Mineral maps of Mars

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Lots of space has been devoted in science journals to results from NASA’s robot rovers on Mars.  Well, haven’t they been exciting?  Iron-oxide “blueberries, a cliff with bedded sediments and some iron-aluminium sulphate in a combined traverse of a kilometre at most: imagine a geologist coming back from a terrestrial field trip costing a year’s GDP of a small poor country and writing a report for the funding agency!  That is a bit cruel, for in planetary exploration the themes are context, context and context, but we did know that Mars is red and orange, which is enough for most of us to feel happy with a lot of iron coloration.  At the same time as the rovers were deployed, the European Space Agency’s Mars Express was going into orbit (so named because it was assembled in something of a hurry).  That bristles with the geoscientist’s other modern tools: those aimed at sensing materials from their electromagnetic spectra.  There is the High-Resolution Stereo Camera that produces images to rival high-altitude aerial photos of the Earth, and with stereoscopic overlap from which accurate models of Mars’ topographic elevation can be calculated, of which more in the next item.  The principal mineral and rock mapping tool is the Observatoire pour la Minéralogie, l’Eau, les Glaces, at l’Activité (OMEGA), that builds on the spectral mapping by NASA’s Thermal Emission Spectrometer deployed by the earlier Mars Global Surveyor and a similar instrument aboard Mars Odyssey.  OMEGA is every remote sensing geologist’s dream machine, because its coverage of the short-wave end of electromagnetic radiation by 350 narrow bands can match spectra reflected from rocks and soils with those measured under laboratory conditions for several hundred important minerals.  Research geologists don’t get much of that quality of data from Earth, mainly because it is commercially successful in mineral exploration, and very expensive (for much of the Earth, such hyperspectral data is not very useful, because vegetation masks most mineral signatuires).  But data are free from Mars Express (or will be when the main investigators have had a reasonable time to satisfy their curiosity) and has a terrestrially useful resolution down to 100m.  They also cover an awful lot of the planet’s surface and should eventually give 100% coverage.. The 11 March 2005 issue of Science devotes 24 pages (p. 1574-1597) to summarising OMEGA results.  Various papers reveal variations in the composition of pyroxenes in the predominantly mafic Martian surface rocks, those minerals, such as the sulphates gypsum and jarosite, which contain water and signs of weathering by water, and an awful lot about water and CO2 ices around the poles.  But this is not the geology in full of course, but driven by the search for potential habitability.  Common rocks are not made of sulphates and ice, but silicates, which can be assessed by multispectral thermal emission data that prove very useful on Earth.  The lack of information about such fundamental divisions of Martian igneous rocks as ultramafic, mafic, intermediate and felsic is a great disappointment, but perhaps the thermal instrument aboard Mars Odyssey will eventually come up with those more mundane goodies.  Oddly, the planetary treasures of Mars are not being revealed by such sophisticated instruments, but by what is still the work horse for a great deal of  geological image interpretation, black and white stereo images.

