New Scientist’s excellent Inside Science pull-out series now includes one on the Earth’s core (Bowler, S. 2000. Journey to the Centre of the Earth. Inside Science #134, New Scientist 14 October 2000). This covers the origin and evolution of the core, how geologists can assess its composition and structure, and the link between motions in the core and the fluctuations in the Earth’s magnetic field. Like all the Inside Science pull-outs, Sue Bowler’s treatment is at a level easily followed by non-Earth scientists but nonetheless informative and up to date.
Author: zooks777
Former Senior Lecturer at British Open University, research in remote sensing of arid lands, groundwater exploration, Precambrian tectonics and geochemistry
Fish ears at the Eocene-Oligocene boundary
About 33.7 Ma ago, at the Eocene-Oligocene boundary marine invertebrates suffered their largest downturn in the Cainozoic. Marine-core oxygen isotope records suggested that this coincided with a major cooling, when East and West Antarctica both possessed ice sheets. Deep ocean water temperatures, recorded by the oxygen isotopes of benthonic forams, fell by 3-4°C, yet surface waters at low latitudes appear to show little detectable change in the isotopics of planktonic forams. Data from cores become less well resolved in time, the older the sediments are, for a variety of reasons. Tying down a climatic cause for the E/O extinction demands much better precision.
From an astonishing piece of ingenuity and technical skill, we are closer to an answer. Lida Ivany and colleagues, from the Universities of Michigan and Syracuse, USA, collected the tiny ear bones or otoliths of fossil fish from a boundary section on the Gulf of Mexico. Because these grow with the fish and contain growth layers, potentially they can give resolution to the level of a single season. The trick is to get samples on a layer by layer basis and then analyse the tiny masses so extracted for oxygen isotopes. That is what the team managed to do (Ivany, L.C. et al. 2000. Cooler winters as a possible cause of mass extinctions at the Eocene/Oligocene boundary. Nature, 407, 887-890). Comparing the fine detail from Eocene and Oligocene fish ears shows that the local climate was much more seasonal in the early-Oligocene. While summer temperatures stayed at much the same level as in the immediately preceding Eocene, early-Oligocene winters were much colder. That would account for the inability of marine core data to detect any significant global cooling, and seasonal contrasts could have knocked out marine invertebrates evolved to more equable conditions.
News and Views in the same issue of Nature includes a fascinating look at these novel data in the context of wider knowledge of what was happening at the E/O boundary (Elderfield, H. 2000. A world in transition… Nature, 407, 851-852
Primordial slime
A timeless phrase from the film One-eyed Jacks is Marlon Brando’s, “You ain’t nothin’ but a ball o’ spit”, to the oppressive and corrupt lawman played by Slim Pickens. Some molecular biologists would come close to agreeing, though not in anyway to mock that fine actor. In Lyn Margulis’ theory of endosymbiotic origin for the Eucarya, of which we are a multicellular one, a candidate for the organism that played host to several others that went on to become eucaryan organelles is a slimy beast. It is Thermoplasma acidophilum, a member of one of the three fundamental domains of living things, the Archaea. Thermoplasma has no proper cell wall, contains DNA with proteins like those which bind nucleic acid in eucaryan cells, and it thrives in burning coal heaps. It is pretty much slime that needs both highly acid and very hot conditions to metabolise, and both result from the spontaneous oxidation of sulphides in coal exposed to air. Its very sliminess makes it worth considering as the original envelope for the baggage of the first Eucarya, so that they could get in. It is also an anaerobic fermenter – a methanogen – on whose waste products aerobic Bacteria might live while protecting the host from oxygen that would be highly toxic to it and perhaps supplying it with useful chemical products. Very roughly, that is how Margulis explained mitochondria, the organelles that are common to all eucaryan life. For a symbiosis to become a cellular unit from which all animals, plants etc descended demands an exchange of genetic material between all the participants, so that they become incapable of independent reproduction.
A few months after gongs were beaten to announce the completion of the human genome sequencing, Andreas Ruepp and colleagues from Germany and the USA laid out the genome of the loathsome Thermoplasma (Ruepp, A. and 9 others 2000. The genome sequence of the thermoacidophilic scavenger Thermoplasma acidiphilum. Nature, 407, 508-513). Thermoplasma, being an “extremophile” is also a candidate for having evolved in the hot environment of sea-floor, hydrothermal vents. It comes equipped with so-called heat-shock proteins, that eucaryan cells have turned to a multiplicity of other uses in their later, cooler, oxygen-loving evolution. The astonishing feature of its genome is that it is either a molecular thief or prone to being burgled. Many of its genes are identical to those in the sequences of other bacteria species whose habitats overlap with that of Thermoplasma. As well as offering little hindrance to large molecules entering it, the archaean seems not to generate enzymes that in many other cells detect and destroy alien DNA. The fact that Thermoplasma shows less affinities with eucaryan genetics than with that of Bacteria, suggests that it probably was not our ultimate ancestor. But that is hardly surprising, since such an organism would have had to share an environment with aerobic ancestors of organelles, one very different from the high temperatures and low pH of Thermoplasma and its fellows. To me, the new information serves to show strongly that an endosymbiotic origin of the Eucarya was indeed possible, given this mixture of larcenous and tolerant metabolism.
