Rhenium fever drives miners into the volcano

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Satellites demand durable components, and for some applications the metal rhenium is irreplaceable.  But it is hard to smelt, as well as being rare.  Its current price of US$1.45 per gram reflects its conventional extraction from gases emitted by roasting molybdenum ore, a by-product of copper mining.  At around one sixth the value of gold and with work beginning in earnest on the US-Russian International Space Station, a sizeable chunk of rhenium promises a quick profit.  For geologists in the economic black hole that was the Soviet Union, rhenium has become a magnet and they are developing possibly the most extraordinary mining venture ever attempted.

Volcanologists of the Russian Institute of Experimental Mineralogy discovered, in 1992, that fumaroles of the volcano Kudriavy in the Kuril Archepelago exhale and precipitate pure rhenium sulphide – the hitherto unknown mineral rheniite.  The vents’ build-ups contain at least ten tonnes of rhenium, and fumarole gases replenish it at a rate of several grammes each day.  As well as mining the vents, even condensing rheniite is an economically attractive proposition.  Even now, scientists of the Moscow-based Institute of Mineralogy, Geochemistry and Crustal Chemistry are building a wooden pyramid to cap one of the vents.  This will funnel fumarole gases into a chemical trap for rhenium, that uses zeolites as an ion extractor.  Future plans, sensibly, focus on concrete or ceramic caps to tap all the fumaroles in Kudriavy’s crater. 

Source:  Jones, N., 2000.  Outrageous fortune.  New Scientist, 26 August 2000, p 24-26

Unravelling Neoproterozoic environments

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The latest Precambrian or Neoproterozoic, from1000 to 544 Ma ago, and especially from 700 Ma to the start of the Cambrian, is the most important episode in the history of biological evolution.  That is the episode during which remains of large, soft-bodied animals (the Ediacaran fauna) first appear and at whose end animals able to secrete hard parts burst onto the scene.  It marks the preparation for the beginning of life as we know it best; the Cambrian Explosion.  This period is remarkable also by its huge climatic upheavals that twice turned Earth into a planetary snowball, when ice masses extended to tropical latitudes.  As if these unprecedented and never repeated big freezes were not sufficient to focus geologists’ undivided attention on the late-Neoproterozoic, seawater became for a time so depleted in oxygen that soluble ferrous iron entered shelf areas to precipitate out as banded iron formations, which had vanished around 2.2 Ga when oxygen first entered the oceans in any amounts.  Neoproterozoic world events opened with all continental lithosphere known to be around at the time consolidated in the mother of all continents, literally called Rodinia from the Russian for motherland.  Rodinia broke up with the as yet unexplained break out of Laurentia from close to its heart.  A massive round of sea-floor spreading saw tiles from the Rodinian mosaic reassembled as the core of the Gondwana supercontinent beginning around 650 Ma ago.  Gondwana played a massive role in subsequent tectonics until it too broke up in the Mesozoic.  These were interesting times, relative to which the Phanerozoic seems somewhat tame, except for its tangible record of life’s ups and downs.

But there is a problem; with magmatic activity sparsely distributed in Neoproterozoic space and time, and a lack of rapidly changing biomarkers, division of events through time and, more important, correlating events from place to place has proved difficult, except in a barely useful and often mistaken way.  Geological accounts of the late-Precambrian have been permissive and provocative, to say the least.  That seems likely to change rapidly.  Frustration centred on the time problem set against the undoubted drama of events had spurred the development other means of stratigraphic division and correlation.