The triumph of the old on Mars

Except perhaps for some of the current generation of geologists, who are immersed in their remote sensing training by false colour images of spectrally revealing multispectral image data, a great many professionals who engage in mapping cut their teeth on what is known simply as photogeology.  And it is simple.  Provided images are taken of an area from different angles, with the simplest of instruments most people’s innate stereoscopic vision enables them to see startling illusions in three dimensions.  Stereoscopy has been to geologists of the mid to late 20th and early 21st centuries what the binoculars were to those earlier scientist who discovered the great nappes of the Alps and thrust belts of the Rockies.  A stereoscope of some kind is the latter-day analogue of that “Swiss Hammer”.  Two stereo images reveal a great deal more than twice the information of one flat image, no matter how detailed.  Using complex software, which converts the parallax differences that enable us to see 3-D to the differences in topographic elevation that cause relative shifts in the position of features on overlapping images creates accurate models of the elevation itself.  That enables quantitative measure of many features related to topography, and allows the images to be viewed in perspective, as if they were indeed captured by binoculars from a high view point.  Results from the Mars Express High-Resolution Stereo Camera (HRSC) have proved able to revolutionise our understanding of the Martian surface.  The 17 March 2005 issue of Nature reports three important new results that stem from HRSC data.  For several years the possibility of glaciers having carved some features on Mars have been suspected from lower resolution elevation data.  Now it is certain from exquisite perspective views of debris aprons that record the flow of smashed rock from large mountains, almost certainly because the debris was once extremely dirty glacial ice (Head, J.W. et al. 2005.  Tropical to mid-latitude snow and ice accumulation, flow and glaciation on Mars.  Nature, v.  434, p. 346-351).  The flows are reminiscent of rock-rich glaciers in the hyper-arid Dry Valleys of Antarctica.  These authors present evidence that suggests that the flows are as young as 130 Ma, and may yet contain water ice.  A second paper also reveals the influence of near-surface ice on Mars (Hauber, E. et al. 2005.  Discovery of a flank caldera and very young glacial activity at Hhecates Tholus, Mars.  Nature, v. 434, p. 356-361).  In its case it seems to have been mobilised by an explosive volcanic eruption, possibly as young as 20 Ma, to produce debris flows and also very well preserved drainage channels at a much smaller scale than those known from Mars’ earliest history.  The drainages might have resulted from subsurface ice melting by high heat flow and emergence of the “groundwater” to carve the meandering channels.  There is an important caution: any dating on Mars depends on assuming a timescale based on counting impact craters and noting their relations to each other and different kinds of surface.  The third paper observes something very different (Murray, J.B. et al. 2005.  Evidence from the Mars Express High Resolution Stereo Camera for a frozen sea close to Mars’ equator.  Nature, v. 434, p. 352-356).  HRSC images reveal an area about the same size as the North Sea that is not only completely flat, but shows features very like those associated with pack ice in the Arctic and around Antarctica.  They are plates whose edges can be fitted together, and in some cases islands have resulted in pressure ridges very like those seen where terrestrial pack ice meets land.  There are even examples of impact craters that have been flooded.  Murray and colleagues attribute all this to a large volume of subsurface water released by very recent volcanism along fissures close to the Martian equator.  Basalt floods had been identified in the region before, but not evidence for a possible sea-sized, frozen lake.  Similar, but not so revealing features elsewhere on Mars have been interpreted as lava rafts that once floated on flood basalts.  Naturally, Mars scientists are very excited about the possibility of a large ice sheet at the equatorial surface, which may be as much as 45 metres deep.  Unfortunately, the observations are from an area not yet covered by spectral data that would resolve whether the surface is ice-rich or more mundane lavas.

Evolutionary rhythms

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The late Jack Sepkoski did a lasting service for those who study life’s record by combing the literature to compile the first and last appearance of each marine fossil genus.  It is from this archive that we have been able to visualise mass extinctions and those less in magnitude numerically.  As well as the “Big Five” there are other die-offs, particularly through the Mesozoic and Cenozoic record.  To some extent the extinction patterns also appear among terrestrial taxa that have been less well documented, partly because few have had Sepkoski’s determination and partly because land organisms leave fewer traces.  It quickly became apparent to him and other palaeontologists that extinction occurred sharply, which is why the biologically-determined division of Phanerozoic time since 542 Ma is so well defined world-wide.  What also emerged from inspection of the time series of genus and family numbers was a pulse in the timing of significant extinctions, which appears to have been between 25 and 30 Ma.  That struck a chord with specialists in volcanic activity, and there is a good correlation between the occurrence of flood-basalt outpourings and extinctions.  But at least one of the five largest extinctions, at the K-T boundary, coincides with abundant evidence for a major impact by an extraterrestrial body.  Planetary scientists then began looking for a pulsed variation in the intensity of bombardment of the Inner Solar System.  There is no tangible evidence of that, although there are theoretical arguments that suggest that the Sun in its ~250 Ma orbit around the galactic centre wobbles through dust arranged in bands close to the galactic plane every 30 Ma.