See also: Cowan, D. 2000. Use your neighbour’s genes. Nature, 407, 466-467
Eve never met Adam
A bit of molecular biology never did Earth scientists any harm, and new research on connectedness in DNA between people now living in different parts of the world sheds new light on the origin of fully modern humans.
All humans are, at most, one tenth of a percent different in their genetic make up; we are ten times more closely related than are chimps from different bands in the forests of West Africa. This low variance almost certainly results from the origin of fully modern humans in very recent times. The well-known comparison between DNA in mitochondria (mtDNA) of people across the world points to a divergence in our “bush” of descent about 140 000 years ago. Because mtDNA passes through the female line, this aspect of modern human origins has been said to stem from a mitochondrial “Eve” living in Africa at the time. This does not mean that only one fully-modern woman was alive at the time, but that lines of descent from others died out subsequently.
The other side of the evolutionary coin is descent worked out through the male line. Molecular biologists have focussed on DNA in Y-chromosomes that only men possess and pass on to their sons. A team at Stanford University in California used cell material from over a thousand men from 24 widely separated regions to investigate relatedness and divergence with the highest precision yet. Their results point to a time of divergence between 50 and 70 000 years ago; half that for female inheritance. While the mismatch certainly knocks creationism and its literal reading of the Old Testament still further out of the park, how the mismatch arose is hard to fathom. One possibility is that a mutation affecting Y-chromosome DNA only imparted such an advantage to the males who carried it that their descendants survived, while those not so favoured had their lines snuffed out. Alternatively, it may simply have been that some important technological discovery, or maybe even a cultural change, such as art that seems to first appear in Africa around 70 000 years ago, gave a very small family group the potential for only their descendants to survive through 3 to 4 000 generations. Whatever, the “bottleneck” through which all our genes passed at the time was in Africa.
Source: Cohen, P. 2000, Eve came first. New Scientist, 4 November 2000, p. 16.
The undead
The notion of bringing to life ancient organisms carries overtones of Jurassic Park, and more scientifically those of contamination by modern organisms. But has it been done? Russell Vreeland and colleagues from West Chester University, USA, claim to have cultured bacteria preserved in fluid inclusions from a Permian salt deposit (Vreeland, R.H. et al. 2000. Isolation of a 250 million-year-old halotolerant bacterium from a primary salt crystal. Nature, 407, 897-900). The stringent conditions of sampling suggest that indeed this is an old bug, as does the fact that it seems to be a salt-tolerant bacterium. However, it is hard to believe that living organic material can survive without apparent damage for so long.
In the accompanying News and Views pages, John Parkes, of the University of Bristol, UK, discusses the ramifications, and that surrounding claimed revival of bee-dwelling bacteria from Miocene amber. Some are worrying. Bacterial spores might survive indefinitely, to be released on an ill prepared world that has lost any shred of resistance to pathogens. Others bring a spark to some dormant ideas, particularly that of life spreading galactically by meteorite transportation.
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Chinese crust in miraculous escape
Ultra-high pressure (UHP) metamorphic rocks from the Yankou region in China have been down a subduction zone to more than 200 km and then rebounded to the surface. Kai Ye, Bolin Cong and Danian Ye of the Chinese Academy of Science in Beijing have worked on barometric indicators from eclogites and garnet peridotites to reach this conclusion (Ye, K. et al. 2000. The possible subduction of continental material to depths greater than 200 km. Nature, 407, 734-736). It is no surprise to learn that basaltic and peridotitic materials have been down a subduction zone, because that is what oceanic lithosphere does continually, though how they return to the surface as intact slabs is problematic.
What is surprising is that such highly compressed rocks are associated with similarly UHP materials that are chemically normal materials of the continental crust. The Yankou rocks now hold the record for deep diving. Sialic subduction is not easy because of its reluctance to reach densities that exceed that of the mantle. That being said, there are growing suspicions that continental materials may contribute to the composition of alkaline magmas formed deep beneath hot spots. If sial does not reach 200 km depth, its density always lies above that of the mantle, and it must be buoyant. Taken deeper, however, the situation reverses because of phase changes that compress silica and feldspar, so that at 300 km depth they become much denser than mantle, and must continue sinking to become potential contributors to later mantle melting.
