about 2200 Ma, and controllers of the Earth’s atmospheric composition and thereby its climate, which remains a mystery.

Timing the uplift of the Tibetan Plateau

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The rise of the huge Tibetan Plateau, with an average elevation of 5 km, presented a major barrier to atmospheric circulation, perhaps one of the largest that has ever existed.  With its latitude close to the down flow of the tropical Hadley cells, it has had an effect on the Asian monsoon in particular, strengthening its effects.  Many climatologists believe that Tibet has played a major role in global climatic change towards the end of the Cainozoic.  So, timing the uplift is critical in assessing the modelled effects in relation to detailed climate records of the Neogene.  This is by no means easy, for the late-Tertiary sediments are terrestrial in origin.

A team of Australian and Chinese geologists focussed on the sedimentary record in the Tarim Basin, north of the Kunlun mountains that form the northern flank of the Tibetan Plateau (Zheng, H.  et al., 2000.  Pliocene uplift of the northern Tibetan Plateau.  Geology, v. 28, p. 715-718).  Sediments there change from redbeds deposited in gently sloping flood plains to coarse debris laid down by flash floods at a rising mountain front; exactly the relationship that records the beginning of uplift in northern Tibet.  Dating this is no easy matter, however.  The technique that the team used is magnetostratigraphy, based on highly sensitive measurements of the polarity of 2500 samples of weakly magnetized sediments.

The change in facies spans a period when the Earth’s magnetic field was reversed – the Gilbert reversed chron – which occurred between 4.5 to 3.5 Ma ago.  The maximum age for the beginning of Tibetan uplift in the north is therefore 4.5 Ma, in the Pliocene.  This contrasts with the accepted age of Oligocene – Miocene for uplift of the Himalaya and southern Tibet, and with models that postulate climatic change that followed it.  Whereas the Tarim Basin today is arid, the sediments indicate that until the Pliocene abundant water flowed at the surface, to deposit great thicknesses of fine alluvium.

Danger of CO2 release in Cameroon

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On 21 August 1986 a huge cloud of carbon dioxide gas was released from Lake Nyos located at 300 metres in the Highlands of Yaounde District of Cameroon. Because carbon dioxide is more dense than air it hugged the ground and flowed down valleys. The cloud travelled as far as 15 miles (25 km) from the lake. It was moving fast enough (as much as 80 kph) to flatten vegetation. 1,700 local people died by suffocation, probably unaware of their plight.   Two years earlier 37 people died similarly in a gas release from nearby Lake Monoun

Lake Nyos is in the Oku volcanic field, and 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.  Being extremely deep (about 200 metres) and with no surface inlet the lake water is strongly stratified, so that CO2-rich water builds up at the bottom.  The gas release must have involved an overturn of the stratification, so that dissolved gas came out of solution as pressure decreased.  What triggered the overturn is hard to establish, but one possibility is that during August (both catastrophes occurred in that month) cold weather cools surface waters so that they sink.  Other possibilities are storms, landslides or earthquakes, but there are no records of any of these preceding either event; they came completely unannounced.

Since 1986, gas levels have built up, and now stand at twice their concentration following the disaster, so danger threatens the local people and their livestock once again.  An international team, headed by George Kling a geologist at Michigan University, USA, has devised a means of venting the gas harmlessly.  This involves sinking 15 centimetre diameter polyethylene pipes to the lake bed.  Once pumping starts, gas bubbles forming as pressure releases will drag the water upwards, as a self-sustaining siphon, similar to the air-lift dredges used in marine archaeology.  Four such pipes would rid the lake of its lethal gas content in two years, and even one would reduce the hazard considerably.

Sources:  Observer, 20 August 2000, University of Michigan (http://www,biology.lsa.umich.edu/~gwk/research/nyos.html)

Inglorious mudstones

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Because they succumb to erosion easily, mudrocks do not outcrop well, except on the coast or in arid lands.  Often they show little if any stratification that field workers can distinguish from the partings imparted by compaction and dewatering, which make shales from them.  Yet they are repositories of a great deal of information (see Earth Pages archives – Methane hydrate – more evidence for the ‘greenhouse’ time bomb).  In hand specimen their two main components, silt-sized quartz grains (<62 microns) and clay minerals (>4 microns) only become distinguishable by chewing!  They are irresolvable using optical microscopes, and detailed work needs scanning electron microscopy.

