Author: zooks777

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

Wars in the Congo and physical resources

Published on by zooks777Leave a comment

The Democratic Republic of Congo (DRC, formerly Zaire) is the most war-torn country in Africa, and has been since Belgium relinquished its largest colony in 1960.  It is also Africa’s most mineral-rich country outside of the Republic of South Africa.  Most of its population, particularly outside of the major cities, has been repeatedly caught up in the most savage conflicts, which have left more than 2 million dead and far more displaced or reduced to conditions of bare survival.  From the civil war following the attempted secession of the most mineral-rich province of Katanga shortly after independence to the present, Congo peoples’ suffering has centred on various groups’ attempts to loot its mineral riches.  Despite the DRCs  strategic importance as a supplier of cobalt and tantalum, for which it is the world’s largest source, and its world-ranking production of copper and zinc, diamonds (up to one third of a ton annually, mainly of industrial quality), and gold (up to 6 tons annually), neither the UN nor those powers currently engaged in Iraq have made any determined effort to end the 40-year plight of its people.

Every geologist suspects that war in the Congo has a direct link to its mineral resources, but until recently its economic basis has remained carefully hidden by the various warring groups, and to some extent by the world mineral industry which ultimately benefits.  Ingrid Samset of the University of Bergen in Norway has reviewed the particular role of diamonds in the recent phases of conflict, that followed the fall of the reviled President Mobutu in May 1997 (Samset, I. 2002.  Conflict of interests or interests in conflict?  Diamonds and war in the DRC.  Review of African Political Economy, v. 93-94, p. 463-480).

Following the occupation of eastern DRC by armies from Rwanda and Uganda in collusion with the anti-Kabila RCD forces, and the sending of troops by Namibia, Angola and Zimbabwe to assist the Kinshasa régime in mid 1998, official figures for production of and revenues from all physical resources fell far more dramatically than for other exportable commodities, such as coffee.  The largest falls involved diamonds and coltan (columbite-tantalite).  Both combine very high value relative to weight (coltan trades at up to US$400 per kilogram) with simple extraction technologies.  Both are mined extensively by artisanal groups, and so are attractive for quick, clandestine looting.  Tantalum is used in making capacitors, specifically for mobile phones, and the boom in the price of coltan followed the vast expansion of cellular phone networks world wide.   Zimbabwe, and to a lesser extent Angola and Namibia have won official concessions for diamond mining in exchange for their military involvement.  The embattled ZANU-PF régime in Harare is probably highly dependent on revenues from Congo diamonds.  In the case of Uganda and Rwanda’s involvement with opposition forces in eastern DRC, the economic aspects of their roles are more difficult to dig out.  Both countries lack diamond or coltan reserves, yet in the case of diamonds, their exports rose by 12 and 90 times, respectively, since the start of their involvement.  Comparing their export values with probable production in the area that they help control, there is a shortfall of about US$13.5 million.  Samset suggests that “missing” diamonds are being used directly as easily “laundered” barter goods in exchange for arms.  In the case of coltan, Rwanda is estimated to have benefited by US$250 million, at the time of the tantalum price peak in 1999-2000, from looting of eastern DRC.  Neither coltan nor diamonds carry signs of their origin (but see Forensic geochemistry to foil “fencing” of conflict diamonds in EPN, June 2002), so tracking looted goods and bringing those involved to account is no easy task.  The state of Israel is heavily involved in the gem diamond trade, as is the Republic of South Africa, and the USA accounted for more than 80% of all industrial diamond exports from the former Zaire.  One of the oddest coincidences was the sudden involvement in peace-making attempts during the Eritrea-Ethiopia war of 1998-2000 of the government of Rwanda, despite its geographic remoteness from that particular conflict and lack of diplomatic experience.

See also: http://www.american.edu/TED/ice/congo-coltan.htm for an analysis of the role of coltan in the DRC conflict.

