Plankton and the end of the Palaeocene-Eocene global warming

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Various geochemical signals show that the Palaeocene-Eocene boundary (at 55 Ma) was a time of global warming superimposed on the general Cainozoic cooling from the ‘hothouse’ of the Cretaceous Period.  Some also point to an enhanced ‘greenhouse’ effect driven by massive methane release from gas hydrates on the sea floor.  Methane, a ‘greenhouse’ gas in its own right, oxidizes to CO2 in the atmosphere, transferring its carbon that eventually ends up in the shells of marine organisms.  It is the carbon-isotope blip at the P-E boundary that points to methane as a source of the warming.  Not only does it appear in the marine C-isotope record from foraminifera shells in cores, but also in the teeth of terrestrial mammals, which means that the carbon reservoirs of both atmosphere and seawater were globally changed.  Using the magnitude of that signal allowed palaeoclimatologists to estimate the amount of methane released – about 1 500 billion tonnes.  On a millennial scale, that is comparable to a rate of warming similar to that currently induced by human activities.

The P-E boundary marks the most dramatic biological changes since the mass extinction 10 million years before at the Cretaceous-Tertiary boundary.  But its underlying control is sufficiently close to what is happening to climate now to form both an object lesson and a means of modelling what may happen if current emissions continue.  One of the important aspects needing scrutiny is how such warming events come to an end.  British and American oceanographers have taken a look at the P-E record in ocean sediment cores, and believe they have come up with an answer, at least in part (Bains, S., 2000.  Termination of global warmth at the Palaeocene/Eocene boundary through productivity feedback.  Nature, v. 407 14 September 2000, p. 171-174).

Most such studies focus on oxygen- and carbon-isotope records in the carbonate of foraminifera shells, revealing ups and downs in seawater temperature and volume of land ice, and of biological productivity and releases of ‘greenhouse’ gases.  Unfortunately, neither isotopic record properly resolves the alternative contributions to variation.  Santo Bains and colleagues add another parameter that helps resolve the influence of biological productivity in the oceans.  Marine organisms, especially plankton, either precipitate barium sulphate (barite) in tiny crystals within their cells or induce its precipitation once they die and decay.  Because barite is not prone to much change by later events on the sea floor, counting its crystals in marine cores is a reliable proxy for the varying abundance of plankton through time.

One strong possibility during major warming events is that ocean circulation becomes sluggish, perhaps stopping altogether.  That slows the re-supply of nutrients to sunlit upper layers, and works to reduce photosynthetic life in the oceans.  The barite record produced by Bains et al. shows the opposite for the P-E events.  For about 40 000 years after the P-E event biogenic barite rose to more than twice its normal abundance.  The ocean biosphere responded to the methane blurt by blooming.  Why it did so is not yet clear, but such a spurt in drawing CO2 into living and dead and buried tissue would work to reverse the warming event.  The barite peak coincides exactly with the oxygen- and carbon-isotope records’ features that signify temperature and the influence of isotopically light carbon from methane released by gas-hydrate breakdown.  It might seem as if life did regulate climate in a geologically rapid manner following the P-E event, to the delight of Gaians.  However, the control over biological productivity is ultimately nutrients, and life has little influence over their supply to the oceans.  Among the possibilities for an essential nutrient bonanza, and increased circulation of the oceans is definitely ruled out during major warmings, are hugely increased rainfall to wash terrestrial sediments and dissolved matter into the oceans, and increased volcanism that would supply fine ash to the distant ocean surface.

Converging on an explanation for the end of a period of global warming is far from showing how this might be achieved for a warming induced by human activities.  That might well prove eventually to be a life-or-death necessity for our species, bearing in mind that the P-E warming was a fatal crisis for many land mammals of the time.

See also: Schmitz, B., 2000.  Plankton cooled a greenhouse.  Nature, v. 407 14 September 2000, p. 143-144.

A new regular pulse in recent climate

Gerard Bond of the Lamont-Doherty Earth Observatory at Columbia University, Palisades, New York has taken his analysis of high-frequency climatic shifts in the last glaciation into the Holocene record.  Previously, Bond had tried to make sense of the sharp fluctuations of the order of a few thousand years that are seen as gravel layers in the uppermost levels of sea-floor cores and in the oxygen isotope records of cores through the Greenlandic and Antarctic ice sheets.  The first signs of short ups and downs in climate were the coarse layers first found by Hartmut Heinrich in the glacial part of the sea-floor record.  Heinrich ascribed them to periodic releases of iceberg armadas as the ice sheets of the last glaciation became unstable.  Bond’s latest work also focuses on Heinrich events, but he has used specific lithologies as markers rather than merely grain-size variations.  In particular, hematite-stained quartzo-feldspathic materials seem likely to have come from altered rocks in east Greenland and Svalbard, far distant from the drill sites whose cores he has examined.  The proportion of reddish grains varies systematically in the cores, some layers coinciding with Heinrich events, but there are many more.  The layers appear roughly every 1500 years.  This periodicity coincides with cycles of dust blown from the Sahara to form layers in cores from the west African coast, so whatever the pulses represent, they are global signals.

Interestingly, the cycles show little sign of change in the period after the melt back that signified the beginning of the Holocene interglacial.  Behind the long-term climatic shifts in glacials and interglacials, that coincide with the 100, 41, 23 and 19 thousand year fluctuations in solar warming of the northern hemisphere, some other process must be put-putting in the background.  The 1500 year cycles may stem from processes that shift heat in the oceans and atmosphere.  A likely candidate is the production of deep currents by sea-ice formation in the northern North Atlantic.  However, detailed calculations of tides suggest a similar pacing that might change the mixing of surface and deep water in the ocean conveyor system.

Whatever the driving force, this periodicity strikes a chord with emerging details of Holocene climate changes from lake-sediments studies and the historic record.  One such recent cooling pulse that might have delivered icebergs to mid-latitudes in the North Atlantic was the Little Ice Age that peaked in the 17th century that saw prolonged stresses on the population of Europe, and major political changes that resulted from such events as the Peasants’ Revolt and repeated famines.

Source:  Pearce, F.  2000.  Feel the pulse.  New Scientist, 2 September 2000, p. 30-33.

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.