In this case it seems as if the slab of Chinese sial was dragged from the lower crust by its attachment to enough basic and ultrabasic rocks that the whole lot broke the buoyancy barrier by their density change at high pressures. Getting back to the surface poses the big problem, the authors proposing that they were rafted by rocks beneath them. Somehow, a large mass of UHP basic-ultrabasic material must have become detached from sialic materials before the combined slab passed the 300 km boundary and became doomed to long-term mantle residence. That would give them and any eclogites remaining attached to them sufficient buoyancy to bob up once again.
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Ups and downs of the “greenhouse” effect
Several gases have the property of absorbing radiation in the wavelength range emitted by the Earth because of its surface temperature, including methane as well as carbon dioxide, the usual culprit. By doing so, they delay the escape of thermal energy through the atmosphere to outer space and give the Earth a higher surface temperature than it would have if they were not present. Because methane oxidizes to CO2 more rapidly than the latter gas’s recycling time, a record of atmospheric carbon dioxide is the best guide to fluctuations in the “greenhouse” effect through the past glacial-interglacial cycles. Bubbles in cores through the ice sheets of Greenland and Antarctica trapped air at the time when snow converted to ice within a few decades after it fell in polar regions.
The publication of data of all kinds from the ice-core drilled beneath the Vostok camp in Antarctica (see Earth Pages archive – Milankovic forcing flawed? July 2000) opened up 420 000 years worth of atmospheric composition shifts. Daniel Sigman and Edward Boyle, of Princeton University and MIT, Massachusetts, review all the bio-geochemical factors that might have contributed to the CO2 time series for the last 4 major climate cycles (Sigman, D. M. & Boyle, E.A. 2000. Glacial/interglacial variations in atmospheric carbon dioxide. Nature, v. 407, p. 859-869).
While work continues to fully grasp this climate forcing function, Sigman and Boyle argue convincingly that the overwhelmingly dominant influence on it is the combined biological and physical carbon “pump” of the ocean around Antarctica.
News from the South
Increasingly, evidence of many kinds points to a dominant influence on climatic ups and down through the last 2.5 Ma by processes in the northern hemisphere. Empirically, at least, the global-climate time series seems to show patterns that closely resemble Milankovic’s predictions of varying insolation at high northern latitudes. For millennial-scale fluctuations, such as Heinrich events and the Dansgaard-Oeschger cycles in ocean and ice-sheet cores respectively, the focus is on changes in deep-water formation in the North Atlantic. The South cannot be set aside, however, and there are two important issues that crop up in October’s publications. One is the extent to which climatic events in the southern hemisphere tracked those in the North, and the other is the role of the southern oceans in the global carbon cycle that underpins the climate-related fluctuations in atmospheric CO2 concentrations.
Both the Greenlandic and Antarctic ice cores show synchronicity of CO2 trapped in air bubbles with the records of local air temperature and global land-ice volume, going back over 400 ka in Antarctica. With more or less constant additions from volcanism, the ups and downs of the primary “greenhouse” gas have to be mediated by removal of carbon in one form or another from the ocean-atmosphere system through the agency of biological processes. Just what process, where it is most active and the controls underlying it form a topic of continual discussion and research. One possibility is variation in the biological productivity of the open oceans, coupled with removal of carbon from the ocean-atmosphere interface.
In terms of size and potential, the Southern Ocean is overwhelmingly the most likely candidate for a control. It is today the largest repository of unused nutrients in surface waters (by comparison with its potential for supporting phytoplankton it is a “wet” desert), but also a major source of deep-water formation that could sequester carbon from the surface environment. The late John Martin suggested that the main control over ocean productivity is soluble iron, currently at low concentrations far from land. The first realistic experiment to verify this involved “seeding” a small area of the equatorial Pacific with iron sulphate in 1995. Sure enough, that provoked a short-lived bloom of microscopic marine plants and local changes in dissolved CO2, but a boost in productivity at low latitudes is unlikely to lead to carbon removal from the surface part of the C-cycle.
Eighteen months ago, a multinational team of 35 ocean scientists conducted a similar experiment off Antarctica at 60°S (Boyd, P.W. et al. 2000. A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization. Nature, 407, 695-702. See also: Chisholm, S.W. 2000. Stirring times in the Southern Ocean. Nature, 407, 685-687). Once again bio-productivity soared by three times, and an input of 9 t of ferrous sulphate into about 50 km2 of ocean triggered an estimated 600 to 3000 t of extra algal carbon production. The “bloom” lasted for at least 6 weeks, being transformed into a swirling ribbon 150 km long. But it did not seem to be absorbed into deep water, merely mixing at the surface. In principle, iron dissolved from dust blown far from land during cold, dry episodes might have drawn down CO2 levels, but it is still uncertain. Yet the dust records trapped in ice cores do show a pronounced negative correlation with both CO2 and climate proxies.
