Month: August 2002

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.

Evidence for slab break-off in subduction zones

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The detachment of lithospheric masses and their falling-off into the mantle, either by delamination of deep lithosphere beneath continents or the breaking of a subducted slab, have become popular means of explaining a variety of unusual phenomena in mountain belts.  In the Himalaya and Tibetan Plateau, such models have been evoked for the formation of odd K-rich basalts in the Eocene and Miocene, and the crustal melting that generated leucogranites around 20 Ma ago along the entire length of the Greater Himalaya.  Taking all the oddities of the Indo-Asian collision zone together does seem to support such a model (Kohn, M.J. & Parkinson, C.D. 2002.  Petrologic case for Eocene slab breakoff during the Indo-Asian collision.  Geology, v. 30, p. 591-594).  However, there is still no tangible direct evidence beneath the region.

Using seismograms for deep-Earth tomography appears to be able to resolve a range of proposed variants of tectonics, as well as the gross behaviour of the deep mantle. The site where two plates are being subducted on the west side of the North Pacific, marked by the Kamchatka peninsula, is pretty odd as well.  Although rates of subduction of both plates are high, the part of Kamchatka at one boundary no longer has active volcanoes, whereas the other does.  In fact one of the volcanoes there holds the world record for magma output.  Up to 5 Ma ago, the whole of Kamchatka was actively volcanic.  An explanation for the sudden halt to volcanism is that the dehydrating slab which provides the essential watery fluid for partial melting of the overlying mantle wedge – the source of subduction-zone magmas – broke away from the subduction zone and “fell” into the mantle 5 Ma ago.  That would have removed the source of hydrous fluid at a stroke.  Seismic tomography now seems to be capable of resolving just such a foundered slab (Levin, V. et al. 2002.  Seismic evidence for catastrophic slab loss beneath Kamchatka.  Nature, v. 418, p. 763-767).  There is no slab beneath the presently inactive volcanoes, whereas it is intact beneath the active ones.  The authors also claim that the seismic structure reveals a more recently foundered piece of lithosphere, whose rapid loss of hydrous fluid helps explain the phenomenally high magma production of the Klyuchevskoy volcano.  Such slab break-off is clearly a potential engine for enormous changes in magmatism, and the first seismic evidence for it is bound to spur a search for more examples.

The Malnourished Earth hypothesis – evolutionary stasis in the mid-Proterozoic

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Proterozoic

Accepted biogeochemical wisdom suggests that about 2000 Ma ago, the terrestrial environment changed from one in which oxygen was a rare free element to an increasingly oxygenated world.  One line of support for this involves the first appearances around that time of redbeds and lateritic palaeosols, that signify a surge in the O2 content of the atmosphere.  The other pointer is the disappearance of banded iron formations (BIFs), suggesting that soluble iron-2 was no longer available in the oceans due to its oxidation near its main source at mid-ocean ridges. The first unambiguous microfossils of eukaryotes, which need oxygen for their metabolism, also appeared some two billion years ago.

There is, however, a different view; that there was a transition between the anoxic world of the Archaean and Early Palaeoproterozoic and that marked by pervasion of atmosphere and hydrosphere by oxygen.  It stems from studies of sulphur isotopes in Proterozoic marine sediments by Donald Canfield of Odense University Denmark (Canfield, D.E., 1998.  A new model for Proterozoic ocean chemistry.  Nature, v. 396, p. 450-453).  Canfield found evidence for steadily increasing sulphate ions in seawater from 2300 Ma, which he suggested would have led to increasing production of hydrogen sulphide in the deep oceans by sulphate-reducing bacteria.  He proposed that it was combination with deep-ocean sulphide ions that shut off the supply of soluble iron-2, essential for the production of shallow-water BIFs.  Today, sulphide precipitation is restricted to hydrothermal vents and most iron is removed by combination with oxygen in sediments on the main ocean floors.  In short, Canfield proposed a transitional ocean akin to the Black Sea, with an oxic near-surface zone but anoxic at depth.  Not only iron would have been removed in sedimentary sulphides, but many other metals, leading to their depletion in seawater.  Ariel Anbar of the University of Rochester and Andrew Knoll of Harvard examine the biological repercussions of this transitional ocean (Anbar, A.D. & Knoll, A.H. 2002.  Proterozoic ocean chemistry and evolution: a bioinorganic bridge?  Science, v. 297, p. 1137-1142).