The geologically instantaneous mixing of isotopes affected by global processes forms the basis for identifying large events that fractionate them in stratigraphic sections everywhere.  That has been the biggest contribution of the oxygen-isotope data in seafloor sediment cores for the Neogene, in which fluctuating volumes of land ice shifted the proportion of 16O to 18O in ocean water, so that features in d18O records become means of fine-tuned correlation world-wide for climate shifts.  Carbon isotopes play a similar role in charting changes in global bio-productivity and burial of dead matter and carbonate hard parts.  Strontium serves to detect changing balances between supply of dissolved material from oceanic magmatism and from erosion of 87Sr-enriched continental crust.  Sulphur isotopes also help chart supply and demand among organic and inorganic processes.  Such chemo-stratigraphic methods were recognised as a lifeline for resolving Precambrian evolution in the late 1980s.  A decade on, painstaking work  has begun to bear fruit, as covered by a Special Issue of the 100th volume of Precambrian Research (v. 100(1), 2000).  Andrew Knoll of the Botanical Museum, Harvard, USA summarises progress (Knoll, A.H., 2000.  Learning to tell Neoproterozoic time.  Precambrian Research, v. 100, p. 3-20), but details of the chemo-stratigraphic approach and what the prominent isotopic markers might mean appear in a paper of monographic proportions from a team at the Department of Earth and Planetary Sciences, Macquarie University, Australia (Walter, M.R. et al., 2000.  Dating the 840-544 Ma Neoproterozoic interval by isotopes of strontium, carbon and sulphur in seawater, and some interpretative models.  Precambrian Research, v. 100, p. 371-433)

Chemostratigraphy seems to resolve the question of how many late-Precambrian icehouse conditions of global significance.  Though some have speculated on as many as 5 or 6 from occurrences of glacigenic rocks, only two match with isotopic signals, one (Sturtian) around 700 Ma and one around 600 Ma (Marinoan).  Both have associated negative d13C excursions in carbonates to the level of mantle carbon, which suggest that life was reduced to a minimum by ‘Snowball Earth’ conditions.  Associated shifts in the proportion of isotopically heavy sulphur are different.  Sturtian glaciation matches with an increase in d34S, a likely product of ocean anoxia, the involvement of light 32S in bacterial reduction of sulphate to sulphide ions, and the burial of iron sulphide at sources of ferrous iron around sea-floor hydrothermal systems.  The anoxia was sufficiently extreme for Fe2+ to dissolve and mix throughout the ocean water column, so that precipitation as ferric oxy-hydroxides burgeoned in shelf seas to form BIFs a little younger than the glacigenic rocks.  Marinoan glaciation, though equally catastrophic for bioproductivity,  did not fully deplete the oceans of oxygen.  Massive peaks of d13C prior to glaciations suggest that intense precipitation of carbonates in the limestones so common in the run-up to frigidity, plus burial of abundant dead organic matter in the case of the Marinoan, dramatically drew down CO2 from the atmosphere.  Life’s recovery after the Sturtian, together with organic burial, boosted oxygen levels, as too following the 600 Ma Marinoan.  Possibly the delivery of huge amounts of glacially ground rock flour added nutrients that helped fuel this biological pump, and an increase in 87Sr/86Sr after the Marinoan could reflect such fertilization.  There is much more in the paper that will fuel advances in ideas of the co-linkage of glaciation and biological evolution – essentially adaptive radiation by the few eukaryotes that survived anoxia and other stresses – and the evidence for large increases in oxygen production that are prerequisites for the origin of large, oxygen-demanding animals in the Ediacaran fauna.  What came as complete surprise to me, a non-specialist, was clear evidence from several well-studied sections for the largest negative d13C excursion in geological history only 2 Ma before the Cambrian Explosion, which took less than a million years to develop..  Other isotopic trends seem to indicate a brief but highly intense global warming that snuffed the Ediacaran animals from the fossil record.  The unique depletion in heavy carbon points strongly to the seabed belching teratonnes of methane in unstable gas hydrate, a product of double selection of 12C by photosynthesizing plankton and methanogen bacteria metabolizing dead planktonic matter within ocean-floor sediments.

