Month: November 2001

Strontium load of Himalayan rivers

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One process connected to long-term climate change is the way that weakly acid rainwater (containing dissolved CO2) weathers silicates in continental rocks, one product being carbonate in soils.  The process should draw CO2 from the atmosphere, thereby reducing its “greenhouse” effect.  The idea is by no means new, but received a boost in the mid 1990’s from Maureen Raymo’s suggestion that fluctuations in the strontium-isotope composition of the oceans through geological time should be a proxy for changes in the rate of continental weathering.  The 87Sr/86Sr of marine carbonates does show clear correlation with long-term climate shifts during the Phanerozoic..

Continental weathering should increase as topographic relief becomes greater through mountain building episodes.  The Himalaya’s rise through the late-Tertiary has been suggested as a major influence over climatic deterioration, partly by its effect on the Asian monsoon and partly as a huge site for the sequestration of atmospheric CO2 by chemical weathering.  Himalayan rivers have enormous flows and equally large sediment and dissolved element loads.  In particular they carry far more strontium than other rivers, and it has a highly radiogenic content of 87Sr.  There are three means of attaining these levels: from average continental crust which has a higher 87Sr/86Sr ratio than oceanic crust (the other main source of seawater strontium); from strontium rich limestones that acquired their isotopic signatures from the ocean when they were deposited; or from sources with unusually high 87Sr/86Sr ratios.  The Himalaya are well known for carbonate sediments, and for granites formed by melting of deeper, older continental material that gives them very high proportions of radiogenic strontium.  Recent work now shows that a significant contribution of highly radiogenic strontium to Himalayan rivers is hydrothermal activity (Evans, M.J. et al. 2001.  Hydrothermal source of radiogenic strontium to Himalayan rivers.  Geology, v. 29, p. 803-806).  Hot springs feeding a major tributary of the Ganges contribute up to 30% of its strontium load, and incidentally a great deal of CO2.  Both result from hydrothermal alteration of deeper rocks, and are unrelated to weathering if the water involved emanates from the deep crust.  It seems that these waters are recycled rainwater, so this is a case of a high-temperature chemical weathering.  Whatever, it further complicates the original notion of linkage between mountain building and climate.

Methane and Snowball Earth

The well-publicized “Snowball Earth “ model for Neoproterozoic glaciogenic rocks that occur at tropical palaeolatitudes has to involve an escape mechanism from global frigidity.  Without some means of warming, the high albedo of widespread ice would have locked the Earth into perpetual glaciation, which of course did not happen.

The main proponents of the model, Paul Hoffman and Dan Schragg of Harvard University suggested a gradual build up of volcanogenic CO2 during “Snowball” conditions, when a dry atmosphere would have retained the “greenhouse” gas instead of its being sequestered to the oceans and carbonate rocks by acid rain and continental weathering.  Gradually, atmospheric temperatures would have risen due to trapping of outgoing, long-wave radiation by CO2.  This simple aspect of the model leads to scenarios where warming overruns once ice sheets disappeared, to give extremely high-temperature conditions.  Using carbon-isotope data from marine carbonates is a means of supporting or refuting this escape mechanism, and also of detecting the influences of other components of the carbon cycle.  Carbonates take up carbon dissolved in seawater without fractionating its different isotopes, and provide measures of the degree to which organic processes did contributed to fractionation.  Cell processes preferentially take up 12C, and if large masses of undecayed organic matter ends up in seafloor sediments, the proportion of “heavier” 13C (indicated by the standardized ratio of the two main isotopes d13C) increases in seawater and the atmosphere.  Carbon of mantle origin, that emerges as volcanic CO2, has a constant d13C of about -5‰.  So these two processes contribute to an isotopic balance, which for most of the Mesozoic and Cenozoic Eras established a d13C of between 0 and +4 ‰ in sea water and limestones.  This is interpreted as a sign that the recent carbon cycle achieved a balance between volcanic additions and organic carbon burial weighted towards trapping of undecayed carbohydrate in sea-floor sediments.  Explanations for broad climate changes since 250 Ma therefore rely more on other mechanisms than on the carbon cycle