Silt to clay proportions in mudrocks are variable. The first is generally taken as an indicator of suspended debris from land masses and its proximity to where the mud accumulated.  The more clay, the further muds were from exposed continents, or so sedimentologists used to assume.   That approach has taken a hard knock from some recent detailed work on Devonian mudrocks (Schieber, J. et al., 2000.  Diagenetic origin of quartz silt in mudstones and implications for silica cycling.  Nature, v.  406 31 August 2000, p 981-985).

Jurgen Schrieber of the University of Texas (Arlington), Dave Krinsley of the University of Oregon and Lee Riciputi of the Oak Ridge National Laboratory in Tennessee used scanning electron microscopy, cathodoluminescence and ion-probe techniques to discriminate between detrital quartz grains and those formed by precipitation of silica from pore water in the original muds.  Those grains that do not luminesce probably formed by silica solution and reprecipitation, and the Devonian mudrocks contain mainly non-luminescent quartz grains.  Oxygen isotope ratios from individual grains confirm this in situ origin.  The researchers had no reason to suspect that their Devonian samples would give such results, and assumptions based on silt to clay ratios from any mudrock are now in doubt.

Worse still, silt in ocean-floor muds, cores of which form the linchpin for Pleistocene climate studies, has been a rough and ready way of estimating wind speeds as climate shifted from glacial to interglacial conditions.  These silts could be precipitates too, and the variations in their proportions may stem from changes in the delivery of dissolved silica from land to the oceans.

See also:  Kemp, A.  2000.   Probing the memory of mud.  Nature, v. 406 31 August 2000, p 951-953

Atmosphere linked to Earth’s rotation

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One of the annoying features of the Earth as a planet is that it engages in a kind of Saint Vitus’ dance.  The best known of its wandering are those involving variations in the eccentricity of its orbit, and the tilt and precession of its axis of rotation.  These follow from the gravitational influences of massive planets elsewhere in the Solar System, and are implicated in the modulation of climatic change through the last 2.5 Ma.  Rather less well-known, and even more aggravating are far more rapid, but geometrically quite small deviations from good behaviour.  One of these is the habit of the spin axis to wander around the geographic poles within a circle roughly 3 to 6 metres across.  It does this every 14 months.  It takes a certain degree of dedication to chart such a tiny planetary tic.  Chandler Wobble is the single claim to fame of its eponymous discoverer.  Seth Carlo Chandler Jr, an American businessman and amateur astronomer, discovered the quirk in 1891 by observing stars with a degree of single-mindedness that might have put a lesser mortal on the couch.  He set out to verify the famous Swiss mathematician Leonhard Euler’s prediction that the Earth ought to wobble every year, and he did.

So minuscule is Chandler Wobble, that keeping it going is something of a vexing problem, for a single jostle’s effect ought to fade away in a few years.  There are innumerable ways of nudging the Earth, and deciding which is sufficiently regular and just right to maintain the wobble is no easy task.  Following in the great tradition of Seth Chandler, Richard Gross of the Jet Propulsion Laboratory compared Wobbling between 1985 and 1996 with the continual but inconstant motions of atmosphere and oceans, as simulated by super-computer modelling of climate.  The forces of winds and currents are simply insufficient to induce the Wobble, but variations in atmospheric and deep-water pressure, together with their positional shifts are, in the manner of Goldilocks and the little bear’s porridge, just about right.  Because changes in water depth are wind-driven (as for instance with the wandering hump in the Pacific’s surface, linked with El Niño), ‘weather’ is the ultimate driving force for Chandler Wobble.

Why devote time to this picayune curiosity?  The answer is to chart more accurately the position of distant spacecraft; not easy when the measuring platform is behaving like a Womble.

Source:  Richard A. Kerr, 2000.  Atmosphere drives earth’s tipsiness.  Science, v. 289, p. 710.

The guts of a sea-floor spreading system

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What goes on beneath constructive plate margins, and ocean ridges has, up to now, been largely a matter of conjecture, blended with the geology of ophiolite complexes obducted onto continents.  Ophiolites are perhaps not such a good model, since the low buoyancy of the basalt capped lithosphere that they represent prevented them from subduction, and stems from unusual conditions.  The bulk of oceanic lithosphere is destined for resorption into the mantle, and it forms at common or garden ridge systems.