Isotopically, the late-Neoproterozoic was chaotic.  Carbon in particular records ups and downs with amplitudes and frequencies that dwarf those of the far-better recorded Phanerozoic, even in later glacial epochs and mass extinctions.  It was two evolutionary developments that probably damped down excursions in carbon isotopes in later times: the stirring of deep-ocean muds by burrowing animals to promote more rapid oxidation of buried organic matter; the increased efficiency of CO2 drawdown by organisms that secreted carbonate hard parts.  Perhaps Precambrian events were not so dramatic after all, equally disturbing events being smudged in the Phanerozoic by the rapid adaptive radiation following the Cambrian Explosion.

My prediction is that this issue of Precambrian Research will become the starting post for a major shift of research into Neoproterozoic and earlier Precambrian sedimentary piles, after two decades of getting things straight in the Mesozoic and Cainozoic.  I feel confident in that, because the stories of Snowball Earth and near extinction of all oxygen demanding life around 700, 600 and now 545 Ma are ones that will, as the Sun might say, run and run.

Plankton and the end of the Palaeocene-Eocene global warming

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Various geochemical signals show that the Palaeocene-Eocene boundary (at 55 Ma) was a time of global warming superimposed on the general Cainozoic cooling from the ‘hothouse’ of the Cretaceous Period.  Some also point to an enhanced ‘greenhouse’ effect driven by massive methane release from gas hydrates on the sea floor.  Methane, a ‘greenhouse’ gas in its own right, oxidizes to CO2 in the atmosphere, transferring its carbon that eventually ends up in the shells of marine organisms.  It is the carbon-isotope blip at the P-E boundary that points to methane as a source of the warming.  Not only does it appear in the marine C-isotope record from foraminifera shells in cores, but also in the teeth of terrestrial mammals, which means that the carbon reservoirs of both atmosphere and seawater were globally changed.  Using the magnitude of that signal allowed palaeoclimatologists to estimate the amount of methane released – about 1 500 billion tonnes.  On a millennial scale, that is comparable to a rate of warming similar to that currently induced by human activities.

The P-E boundary marks the most dramatic biological changes since the mass extinction 10 million years before at the Cretaceous-Tertiary boundary.  But its underlying control is sufficiently close to what is happening to climate now to form both an object lesson and a means of modelling what may happen if current emissions continue.  One of the important aspects needing scrutiny is how such warming events come to an end.  British and American oceanographers have taken a look at the P-E record in ocean sediment cores, and believe they have come up with an answer, at least in part (Bains, S., 2000.  Termination of global warmth at the Palaeocene/Eocene boundary through productivity feedback.  Nature, v. 407 14 September 2000, p. 171-174).

Most such studies focus on oxygen- and carbon-isotope records in the carbonate of foraminifera shells, revealing ups and downs in seawater temperature and volume of land ice, and of biological productivity and releases of ‘greenhouse’ gases.  Unfortunately, neither isotopic record properly resolves the alternative contributions to variation.  Santo Bains and colleagues add another parameter that helps resolve the influence of biological productivity in the oceans.  Marine organisms, especially plankton, either precipitate barium sulphate (barite) in tiny crystals within their cells or induce its precipitation once they die and decay.  Because barite is not prone to much change by later events on the sea floor, counting its crystals in marine cores is a reliable proxy for the varying abundance of plankton through time.

One strong possibility during major warming events is that ocean circulation becomes sluggish, perhaps stopping altogether.  That slows the re-supply of nutrients to sunlit upper layers, and works to reduce photosynthetic life in the oceans.  The barite record produced by Bains et al. shows the opposite for the P-E events.  For about 40 000 years after the P-E event biogenic barite rose to more than twice its normal abundance.  The ocean biosphere responded to the methane blurt by blooming.  Why it did so is not yet clear, but such a spurt in drawing CO2 into living and dead and buried tissue would work to reverse the warming event.  The barite peak coincides exactly with the oxygen- and carbon-isotope records’ features that signify temperature and the influence of isotopically light carbon from methane released by gas-hydrate breakdown.  It might seem as if life did regulate climate in a geologically rapid manner following the P-E event, to the delight of Gaians.  However, the control over biological productivity is ultimately nutrients, and life has little influence over their supply to the oceans.  Among the possibilities for an essential nutrient bonanza, and increased circulation of the oceans is definitely ruled out during major warmings, are hugely increased rainfall to wash terrestrial sediments and dissolved matter into the oceans, and increased volcanism that would supply fine ash to the distant ocean surface.