Millennial-scale climate shifts are best known from the area around the North Atlantic. The most recent of these, and the most dramatic, was a sudden reversal from the warming trend out of the last glacial maximum around 13 ka ago, which lasted around 1800 years. This is recorded in many ways everywhere around the North Atlantic, and takes the name Younger Dryas (YD) from the associated increase in sediment cores of pollen of the cold-resistant mountain avens (Dryas octopetala). For some years there have been reports of a YD signal in climate records from the southern hemisphere, and some suggesting it was not felt there at all, the most detailed counter-evidence being the lack of the YD signal in Antarctic ice cores (ascribed by some to climatic inertia of the ice-bound continent).
The YD interrupted warming and wetting in the lead-up to the Holocene interglacial, so its signal ought to be easy to verify or rule out, simply because no later glacial advances have obliterated suitable investigation sites and many lakes at high altitudes and latitudes formed about that time. The problem for southern-hemisphere work has been a lack of precise dates. Southern Chile proves to be an excellent place to check, because lakes there go back further and contain evidence for many glacial advances and retreats (Bennett, K.D. et al. 2000. The last glacial-Holocene transition in Southern Chile. Science, 290, 325-328). Moreover, the sediment cores provide sufficient high-precision dates to construct a believably detailed time scale. Bennett and co-workers show that during the YD Chilean glaciers were retreating rather than advancing. That seems to knock the idea of “teleconnections” spanning both hemispheres for this particularly dramatic event, although its signal extends to the north Pacific. Like the mountain avens, however, disputing palaeoclimatologists are a hardy lot. It could be that the site of Bennett and colleagues work was far from a boundary between pollen-shedding species that was sensitive to climate change, despite the excellence of their record (see also Rodbell, D.T. 2000. The Younger Dryas: cold, cold everywhere? Science, 290, 285-286).
No escape from global warming?
Palaeoclimatology is well-funded because it is believed to shed light on the likely consequences of anthropogenic warming caused by CO2 emissions, and perhaps even technical solutions that allow us to continue burning fossil fuels. There is no doubt that throwing money at the range of associated phenomena and data has produced many astonishing findings and connections for the last 2.5 Ma. There is now sufficient high-quality data to reviewing them in their proper context; that of the climatic aspect of the “human condition”. That is the task that yet another multinational group of scientists set themselves at an International Biosphere-Geosphere Programme (IBGP) workshop at the Royal Swedish Academy of Science in November 1999 (Falkowski, P. and 16 others 2000. The global carbon cycle: a test of our knowledge of earth as a system. Science, 290, 291-296).
The workshop used two generalized outcomes of many years of work on Antarctic ice cores: the variation over more than 400 ka of CO2 in trapped air bubbles with temperature shifts; the frequencies and amplitudes of changes in atmospheric CO2. They compare these with human effects over the last 200 years. A great deal of discussion and qualification surrounds the workshop’s conclusions, but they are stark and simple. Anthropogenic change falls way outside that induced by natural processes (whatever they are), and its period bears no relationship to those involved in short- to long term processes. Despite the seeming attraction of technical fixes, such as boosting ocean productivity and the deep-water carbon sink (above), and intervention in terrestrial plant processes to increase CO2 sequestration from the atmosphere, both face the likelihood of weakening natural feedbacks due to the massive change that has taken place. Indeed, the consequences of strategies of these kinds aimed at mitigating climate change cannot be known in advance. This grim conclusion stems from the fact that no matter how well we get to know the climate system of the past, it is no longer what it was. Even a complete halt to all anthropogenic emissions now cannot reverse the trend in the short to medium term.
The group suggests Earth’s entry into a new Epoch (the Anthropocene) of uncertainty, but brimming with growing knowledge. To them, this seeming paradox must not be “used as an excuse to postpone prudent policy decisions based on the best information available at the time”. They also highlight the disciplinary compartmentalization of research that hinders a “proper” understanding of the Earth system. I suppose what they are getting at is the continuing ethos of Descartes’ 400-year old reductionism in science, yet surely their call for a “systems approach” is merely dressing up reductionist empiricism in a more complicated guise; hurling yet more intricate maths at the problem. That is indeed the goal of climate modelling and has been for well over a decade. Perhaps the solution lies not in descriptive retrospection by scientists and in “policy”, but with society as a whole that now begins to confront the mismatch between several thousand years of divided human activity with the rest of the world.