Gut bacteria and human migration

Published on by zooks7771 Comment

Our churning bowels and stomach mimic a variety of inorganic environments in which a large range of bacteria have thrived for hundreds, if not thousands of million years.  The stomach has low pH thanks to hydrochloric acid, sufficiently strong to make limestone fizz should you be unfortunate enough to throw up while collecting fossils.  Parts of the gut are highly reducing, so that humans contribute their bit to global warming through the action of our symbiotic methanogen bacteria, although much less so than ruminant mammals which are major methane producers.  We also host sulphate-sulphide reducing bacteria, with sometime spectacular effects in enclosed spaces.  The animal gut has been around for quite long enough for internal bacteria to evolve and adapt to the dietary habits of their hosts, mostly as symbionts.  However, some are pathogenic and infective.  One pathogen in particular is not infective, so its effects have remained undetected until recently.  It is now known that a major cause of gastric and duodenal ulcers, and digestive-tract cancers is the Gram-negative bacterium Helicobacter pylori.  Massive doses of acid suppressants and bactericides effect miraculous cures on individuals who have had decades of misery from stomach pain.  Now that the culprit has been fingered, you will not be surprised to learn that its DNA has been studied in some detail.  The results are surprising  (Falush, D. and 17 others 2003.  Traces of human migrations in Helicobacter pylori populations.  Science, v.  299, p. 1582-1585).  Helicobacter is extraordinarily diverse, and regionally distinctive.  Because it is pervasive, but not infective, the bacterium travels along with populations of its hosts, and is therefore a potential tool in tracking migrations.  There are 7 geographically distinct H. pylori groups today, and their genetic structure can be traced to ancestors in Africa, Central and East Asia.  Their geographic distribution matches those of human genetic and linguistic patterns, which have been attributed to the colonization of Polynesia and the Americas, to Neolithic migrations of agricultural peoples into Europe from the near-East, the expansion of Bantu-speaking people in Africa and to the slave trade.

Neanderthal review

The last ten years has seen enormous developments in understanding the first Europeans. So, a review of how they lived, how they differed from us, how they might have thought and how they came to an end shortly after our immediate ancestors turned up is very welcome (Klein, R.C. 2003.  Whither the Neanderthals?  Science, v.  299, p. 1525-1527)

The first volcanologists?

If there is ever a chance, the site that I would most like to visit is that discovered by Mary Leakey near Olduvai Gorge in Tanzania.  A bedding surface in volcanic ash records footprints of two adult australopithecines and a juvenile who trudged together through fresh debris from a nearby volcanic eruption.  The earliest and irrefutable confirmation of bipedalism, the tracks are also among the most poignant in the fossil record of humanity.  Did this family survive the tragedy?  The trackway is now covered to guard against erosion and theft.  Altogether less heart-rending are younger footprints in an ash layer from the Roccamonfina volcano in Italy (Mietto, P. et al. 2003.  Human footprints in Pleistocene volcanic ash.  Nature, v. 422, p. 133), long known to locals as “devils’ trails”.  The ash formed on the slopes of the volcano, as a pyroclastic flow, and the fossilised trail slopes at up to 80º.  Because the ash is about 350 thousand years old, whoever made the prints were not fully modern humans, but probably ancestors of Neanderthals (H. heidelbergensis).  The individuals had quite small feet, and may well have been children.  The tracks come down the slope, both zig-zagging and showing occasional hand prints to steady the descent.  They give the impression that whoever made them was not escaping an eruption, but having fun, much as kids today cannot resist hurling themselves down sand dunes and snow slopes.  There is another possibility: curiosity drove them up the volcano after products of an eruption had cooled.  Volcanologists cannot resist doing that either, and, as today, maybe they went up a little too early for comfort and had to leap for their lives.

See also:  Muir, H. 2003.  Earliest human footprints preserve prehistoric trek.  New Scientist, 15 March 2003, p. 15.

Earlier date for first suspected animals

Published on by zooks777Leave a comment

The earliest indisputable traces of metazoan animals are quite literally that – the impressions of soft-bodied organisms preserved as the Ediacaran fauna of Australian and other late-Neoproterozoic sediments dated around 565 Ma.  However, the profound differences in genetic make-up of existing animal phylla, which clearly at the time of the Cambrian Explosion, have been expressed as indicators of animals’ origins more than a billion years ago.  Consequently, the discovery in 1998 of what appeared to be non-Ediacaran trace fossils in the Neoproterozoic Vindhyan Supergroup of India triggered considerable interest.  The problem with many of India’s Precambrian sediments is their lack of precise and verifiable dates.  Occurrences of the sedimentary silicate glauconite in the Vindhyan prompted use of the K-Ar method, which suggested that they were pre-1100 Ma, but that is a notoriously unreliable technique.  Part of the lower Vindhyan succession contains poorcellanites that show textural evidence for having originated at ignimbrites, and they contain zircons of volcanic origin.  Once sampled, it was only a matter of time before precise single-zircon U-Pb dates became available.  In fact, two teams published simultaneously in the February issue of Geology, and gave similar ages from different places (Ray, J.S. et al. 2002.  U-Pb zircon dating and Sr isotope systematics of the Vindhayan Supregroup, India.  Geology, v. 30, p. 131-134;  Rasmussen, B. et al, 2002. 1.6 Ga U-Pb zircon ages for the Chorhat Sandstone, lower Vindhayan, India: Possible implications for early evolution of animals.  Geology, v. 30, p. 103-106).  The first paper gave an age of 1631 Ma for strata immediately beneath the supposedly fossiliferous formation, whereas the second bracketed it between 1628 and 1600 Ma for rocks beneath and above it.