Extinctions are not, of course, the only features of the fossil record.  Primarily it charts variations in diversity, of which suddenly lowered numbers are one aspect in broader fluctuations.  Each extinction eventually precedes an increase in diversity as adaptive radiation from surviving taxa fills ecological niches left vacant or under-populated.  That part of the record has its fascinations, as complexity seems to have emerged in three great pulses, through the Palaeozoic, Mesozoic and Cenozoic Eras, each producing more diverse forms than its predecessor.  There are also slackenings in the pace and periods of apparent stasis.  Getting to numerical grips with the full record requires analysis that uses similar mathematical techniques to that which unlocked proof of Milankovich’s theory of astronomical pacing of climate from finely calibrated oceanic-sediment records.  It is possible to analyse time series in terms of discrete frequencies from which the curves can be reconstructed.  Physicists Robert Rohde and Richard Muller of the University of California have used this Fourier analysis on the 36 thousand strong catalogue published after Sepkoski’s death, with some recalibration of the time scale and some pruning of data – they removed genera with only a single record or whose age is poorly known (Rohde, R.A. & Muller, R.A. 2005.  Cycles in fossil diversity.  Nature, v. 434, p. 208-210).  There are definitely distinct frequencies that dominate the record, and they cannot be present by chance, although that is a purely statistical view.  But to their surprise, and everyone else’s, they are completely unexpected ones at 62 and 140 Ma.  It is proving exceedingly difficult to come up with plausible Earthly or extra-terrestrial explanations.  There are two interesting features: the 62 Ma periodicity dominates the record of relatively short-lived genera; and the “Big Five” seem to fit neatly into the patterns of diversity, albeit at unequally spaced intervals, when the effects of background fluctuations have been removed.  That filtering may allow for increasing preservation towards recent times.  One major control over diversity is, logically, a mixture of the number of potential niches and their geographic isolation, and both are probably related to plate tectonic activity.  Unfortunately, fluctuations in 2 and even 3 geographic dimensions have only the broadest calibration to time.  Added to that is the complex way in which global sea level has changed with time.  So we can expect a great deal of head scratching, and it may come as a relief that the crowing of some volcanologists and impact theorists may have been silenced at a single stroke!

See also:  Kirchner, J.W. & Weil, A. 2005.  Fossils make waves.  Nature, v. 434, p. 147-8.

Making sense of glacial-interglacial cycles?

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The competing periodicities of  the three astronomical “drivers” of climate – orbital eccentricity (~100 ka), axial obliquity (~40 ka) and axial precession (~20 ka) – lie behind several models for the climate changes of the last 0.7 Ma.  Taking in the theories that sway towards the influence of variables in the Earth system itself, around 30 models have some currency at present.  Since climate forecasters have to take account of which factors drive climate in the absence of human emissions, as well as piece together their own particular models, it is easy to see how critics of global warming get a wide hearing: compared with creationists, they have it easy!  Is there any way of resolving what is quite bluntly a theoretical mess?  It is a mess simply because the available data are so complex, and in the case of both main sources, ocean-floor sediments and ice cores, not only are their devils in the detail, but there are whopping contradictions, such as the mismatches in timing between the Greenland and Antarctic ice cores.  Add all the other sources, such as stalactites, tree rings etcetera, together with caveats like the difficulty in time calibration using 14C dating,  and the volume of diverse records become bewildering.  It is tempting that a reversion to a statistical approach, that includes more bells and whistles than hitherto (see Evolutionary rhythms below), can resolve matters.  Peter Huybers and Carl Wunch, of Woods Hole Oceanographic Institution and MIT, have tried that for pacing of the last 0.7 Ma of climate cycles (Huybers, P. * Wunsche, C. 2005.  Obliquity pacing of the late Pleistocene glacial terminations.  Nature, v. 434, p. 491-494).  Generally accepted “wisdom” holds that the last 7 glacial-interglacial cycles are paced by ~100 ka eccentricity forcing, even though it has the weakest effect on solar heating, by a very long way.  But there are smidgens of evidence for some interaction between that and the much stronger influence of changes in the Earth’s axial tilt or obliquity.  Huybers and Wunsch go for the Popperian rigor of first defining a null hypothesis, that obliquity has no effect, and then designing a test.   It isn’t easy to decide how the contrary hypothesis that it does can be evaluated though.  The clearest features in all climate records are the ends of glacial epochs or termination: they are sudden, sharp and generally look the same.  Most other features have some kind of pattern, but little consistent comparability.  Using the most advanced statistical techniques, which employ many iterations to test for stability in statistical models, they can show that the null hypothesis fails.  The positive result is that the time between terminations that are repeatedly modelled  falls into two envelopes, around 120 and 80 ka, which simple arithmetic shows are divisible by 40 ka.  But how can axial obliquity only have an effect every two of three of its cycles, while a single cycle does not appear in the time-series; is it nature skipping beats somehow.  One means that the authors suggest is that the underlying pace of eccentricity can effect the temperature at the base of ice sheets, depending on their thickness.  If they are thin, then the heating is insufficient to trigger ice-sheet collapse because the base is very cold, whereas if ice is thick the effects of thermal conductivity and heat flow makes the ice base warmer and more subject to perturbation beyond its failure limit.  It was at this point that I gave up, but wish the authors good luck in promoting their possibly unifying hypothesis for what finishes off glacial epochs…..