Daniel Sigman and Edward Boyle, of Princeton University and MIT, USA, usefully review the whole issue of varying CO2 through the 420 ka Antarctic ice-core record, together with its environmental buffering (Sigman, D.M. and Boyle, E.A. 2000. Glacial/interglacial variations in atmospheric carbon dioxide. Nature, 407, 859-869). Their article helps see the views of Falkowski et al. from a broad and detailed context, and links to News from the South (above), because Sigman and Boyle conclude that while the pacing of climate change tracks the combined effects of orbital processes on solar energy input at high northern latitudes the “greenhouse” effect changes because of biological and physical processes in the Southern Ocean that surrounds Antarctica.
The nudge of noise
The emergence of a signal in the climate shifts through the last ice age and the Holocene with a roughly 1 000 to 1 500 year period (see Earth Pages Archive, A new regular pulse in recent climate, September 2000) finds no link with processes linked to Earth’s orbital behaviour. It must be generated within the Earth system itself. That being said, there is a lot of debate over what precisely is involved. It’s safe to say that debate will continue.
However, another factor might well be involved; one that is as much to do with statistics as with phenomena with sufficient power to flip climate patterns. Random noise is everywhere in nature. If strong enough at a critical time, such stochastic noise might resonate with an otherwise weak, periodic phenomenon to give it sufficient push that it shows up in a climate change. Let’s say that there is some weak pulsation that bears on climate – not really known with certainty, but having a 1 500 year period. If resonance with noise was involved, we might expect to see 1 500, 3 000, 4 500 year periods in the climate record (1-, 2- and 3-cycle shifts), with the first more common than the last two – that is how the statistics should work. The fact that short-term climate pulses (the stadial-interstadial events) cluster around 1 000 to 1 500 years might indicate that random noise is implicated. However, only the last 120 000 years of climate data have sufficient precision for such statistical analyses, so it might be fortuitous.
The same nudge of randon climatic noise has also been called on to explain the jump from a roughly 41 000 year cyclicity to the present one of 100 000 years about 700 000 years ago. The first correlates with the period of changes in the Earth’s axial tilt, and the second with changes in the eccentricity of its elliptical orbit. The effect of orbital variations on the energy received from the Sun is so very small that it cannot have much of an effect on climate by itself, but changes related to axial tilt are ten times bigger. The change in behaviour seven ice ages ago is therefore hard to explain, without the nudge of noise.
Source: Kerr, R.A. 2000. Does a climate clock get a noisy boost. Science, v. 290, p. 697-698.
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Heads or tails?
The basalt floods draped over some great continental plateaux and considerable areas of the ocean floor, ocean islands far from plate boundaries and the volcanic provinces sitting at the ends of various oceanic island chains are with little doubt the product of plume-like masses rising from great depth in the mantle. What is not so well agreed is just what bit of a plume underwent partial melting to make the magma, the depth at which that took place and the prevailing temperature. There is some support for plumes that rise from a mantle transition zone about 700 km down, where there is an abrupt increase in temperature. Such plumes form a hot head when they impact the lithosphere, and that should be the source for magma. Plumes that rise from the core mantle boundary, should in theory have heads that are cooler than their tails, and which grow hugely by being stalled at the 700 km discontinuity. The two combined might form little plumes that rise from a big head at 700 km that spreads laterally. Nicholas Arndt gives a neat summary of these unseen ramifications in a recent issue of Nature (Arndt, N. 2000. Hot heads and cold tails. Nature, 407, 458-461).
Arndt was moved to make his comments by evidence from Namibian flood basalts from the 128-138 Ma old Paraná-Etendeka large igneous province (Thompson, R.N. and Gibson, S.A. 2000. Transient high temperatures in mantle plume heads inferred from magnesian olivines in Phanerozoic picrites. Nature, 407, 502-506). Thompson and Gibson found highly magnesian olivine crystals, among more normal ones, in basaltic dykes that cut the Etendeka basalts. The more Mg-rich an olivine is the more primitive (the more like the composition of the mantle) the magma from which it crystallized. They calculate that these anomalous olivines equilibrated with a magma with 24% MgO (compared with the <10% of most basalts) – probably a komatiite. But they are in much more evolved basalts, so they suggest that a primitive magma at the hot head of a plume that hit the lithosphere itself underwent fractional crystallization to produce plain basalt. They draw from that the conclusion that the plume head was 300-400°C hotter than the surrounding mantle – as expected in the first plume model above. Arndt is sceptical, partly because there are so many unknowns about the source region and partly because there are many other possible explanations. He suggests more similar work and other kinds of geochemical research on large igneous provinces in general. To that might be added looking for some of the possible mechanical consequences of hot or cool plume heads.