If the structures preserved in the Chorhat Sandstone do prove to be true trace fossils, there will be little doubt that animals appeared at least three time earlier than the previous fossil-based estimate, more in line with the molecular evidence.  However, the structures are disputed, and there is another oddity about the palaeontology of the Vindhyan.  Limestones that conformably overly the 1600 Ma dated horizon have been reported to contain brachiopods and “small, shelly faunas” typical of the earliest Cambrian elsewhere.  Since the limestones are only a few hundred metres higher in the Vindhyan sequence, and contain 87Sr/86Sr isotope ratios that are appropriate for Neoproterozoic seawater, brings their content of Cambrian fossils into doubt.  Clearly, a great deal more work is needed to resolve the significance of the Vindhyan finds, particularly establishing accurate, basin-wide stratigraphic correlation.

Are mass extinctions artefacts of sampling bias?

Evidence for mass extinctions comes from inventories of fossil species, genera and families collected from the sedimentary record.  There has always been a geographic bias in this sampling towards more accessible areas and those with the greatest number of palaeontologists, i.e. towards rich countries.  Increasing grants for expeditions to remote areas and the slow growth in numbers of specialists in less well-endowed countries does smooth out the bias.  However, because of many factors, including ups and downs in sea level and the effects of orogeny on rates at which deformed sediments have been eroded, the stratigraphic record itself does not accurately represent time with exposed rocks.

The data on which extinction records rest are those compiled by the late Jack Sepkoski, yet until recently there has been little attempt to weight them according to stratigraphic record, although much statistical re-evaluation has gone on (e.g. The “Big Five” become the “Big Three”? Earth Pages of January 2002).  This stratigraphic evaluation to some extent pulls the rug from under those who speculate on the causality of extinction (Peters, S.E. and Foote, M.  2002.  Determinants of extinction in the fossil record.  Nature, v. 416, p. 420-424).  A great many ups and downs in the fossil record do seem to depend on the amount of exposed sedimentary rock.  Widespread gaps in the sedimentary record result in spurious and abrupt ends to evolutionary lineages; pseudo-extinctions.  Although the period- and era-ending extinctions seems still to be statistically valid, those at stage boundaries are suspect.  One of the lessons to be learned is that the previous good correlation between sea-level change and extinction and origination rates is particularly suspect, as eustasy is a first-order contributor to chages in sedimentary deposition and preservation.

Doubt cast on earliest bacterial fossils

In autumn 1996 two of the most blatant hyperboles in the recent history of the Earth sciences hit the world’s headlines; two groups of scientists, one from the USA, the other British, announced their discovery of fossil life forms in meteorites reputed to have originated on Mars.  The evidence was in the form of organised structures revealed by scanning electron microscopy.  Subsequently, most biologists and palaeontologists concluded that the case was, in the manner of the third possible verdict in Scottish courts, “not proven”.  Kindly scientists regarded the hype as being prematurely optimistic.  However, critical attention focussed on the announcements because they claimed first discovery of extraterrestrial life.  If one finds a mammoth while digging a ditch, there is some cause for celebration, and the world will believe and congratulate the finder, for the mammoth is unmistakable.  That is not the case for fossilized micro-organisms.  In 1993, William Schopf of UCLA, and co-workers, announced their discovery of the oldest known fossil bacteria in 3465 Ma cherts in a greenstone belt near Marble Bar in Western Australia.  They were microscopic wisps of carbonaceous material, that a trained eye might resolve into filaments made of bacterial cells.  Since the most common living filamentous bacteria are photosynthetic cyanobacteria, that bear close resemblance to sketches of the ancient structures, Schopf and colleagues performed the palaeontological equivalent of Aristotle’s syllogism, by declaring that indeed some of the structures were blue-green bacteria.  In what was generally regarded as an anoxic Archaean world, it seemed there were organisms working to oxygenate the environment.  Various lines of evidence, such as the isotopic composition of carbon in Archaean sediments, were later claimed by others to support such an early arrival of cyanobacteria, that eventually transformed the atmosphere and the conditions for life, so that oxygen-demanding Eucarya, such as ourselves, might evolve and diversify.