Curiously low-velocity material at the core-mantle boundary (CMB)

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One of the oddities of the deep Earth is the presence of zones of the order of 1 to 10 km thick close to the boundary between the lower mantle and the outer core that have seismic wave speeds well below those expected at such depths.  Because wave speed is inversely proportional to density, the chances are that they are “ponds” of extremely dense solid materials.  Denser in fact than basalt might become in the form of eclogite, even compressed appropriately to these extreme depths.  The zones have been a puzzle, but that seems to have been resolved by work from University College, London (Dobson, D.P. & Brodholt, J.P. 2005.  Subducted banded iron formations as a source of ultralow-velocity zones at the core-mantle boundary.  Nature, v. 434, p. 371-374).  The densest materials found commonly at crustal levels are iron oxides and hydroxides, but today they are disseminated through much larger volumes or quartz-rich sediments.  Up to about 1.8 billion years ago, they were produced in huge abundance in sedimentary rocks, along with interbedded cherts, to form banded iron formations (BIFs).  That is widely agreed to have been a phenomenon only possible when the ocean was oxygen free so that iron could be dissolved in the oceans, and that they were precipitated when that Fe(II) came into contact with oxygen being produced by photosynthesising blue-green bacteria in shallow water.  Without any shadow of doubt, BIFs are the densest sediment that the Earth has ever produced, with a 50:50 mix of iron oxide and chert having a density of 3900 kg m-3 at near-surface pressures, compared with 3100 for the upper mantle.  Long ago, Bob Newton of the University of Chicago reckoned that they “didn’t oughta be around still”: Precambrian BIFs are so vast and so dense that they are even more likely to be subducted than oceanic basalt converted to eclogite.  And they would not even need to be metamorphosed to do that.  So, it has taken a long time for someone to cotton on to Newton’s typical prescience.  Quite possibly, BIFs were a tectonic driving force at a time when the basalt-eclogite transformation was thermodynamically unlikely. Dobson and Brodholt observe that BIF density can only get larger (much larger; 6600 kgm-3 at CMB pressure) if they sink  This is a nice hypothesis, for BIFs fit the bill exactly for the ultra-low velocity zones, and carries some interesting corollaries.  BIFs contain a great deal of oxygen, in fact probably the entire productivity of the early Precambrian biosphere: that would have a biogenic isotope signature.  Could that be added to any plume material emanating from the CMB?  Equally, BIFs contain unusually high concentrations of transition metals, and there is another possibility for deep-mantle geochemists to juggle with. The authors also observe that iron-oxides have high electrical conductivity compared with silicates, and ponder on the electromagnetic consequences of that so close to the core.  One thing seems certain; iron oxides probably would not melt, but, depending on the amount of oxygen in the core, they might dissolved in the molten outer core.