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Cashing in on T. rex
In the United States’ legal system I believe there is a statute of limitations. It doesn’t apply to the Cretaceous Period. More precisely, the most complete and fierce-looking specimen of a Tyrannosaurus rex skeleton has been the subject of legal wrangles from the moment she – a female named Sue after Sue Hendrickson of the Black Hills Institute of Geological Research (BHIGR), South Dakota who found her – was excavated. The legal saga is the subject of a new book by a lawyer, Steve Fiffer (Tyrannosaurus Sue, Freeman, New York, ISBN 0-7167-4017-6). The trouble started when the owner of the land on which Sue was discovered in 1990 was paid a paltry US$5000 for the privilege of seeing the awful fossil removed. The rancher’s subsequent claim on her was matched by another from the Cheyenne River Sioux, because the owner had placed his land in trust with the US Department of the Interior, and that conveys certain advantages to Native Americans…. The plot indeed thickened. The FBI and the local sheriff pounced on the hapless saurischian in 1992, and the National Guard supervised her impoundment, pending due process of law. Five years of hearings and criminal proceedings later – a raft of 148 felonies and 6 misdemeanours fell on the owners of BHIGR and one was jailed for 18 months – Sue became probably the oddest lot at Sotheby’s auction rooms. To add further insult, the auction price of US$8.36 million was partly raised by Disney and McDonald’s, and the landowner made US$7.6 million after commission. Sue now entertains in Chicago’s Field Museum of Natural History.
Source: Pojeta, J., 2000. Fossils, G-men, money and museums. Science, v. 289 8 September 2000, p. 1695-1696.
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Molecular ‘fossils’ and the emergence of photosynthesis
The most familiar photosynthesis is that associated with green plants, members of the Eucarya, in which organelles known as chloroplasts play a crucial role. Lyn Margulis’ theory of endosymbiotic incorporation of various bacteria in the origin of the eukaryote cell, sees cyanobacteria as the most likely progenitors of chloroplasts in plants. Aspects of the genetic material in chloroplasts are sufficiently similar to that of blue-green bacteria to make this a robust view. Tracking down when that melding of bacterial ancestors took place is a difficult task, both for molecular biologists and palaeontologists, partly because the record of cell material similar to that of cyanobacteria goes cold about 2.5 billion years ago.
Stromatolites, which today grow through the action of cyanobacteria excluding calcium from their cells in hypersaline environments, go back into the Archaean 3.46 billion years ago, but there is no guarantee that stromatolite forms were always confined to oxygenic photosynthesisers. However, the manner in which photosynthesis by blue-greens fractionates carbon isotopes possibly gives a signal in the d13C record of ancient hydrocarbons. Sadly, the overlaps between carbon-isotope fractionation oxygenic photosynthesisers, chemoautotrophs and anoxygenic photoautotrophs are too broad for this kind of study to give a definitive answer. Nonetheless, some researchers have claimed an Archaean origin for the cyanobacteria using this approach.
The advance of molecular biology, which compares gene sequences among living organisms to seek degrees of relatedness (phylogenies), steadily moves towards widely accepted molecular “clocks” that might resolve the timing of emergent life processes. A joint US-Japan team of molecular biologists have compared the photosynthetic genes of two modern photoautotrophs – green sulphur and green nonsulphur bacteria, neither of which are oxygen producing – with those of other photosynthetic bacteria (Xiong, J. et al., 2000. Molecular evidence for the early evolution of photosynthesis. Science, v. 289 8 September 2000, p. 1724-1730). Their results firmly place oxygenic photosynthesis, as in cyanobacteria, as descendent from earlier anoxygenic photoautotrophy, purple bacteria likely being the first to emerge by developing pigments capable of using solar energy to fuel proton pumping across cell walls. Jin Xiong and co. do not derive any timing for this phylogeny, but palaeobiologists are suggesting from their evidence that the six major photosynthetic bacterial lineages were around in the mid-Archaean (2.8 to 3.0 billion years ago) and maybe earlier. This comes nowhere close to the greater antiquity of stromatolites, but tagging purple bacteria as the first photosynthetic organisms, albeit not producing oxygen, gives a helping hand. Organic molecules originating in them are sufficiently distinct to already have shown up in kerogen from ancient shales, and such precursors to petroleum are present in Archaean sediments.