Converging on an explanation for the end of a period of global warming is far from showing how this might be achieved for a warming induced by human activities.  That might well prove eventually to be a life-or-death necessity for our species, bearing in mind that the P-E warming was a fatal crisis for many land mammals of the time.

See also: Schmitz, B., 2000.  Plankton cooled a greenhouse.  Nature, v. 407 14 September 2000, p. 143-144.

A new regular pulse in recent climate

Gerard Bond of the Lamont-Doherty Earth Observatory at Columbia University, Palisades, New York has taken his analysis of high-frequency climatic shifts in the last glaciation into the Holocene record.  Previously, Bond had tried to make sense of the sharp fluctuations of the order of a few thousand years that are seen as gravel layers in the uppermost levels of sea-floor cores and in the oxygen isotope records of cores through the Greenlandic and Antarctic ice sheets.  The first signs of short ups and downs in climate were the coarse layers first found by Hartmut Heinrich in the glacial part of the sea-floor record.  Heinrich ascribed them to periodic releases of iceberg armadas as the ice sheets of the last glaciation became unstable.  Bond’s latest work also focuses on Heinrich events, but he has used specific lithologies as markers rather than merely grain-size variations.  In particular, hematite-stained quartzo-feldspathic materials seem likely to have come from altered rocks in east Greenland and Svalbard, far distant from the drill sites whose cores he has examined.  The proportion of reddish grains varies systematically in the cores, some layers coinciding with Heinrich events, but there are many more.  The layers appear roughly every 1500 years.  This periodicity coincides with cycles of dust blown from the Sahara to form layers in cores from the west African coast, so whatever the pulses represent, they are global signals.

Interestingly, the cycles show little sign of change in the period after the melt back that signified the beginning of the Holocene interglacial.  Behind the long-term climatic shifts in glacials and interglacials, that coincide with the 100, 41, 23 and 19 thousand year fluctuations in solar warming of the northern hemisphere, some other process must be put-putting in the background.  The 1500 year cycles may stem from processes that shift heat in the oceans and atmosphere.  A likely candidate is the production of deep currents by sea-ice formation in the northern North Atlantic.  However, detailed calculations of tides suggest a similar pacing that might change the mixing of surface and deep water in the ocean conveyor system.

Whatever the driving force, this periodicity strikes a chord with emerging details of Holocene climate changes from lake-sediments studies and the historic record.  One such recent cooling pulse that might have delivered icebergs to mid-latitudes in the North Atlantic was the Little Ice Age that peaked in the 17th century that saw prolonged stresses on the population of Europe, and major political changes that resulted from such events as the Peasants’ Revolt and repeated famines.

Source:  Pearce, F.  2000.  Feel the pulse.  New Scientist, 2 September 2000, p. 30-33.

End-Permian devastation of land plants

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The mass extinction that marks the boundary between the Palaeozoic and Mesozoic Eras snuffed out more than 90% of marine animal species and about 70% of terrestrial vertebrates.  The most complete record of the Permian-Triassic boundary is in marine sediments atop an obducted ophiolite in Japan (Isozaki, Y., 1997.  Permo-Triassic boundary superanoxia and stratified superocean: records from lost deep sea.  Science, v. 276, p. 235-238).  These record a 20 million-year period when deep ocean water was lacking in oxygen, and the anoxia reached extreme conditions for about 4 million years across the boundary.  All the palaeontological signs are that shallow marine faunas dwindled slowly in the 10 million years before the P-T event.  Carbon isotopes from hydrocarbon-rich boundary strata in Canada suggest that over a period of  only 1000 years the oceans were almost devoid of life.  The open oceans had become dead from top to bottom; a scenario graphically expressed by Ken Hsu as a “Strangelove” ocean.  Whatever the pace of preceding extinctions the boundary event was a catastrophe, and the Japanese and Canadian sections suggest that maybe a half-million years passed before surviving organisms began to recover and diverge.