One way of modelling magmatism at ridges is through geochemical analysis of mid-ocean ridge basalts matched with topographic and structural detail of the ridge itself, but this is a blurred approach.  It shows that part of the process must involve ponding of magma in chambers at shallow levels beneath the ridges.  The other aspect is the form taken by the mantle that must rise to undergo adiabatic partial melting.  For fast-spreading ridges, such as the East Pacific Rise, there are two such models: constraint of rising mantle in two-dimensional sheets descending from beneath the ridge itself; three-dimensional plumes of mantle from which magma migrates laterally to ridge segments.  Amplifying geochemical-structural models needs a better idea of the actual processes and the geometries that they take.  A means of getting this information is to use a technique well-honed by petroleum exploration; 3-D seismic reflection profiling.

A consortium of geophysicists from the universities of California and Cambridge used this costly method, involving 200 profiles, to look at 400 km2 of the East Pacific Rise at 9°N (Kent, G.M. and 10 others, 2000.  Evidence from three-dimensional seismic reflectivity images for enhanced melt supply beneath mid-ocean-ridge discontinuities.  Nature, v. 406, p. 614-618).  Melts have about half the seismic velocity of solid rock, and so boundaries between melt and solid show up with better contrast on seismic records than do boundaries in piles of sedimentary rocks.  The surprising result is that instead of vertically extensive magma chambers, expected from either hypothesis, melt occurs in a narrow, continuous sill-like body beneath the ridge.  This connects to a plunging tongue that is probably the path taken by magma from the zone of partial melting in the mantle.  The sill itself occurs at a fixed depth below that predicted from ophiolite studies for the level at which vertical sheeted dykes form the lower part of the petrologically defined crust.  This suggests that the magma simply cannot rise en masse to inject along extensional fissures as the lower crust fails, the sheeted dyke layer acting like a seal in the flow of petroleum in sedimentary basins.  Instead, it seems more likely that magma ekes out as rising rivulets that follow the base of the dyke layer until the reach dilatations at the ridge.

Although results from this study are inconclusive as regards the two models for rising mantle, the detail that it reveals augurs well for further 3-D surveys of ocean magmatism that will complement seismic tomography of the deep mantle.

Flood basalt events and mass extinctions

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Searching for sudden events that might explain the disappearance of sizeable proportions of fossil taxa is a growing cottage industry among geologists.  Until 1980, with Alvarez’ discovery of geochemical evidence for a comet or asteroid impact at the Cretaceous-Tertiary boundary, such tumbles in life’s diversity and volume were merely palaeontological markers which geologists chose to divide the stratigraphic column of the Phanerozoic into Periods and Stages.  Mass extinctions now take on a much greater importance through the hunt to explain them.  The popular vision of herds of dinosaurs writhing in the inferno following the Chixculub bolide strike at the K-T boundary dwarfs to a large degree the equally certain knowledge that at the same time vast basalt floods in what is now north-western India may have had an equally doleful outcome.

Super-large volcanic events, akin to the Deccan Traps, are a great deal simpler to spot than the subtle signs of impacts in the rock record.  Improved precision in dating such basalt piles shows that three of the “Big Five” mass extinctions occurred within the 1 to 2 million-year life spans of flood-basalt paroxysms: the Deccan Traps at the K-T; The Parana Basalts at the Triassic-Jurassic; and the Siberian Traps at the Permian-Triassic boundaries.  A similar correlation exists for the lesser Palaeocene-Eocene boundary event at 55 Ma, which implicates the North Atlantic large igneous province responsible for flood basalts in north-west Scotland and Greenland.

The scales have tilted further towards a terrestrial cause for mass death with the recent discovery that the Karoo and Ferrar flood-basalt provinces of South Africa and Antarctica formed at a time (183.6+ 1 Ma) that brackets a lesser extinction event in the early Jurassic Period.  Jósef Pálfy of the Hungarian Natural History Museum and Paul Smith of the University of British Columbia (Pálfy, J.  and Smith, P.L., 2000. Synchrony between Early Jurassic extinction, oceanic anoxic event, and the Karoo-Ferrar flood basalt volcanism. Geology; v. 28, p. 747–750) use U-Pb dating of thin volcanic ash layers in the Jurassic sedimentary pile of North America to calibrate the ages of individual ammonite Zones of the Pliensbachian and Toarcian Stages of the Jurassic.  At that time, about 25 % of organisms at the family level became extinct globally over a period of about 4 million years – the Pliensbachian-Toarcian event was not abrupt.  The record in the British Jurassic for extinction of marine animal species shows a marked change at around 183 Ma, within the time span of the Karoo-Ferrar eruptions.