The most comprehensive study of Neoproterozoic carbon (Walter, M.R. et al. 2000.  Dating the 840-544 Ma Neoproterozoic interval by isotopes of strontium, carbon and sulfur in seawater, and some interpretative models.  Precambrian Research, v. 100, p. 371-433) does indeed show dramatic see-sawing of d13C through supposed “Snowball” events, from highly positive values (<+10‰) before glaciogenic sedimentation to highly negative (>-10‰) in the immediate aftermath.  However, few data were available from within glaciogenic sediments, and resolution is insufficient to detect tell-tale trends.  The key approach needs detailed carbon isotopes through a single event, and such data appeared recently for the famous Neoproterozoic glaciogenic-cap carbonate sequence of Namibia (Kennedy, M.J. et al. 2001.  Are Proterozoic cap carbonates and isotopic excursions a record of gas hydrate destabilization following Earth’s coldest intervals.  Geology, v. 29, p. 443-446)

Kennedy et al. measure d13C in carbonate cements in the glaciogenic diamictites, in overlying cap carbonates and in cement to later clastic rocks.  Interestingly, there is little sign of a gradual decrease in 13C through the glaciogenic rocks.  Constant oceanic carbon composition would be expected if no volcanic CO2 entered seawater during frigid, dry conditions, and living processes were minimal.  In the cap carbonates d13C plummets from +3‰ to -4‰.  One simple explanation would be massive “rain-out” of volcanic CO2 (d13C of -5‰) that had built up in the air during the “Snowball” episode.

Whizz-bang at end of Permian

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Relating mass extinctions to the effects of impacts by comets or asteroids is now a major industry, and a great number of geologists who sneered at early suggestions of extraterrestrial influences over evolution are finding ever new ways to cook and eat their headgear.  Oddly, however, many of those who bore the brunt of such mean-spirited, and somewhat premature scorn still cling to the safe old K-T event.  Soon all the thin K-T boundary material will have been consumed by these cautious, if meticulous scientists.  Thankfully, some have ventured to seek evidence for other catastrophes that came out of the blue. In comparison with the end-Permian extinction, the K-T event is a mere bagatelle.  However, attaching it to an extraterrestrial cause has proved difficult.  It has attracted as many opponents of impact theories as “whizz-bang” aficionados, with much talk of the effects of sea-level changes, volcanism, ocean anoxia and climate shift.  They may be in for a big surprise.

The Permian-Triassic boundary in Meishan, China is at first sight a nondescript sequence of shallow marine strata, albeit complete.  The last occurrence of Permian marine genera there, with typical signs of mass extinction, coincides with a 20-fold increase in nickel concentrations.  Closer examination reveals other brusque geochemical and mineralogical anomalies, including magnetic grains of iron-silicon-nickel alloy, but no iridium anomaly (the popular target for detecting asteroidal impact horizons) or examples of shocked quartz and feldspar (Kaiho, K. et al. 2001.  End-Permian catastrophe by bolide impact: Evidence of a gigantic release of sulfur from the mantle.  Geology, v. 29, p. 815-818).  Most significant is a sudden drop in 34S due to a large increase in the amount of isotopically light sulphur in the environment.  Kaiho et al. attribute this to vast emission of sulphur from the mantle.  A coincident fall in the 87Sr/86Sr ratio could also result from entry into the oceans of lots of mantle-derived strontium.

The P-Tr boundary also coincides with the time of eruption of the largest continental flood-basalt province, the Siberian Traps.  No doubt other scientists will seek to account for the chemical anomalies at Meishan as distant effects of the Siberian volcanism alone, as they have for the K-T boundary anomalies because of their coincidence with Deccan volcanism.  The authors prefer to suggest a causal link between impact and massive volcanism.

Surviving the Archaean with a UV jacket

Earth’s dominance, for at least the last half billion years or so, by oxygen-dependent and oxygen producing life forms stems from the evolution of photosynthetic organisms whose cell metabolism involves breaking the strong bonds in water molecules with solar energy.  Chemo-autotrophic life that exploits other energy sources has been consigned to niches that are very much narrower than they were at the biosphere’s outset.  The earliest primary producers using oxygenic photosynthesis were the cyanobacteria – arguably the predecessors of modern plants’ chloroplasts, in Lyn Margulis’ endosymbiotic model for the origin if the Eucarya.  Carbon isotopes from the early Archaean do suggest their presence close to the start of recordable geological history, and at around 3.5 Ga the first known stromatolites were almost certainly secreted by blue-green bacteria (See Carbonates and biofilms, Earth Pages August 2001).