The much-studied K-T boundary’s association with abundant evidence for an associated giant impact, prompted geologists to look for a similar story for the near end of Earth’s life 190 million years earlier.  Supporting evidence has yet to emerge, although the boundary includes the period when huge volumes of continental flood basalts poured over what is now Siberia.

Terrestrial records are far less easy to divide into fine time divisions, partly because they record both deposition and erosion, and partly because fossils are less well-preserved than in marine sediments.  Continental sediments spanning the P-T boundary are particularly frustrating, because of the wide extent of arid to semi-arid conditions then.  The Karoo Basin of South Africa does record wonderfully the fate of vertebrates (only 6 out of 44 genera survived the  boundary event), but less so that of plants.  Abrupt changes in plant-life are equally as important as those of animals, simply because they are at the base of the terrestrial food chain.  One way of addressing vegetation shifts of the most general kind is to look for evidence of how river systems changed their patterns of deposition, and this is what a team from the University of Washington (Seattle) and the South African Museum have done in the Karoo Basin (Ward et al., 2000.  Altered river morphology in South Africa related to the Permian-Triassic extinction.  Science, v. 289 8 September 2000, p. 1740-1743).

Peter Ward, David Montgomery and Roger Smith examined sedimentary structures produced by river channels in the sandstone members of the Karoo sedimentary pile.  Permian rivers seem to have flowed in distinct, meandering channels, whereas those of Triassic age laid down sands that show consistent evidence for intricately braided  channel systems.  The shift from one to the other type falls right at the P-T boundary.  Meanders of large river channels typify land surfaces with abundant vegetation that binds alluvium.  Where vegetation cover is sparse, there is little to constrain river flow and alluvial erosion, and wide braided river courses develop.  The authors conclusion is that vegetation suffered a catastrophic die off at the P-T boundary, leaving formerly lush plains as sandy wastes.  Such a loss of plants that would previously have contributed to balancing the atmosphere’s CO2 levels and the proportions of light and heavy carbon isotopes in the global environment would have helped produce the “Strangelove” signal in the ocean sediments.  The land was seared, and evidence from similar sediments in Australia and Antarctica suggests a global loss of plant life.  Incidentally, the boundary in many places shows a leap in the abundance of fungal spores, so the Mesozoic began with decay on a grand scale.

See also: Kerr, R.A., 2000.  Biggest extinction his land and sea.  Science, v. 289 8 September 2000, p. 1666-1667.

Carbon isotopes of individual microfossils

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Organisms at the base of the food chain, autotrophs that synthesise biological compounds directly from carbon dioxide, water and other fundamental materials in their environment, favour incorporating the lighter of the two common isotopes of carbon, 12C, as opposed to 13C.  Consequently, one of the prime signatures of life in the carbon found in rock is a depletion in 13C, usually expressed as d13C with a negative value.  It is this signature that has allowed the origin of life to be pushed back almost to the age of the oldest rocks on Earth (around 3.9 billion years ago) from carbon isotope studies of carbonaceous compounds (kerogen) in ancient sediments.

Different organisms alive today, particularly among the ecologically diverse bacteria, use different biochemical reactions in synthesising living material.  Each of these have different effects on d13C.  Potentially these differences could be used to identify roughly the kinds of bacteria that lived in the distant past.  Up to now, however, isotopic studies of organic carbon have only been possible for bulk extracts from rock.  That enables some bold conclusions, such as the current suggestion that oxygen-producing blue-green bacteria were around 3.5 billion years ago, but whole-rock results are ambiguous because of mixing of carbon originating from different metabolic pathways. 