Iron and molybdenum are crucial elements for eukaryotes, albeit only in small quantities, because they are central to the enzymes that fix nitrogen.  Insufficient quantities would put early eukaryotes at an evolutionary disadvantage to prokaryote life.  Moreover it would reduce ocean productivity.  This, they propose, can help explain the lack of evolution among eukaryotes until the late Proterozoic.  The carbon isotope record of seawater (derived from limestones) shows a strange pattern that supports a period of biological stasis from 2000 to about 1200 Ma.  From the end of the Archaean until 2 billion years ago, there are huge fluctuations (to highly positive and negative values) in the proportion of heavy 13C, and so too in the Neoproterozoic.  The period in between shows no significant carbon-isotope fluctuation, d13C remaining at around zero, which Anbar and Knoll attribute to very low biological productivity.  In their model, it was the release of massive amounts of metals by continental erosion during the “Snowball Earth” glacial periods of the Neoproterozoic that was able to kick start life, especially that of the eukaryotes.  Emergence of the efficient, multicelled algal photosynthesizers drove up oxygen levels, eventually to oxygenate the deep oceans.

A cautionary note needs to be thrown in, however, especially when using analogies with the modern Black Sea (see Analogue of Archaean carbon cycle in Black Sea reefs).  Biogenic carbonates on the Black Sea bed show huge negative excursions in their d13C, because organisms that formed them metabolized methane, thereby incorporating methane’s strong depletion in heavy carbon.  As well as there being little direct evidence for Anwar and Knoll’s idea, the methane part of the carbon cycle needs to be factored into interpretations of the carbon-isotope record.

See: Kerr, R.A. 2002.  Could poor nutrition have held life back?  Science, v. 297, p. 1104-1105.

Isotopic evidence for early life may be from metamorphic processes

Controversy has surrounded reports of carbon-isotope evidence from the oldest recognisable sedimentary rocks that can be interpreted as signs of life 3800 Ma ago.  The problem is that the data came from carbon trapped in resistant minerals, such as apatite, in the metamorphosed Isua supracrustal rocks of west Greenland.  A detailed study of carbon in various forms in the Isua metasediments (van Zullen, M.A. et al. 2002.  Reassessing the evidence for the earliest traces of life.  Nature, v. 418, p. 627-630) strongly suggests that the isotopic evidence for life is flawed.  It seems likely that both graphite and carbonates in the Isua rocks originated by chemical reactions that took place during metamorphism; they are probably metasomatic in origin.  The wide range of d13C values found in both graphites and carbonates could have formed by isotopic exchange between graphite and carbonate during metamorphism.  Graphite inclusions in apatite, the source of carbon isotopes claimed to reflect the earliest biological activity, are petrographically no different from inclusions in other minerals.  Indeed, the sample originally used to suggest the isotopic influence of early life is of metasomatic origin.

All is not lost, however, for graphite that is highly depleted in heavy carbon-13 (a sign, albeit ambiguous, for organic processes) also occurs in turbidites that show graded bedding.  These rocks show no petrographic signs of metasomatism, and may contain signs of life.  Ominously, the US, Norwegian and Estonian co-workers, having looked in detail at carbon found in low concentration within BIFs and cherts from Isua, conclude that at least some is recent organic matter that groundwater flow has carried into the rocks.

Bizarre impact structure beneath North Sea

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The increasing use of finely-resolving 3-D seismic surveys in offshore exploration for hydrocarbons reveals exquisite detail of structure in strata beneath the sea floor.  So it is no surprise that oil-company geophysicists are able to image features that would otherwise remain hidden to researchers in universities.  If such discoveries are of little interest commercially, their finders are free to publish.  During routine surveys in the southern North Sea, an array of seismic profiles gradually built up a picture of something more reminiscent of the surface of an icy moon of Jupiter than a sequence of basinal sediments (Stewart, S.A. & Allen, P.J.  2002.  A 20-km-diameter multi-ringed impact structure in the North Sea.  Nature, v. 418, p. 520-523).  The circular feature found in strata at the top of the Cretaceous, might have been passed off as the product of deeper rise of salt diapirs from the widespread Permian evaporites of the North Sea basin, but for several features.  The surveys revealed no signs of the low-density Permian salt having bulged upwards below the structure, and disruption stops at depth.