This correlation ties in well with the Toarcian ocean-anoxia event, recorded in the British and Swedish Jurassic (see Earth Pages archives – Methane hydrate – more evidence for the ‘greenhouse’ time bomb) which seems to have coincided with a huge gush of methane into the atmosphere, released by methane hydrate layers in ocean-floor sediments.  Methane, a greenhouse gas in its own right, oxidizes to carbon dioxide.  What may have happened is that the Karoo-Ferrar volcanism injected massive amounts of CO2, leading to global warming.  This, transmitted to deep ocean water, could have triggered breakdown of methane hydrate to give a massive positive feedback to global climate.  The heat itself might have driven species and families to extinction, or changed ocean circulation to induce stagnation and anoxia.

Important as Pálfy and Smith’s findings are, they by no means resolve the complexities of interwoven terrestrial events.  The 90 million-year old Cenomanian-Turonian ocean-anoxia and extinction event had an associated methane burst, but no flood basalts.  That at the Palaeocene-Eocene boundary has no associated anoxia.  The largest basalt flood known, beneath the Pacific to form the Ontong-Java Plateau about 120 Ma ago, induced methane release and anoxia, but has no associated extinction peak

Despite well-funded attempts to link mass extinctions, other than the K-T event, to impacts, there is little tangible sign of such a connection using precise radiometric dating.  Still, the focus of high-profile stratigraphic research is on boundaries rather than what lies between them.

Putting numbers on ecological effects

In the same issue of Geology a team of American palaeoecologists (Droser, M.L.. Bottjer, D.J., Sheehan, P.M. and McGhee, G.R., 2000. Decoupling of taxonomic and ecologic severity of Phanerozoic marine mass extinctions. Geology; v. 28, p. 675–678) assess the degree to which ecologies change after mass extinctions.  They focus on the Late Ordovician and Late Devonian events (two of the “Big Five”).  Although both involved similar levels of loss of taxonomic diversity (about 22% decline in marine families), marine ecosystems underwent no significant change after the Ordovician event.  Following that towards the end of the Devonian, however, marine ecology changed drastically.  One example is reefs colonized by tabulate corals.  The early corals were devastated by both extinctions, losing about 75% of taxa.  Coral-rich reefs continued after the Ordovician, but virtually disappear from marine ecosystems after the Devonian, until much later in geological time.  The most likely explanation for this is that Palaeozoic reefs formed mainly from organisms known as stromatoporoids, which gave the 3-D structure required for tabulate corals.  Stromatoporoids lost 50% of their diversity after the Devonian event, and did not recover as reef-formers.  The main implication of this study is that the effects of extinctions do not simply depend on the quantity of taxa that are snuffed out, but on specific components of the ecosystems involved.

Cretaceous beetle attack

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Following last month’s item on vast moth beds in Denmark, yet another bizarre result of painstaking palaeontological research has surfaced in the July 14 issue of Science.

Palaeobotanist Peter Wilf of the University of Michigan, and colleagues, have collected extensively from the late-Cretaceous and early Tertiary terrestrial sediments in Wyoming and North Dakota.  Among their specimens of early angiosperm (flowering plants) leaves are a number showing evidence of insect damage.  The authors matched chew marks on what are probably ancestral leaves of the ginger plant with those of living beetles.  Amazingly, Wilf and co. showed that the damage is near-identical to that created by larvae of rolled-leaf beetles that still prey on the ginger plant (Wilf, P. et al., 2000.  Timing the radiations of leaf beetles: hispines on gingers from latest Cretaceous to Recent.  Science, v. 289, p 291-294).  Larvae take up residence in the curled, young leaves of gingers.  The young beetles then chew leaf tissue in highly distinctive patterns.  Only when the leaves unfurl do the bite marks reveal themselves, the beetles being long gone.

Curled-leaf beetles are extraordinarily loyal to their favourite plants among the gingers and heliconias, so that beetle-plant pairings generally involve only one beetle- and one plant species   Quite probably beetles and other insects underwent an evolutionary explosion at the time of the radiation of the angiosperms, because of the diversity of forms and metabolic pathways followed by flowering plants compared with other members of the Plant Kingdom.  The find helps confirm the hypothesis proposed by insect evolutionist Brian Farrell of Harvard University, that most plant-eating beetles evolved in parallel with flowering plants.