The interest in the emergence of photosynthesis is understandable, because of the huge increase in opportunities that it presented, by comparison with chemoautotrophic metabolism that seems likely to have been the first life strategy. The latter depends on chemical tricks with reduced materials, such as S, Fe2+ and methane delivered by sea-floor hydrothermal vents. Assuming appropriate rates for Archaean magmatism, that could sustain about 1012 moles of carbon fixing in cells per year. The anoxygenic photosynthetic pathway would have multiplied that by ten times. However, it is oxygenic photosynthesis that exploded life’s potential for interaction with the inorganic world, and that stemmed from the chemical-physical process at the root of what blue-greens did. The essence of oxygenic photosynthesis is that the pigments (like chlorophyll in plants) involved in transforming photon energies into electron flows, which are essential in the reduction of CO2 and water to carbohydrates, actually break the very strong bond between hydrogen and oxygen in water; that is why it releases free oxygen as a by-product. That feat involves a combination of the processes used by green sulphur and purple bacteria, which in itself implies the later emergence of cyanobacteria as confirmed by Xiong et al’s work. By using water molecules in this way, however, oxygenic photosynthesis opened up the whole near-surface of the hydrosphere, increasing potential bioproductivity by a further two or three orders of magnitude at least. It can be said that such a development truly brought life onto the front stage from hiding in obscure nooks and crannies. But we still have little precise idea of when that happened.
See also: Des Marais, D., 2000. When did photosynthesis emerge on Earth? Science, v. 289 8 September 2000, p. 1703-1705.
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Rhenium fever drives miners into the volcano
Satellites demand durable components, and for some applications the metal rhenium is irreplaceable. But it is hard to smelt, as well as being rare. Its current price of US$1.45 per gram reflects its conventional extraction from gases emitted by roasting molybdenum ore, a by-product of copper mining. At around one sixth the value of gold and with work beginning in earnest on the US-Russian International Space Station, a sizeable chunk of rhenium promises a quick profit. For geologists in the economic black hole that was the Soviet Union, rhenium has become a magnet and they are developing possibly the most extraordinary mining venture ever attempted.
Volcanologists of the Russian Institute of Experimental Mineralogy discovered, in 1992, that fumaroles of the volcano Kudriavy in the Kuril Archepelago exhale and precipitate pure rhenium sulphide – the hitherto unknown mineral rheniite. The vents’ build-ups contain at least ten tonnes of rhenium, and fumarole gases replenish it at a rate of several grammes each day. As well as mining the vents, even condensing rheniite is an economically attractive proposition. Even now, scientists of the Moscow-based Institute of Mineralogy, Geochemistry and Crustal Chemistry are building a wooden pyramid to cap one of the vents. This will funnel fumarole gases into a chemical trap for rhenium, that uses zeolites as an ion extractor. Future plans, sensibly, focus on concrete or ceramic caps to tap all the fumaroles in Kudriavy’s crater.
Source: Jones, N., 2000. Outrageous fortune. New Scientist, 26 August 2000, p 24-26
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Unravelling Neoproterozoic environments
The latest Precambrian or Neoproterozoic, from1000 to 544 Ma ago, and especially from 700 Ma to the start of the Cambrian, is the most important episode in the history of biological evolution. That is the episode during which remains of large, soft-bodied animals (the Ediacaran fauna) first appear and at whose end animals able to secrete hard parts burst onto the scene. It marks the preparation for the beginning of life as we know it best; the Cambrian Explosion. This period is remarkable also by its huge climatic upheavals that twice turned Earth into a planetary snowball, when ice masses extended to tropical latitudes. As if these unprecedented and never repeated big freezes were not sufficient to focus geologists’ undivided attention on the late-Neoproterozoic, seawater became for a time so depleted in oxygen that soluble ferrous iron entered shelf areas to precipitate out as banded iron formations, which had vanished around 2.2 Ga when oxygen first entered the oceans in any amounts. Neoproterozoic world events opened with all continental lithosphere known to be around at the time consolidated in the mother of all continents, literally called Rodinia from the Russian for motherland. Rodinia broke up with the as yet unexplained break out of Laurentia from close to its heart. A massive round of sea-floor spreading saw tiles from the Rodinian mosaic reassembled as the core of the Gondwana supercontinent beginning around 650 Ma ago. Gondwana played a massive role in subsequent tectonics until it too broke up in the Mesozoic. These were interesting times, relative to which the Phanerozoic seems somewhat tame, except for its tangible record of life’s ups and downs.
But there is a problem; with magmatic activity sparsely distributed in Neoproterozoic space and time, and a lack of rapidly changing biomarkers, division of events through time and, more important, correlating events from place to place has proved difficult, except in a barely useful and often mistaken way. Geological accounts of the late-Precambrian have been permissive and provocative, to say the least. That seems likely to change rapidly. Frustration centred on the time problem set against the undoubted drama of events had spurred the development other means of stratigraphic division and correlation.