Did the earliest agriculture kick-start global warming

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Most climate scientists encourage us to believe that planetary warming caused by gas emission from our energy intensive life style is both new and an inevitable context for our future.  Yet, one leading authority on past climates, William Ruddiman of the University of Virginia, reminds us that it isn’t only cars and power stations that release warming gases (Ruddiman, W.F. 2005.  How did humans first alter global climate. Scientific American, v. 292 March 2005, p. 34-41).  New evidence from air bubbles in the Vostok core through Antarctic ice shows a strange deviation of atmospheric CO2 around 8000 years ago, from a downward trend in the early Holocene to one that relentlessly rises to the levels that characterised the recent pre-industrial world.  At around that time early agriculturalists in Europe and China began to chop down forest to make fields, thereby releasing the carbon content of felled trees to the atmosphere as CO2.  By 5000 years before present, rice cultivation in East Asia had begun the release of methane from waterlogged paddy fields, and the methane content of ice bubbles reveals a reversal of methane decline at that time exactly..  Ruddiman’s view is that the release of both “greenhouse” gases reversed a natural cooling trend, and that growing populations sustained growth in atmospheric CO2 (methane is quickly oxidised in the atmosphere). Comparing the rising CO2 of the Holocene with its records in ice-bubble for the previous three interglacials, shows that in each previous case the gas rose to a maximum early in the interglacials and then declined steadily.  The invention of agriculture and its spread from around 11000 years ago in the Near East, he claims, could have staved off the onset of global cooling and the climatic descent into another glacial epoch, by eventually adding 40 parts per million of CO2 to the air.  To support his hypothesis Ruiddiman compares the more recent ice-core records with historic catastrophes, mainly plagues that wiped out substantial proportions of the word population .  Sure enough, there are falls in CO2 at the time of each major plague; that between 540 to 542 AD in Europe, the Black Death of the Middle Ages, and the reduction of the population of the Americas by maybe 90% when “Old World” diseases such as smallpox and measles met no resistance among native peoples.  In many respects Ruddiman’s ideas seem plausible, until we see the data.  The problem with ice core data is that its resolution degrades through time, and before 70000 years ago, no annual layers are preserved in glacial ice.  Moreover, records from different Antarctic cores differ wildly for the historic period and Ruddiman does not show the record from Greenland ice.  Finally, records of ice volume and ice-cap temperatures, derived from marine and glacial oxygen isotope records, show that each previous interglacial involved very different fluctuations in many other climate-related parameters.  If nothing else, Ruddiman’s  ideas will be challenged and the issue will “run and run” until the next “big thing”.

Tiny Indonesian hominids get the SciAm treatment

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The tiny adult remains of Homo floresiensis reported in 2004 (see The little people of Flores, Indonesia, November 2004 issue of EPN) astonished the palaeoanthropological community more than any discovery since René Dubois’ found the first H. erectus remains on nearby Java almost a century ago.  Their recent geological age (about 13 ka), together with evidence for cohabiting the island of  Flores with fully modern humans and legends of the ebu gogo – “the grandmother who eats anything” spice up the find no end.  So it is not surprising that Scientific American has commissioned an excellent popularised account of where things stand with the little people only a few months after the discovery was announced in Nature (Wong, K. 2005.  The littlest human.  Scientific American February 2005 issue, p. 40-49).  It is not just the sheer tinyness of Homo floresiensis that draws our attention, but the fact that with a brain no larger than 2 Ma old australopithecines, the species crafted tools that are far more sophisticated than those of their most likely ancestor, H. erectus.  They also found their way across a seaway that could never have dried out during glacial maxima, used fire, and just as important survived competition with fully modern humans for around 20 ka.  Yet, as the article is at pains to point out, the find is so new that it is easy for specialists to kid themselves into believing a great deal more than may eventually turn out to be likely.  With two cultures on one small island, there may well have been mixing of artefacts, and also occupation of the site – a large cave – by both over the long period when they shared the island.  Opinion of many leading figures in the field is related by Kate Wong, and it is very clear that there is a lot of puzzlement.