To thrive and colonise ocean surface waters, the shallows and perhaps even the continental surface – their water-splitting, solar powered metabolism opened up those opportunities – cyanobacteria, more than any other prokaryotes, had to resist massive damage from ultraviolet radiation.  Lack of atmospheric oxygen, and therefore ozone, left Earth’s surface with no shield to the most biologically damaging, short-wave UV.  Despite the fact that modern “blue-greens” can survive climatic extremes from the frigidity of Antarctica’s Dry Valleys to superheated water in hot springs, as regards UV damage they are wimpish.  This is partly due to its bleaching effect on the light-harvesting pigment on which chlorophyll depends.  Cyanobacteria cells do have some biochemical protection against radiation damage, but it is of no avail when bathed in the “hardest” UV likely to have characterized Archaean surface environments.

A widely held view is that “blue-greens” survived and prospered because of another function common to many single-celled organisms; their tendency to promote nucleation of inorganic compounds outside their cell walls.  Stromatolites themselves are good examples of the production of biofilms, being made of minute laminae of carbonates, whose secretion helps cyanobacteria avoid calcium stress.  In modern hot springs that contain dissolved silica, these organisms often help generate sinters made of silica.  A team from the University of Leeds (Phoenix, V.R. et al.  2001.  Role of biomineralization as an ultraviolet shield: Implications for Archaean life.  Geology, v. 29, p. 823-826) has performed controlled experiments on living cyanobacteria from Icelandic hot springs to check their defences against short-wave UV.  With a biofilm screen (in the experiment they used wafers made from associated iron-silica sinter, as well as colonies with a biofilm) the organisms easily survived and continued to photosynthesize.  Exposed “naked” they succumbed after only a few days exposure.  It seems that traces of iron incorporated in the films dramatically enhance the UV-screening, without reducing photosynthesis.  Archaean iron-rich cherts are massively abundant in banded iron formations, and the first definite remains of cyanobacterial cells come from such silica-rich material.  However, the ubiquitous stromatolites in limestones of early Precambrian times are the main signs of life.  It remains for the UV-screening properties of carbonate biofilms to be assessed.

New phyllum from Chinese Cambrian

Incompleteness of the fossil record is partly a result of the bias towards organisms with hard parts and against soft tissue, during sedimentary processes.  For preservation of soft-bodied animals, together with that of intricate parts of the usual fossils, palaeontologists look to site where preservation is exceptionally good – lagersttätten.  An example is the Solenhöfen Limestone, famous for Archaeopterix.  Mudstones formed under highly reducing conditions, which excluded bacteria that complete oxidize flesh, provide similar opportunities.  Work through the last two decades by Simon Conway Morris of the University of Cambridge has resulted in working and interpretative methods that permit extremely detailed analysis of physiologies, beginning with the most famous lagersttätte, the Middle Cambrian Burgess Shale of British Columbia.  Conway Morris and others unearthed beasts so strange that they had little choice other than to erect new Linnaean Classes and Phylla to classify them.  Equally as important, such sites help fill in the details of early members of those which survive today, including the elusive penis worms.

Conway Morris has been part of a team based at the Northwest University in Xi’an China, which has discovered lagersttätten in the Lower Cambrian, closer in time to the explosive development and radiation of animals at the end of the Precambrian.  Once again, unsuspected novelty has turned up (Shu, D.-G. et al.  2001.  Primitive deuterostomes from the Chenjiang lagersttätte (Lower Cambrian, China).  Nature, v. 414, p. 419-424).  Along with excellent examples of agnathan fish and many familiar soft-bodied animals, the prize in this case are remains that warrant a new, extinct Phyllum, the Vetulicolia.  The organisms are small but complex, with two main body chambers that reveal mouth, innards and gill slits.  The last helps place them within the deuterostomes; an “umbrella” that groups chordates (sea squirts and vertebrates) and echinoderms (they have lost such slits, but are genetically closer to chordates than any other group).  Critical to the evolutionary significance of the vetulicolians is a groove that floors what is interpreted as the anterior part of their alimentary canals.  Such a groove characterizes the pharynx of chordates, where it serves as “gutter” for various glands – the endostyle, also involved with iodine in metabolism.  If the vetulicolian groove is an endostyle, then they are chordates.  However, lacking an axial stiffening rod (notochord of the chordates in general, and vertebral column in vertebrates) they must be primitive.  Occurring with true vertebrates, in the form of jawless fish, the vetulicolians are a relic of some earlier stage in vertebrate evolution.  Shu et al. take the cautious view that they are early deuterstomes from which echinoderms and chordates emerged – close to the fundamental division among animals into deuterostomes and protostomes.