There is one snag with the Marble Bar chert.  It almost certainly formed by hydrothermal activity on the Archaean ocean floor; deep and dark.  Photosynthesis using solar energy would be unlikely.  Re-examination of the putative fossil filaments, using both microscope and Raman spectroscopy (means of estimating C/H ratios from spectra excited from carbonaceous matter by a laser) has raised a minor storm.  Martin Brazier of Oxford University and colleagues from Britain and Australia question the biological origin of the structures (Brazier, M.D. et al. 2002.  Questioning the evidence for Earth’s oldest fossils.  Nature, v. 416, p. 76-81).  Amazingly, one of their observation while examining Schopf’s original material with a high powered microscope was that by racking the objective up and down to visualize the structures in 3-D, most showed to be highly irregular smears of carbonaceous stuff.  Only one position provided life-like shapes.  While Brazier et al. do not deny that life was around in the chert-forming hot spring – probably chemautotrophic prokaryotes – they are convinced that Schopf’s structures are artefacts formed by hydrothermal reworking of degraded organic molecules.  In a rejoinder, Schopf and US colleagues accept the deep-water, hydrothermal origin of the cherts and concede that none of the structures are blue-green bacterial cells, but still maintain that they are biogenic (Schopf, J.W. et al. 2002.  Laser-Raman imagery of Earth’s earliest fossils.  Nature, v. 416, p. 73-76).  The earliest undisputed fossil micro-organisms are almost 1.4 billion years younger than those of Marble Bar.  They are from cherty layers in banded iron formations, formed probably in shallow water by the combination of oxygen produced by cyanobacteria with dissolved ferrous iron.  The Archaean contains plenty of BIFs, and perhaps a search for the oldest biotas in them would give more definite results.

See also:  Kerr, R.A. 2002.  Earliest signs of life just oddly shaped crud?  Science, v. 295. P. 1812-1813.

Antarctic melting and northern hemisphere deglaciation

Published on by zooks777Leave a comment

There is a large body of opinion, supported by plenty of circumstantial evidence, that the end of the last glacial maximum around 20 ka was controlled by processes that operated in the North Atlantic and its seaboard.  A favoured mechanism is the re-establishment of thermohaline circulation involving North Atlantic deep water that dragged surface water northwards from the tropics, to set up the Gulf Stream.  Temporary shut-down of thermohaline flux, probably by massive release of freshwater to the North Atlantic from melting of ice sheets, is widely understood to have triggered the sudden reversal to frigid conditions in the Younger Dryas around 11.5 ka.  The largest warming pulse in the northern hemisphere, between 14.6 to 14.0 ka, is recorded by a sudden increase in d18O of ice in the Greenland cores, and is known as the Bølling-Allerød warm interval.  Around that time, sea level rose by 20 m in a few hundred years, and that involved production of fresh glacial meltwater at a rate equivalent to the continual flow of five rivers the size of the Amazon.  Such rapid sea-level rise drowned coastlines and in some areas killed coral reefs.  On such drowned reef in the Caribbean gave a date of 14.2 ka, which since 1989 has been the only indicator of precise timing for the massive influx of meltwater to the oceans.  The date is within the Bølling-Allerød, hence the link between warming and events around the North Atlantic.  That central hypothesis is now under threat, following the dating of drowned coral reefs on the Sunda Shelf at 14.7 ka, and a re-evaluation of the Caribbean data. (Weaver, A.J. et al. 2003.  Meltwater pulse 1A from Antarctica as a trigger of the Bølling-Allerød warm interval.  Science, v. 299, p. 1709-1713).

Using the revised ages and climate modelling, Andrew Weaver and colleagues from the Universities of Victoria and Toronto, Canada and Oregon State University see the massive ice-melting as the precursor to the Bølling-Allerød warm interval and deglaciation of lands around the North Atlantic.  A more plausible source of freshwater influx is a major melting event in Antarctica, so warming in the south may well have driven that of the northern hemisphere.

See also: Kerr, R.A. 2003.  Who pushed whom out of the last ice age.  Science, v. 299, p. 1645.

Digital library of educational web sites

Published on by zooks777Leave a comment

The Digital Library for Earth Systems Education now has a site that can act as a gateway to thousands of Earth Science web sites that provide educational materials from school to professional level (www.dlese.org) .  There is a comprehensive range of topics, and a potentially useful enhancement is part which announces conferences and discussion groups.

Devonian art

The Devonian Period witnessed sudden diversification of vertebrate animals, including the first amphibians.  The oceans were teeming with other life forms.  Museums have for a long while commissioned artists to create dioramas of ancient seas that express part of their diversity.  Using gaming software, British artist Christian Darkin is in the process of developing an interactive Devonian diorama, eventually intended for museum use.  His work in progress can be viewed at   www.geocities.com/christiandarki/fish.htm .  You will need the Macromedia Shockwave plug in, downloadable direct from Darkin’s site, and a higher speed connection than with a normal modem.  However, the experience is dramatic and informative.