Being able to analyse carbon isotopes from individual fossil cells is a major breakthrough, and a team of palaeobiologists from the universities of California and Regensburg, Germany has done just that (House, C.H. et al., 2000.  Carbon isotope composition of individual Precambrian microfossils.  Geology, v. 28, p. 707-710).  They used an ion microprobe that allowed the discovery of biological carbon encapsulated in resistant materials from 3.8 billion-year old metamorphosed iron formations from West Greenland.  That involved probably mixed carbon of biological origin.  In the new work, the isotopic analyses are from individual bacterial cells preserved in 850 and 2100 Ma banded iron formations, and suspected to be blue-green bacteria.  The results clearly distinguish one metabolic pathway – the Calvin cycle used by blue-greens – from other possibilities.

Tangible bacterial fossils go back, albeit rarely, to more than 3 billion years ago.  It is the older life forms that are most intriguing, because by 2100 Ma ago the Earth’s atmosphere had become oxygen bearing, thereby allowing the rise of the Eucarya from which we stem.  Older material might give clues to the more primitive Bacteria and Archaea that were the exclusive rulers of the biosphere before about 2200 Ma, and controllers of the Earth’s atmospheric composition and thereby its climate, which remains a mystery.

Timing the uplift of the Tibetan Plateau

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The rise of the huge Tibetan Plateau, with an average elevation of 5 km, presented a major barrier to atmospheric circulation, perhaps one of the largest that has ever existed.  With its latitude close to the down flow of the tropical Hadley cells, it has had an effect on the Asian monsoon in particular, strengthening its effects.  Many climatologists believe that Tibet has played a major role in global climatic change towards the end of the Cainozoic.  So, timing the uplift is critical in assessing the modelled effects in relation to detailed climate records of the Neogene.  This is by no means easy, for the late-Tertiary sediments are terrestrial in origin.

A team of Australian and Chinese geologists focussed on the sedimentary record in the Tarim Basin, north of the Kunlun mountains that form the northern flank of the Tibetan Plateau (Zheng, H.  et al., 2000.  Pliocene uplift of the northern Tibetan Plateau.  Geology, v. 28, p. 715-718).  Sediments there change from redbeds deposited in gently sloping flood plains to coarse debris laid down by flash floods at a rising mountain front; exactly the relationship that records the beginning of uplift in northern Tibet.  Dating this is no easy matter, however.  The technique that the team used is magnetostratigraphy, based on highly sensitive measurements of the polarity of 2500 samples of weakly magnetized sediments.

The change in facies spans a period when the Earth’s magnetic field was reversed – the Gilbert reversed chron – which occurred between 4.5 to 3.5 Ma ago.  The maximum age for the beginning of Tibetan uplift in the north is therefore 4.5 Ma, in the Pliocene.  This contrasts with the accepted age of Oligocene – Miocene for uplift of the Himalaya and southern Tibet, and with models that postulate climatic change that followed it.  Whereas the Tarim Basin today is arid, the sediments indicate that until the Pliocene abundant water flowed at the surface, to deposit great thicknesses of fine alluvium.

Danger of CO2 release in Cameroon

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On 21 August 1986 a huge cloud of carbon dioxide gas was released from Lake Nyos located at 300 metres in the Highlands of Yaounde District of Cameroon. Because carbon dioxide is more dense than air it hugged the ground and flowed down valleys. The cloud travelled as far as 15 miles (25 km) from the lake. It was moving fast enough (as much as 80 kph) to flatten vegetation. 1,700 local people died by suffocation, probably unaware of their plight.   Two years earlier 37 people died similarly in a gas release from nearby Lake Monoun

Lake Nyos is in the Oku volcanic field, and is one of several maars produced by one-off explosive events in the recent past.  Isotopic analyses of gas remaining dissolved in the lake show that the CO2 is of volcanic origin.  The lakes are fed by springs on their beds, which is where the CO2 enters.  Being extremely deep (about 200 metres) and with no surface inlet the lake water is strongly stratified, so that CO2-rich water builds up at the bottom.  The gas release must have involved an overturn of the stratification, so that dissolved gas came out of solution as pressure decreased.  What triggered the overturn is hard to establish, but one possibility is that during August (both catastrophes occurred in that month) cold weather cools surface waters so that they sink.  Other possibilities are storms, landslides or earthquakes, but there are no records of any of these preceding either event; they came completely unannounced.