(See also:  Gee, H.  2001.  On being vetulicolian.  Nature, v. 414, p. 407-408)

EarthScope

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North America, particularly its west coast, is the best studied natural laboratory for active tectonics.  Nonetheless, the downturn in Earth Science funding in the USA has threatened an ambitious project aimed at consolidating knowledge of plate interactions there.  Nature (15 November 2001, p. 241) reports that the EarthScope initiative now has strong backing from the US National Academy of Sciences.

EarthScope has 4 elements: a mobile grid of seismometers; an observatory to monitor movement of plates below the NW Pacific Ocean; a programme aimed at drilling into the San Andreas Fault System; an interferometric radar satellite that will accurately measure ground movements in relation to tectonic and volcanic features.  The total cost is around $400 million, shared equally between NASA and the National Science Foundation, if the funding proposal wins acceptance.

Information from:  http://www.earthscope.org

Continental tectonics of eastern Eurasia

Interferometric radar remote sensing provides high precision information on Earth motions associated with earthquakes (Radar analysis of Turkish earthquake, Earth Pages August 2001), but depends on “before and after” imaging.  Continental tectonics is not just the outcome of occasional large movements on major faults, but of strains that continually occur throughout the lithosphere.  Global positioning satellites provide means of precise location, particular when operated in differential mode, in which field-station signals are matched to those at fixed, geodetically precise base stations.  Precisions to within centimetres or better are now commonplace at low cost.  Structural geologists have been using GPS receivers for over a decade to check on the annual rates of plate motion across major structures such as the Alpine Fault of New Zealand and spreading centres such as that exposed on land in Iceland.  In the 19 October issue of Science, such geodetic analysis of tectonics leaped by an order of magnitude.

The jewel in the crown of continental tectonics is eastern Eurasia, where the active collision of the Indian sub-continent with Asia drives a huge array of very large faults that separate rigid blocks and others, such as the Tibetan Plateau, that are deforming en masse.  The spreading power of the Carlsberg and Central Indian Ridges is dissipated in motion of continental crust spanning 30° of latitude and 60° of longitude.  Chinese scientists and their collaborators from the US universities of Alaska and Colorado have measure GPS positions at 354 stations throughout China, every one or two years for the last decade.  Their analysis of the interim results (Wang, Q. et al.  2001.  Present-day crustal deformation in China constrained by global positioning system measurements.  Science, v.  294, p. 574-577) helps confirm or modify ideas about crustal motions that stemmed from seismic first-motion studies and regional field evidence.  More than a third of the tectonic power accounts for crustal shortening within the Tibetan Plateau.  While the western part of the huge system involves consistent motion towards the north-north-east, driving into Eurasia’s hinterland, the “free-edge” of eastern China  and Indo-China seems to encourage the escape tectonics first proposed by Molnar and Tapponier.  That involves a massive clockwise rotation around the East Himalayan Syntaxis, which takes up a great deal of motion.  Whereas Molnar and Tapponnier proposed the shoving of south-eastern China oceanwards by the “escape” of Tibet, Wang et al’s measurements reveal that its motion to the east is only between one third and a quarter that of the adjacent east Tibetan Plateau.  The lack of any sign that Tibetan crust is overriding that of south-east China, or that the latter is being shortened, may suggest that escape is funnelled around the East Himalayan Syntaxis into Burma and South-East Asia.

Fate of the Neanderthals

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Chris Stringer and William Davies report on two recent conferences about the Neanderthals in the 25 October issue of Nature (Stringer, C. and Davies, W 2001.  Those elusive Neanderthals.  Nature, v. 413, p. 791-792).  Debate continues on what happened to them, and why.  Assimilation by gene flow remains a possibility with a few researchers, despite the mismatch between fragmental Neanderthal DNA and that from modern people, and the inability to get Cro-Magnon genetic material is vexing.  Acculturation – the influence of the behaviours of groups on one another – is also an unresolved issue.  At the centre of that particular debate are tools associated with late-Neanderthal sites that bear close resemblance to those of early Cro-Magnons; the so-called Châtelperronian.  The problem is precision and accuracy of dating the material, which, of course, constitute the palaeoanthroplogist’s Sword of Damocles.  Dating using the decay of 14C has long been a right old mess, what with variations in the cosmogenic productivity of the isotope, and the tendency of common bone samples to pick up stratigraphically younger carbon from humic acids in soils.  Charcoal is the material of choice, but in the case of Châtelperronian artefacts only associated bone seems to be available.  Help might be on the way in resolving inaccuracy that stems from variable 14C productivity by using marine-core data to calibrate terrestrial 14C dates to calendar years (the “CalPal” curve).  It does, however, seem to be peeking over the horizon at present.