Silica in BIFs

Published on by zooks7771 Comment

Following close on the heels of the hypothesis that iron in Precambrian banded iron formations was precipitated by bacteria (see BIFs and bacteria in February 2003 issue of Earth Pages News) is an account of the origin of silica that makes up roughly half the banding (Hamade, T. and 4 others 2003.  Using Ge/Si ratios to decouple iron and silica fluxes in Precambrian banded iron formation.  Geology, v. 31, p. 35-38).  The rare-earth elements and Nd isotopes in the iron-rich layers suggests that they probably originate from ocean-floor hydrothermal activity.  How their cherty layers formed has largely been overlooked.  Before the Cambrian Explosion there were no organisms that secreted silica in their skeletons.  Consequently, the dissolved silica content of Precambrian oceans was probably much higher than now.  Because silica becomes highly soluble only under very alkaline conditions, it may have been close to saturation in Precambrian seawater.  Quite small changes in seawater chemistry would result in its precipitation as fine-grained chert.  But the main issue is where the dissolved silica came from.

Hamade et al. examined the amount of germanium in the cherts, because it is in the same group of elements and acts as if it were a heavy isotope of silicon.  So it follows Si very closely in its distribution.  Alteration of mafic rocks by sea-floor hydrothermal activity dissolves Si, and so does weathering of continental materials; there is a dual source of seawater Si.  However, basalts have more than 10 times as much Ge as do granitic rocks, and the Ge/Si ratio is a good guide to the dominant source of Si.  Cherts in the BIFs from the famous Hamersley basin in Western Australia have Ge/Si ratios that increase with the amount of iron.  The most silica-rich BIFs seem to have formed from waters derived from continental areas, whereas the iron-rich varieties have a sea-floor hydrothermal signature.  The authors conclude that these BIFs formed on a continental shelf subject to regular, periodic upwellings of deep ocean water.

When did southern Tibet get so high?

Published on by zooks777Leave a comment

For about a decade it has been suggested that the Tibetan Plateau, which rises to more than 5000 metres, has a profound effect on climate.  This may be partly due to the way such a high and enormous area deflects regional wind patterns, but largely to its profound interconnection with the South Asian monsoon.  When such a circulation barrier arose is critical to understanding how it relates to climate evolution in the latter part of the Cenozoic.  There are various suggestions, based on aspects of its structural and magmatic evolution.  Theory suggests that the southern part came into being in Eocene times, possibly because a segment of the lithosphere beneath broke off to subside into the mantle – there are volcanic rocks whose chemistry does suggest such a mechanism.  About 8 Ma ago the southern Plateau began to spread laterally, producing a series of N-S extensional basins, which suggests that by then sufficient gravitational potential had accumulated to make the thickened crust unstable.  About that time various signatures arose in foraminifera of the Indian Ocean and sediments derived by erosion, which suggest that the monsoon increased in intensity.

When the Plateau attained sufficient elevation above sea level to start spreading sideways and affect atmospheric circulation largely rests on these theoretical judgements.  For the ideas to firm up needs some means of estimating topographic elevation, which is not easy to do.  One way is to use plant remains that can give clues, either because the species involved are sensitive to elevation today, or the morphology of their leaves shows signs of physiological adaptation to elevation.  The first is ruled out in old sediments, simply because the species present are now extinct..  Plants metabolism is dependent on diffusion of water and CO2 into their leaves during photosynthesis, and features, such as stomata density, give clues to the conditions for such diffusion.  Luckily, sediments from southern Tibet do contain well-preserved plants, and a multinational group led by Bob Spicer of the British Open University have attempted to assess palaeo-elevation for the time at which they were deposited (Spicer, R.A. and 7 others 2003.  Constant elevation of southern Tibet over the last 15 million years.  Nature, v. 421, p. 622-624).  Their method relies on linking leaf morphology to a property of the atmosphere, known as moist static energy (MSE), through estimates of atmospheric enthalpy from the leaves.  That is not the end of the estimation, because MSE needs to be related to elevation and the only way is to use climatic modelling for the past.  Whatever, Spicer and colleagues reckon that 15 Ma ago their sampling site was more or less at the same elevation as today, around 4.5 km above sea level.  If true, they have established that the south part of the Plateau was already in existence during the Middle Miocene.  Being so convoluted, despite its apparent precision, the leaf analysis method does need independent confirmation.  There is a much easier and arguably more reliable method, based on the change in the size of bubbles formed by gas escaping from lavas, according to atmospheric pressure (see Cunning means of estimating uplift in November 2002 issue of Earth Pages News).  There are lavas in southern Tibet that date from Cretaceous times, including some about a million years younger than the plant remains.