The oldest modern humans

For a long time it has been known that the “front line” between fully modern humans and European Neanderthals was in the Middle East, with fluctuating occupation of highly productive sites since around 100 ka.  It is also well established that the ancestors of all of us outside Africa began to migrate some 70 to 80 thousand years ago, the signs being that the pressure was drying of the continent as global climate cooled.  The route take is not at all well defined, but one possibility is across the Straits of Bab el Mandab at the entrance to the Red Sea as islands became exposed when sea level began to fall.  So, fully modern humans originated in Africa, but where and when?  Unsurprisingly because of the intensity of research there since the discovery of Lucy, the Afar Depression of Ethiopia has provided most remains of H. sapiens sapiens.  Volcanic ash layers in sediments that contain specimens there give ages up to about 160 ka.  But Ethiopia has other hominid-rich sequences, including ones that have yielded anatomically modern humans.  The most notable is the Late Pleistocene Kibish Formation of the Omo River basin in southern Ethiopia, a deltaic sequence that formed when Lake Turkana had higher levels.  Human remains occur in the lower part of the Kibish Formation, and as luck would have it, they occur between two volcanic ash horizons and can be accurately dated (McDougall, I et al. 2005.  Stratigraphic placement and age of modern humans from Kibish, Ethiopia.  Nature, v. 433, p. 733-736).  For the moment, they are the oldest proper humans at 195 ka.  That age has interesting connotations as regards the climatic conditions of their lives.  The Omo basin shares watersheds with drainages into the Blue and White Nile system.  At 195 ka increased deposition of organic matter characterised the sediments beneath the Nile delta, which suggests greatly increased rainfall in the uppermost reaches of the Nile system.  That coincides with the onset of deposition of the Kibish Formation when Lake Turkana stood much higher than at present.  The area would have been lush.

Prize for solving the world arsenic crisis

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Almost every month there are announcements of yet more areas of the world that face hazards from natural contamination of groundwater by release of arsenic from whichever minerals host it in sediments.  In Bangladesh alone, the WHO estimates that tens of million people are at risk.  Although large tracts of the US and other rich countries do have arsenic levels in groundwater that are above the maximum recommended for safety, the crisis is one that most severely affects some of the world’s poorest and most populous countries. To help solve this massive public health problem, the National Academy of Engineering is offering the Grainger Challenge Prize for Sustainability, a sum of US$1 million, to the individual or individuals who design and create a workable and cheap water treatment system that anyone can use for arsenic-contaminated groundwater in Bangladesh, India, Nepal, and other developing countries.  The most likely cheap remedy lies in the use of iron hydroxide as a means of absorbing dissolved arsenic, but several other candidates, including coal fly ash and limestone, together with biological precipitation, have recently begun tests.

Incidentally, the action in the UK against the Natural Environmental Research Council, for negligence in failing to analyse for arsenic in Bangladesh groundwater in the early 1990s, on behalf of 400 Bangladeshis affected by arsenic poisoning, recieved the legal go ahead to appeal against an earlier decision by a British court to throw out their case.  My thanks to John McArthur of University College London for this news.

Drilling into the San Andreas Fault?

It seems that in order to really get a feel for the physical and chemical processes involved in faulting, drilling into an active one is a good idea, or at least that seems to be the driving motive behind the SAFOD (San Andreas Fault Observatory at Depth) project of the US Geological Survey (Cohen, P. 2005.  Journey to the centre of a quake.  New Scientist, 5th  February 2005 issue, p. 42-45).  It might make sense, because pressures of pore fluids near active faults seem likely to exert some influence over whether a fault segment moves or not.  Overpressured fluids can serve to lubricate the otherwise sticky fault surface.  In the case of the San Andreas, activity is fragmented.  Detailed monitoring of microseismicity near Parkfield, California revealed that a mere 100 x 100 metre patch on the fault plane was responsible for much of the activity.  It lies about 3 km down, just within reach of oil-drilling technology.  In fact the Parkfield segment is one of the shallowest active zones on the whole fault..  There are already holes in place, drilled to 2 km to host monitoring instruments, and new drilling methods eventually will allow sideways puncturing of the fault plane so as to install more.  But even sophisticated drilling is still largely a blind operation, which inevitably hits snags, and there have been several in the SAFOD project.  One severed communications with existing instruments.  The general idea behind SAFOD is that fault displacements propagate from small “nucleation” sites.  The length of the fault that undergoes displacement during one movement is generally correlated with the magnitude of the resulting earthquake.  Parkfield seems to be such a nucleation site, but since the earthquakes associated with it are of small magnitude chances are that interfering with it will not accidentally release a large one.  The benefits, set against the risks and undoubtedly high costs, are mainly that even the tiniest motions can be monitored.  Surface monitoring of course cannot investigate pore fluids and other phenomena, and nor can it detect events less than magnitude 0.5, whose energy is absorbed by rock before it can reach the surface.  By monitoring what happens in events with a range of small magnitudes, it ought to be possible to develop earthquake theory to the point where at least the role of fluid pressures, the feedback between earth vibrations set off by one event and movements on a later one, and the effects of mineralogy on friction that resists movement can be assessed.  Whatever, once in place, the wait for useful results to accumulate could be a long one, so SAFOD is planned for a 15 year lifetime