Since 1986, gas levels have built up, and now stand at twice their concentration following the disaster, so danger threatens the local people and their livestock once again.  An international team, headed by George Kling a geologist at Michigan University, USA, has devised a means of venting the gas harmlessly.  This involves sinking 15 centimetre diameter polyethylene pipes to the lake bed.  Once pumping starts, gas bubbles forming as pressure releases will drag the water upwards, as a self-sustaining siphon, similar to the air-lift dredges used in marine archaeology.  Four such pipes would rid the lake of its lethal gas content in two years, and even one would reduce the hazard considerably.

Sources:  Observer, 20 August 2000, University of Michigan (http://www,biology.lsa.umich.edu/~gwk/research/nyos.html)

Inglorious mudstones

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Because they succumb to erosion easily, mudrocks do not outcrop well, except on the coast or in arid lands.  Often they show little if any stratification that field workers can distinguish from the partings imparted by compaction and dewatering, which make shales from them.  Yet they are repositories of a great deal of information (see Earth Pages archives – Methane hydrate – more evidence for the ‘greenhouse’ time bomb).  In hand specimen their two main components, silt-sized quartz grains (<62 microns) and clay minerals (>4 microns) only become distinguishable by chewing!  They are irresolvable using optical microscopes, and detailed work needs scanning electron microscopy.

Silt to clay proportions in mudrocks are variable. The first is generally taken as an indicator of suspended debris from land masses and its proximity to where the mud accumulated.  The more clay, the further muds were from exposed continents, or so sedimentologists used to assume.   That approach has taken a hard knock from some recent detailed work on Devonian mudrocks (Schieber, J. et al., 2000.  Diagenetic origin of quartz silt in mudstones and implications for silica cycling.  Nature, v.  406 31 August 2000, p 981-985).

Jurgen Schrieber of the University of Texas (Arlington), Dave Krinsley of the University of Oregon and Lee Riciputi of the Oak Ridge National Laboratory in Tennessee used scanning electron microscopy, cathodoluminescence and ion-probe techniques to discriminate between detrital quartz grains and those formed by precipitation of silica from pore water in the original muds.  Those grains that do not luminesce probably formed by silica solution and reprecipitation, and the Devonian mudrocks contain mainly non-luminescent quartz grains.  Oxygen isotope ratios from individual grains confirm this in situ origin.  The researchers had no reason to suspect that their Devonian samples would give such results, and assumptions based on silt to clay ratios from any mudrock are now in doubt.

Worse still, silt in ocean-floor muds, cores of which form the linchpin for Pleistocene climate studies, has been a rough and ready way of estimating wind speeds as climate shifted from glacial to interglacial conditions.  These silts could be precipitates too, and the variations in their proportions may stem from changes in the delivery of dissolved silica from land to the oceans.

See also:  Kemp, A.  2000.   Probing the memory of mud.  Nature, v. 406 31 August 2000, p 951-953

Atmosphere linked to Earth’s rotation

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One of the annoying features of the Earth as a planet is that it engages in a kind of Saint Vitus’ dance.  The best known of its wandering are those involving variations in the eccentricity of its orbit, and the tilt and precession of its axis of rotation.  These follow from the gravitational influences of massive planets elsewhere in the Solar System, and are implicated in the modulation of climatic change through the last 2.5 Ma.  Rather less well-known, and even more aggravating are far more rapid, but geometrically quite small deviations from good behaviour.  One of these is the habit of the spin axis to wander around the geographic poles within a circle roughly 3 to 6 metres across.  It does this every 14 months.  It takes a certain degree of dedication to chart such a tiny planetary tic.  Chandler Wobble is the single claim to fame of its eponymous discoverer.  Seth Carlo Chandler Jr, an American businessman and amateur astronomer, discovered the quirk in 1891 by observing stars with a degree of single-mindedness that might have put a lesser mortal on the couch.  He set out to verify the famous Swiss mathematician Leonhard Euler’s prediction that the Earth ought to wobble every year, and he did.