Precambrian warmth and methane

Methane is a more efficient “greenhouse” gas than CO2, but it soon oxidises in the presence of oxygen.  During the Phanerozoic there have been several massive releases of methane, probably from gas hydrates in deep-ocean sediments, which produced warming spikes that decayed away quickly in geological terms.  Before there was much, if any, oxygen in the atmosphere, methane could linger and add to the retention of heat by carbon dioxide and water in the atmosphere.  One of the longest running disputes in environmental geochemistry concerns when oxygen levels became significant in the Precambrian, and what they were compared with later times.  Whether the Earth was warm or cold has a bearing on this.  Cosmological theory suggest that stars similar to the Sun progressively grow more energetic with time.  Without some kind of greenhouse effect, the Earth would have been condemned to frigidity from its outset.  Even today, with a more radiant Sun, only atmospheric retention of solar heat keeps overall temperature from being well below freezing.  The further back in time, the greater the “greenhouse” effect would have to have been to stave off complete ice cover and a runaway “icehouse”.  Methane almost certainly played a part in this once methane generating organisms evolved, up to about 2200 Ma, when there are signs (continental redbeds and soils rich in iron oxides) that atmospheric oxygen was appreciable.  However, warmth prevailed for about 1.5 billion years thereafter, until the plunges into frigid conditions of the so-called “Snowball Earth” period from about 700 to 550 Ma.  Somehow, the greenhouse effect lingered.

Alexander Pavlov of the University of Colorado, and colleagues from Pennsylvania State University have addressed the implications of this continued warmth in terms of maximum oxygen levels needed to avoid complete oxidation of methane releases (Pavlov, A.A. et al. 2003.  Methane-rich Proterozoic atmosphere?  Geology, v. 31, p. 87-90).  Today, more than 90% of all methane production beneath the ocean floor is consumed by bacteria, depending on the amount of dissolved oxygen and sulphate ions (for aerobic and anaerobic methanotrophs).  There is plenty of evidence that deep Precambrian ocean water was anoxic, so a great deal more methane would have emerged from them.  That it was also poor in sulphate ions is shown by their low levels in solid solution with carbonates and Proterozoic sulphur isotopes in marine sediments.  The authors argue that this signifies low atmospheric oxygen levels, around 5 to 18 percent of modern concentrations.  The scene may have been set for an excess of methane production over its oxidation, thereby keeping the “greenhouse” warming above the levels when glaciation would have been widespread..  If so, something completely upset this balancing act in the Neoproterozoic, to drive down temperatures several times – the “Snowball Earth” events.  The trigger may have been a boost in oxygen production and retention in the atmosphere.

El Niño in the Eocene

The oceanographic-climatic phenomenon in the equatorial Pacific, known as the El Niño-Southern Oscillation (ENSO), now seems to be major force in driving climate shifts far afield, such as the current drought in the Horn of Africa.  Its cyclicity relieves the suffering brought by El Niño events, yet the processes may well be highly unstable.  Some believe that it is only a matter of time before ENSO reverts to a permanent El Niño condition, with disastrous consequences.  Such a stabilisation in the past may have resulted in warming at high latitudes that permitted lush vegetation in near-polar regions, during the Cretaceous and the Eocene.  The Eocene was much warmer than now, as a result of a massive release of methane from seafloor sediments around 55 Ma.  So it makes sense to look at its climate record to check for a permanent El Niño.  Matthew Huber and Rodrigo Caballero of the University of Copenhagen have compared climate records from annually layered lake sediments from the Eocene of Germany and Wyoming in the western USA with climate models to test the hypothesis (Huber, M. & Caballero, R. 2003.  Eocene El Niño: Evidence for robust tropical dynamics in the “hothouse”.  Science, v. 299, p. 877-881).  The climate data from the lake sediments (thickness variations in annual layers) show clear signs of a roughly 5-year cycle of climate change, attributed to an Eocene ENSO.  This tallies nicely with simulations for the Eocene continent-ocean set-up.  Although the authors claim that their findings refute the hypothesis that global warming tends to shut down ENSO, which is a comforting thought, Eocene ocean and air circulation was not the same as now by any means.  There have been interglacial periods during the Pliocene to present climate system in which temperatures exceeded those of the Holocene.  Surely, annually layered sediments from those times will provide a better test.