Another nail in the coffin for fossil fuels

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The 30- or more year long debate about anthropogenic climate change resulting from the ‘greenhouse’ effect of carbon dioxide releases by fossil-fuel burning has grown sharper in recent years.  Some specialists have cast doubt on climatologists’ ability to unravel human effects on the undoubted rise in mean surface temperature over the last 150 years from underlying fluctuations that stem from natural processes.  Considering the number of forcing factors, both large and small and with different periodicities, such doubts are valid, even though they may well be overemphasised in order to support continued and rising use of petroleum and coal.

The climate record for the last millennium is known to have been one of considerable change, involving a mediaeval warm period during its first half and the so-called ‘Little Ice Age’  from around 1600 to the mid 19th century.  Even within the recent warming trend there have been climatic ups and downs in the northern hemisphere, such as the mid-20th century warm period and several documented examples of cooling associated with major volcanism than punched aerosols into the stratosphere.

Thomas Crowley of Texas A&M University (Crowley, T.J., 2000.  Causes of climate change over the past 1000 years.  Science, v. 289, p 270-277) has attempted to isolate the known natural forcing functions and model their individual effects on mean surface temperatures in the northern hemisphere, to isolate meaningful signs of human effects from the various records temperature changes.  Even charting the changes is no simple task, as most records are local to regional, rather than valid for the whole hemisphere.  The most comprehensive climate data model uses proxy indicators from ice cores, tree-ring studies and coral time series, scaled to instrumental temperature records for the century from 1860-1965.  Uncertainty increases backwards with time, to around + 0.3°C in year 1000 AD.  This is somewhat greater than the fluctuations recorded in the temperature reconstructions, that Crowley and others have derived by a complex statistical method that fits a smoothed trend to the temperature fluctuations modelled from proxy data.

The approach used in Crowley’s analysis is to identify each likely, natural factor that influences energy balance in the northern hemisphere, and then to model the trends that they produced, and should continue to be producing.  There are two factors that are significant in millennial to shorter timescales: influences of volcanic aerosols, as timed by ash layers in ice-sheet cores; and variations in solar output based on 10Be and 14C variations in ice cores and tree rings (solar radiation generates both at the top of the atmosphere).  Possible anthropogenic forcing factors are numerous, including industrial aerosols and emitted gases other than the usual suspect, carbon dioxide.  The end product is a temperature time series which removes the effects of all known forcing factors, except those connected with ‘greenhouse’ gases, from the northern-hemisphere temperature model.  Until the early 19th century, this hovers close to zero variation, and then starts an upward rise to a value of about 0.75°C by now.

Crowley’s modelling adds significantly to the case for a detectable economic influence on climatic warming, by showing that known natural forcings cannot account for the rising trend since the start of the Industrial Revolution.  It does not prove the case, and leaves several features in recent climatic change unexplained, notably the cooling in the late 19th century.  Also, this approach does not exhaust all possibilities involved in climate shifts, such as linked fluctuations in energy movement by ocean-atmosphere circulation that work to make some regions experience cooling while others warm.  Such processes might respond to variations in solar-energy input by a kind of complex resonance, which remains to be looked at.

See also 20 percent more oil in the ground

Methane hydrate – more evidence for the ‘greenhouse’ time bomb

Where ocean water is more than 400 metres deep and bottom temperatures fall below 1 to 2°C methane and water can freeze to form crystals of methane hydrate.  These efficiently absorb more methane in a gas-solid solution, known as a clathrate.  Being lighter than seawater, methane clathrates do not carpet the ocean floor, but occupy pore spaces in sediments.  Under the anaerobic conditions of marine sediments, bacteria break down buried organic matter to release methane.  Build up of the gas in clathrates forms distinct reflecting horizons seen on many seismic sections of marine basins.  Estimates suggest that methane clathrates contain around the same amount of buried carbon as all fossil fuels lumped together.  Since methane is a powerful ‘greenhouse’ gas when released into the atmosphere, breakdown of the clathrates is a potential mechanism for global warming.