The feature consists of at least 10 concentric rings extending to 20 km diameter, and at its centre is a bowl-shaped depression around a clear peak.  Not only is it an impact structure, but one of a particular class known as multi-ringed basins.  Those known from the Moon, are vastly bigger and are thought to have formed by such immense energy that the lunar surface rippled to fail along large concentric faults.  Lunar and terrestrial craters of the size of the North Sea structure usually have no concentric structure, being circular pits with rims and occasionally a central peak cause by rebound of the crust after impact.  The only similar features known are from moons of the Giant Planets that are made mostly of ice.  It is surprising that the North Sea example closely resembles them.  Modelling of such craters on Callisto suggests that they form when surface materials are underlain at depth by weaker ones; possibly an ice-liquid slush on ice moons.  The North Sea impact was into the Upper Cretaceous Chalk, whose upper strata are more homogeneous than those at deeper stratigraphic levels, which contain layers of mudstone.  Had impact occurred while the strata were not completely lithified, then the clays would have allowed inward movement to fill the crater excavated by impact, the more rigid upper Chalk having fractured during this movement.

Whether or not the impact accompanied the Chicxulub crater, implicated in the end-Cretaceous mass extinction, is not certain, although it does seem to predate Tertiary sedimentation in the North Sea.  There are probably many more impact structures on the sea floor, buried by marine sediments, but only in hydrocarbon-rich basins are they likely to be unmasked by seismic surveys.

Evidence builds for major impacts in Early Archaean

Following the discovery that anomalous tungsten isotope compositions of some Early Archaean rocks suggest a major component of extraterrestrial material in them (See Earth Pages News, August 2002, Tungsten and Archaean heavy bombardment), geochemists from Louisiana State and Stanford universities report evidence of debris from very large impacts in the same period (Byerly, G.R. et al. 2002.  An Archean impact layer from the Pilbara and Kaapvaal cratons.  Science, v. 297, p. 1325-1327).  Their case rests on the occurrence of layers of rock containing spherules of what formed as molten silicate droplets, in Early Archaean greenstone belts of the Barberton and Warrawoona areas of South Africa and Australia.  Zircons from a single layer in both areas yield identical ages of 3470 Ma, suggesting that the layers formed during a single impact event.  The authors speculate that a major unconformity in the Archaean of the Pilbara province in Australia, which is around the same age, may be the result of tsunamis induced by the impact.  It seems as if the responsible impact had a global effect, and may have released 1 to 2 orders of magnitude more energy than that responsible for the K/T event.  Judging by the lunar cratering record, this and previous finds help confirm expectations of similar bombardment on Earth during the Early Archaean.

Very early differentiation of planetary bodies

The radioactive decay of 182hafnium to 182tungsten seems likely to resolve the influence of impacts on the Earth ‘s evolution (See Earth Pages News, August 2002, Tungsten and Archaean heavy bombardment).  It is even more useful in refining ideas about the evolutionary pace of the parent bodies of meteorites.  The half-life of 182Hf is only 9 million years (all of it has decayed away in the Solar System by now), so the amount of radiogenic 182W associated with hafnium in a meteorite is a guide to pervasive geochemical processes early in the history of their parent bodies.  Hafnium has an affinity for silicates, whereas tungsten is siderophile and likely to enter planetary cores, should they form.  Because 182Hf decays so quickly, it is not easy to work out its original abundance, relative to stable 180Hf, in the source material for the Solar System.  That is a prerequisite for estimating when the hafnium-tungsten differentiation took place in a planetary body.  Two papers in the final August 2002 issue of Nature agree on this initial ratio (Yin, Q. et al. 2002.  A short timescale for terrestrial planet formation from Hf-W chronometry of meteorites.  Nature, v. 418, p. 949-952.  Kleine, T. et al. 2002.  Rapid accretion and early core formation on asteroids and the terrestrial planets from Hf-W chronometry.  Nature, v. 418, p. 952-955), which has important connotations; it is less than half the previously assumed value.  They determined this initial ratio using Hf-W data from independently dated carbonaceous-chondrite meteorites, whose parent bodies were never fractionated.