Letting Cameroon’s soda-pop lakes go flat

Published on by zooks777Leave a comment

The April 2001 issue of Earth Pages News (Taming Lake Nyos, Cameroon) announced attempts to release CO2-rich water from the bottom of the notorious Lake Nyos, by setting in motion a sort of soda siphon.  A massive discharge of gas from Lake Nyos in 1986 killed 1700 local people, possibly after a small earthquake and landslide disturbed the bottom water.  Nearby Lake Monoun had already asphyxiated 37 people two years previously.  Both lakes are stagnant, and carbon dioxide released by exhalation from deep magma chambers dissolves under pressure in their deepest levels.  If the water rises, then it belches out dissolved gas, with potentially disastrous results.  Taming these killer lakes by bringing gas-rich water up pipes works because as the gas bubbles out of solution it rushes up the pipe dragging water with it, to create a fountain.  This is slowly relieving the danger of Lake Nyos, and there have been no problems caused by disturbing the deep water by the pipe’s presence, so far.  A French team from the University of Savoie is now installing a similar device in Lake Monoun, which poses a greater threat than Nyos, because the gas-rich water is only 60 metres down.  Potentially far more dangerous are the lakes of the East African Rift system, where magma exhalation is far more widespread and seismicity more common.  Lake Kivu, near Goma on the border between Rwanda and the Democratic Republic of Congo, threatens far more people with a massively greater threat, which also includes huge volumes of buried methane.  Luckily, the lava flow there during early 2002 did not reach the gas-rich level.  The experience from Cameroon promises an eventually easing of the dangers elsewhere.

Source: Krajick, K. 2003.  Efforts to tame second African “killer lake” begin.  Science, v. 299, p. 805.

Hydrogeology of sea-floor cooling

Published on by zooks777Leave a comment

Much of the Earth’s internal heat production escapes from the ocean floor, by a combination of direct cooling of new lavas at ridges, hydrothermal pumping of seawater through oceanic crust and conduction.  These processes are responsible for the increase in density of oceanic lithosphere that causes the ocean floor to gradually deepen away from spreading axes, thereby adding a gravitational force (ridge-slide force) to help drive plate tectonics.  The cooling also ensures that oceanic lithosphere is sufficiently cool at destructive margins for metamorphic processes in subduction zones to increase its density above that of the mantle, thereby largely driving plate tectonics through slab-pull force.  More than 70% of internal heat loss through the oceans is dissipated through crust that is younger than 1 Ma.  Much of that emanates from huge hydrothermal geysers, about which a great deal has been revealed in recent years.  What of the other 30% that escapes through older crust?  The older it is, the more it is literally blanketed by sediments that should act to block circulation of seawater, because they are so fine grained and impermeable.  It might seem as if heat lost would have to be by conduction alone.  That is not sufficient to explain the shape of the ocean basins.  However, some recent work near the Juan de Fuca Ridge in the NE Pacific by a team from the USA, Canada and Germany (Fisher, A.T. and 12 others 2003.  Hydrothermal recharge and discharge across 50 km guided by seamounts on a young ride flank.  Nature, v. 421, p. 618-621) shows that basic principles of hydrogeology guide seawater to increase heat loss.  Outflow is not through the sedimentary cover, but through seamounts, which are outcrops of the underlying igneous part of the crust.  Like many springs on land, the water that flows from them can come from far afield.  The sedimentary cover acts as an aquiclude, making the crystalline crust a confined aquifer, but for any flow to operate water must infiltrate the ocean floor.  Fisher and colleagues have found that some seamounts have higher heat flow than others, and are sites of outflowing warm water.  Some have anomalously low heat flow and may well be sites where seawater is infiltrating.  Dating outflowing water using 14C reveals that it is very young, and must have flowed rapidly, yet in their study area there are no signs of significant recharge through the sediments.  One seamount, 50 km from another which discharges water is the only likely source.  So, it seems as if the distribution and number of sea mounts on the oceanic part of a plate might bear greatly on the processes that eventually take place when the plate is subducted.  “Pimply” plates could have cooled more than smooth plates with an unbroken blanket of inefficiently conductive sediments.

Young age for “Mungo Man”

Published on by zooks777Leave a comment

In the February 2001 issue of Earth Pages news, I commented on the extraordinary feat of Australian geneticists’ having extracted mitochondrial DNA from fossil Australians that date back perhaps 60 thousand years (Out of Africa hypothesis confounded?). The oldest not only represents the earliest Australian yet found, but turned out to be very different from that of later inhabitants (Adcock, G.L. et al. 2001.  Mitochondrial DNA sequences in ancient Australians: Implications for modern human origins.  Proceedings of the National Academy of Sciences, v. 98, p. 537-542).  That was “Mungo Man”, named after an archaeological site near Lake Mungo in western New South Wales.  At the time of publication, the date associated with the level in which the skeleton had been found was about 60 ka).  This was so early relative to the evidence for a 70 ka estimated age for the last common male ancestor of DNA in modern humans’ Y chromosomes (one pin in the Out of Africa Hypothesis), that multi-regionalists reckoned that it supported their ideas.  Oddly, the dating, based on thermoluminiscence of quartz, which records the time since grains were last exposed to daylight, used material from 400 metres away from the burial.