One of the alternative processes that might have snuffed out Neanderthals is climate change.  High-resolution marine records are not too useful in that regard, because they reflect global processes, and Neanderthal demise was a regional issue.  Pollen records from lake sediments in Italy now reveal the intricacies of European climate during the critical period around 30 ka.  It was time of rapid fluctuations in tree cover.  However, similar rapid vegetation shifts occurred long before modern human influx, and the Neanderthals survived them.  One possibility, allied to the competitive-disadvantage hypothesis, is that Cro-Magnons brought a steppe culture with them, which allowed them to occupy open country more successfully than Neanderthals with a woodland culture.

The topic is stymied by imprecise dating (it can be as bad as ± 4 ka), so that open-season for speculation is protracted.  There is a reluctance to consider extinction through epidemic diseases brought by newcomers, and against which Neanderthals had no immunity.  Disease has played such a huge role in population crashes throughout recorded history, that for it not to be at the forefront is curious.  It is a widely supported hypothesis for extinction of large mammals that coincided with first entry by modern humans into the Americas ( see Late Pleistocene mass extinction – July 2001 Earth Pages).  That would have had to involve jumps between species, rather than simple transmission of killers such as measles between genetically very similar populations of humans.

En route out of Africa

Finds of H. erectus and artefacts in China and Georgia date back as long ago as 1.8 Ma; the earliest signs of massive diffusion of early humans protected by their culture from entirely new climates and surroundings.  The great question is, “Which way did they go?”  To many palaeoanthropologists, obstacles presented by the Arabian Desert and Caucasus Mountains, favoured exit from Africa via the Straits of Bab el Mandab (closed at that time) and coastal diffusion.  It now seems that movements of early humans did reach the Levant at a very early date.  Ron Hagai and Shaul Levi have produced strong evidence for H. erectus’ presence in the Dead Sea rift at around the same time (Hagai, R. and Levi, S.  2001.  When did hominids first leave Africa?: New high-resolution magnetostratigraphy from the Erk-el-Ahmar Formation, Israel.  Geology, v. 29, p. 887-890).  They found that sediments enclosing primitive, Oldowan tools (but no skeletal remains) accumulated during the period between two magnetic polarity reversals.  With other evidence, these correlate with the Olduvai subchron from 1.96 to 1.78 Ma.  Definitely a “first” for the Middle East, but by no means proof that this lay on the route to wider colonization, even at Dmanisi, across the Caucasus in Georgia.  Little would prevent easy diffusion from East Africa along the proto-Nile or the Red Sea coast to reach the Dead Sea rift, but the obstacles to the north and east of Israel would have been far greater for poorly clad and equipped Erects.

Experimental satellite to have extended mission

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The Earth Observing-1 (EO-1) satellite, launched by NASA in late 2000, carries two remote-sensing instruments that may become operational devices in the future, given a proven track record on EO-1 and, of course, sufficient funding.  One, the Advanced Land Imager (ALI) is a test bed for sensors earmarked for the follow-on to the current Landsat-7 Enhanced Thematic Mapper+ (ETM+).  As well as the existing ETM+ six bands, ALI covers three others close to existing bands.  Whether by design or good fortune, two of these help define the important VNIR broad absorption by ferric iron minerals, neglected in remote sensing since the early days of the Landsat Multispectral Scanner.  Like the ETM+, ALI also carries a panchromatic band that spans the visible range, and which is aimed at providing a means of sharpening detail in images.  On ALI, however, this band has an improved resolution of 10 metres as opposed to the current 15.

More innovatory is the Hyperion instrument, a hyperspectral device that spans the visible to short-wave infrared range with 242 bands that are 10 nanometre wide.  Hyperion is comparable with airborne hyperspectral devices, such as AVIRIS.  In the experiment it captures data swathes that 7.7 km wide, made up from 256 pixels with a resolution of 30 m.  After initial difficulties with allowing for atmospheric effects  on the data, newly calibrated Hyperion data closely mimic mineral spectra.

Early work on EO-1 data in many fields, including geology, has proved sufficiently promising that NASA has given the mission a year-long extension.  Although data are restricted to only a few target areas suggested by the investigators, the extension is good news.  It is a reassurance about continuity of the Landsat programme, and a tantalising indication that the ill-fated hyperspectral Lewis satellite may be resurrected.

Information from: http://eo1.gsfc.nasa.gov/