The two research groups, from Harvard University and the French Laboratoire des Sciences de la Terre, and the universities of Münster and Köln, Germany, respectively, use the new initial ratio to estimate the age of core formation from a range of meteorites.  Their estimates dramatically shorten the time between original accretion and core formation in a variety of bodies whose Hf-W isotopes have been studied previously.  The parent of the eucrite class of meteorites, probably the asteroid Vesta, differentiated within only 3 to 4 Ma, whereas the cores of the Earth and Mars took a little longer – about 29 and 13 Ma respectively.  In geological terms, accretion and core formation probably accompanied one another.  Of course, such estimates based on isotopic decay systems assume that the initial ratios existed at the time of accretion.  That may not be valid if the pre-Solar nebula took millions of years to evolve to the stage of self-collapse under gravity, which is the prerequisite for the formation of a planetary system.  However, there is evidence from short-lived decay systems involving other radioactive isotopes, such as 26Al, in meteorites, that points to the influence of a nearby supernova that triggered the formation of our Solar System.  Such an event is required to synthesize short-lived isotopes anyway.  Moreover, the shock from a supernova could accelerate collapse to mere few tens of thousand years.

See: Cameron, A.G.W. 2002.  Birth of a Solar System.  Nature, v. 418, p. 924-925.

Biofilms and BIFs

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Biomineralization is a growing topic that ranges from life’s influence on the production of economic deposits of metal ores to even the suspicion that it might play a role in Alzheimer’s syndrome.  The most common, and enduring evidence of the influence of micro-organisms in making rocks are stromatolites made of carbonates that blue-green bacteria have secreted, perhaps from as early as 3500 Ma ago.  Something similar, though it involves eukaryotic algae, is the formation of tufa or travertine where springs emerge from limestones.  Many a child, including my young self, consigned a cuddly toy to “petrifying” springs, such as Mother Shipton’s Well in Knaresborough, Yorkshire.  Few retrieved them, which is why there aren’t many rock-like Teddies around..  Another childhood memory, that bears on biomineralization, is a spring surrounded by orange and brown slime that we supposed was so deadly that only bathing in helicopter fuel would ward off a dreadful end brought on by the faintest splash of the loathsome gunk.  It is a great surprise to learn that such ochreous springs, common where coal mines drain to the surface, might hold a key to the formation of Precambrian banded iron formations (BIFs) (Brake, S.S. et al. 2002.  Eukaryotic stromatolite builders in acid mine drainage: implications for Precambrian iron formations and oxygenation of the atmosphere.  Geology, v. 30, p. 599-602).

Groundwater that has passed through iron-sulphide bearing rocks, becomes both acid and charged with iron-2 after oxidation of pyrite.  It is high acidity and low Eh that dissolves toxic heavy metals and arsenic, rather than their iron content, that make springs of such waters so hazardous to small boys bent on careers as hydraulic engineers (check their shins and fingers for the lingering water blisters that are a sure sign of the onset of arsenic poisoning).  It seems that Euglena, a common “animalcule” in such springs that is easily seen with a cheap microscope, is an ochre (iron-3 hydroxides and sulphates) forming agent.  It is an acid-tolerant, oxygenic photosynthesizer that builds slimy mats.  Given time and substantial supplies of dissolved iron, Euglena actually builds hard structures reminiscent of stromatolites.  Brake and colleagues from Indiana State and Kansas universities, and the Colorado School of Mines, studied Euglena from coal-mine drainages under lab conditions, and provide details of their metabolism.  The modern iron-stromatolites are so like some variants of BIFs from the Archaean and Palaeoproterozoic, when they were at their acme, that the authors suspect their origins in biofilms formed by prokaryotic organisms with similar metabolism to the more complex Euglena.  Until their work, most geologists regarded BIFs as products of inorganic precipitation of iron-3 compounds and silica when iron-2 rich seawater met oxygen produced by photosynthesizing cyanobacteria.  Indeed they speculate that the biofilm makers could have been early eukaryotes, despite the first unambiguous evidence for nucleus-bearing organisms being no older than 2100 Ma.  If they are correct, then such communities would have needed free oxygen, and would themselves have contributed to oxygen build-up in the early atmosphere.