In the last few years, thermoluminescence dating has improved.  Using an optically stimulated variant to date sand grains from Mungo Man’s burial, James Bowler and associates from Australia have resolved the problem (Bowler, J.M. and 6 others 2003.  New ages for human occupation and climatic change at Lake Mungo, Australia.  Nature, v. 421, p. 837-840).  The burial was 40 ka ago, late enough for migrations spreading from Africa around 70 ka to have reached Australia.  Bowler and colleagues suggest that first colonisation of Australia was perhaps around 50 ka.  The date also support two other much debated ideas, that humans’ arrival resulted in their eating to extinction most of the large animal species in Australia, and by using scrub burning on a large scale to drive game in the “red centre”, changed the climate to its present arid state.  Mind you, climate change may have been coincidental and arose from global cooling and low-latitude drying as northern ice sheets began to spread in earnest.  Possibly climatic stress drove the first Australians to adopt fire as a hunting tool.  What the new work does not do is set to rest the suspicions for even earlier occupation recorded by artefacts and even stone markings that may be art.  Some workers have suggested that these may date to more than 100 ka, although without a clue as to the creators.

See also:  Young, E. 2003.  Mungo Man has his say on Australia’s first humans.  New Scientist, 22 February 2003, p. 15. 

Darwinian evolution of humans challenged by Y-chromosome data?

This section is usually reserved for items that predate historic times.  However, new work on genetic markers in the Y-chromosomes of Central Asian (from the Pacific to the Caspian Sea) men has revealed an astonishing feature.  Of the 2123 individuals who donated swabbed tissue for Y-chromosome DNA sequencing 8% have almost identical patterns of markers.  Scaled up to the regional population, the data suggest that about 16 million men in the area show this peculiar similarity – about 0.5 % of all living males.  The authors of the study (based in Mongolia, Uzbekistan, China, the UK and Italy) make a strong case for the direct male lineage of this living population having started in Mongolia 1000 years ago, and really getting underway with Genghis Khan’s imperial exploits in the 13th century (Zerjal, T and 22 Others 2003.  The genetic legacy of the Mongols.  American Journal of Human Genetics, v. 72, p. 717-722).  For the line to have remained so dominant requires “social engineering” on an almost superhuman scale.  Not only must Genghis himself have been the “stud” he is reputed to have been, together with his contemporary, close male relatives and their direct male descendants, but unrelated men of the time in that region must somehow have been excluded from access to local women.  History suggests that was ensured by massacre and bondage on a vast scale throughout the history of the Mongol Empire.

Markers in Y-chromosome DNA arise through mutation, and are highly unlikely to carry any kind of genetically determined trait, least of all a predilection for pillage, murder and rape!  Complex analysis of the distribution of genetic markers in populations leads to ideas about how they arose, their relatedness to other markers, and an estimate of their age relative to one another.  Study of Y-chromosome markers helps understand when a male lineage began.  One such marker is estimated to have first appeared about 70 thousand years ago (see Eve never met Adam in Earth Pages News, November 2000) and occurs in all analysed modern men, giving rise to the notion of a last common male ancestor living around that time.  That all modern males are descended from him suggested some kind of evolutionary “bottleneck” at that time, through which only a very small, related group’s were fit, in the Darwinian sense, to pass.  Maybe some other mutations conferred that fitness.  Perhaps some universal calamity reduced human population to only one or two small bands; chance rather than genetic determinism..  The third suggestion was that a small group’s development of a new technology conferred the potential for them to have progeny that survived to breed successfully for generation after generation, thereby coming to dominate the small populations of the pre-agricultural period.  The last would have had little to do with Darwinism, arising from a cultural change that had a dramatic effect.  The Genghis-related Y-chromosome discovery raises another possibility, that of social and sexual dominance of some “Big Man” through political achievement and ruthlessness; aspects of conscious social being and culture, and indeed economics and technology.  Tool makers and users who passed their skills down the generations are quintessentially human, and have increasingly developed with a cultural “cushion” from purely unconscious, natural processes for 2.5 million years.  Surely, some kind of “Big Man” (and possibly “Big Woman”) hypothesis has a place in thinking about human evolution as a whole.