In 1995, evidence began to emerge that 55 million years ago, at the Palaeocene-Eocene boundary, a pulse of global warming probably stemmed from catastrophic release of methane from ocean-floor clathrates.  The signs lay in the proportions of 12C to 13C in organic matter within marine sediments of that age.  Since organisms selectively take up the light isotope of carbon, when organic matter becomes buried, seawater becomes enriched in 13C.  Buried carbon, both in organic molecules and in marine carbonates, takes on the isotopic signature of seawater at the time.  This means that the ups and downs of carbon burial leave an imprint in the carbon-isotope record of marine sediments.  When warming of deep-ocean water or a pressure release caused by a lowering of sea-level releases biogenic methane from clathrates, its high 12C content quickly appears in the carbon in seawater as a whole, by oxidation to CO2 and solution.  This reduces the proportion of heavy to light carbon in both buried organic matter and marine carbonates, forming a downward ‘spike’ in the carbon-isotope record.  Other processes can produce the same effect, such as increased release of volcanic CO2, which is also isotopically light, or a collapse of the marine biosphere.  So 12C ‘spikes’ need to be matched with other evidence.

Co-workers from Britain and Denmark have just reported in detail on just such an excursion that took place about 183 Ma ago, during a period of massive burial of carbon in the Jurassic Period (Hesselbo, S.P. et al., 2000.  Massive dissociation of gas hydrate during a Jurassic ocean anoxic event.  Nature, v. 406, p. 392-395).  The Early Toarcian was a period where circulation in the deep ocean stopped, to give anoxic conditions, ideal for burial of dead organisms.  While that lasted, important hydrocarbon-rich source rocks for petroleum reserves were laid down, and the carbon isotopes in sea water became unusually ‘heavy’.  Several stratigraphic sections show evidence for a 12C ‘spike’ in the middle of this period of general 13C enrichment of the oceans.  Hesselbo and co-workers isotopically analysed Toarcian mudstones exposed on the Yorkshire coast in England, which contain both marine matter and fragments of wood formed in terrestrial ecosystems.  Both show 12C enrichment, and that means that the entire carbon cycle at the time was somehow perturbed.  With no sign of massive extinction, the signature pointed either to a methane release or to hugely increased volcanism.

Although there was strong volcanic activity on Earth during the Toarcian, it came nowhere near being able to generate the anomaly.  Also the ‘spike’ occupies at most a period of a few tens of thousand years.  Only catastrophic release of methane from clathrates, equivalent to 20% of those estimated to be present today, is able to account for the anomaly.  Why it happened is nor certain; it may have been a result of either increased temperature of deep-ocean water by general global warming, to which it added, or perhaps too great a release of methane by decay in organic-rich sediments to be taken in by clathrates.  Another trigger, for which evidence is lacking at present, is through a comet impact in an ocean basin.

Methane hydrate layers in the oceans pose an ever-present threat today, because of their extreme sensitivity to temperature and pressure.  Some scientists believe that small releases may lie behind inexplicable disappearances of ships due to the drop in bulk density of seawater frothed by bubbles.  Also many areas of shallow seas are pockmarked by vents marking methane release when sea level stood lower during glacial epochs, and at least one methane spike in ice-core records can be correlated with a massive submarine landslide off western Norway.

Silica as a control over atmospheric CO2 levels

Today the oceans far from land are the equivalent of deserts, having very low biological productivity.  This is not due to a lack of the main nutrients, potassium, nitrogen and phosphorus, or to too little sunlight for photosynthesis.  For some time, marine specialists have suggested that the culprit is too little soluble iron – a micronutrient at the core of pigments and the enzyme RuBisCO, on which photosynthesis and the fundamental Calvin cycle depend.  The halving of atmospheric CO2 levels during glacial maxima is widely believed to reflect more efficient ocean bioproductivity and thus burial of dead organic matter.  The idea that general dryness and windiness during glacial epochs delivered soluble iron to the remote ocean surface is one means of explaining this.  However, it took CO2 8 000 years to rise to pre-industrial levels after the last dusty period when ice sheets reacehd their maximum extent, whereas iron lingers in seawater for only a few tens of years at most.  Dust carries far more silica than soluble iron, and SiO2 resides for 15 000 years or so.  This encourages the blooming of silica secreting diatoms in competition with calcium-carbonate secreting plankton.  Carbonate production by cells actually generates CO2, so less carbonate secreters relative to those producing silica shells means that tendency is offset by a greater contribution to buried carbon from dead silica secreters (Source:  Tréguer, P and Pndaven, P., 2000.  Silica control of carbon dioxide.  Nature, v. 406, p. 358-359)