The geologically instantaneous mixing of isotopes affected by global processes forms the basis for identifying large events that fractionate them in stratigraphic sections everywhere. That has been the biggest contribution of the oxygen-isotope data in seafloor sediment cores for the Neogene, in which fluctuating volumes of land ice shifted the proportion of 16O to 18O in ocean water, so that features in d18O records become means of fine-tuned correlation world-wide for climate shifts. Carbon isotopes play a similar role in charting changes in global bio-productivity and burial of dead matter and carbonate hard parts. Strontium serves to detect changing balances between supply of dissolved material from oceanic magmatism and from erosion of 87Sr-enriched continental crust. Sulphur isotopes also help chart supply and demand among organic and inorganic processes. Such chemo-stratigraphic methods were recognised as a lifeline for resolving Precambrian evolution in the late 1980s. A decade on, painstaking work has begun to bear fruit, as covered by a Special Issue of the 100th volume of Precambrian Research (v. 100(1), 2000). Andrew Knoll of the Botanical Museum, Harvard, USA summarises progress (Knoll, A.H., 2000. Learning to tell Neoproterozoic time. Precambrian Research, v. 100, p. 3-20), but details of the chemo-stratigraphic approach and what the prominent isotopic markers might mean appear in a paper of monographic proportions from a team at the Department of Earth and Planetary Sciences, Macquarie University, Australia (Walter, M.R. et al., 2000. Dating the 840-544 Ma Neoproterozoic interval by isotopes of strontium, carbon and sulphur in seawater, and some interpretative models. Precambrian Research, v. 100, p. 371-433)
Chemostratigraphy seems to resolve the question of how many late-Precambrian icehouse conditions of global significance. Though some have speculated on as many as 5 or 6 from occurrences of glacigenic rocks, only two match with isotopic signals, one (Sturtian) around 700 Ma and one around 600 Ma (Marinoan). Both have associated negative d13C excursions in carbonates to the level of mantle carbon, which suggest that life was reduced to a minimum by ‘Snowball Earth’ conditions. Associated shifts in the proportion of isotopically heavy sulphur are different. Sturtian glaciation matches with an increase in d34S, a likely product of ocean anoxia, the involvement of light 32S in bacterial reduction of sulphate to sulphide ions, and the burial of iron sulphide at sources of ferrous iron around sea-floor hydrothermal systems. The anoxia was sufficiently extreme for Fe2+ to dissolve and mix throughout the ocean water column, so that precipitation as ferric oxy-hydroxides burgeoned in shelf seas to form BIFs a little younger than the glacigenic rocks. Marinoan glaciation, though equally catastrophic for bioproductivity, did not fully deplete the oceans of oxygen. Massive peaks of d13C prior to glaciations suggest that intense precipitation of carbonates in the limestones so common in the run-up to frigidity, plus burial of abundant dead organic matter in the case of the Marinoan, dramatically drew down CO2 from the atmosphere. Life’s recovery after the Sturtian, together with organic burial, boosted oxygen levels, as too following the 600 Ma Marinoan. Possibly the delivery of huge amounts of glacially ground rock flour added nutrients that helped fuel this biological pump, and an increase in 87Sr/86Sr after the Marinoan could reflect such fertilization. There is much more in the paper that will fuel advances in ideas of the co-linkage of glaciation and biological evolution – essentially adaptive radiation by the few eukaryotes that survived anoxia and other stresses – and the evidence for large increases in oxygen production that are prerequisites for the origin of large, oxygen-demanding animals in the Ediacaran fauna. What came as complete surprise to me, a non-specialist, was clear evidence from several well-studied sections for the largest negative d13C excursion in geological history only 2 Ma before the Cambrian Explosion, which took less than a million years to develop.. Other isotopic trends seem to indicate a brief but highly intense global warming that snuffed the Ediacaran animals from the fossil record. The unique depletion in heavy carbon points strongly to the seabed belching teratonnes of methane in unstable gas hydrate, a product of double selection of 12C by photosynthesizing plankton and methanogen bacteria metabolizing dead planktonic matter within ocean-floor sediments.
Isotopically, the late-Neoproterozoic was chaotic. Carbon in particular records ups and downs with amplitudes and frequencies that dwarf those of the far-better recorded Phanerozoic, even in later glacial epochs and mass extinctions. It was two evolutionary developments that probably damped down excursions in carbon isotopes in later times: the stirring of deep-ocean muds by burrowing animals to promote more rapid oxidation of buried organic matter; the increased efficiency of CO2 drawdown by organisms that secreted carbonate hard parts. Perhaps Precambrian events were not so dramatic after all, equally disturbing events being smudged in the Phanerozoic by the rapid adaptive radiation following the Cambrian Explosion.
My prediction is that this issue of Precambrian Research will become the starting post for a major shift of research into Neoproterozoic and earlier Precambrian sedimentary piles, after two decades of getting things straight in the Mesozoic and Cainozoic. I feel confident in that, because the stories of Snowball Earth and near extinction of all oxygen demanding life around 700, 600 and now 545 Ma are ones that will, as the Sun might say, run and run.
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Plankton and the end of the Palaeocene-Eocene global warming
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.
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