So minuscule is Chandler Wobble, that keeping it going is something of a vexing problem, for a single jostle’s effect ought to fade away in a few years.  There are innumerable ways of nudging the Earth, and deciding which is sufficiently regular and just right to maintain the wobble is no easy task.  Following in the great tradition of Seth Chandler, Richard Gross of the Jet Propulsion Laboratory compared Wobbling between 1985 and 1996 with the continual but inconstant motions of atmosphere and oceans, as simulated by super-computer modelling of climate.  The forces of winds and currents are simply insufficient to induce the Wobble, but variations in atmospheric and deep-water pressure, together with their positional shifts are, in the manner of Goldilocks and the little bear’s porridge, just about right.  Because changes in water depth are wind-driven (as for instance with the wandering hump in the Pacific’s surface, linked with El Niño), ‘weather’ is the ultimate driving force for Chandler Wobble.

Why devote time to this picayune curiosity?  The answer is to chart more accurately the position of distant spacecraft; not easy when the measuring platform is behaving like a Womble.

Source:  Richard A. Kerr, 2000.  Atmosphere drives earth’s tipsiness.  Science, v. 289, p. 710.

The guts of a sea-floor spreading system

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What goes on beneath constructive plate margins, and ocean ridges has, up to now, been largely a matter of conjecture, blended with the geology of ophiolite complexes obducted onto continents.  Ophiolites are perhaps not such a good model, since the low buoyancy of the basalt capped lithosphere that they represent prevented them from subduction, and stems from unusual conditions.  The bulk of oceanic lithosphere is destined for resorption into the mantle, and it forms at common or garden ridge systems.

One way of modelling magmatism at ridges is through geochemical analysis of mid-ocean ridge basalts matched with topographic and structural detail of the ridge itself, but this is a blurred approach.  It shows that part of the process must involve ponding of magma in chambers at shallow levels beneath the ridges.  The other aspect is the form taken by the mantle that must rise to undergo adiabatic partial melting.  For fast-spreading ridges, such as the East Pacific Rise, there are two such models: constraint of rising mantle in two-dimensional sheets descending from beneath the ridge itself; three-dimensional plumes of mantle from which magma migrates laterally to ridge segments.  Amplifying geochemical-structural models needs a better idea of the actual processes and the geometries that they take.  A means of getting this information is to use a technique well-honed by petroleum exploration; 3-D seismic reflection profiling.

A consortium of geophysicists from the universities of California and Cambridge used this costly method, involving 200 profiles, to look at 400 km2 of the East Pacific Rise at 9°N (Kent, G.M. and 10 others, 2000.  Evidence from three-dimensional seismic reflectivity images for enhanced melt supply beneath mid-ocean-ridge discontinuities.  Nature, v. 406, p. 614-618).  Melts have about half the seismic velocity of solid rock, and so boundaries between melt and solid show up with better contrast on seismic records than do boundaries in piles of sedimentary rocks.  The surprising result is that instead of vertically extensive magma chambers, expected from either hypothesis, melt occurs in a narrow, continuous sill-like body beneath the ridge.  This connects to a plunging tongue that is probably the path taken by magma from the zone of partial melting in the mantle.  The sill itself occurs at a fixed depth below that predicted from ophiolite studies for the level at which vertical sheeted dykes form the lower part of the petrologically defined crust.  This suggests that the magma simply cannot rise en masse to inject along extensional fissures as the lower crust fails, the sheeted dyke layer acting like a seal in the flow of petroleum in sedimentary basins.  Instead, it seems more likely that magma ekes out as rising rivulets that follow the base of the dyke layer until the reach dilatations at the ridge.

Although results from this study are inconclusive as regards the two models for rising mantle, the detail that it reveals augurs well for further 3-D surveys of ocean magmatism that will complement seismic tomography of the deep mantle.