More information on SAFOD is available at http://www.earthscope.org/safod/index.shtml

Age range of early fossil treasure trove

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The Doushantuo Formation of southern China dates from just before the Cambrian Explosion, and has become a source of astonishing information about animals that preceded the appearance of those with hard parts.  It contains fossil embryos, algae, achritarchs, and small bilaterians that are purportedly the Earth’s earliest animals.  Moreover the formation rests on the cap carbonates of a diamictite reckoned to represent a late Neoproterozoic glacial epoch, and provides a variable trend of carbon-isotope variation that extends up to the base of the Cambrian in southern China.  Because the sequence contains a number of volcanic ash beds it is potentially dateable.  Using a single-zircon U-Pb method, Daniel Condon of MIT and colleagues from the Chinese Academy of Science have established the ages of both top and base of the Doushantuo Formation with considerable precision (Condon, D. et al. 2005. U-Pb Ages from the Neoproterozoic Doushantuo Formation, China.  Science Express, 24 February 2005).  Sedimentation is bracketed between 635 and 550 Ma, the oldest age coinciding with that for the Ghaub tillite in Namibia.  Time-calibration of the carbon-isotope record allows it to be matched with others in Namibia, Oman and Newoundland.  There is one snag; within the sequence is a formation boundary that signifies non-deposition, which the authors correlate with a glacial epoch recognised in Newfoundland (the Gaskiers diamictite), citing sea-level withdrawal as the cause of non-deposition in China.  The well-constrained correlation suggests a major, global increase in the burial of 12C that produced a marked negative excursion in d13C that spans around 90% of the Ediacaran Period that saw the rise of large soft-bodied animals shortly before the emergence of shelly faunas.  The interpretation placed by the authors on this signature of burial of dead organic matter, which relates to no sign of glaciation, is that it would have elevated oxygen levels in the Late Neoproterozoic oceans.  That might have increased productivity by primitive eukaryotes, and possibly opportunities for predation.  The uppermost part of the Doushantuo Formation broadly coincides with the first appearance of complex trace fossils and mollusk-like bilaterians, and elsewhere there are signs of the first reef formation by weakly calcified metazoans at around that time.  Clearly, it is well-dated sections such as these that may hold the key to what exactly prompted the general secretion of skeletal material; the hallmark of the 10 Ma later explosion in fossil animals.

No graphite in Akilia apatites, no sign of life?

In the first EPN of 2005 evidence was reported that weighed against a sedimentary origin for the ~3.8 Ga ironstones of West Greenland from which isotopically light carbon had been claimed to indicate the earliest signs of life (see Iron isotopes enter the Archaean life debate January 2005 EPN).  The original work that claimed a biological signature in carbon from the oldest known metasedimentary rocks focussed on carbon-isotope analyses of apatites in them, in the belief that they would have withstood intense metamorphic alteration because of the resistance of that mineral to chemical reactions.  Following close on the heels of that revelation comes one a great deal more worrying for aficionados of biogeochemistry.  Geoscientists from Estonia, France, the US and Sweden have systematically made petrographic observations on apatite grains from the rocks of the Akilia Association, including those originally reported as carrying geochemical signs of life existing at that time (Lepland, A. et al. 2005.  Questioning the evidence for Earth’s earliest life – Akilia revisited.  Geology, v. 33, p. 77-79).  Of the 190 individual apatite grains examined in 17 rocks, not one showed the slightest trace of carbonaceous material.  It seems that apatite is unlikely to have been the host for the low d13C that caused such a stir in palaeobiological circles when it was first announced, and may well not be a good place to look for biomarkers.  It also throws into question what did produce the signal.  If it was the bulk rock, then the depletion in 13C could have resulted from temperature induced isotopic fractionation.  Another possibility is that the samples were contaminated with modern biological materials, despite the precautions taken to avoid that.

 

Warming may have triggered Northern Hemisphere glaciation

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While I write this issue of EPN it is supposed to be early spring outside, and that is clearly what the ducks reckon as well – they are beginning to, er um, frolic.  But there has been two weeks of snow and frost.  Britain and the rest of Europe owe the frigid snap to cold air spilling westwards from northern Asia; the influence of the Siberian winter high-pressure area.  Although somewhat lost in the recent kerfuffles about whether or not global warming is a fact or a misreading of data, the inevitable build up of mid-continental cold dense air in winter might have interesting consequences, should climate warm.  Normally, areas far from the oceans remain dry as well as getting very cold through radiative heat loss in winter.  When spring comes, such snow as there is soon disappears and the extremes of cold are replaced by surprisingly high summer temperatures, as anyone who has visited Siberia or Northern Canada will know.  Should moist air find its way into such areas during winter, vastly more snow would fall.  Its melting would take longer, and more solar radiation would be reflected back to space in spring.  Such an albedo feedback could induce generalised cooling.  Now evidence has emerged that the earliest known growth of land ice in North America was linked to warming of the ocean from which winds blew over it (Haug G.H. et al. 2005.  North Pacific seasonality and the glaciation of North America 2.7 million years ago.  Nature, v. 433, p. 821-825).  In fact it is axiomatic that growth of continental ice sheets requires a supply of moisture and snow that exceeds the rate of summer melting and ablation, as well as cold winters.

Most theorising about the onset of Northern Hemisphere glaciation has centred on changes in North Atlantic circulation due to closures of the straits where the Isthmus of  Panama now links North and South America, and the start of southward deep-water circulation from the latitude of Iceland.  In fact both are known to have preceded the last Ice Age by a good 2 Ma.  The actual start around 2.7 Ma coincided with an increase in obliquity of the Earth’s orbit that would have led to periods with cold northern summers.  Without abundant mid-continent snowfall, that in itself would not have set ice sheets forming in earnest.  The multinational team of oceanographers studied sea-floor sediment cores from the sub-Arctic Pacific.  To their surprise, sea-surface temperatures provided by evidence from planktonic organisms show evidence at 2.7 Ma for on the one hand cooling of the sea surface (from foraminifer oxygen isotopes) yet considerable warming on the other (from organic chemicals secreted by coccolithophores).  Resolving this paradox requires a careful assessment of the ecological behaviour of the two groups of organisms.  The authors’ explanation involves the onset of density stratification in the North Pacific, so that the surface warmed quickly in summer, retaining warmth during autumn, and warmed slowly in spring from its minimum temperature.  Both result from the high thermal inertia of water.  The productivity of silica-secreting diatoms plummeted to a fifth of its earlier levels at 2.7 Ma as well, explained by ocean stratification reducing the supply of nutrients from deep water upwellings.  Intuitively, a warm sea upwind of the North American continental interior should have generated high snowfall in late autumn and winter.  Haug and colleagues modelled the contrasting effects of an ocean with water overturn and mixing with one that tends to become stratified, to simulate snowfall over the North American Arctic.  From a situation in the Pliocene with snowfall over Greenland and the Arctic islands, the scenario shifts to heavy snow over the whole Arctic in the earliest Pleistocene.  It seems that the trigger for the Great Ice Age was a hemisphere away from the “usual culprit”, the North Atlantic, although its vagaries, once glacial cycles were underway, probably controlled the details thereafter.

Cyber-tourism

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There are so many places one might wish to visit for their scenery and physical geography, yet only limited resources and, of course, time.  The availability of high-resolution satellite images, together with free data that show variations in topographic elevation newly released from the Shuttle Radar Topography Mission, enables realistic simulations of just about anywhere.  William Bowen of the California State University has exploited this opportunity to give all-comers a view of most parts of the land surface, as if they were looking obliquely downwards from a high-altitude aircraft – geogdata.csun.edu/world_atlas/index.html

Mineral wealth

Although there are many glossy books about museum quality mineral specimens, as well as being expensive they often only cover a selection of those minerals known to science.  One of the beauties of the web is that a site can cram in as many pictures and ancillary data as its server permits, and anyone can browse what is on offer.  One such site has been set up by consulting geologist David Barthelmy, which not only illustrates more than 2000 different minerals (half the site’s content of 4300) but allows users to examine their molecular structure interactively – webmineral.com