Influence of continental weathering on climate boosted

Since the resurrection of Chamberlin’s idea that the rate of chemical weathering of continental crust helps regulate atmospheric CO2 by Maureen Raymo, the hypothesis has not yet been supported by convincing geochemical evidence.  There is such a lag between changes in ocean chemistry and evidence for global climate change, that correlations are flimsy.  The need is for a proxy for weathering of the land surface that resides in seawater for a geologically very short period.  Such an element is osmium (Os), which passes from river water through the oceans to sea-floor sediments in about 25 thousand years, so changes in its abundance in sediments ought to match the pace of any climatic shifts.  In principle, there are two main sources for elements in seawater, from sea-floor hydrothermal alteration of oceanic crust, and from continental weathering.  The first can be considered to be more or less constant, except on time scales of tens of million years.  Continental weathering is a response to climate change, and keeps pace with it.  Researchers at the UK Open University and the University of Köln in Germany analysed samples for osmium and carbon isotopes through a sequence of Jurassic mudstones on the NE coast of England (Cohen, A.S. et al. 2004.  Osmium isotope evidence for the regulation of atmospheric CO2 by continental weathering.  Geology, v. 32, p. 157-160).  The carbon isotopes show a sudden drop in d13C within a very hydrocarbon-rich unit famous for it contribution of jet (oil-rich lignite) to Victorian funereal jewellery.  This negative excursion is recognisable world-wide at around 180 Ma.  The most likely explanation is a monstrous blurt of methane from destabilised gas hydrate on the Jurassic sea floor (see Methane hydrate – more evidence for the ‘greenhouse’ time bomb, August 2000 issue of EPN).  The Jet Rock of the Whitby coast therefore preserves a nice example of sudden climatic change, and by the end of its deposition carbon isotopes returned to Jurassic background values.  Methane, a powerful “greenhouse” gas, is rapidly oxidised to CO2 in the atmosphere, so reducing its initial warming effect, but climate would have been hotter for some time afterwards until the excess CO2 was drawn down somehow.  Interestingly, the Jet Rock also shows a sudden leap in the abundance of 187Os, reflected in the 187Os/186Os ratio of the samples, and an upward step in the value of the 87Sr/86Sr ratio – one of the fastest rises known.  The latter is generally assigned to an increase in continental weathering, since continental crust contains more radiogenic 87Sr than does oceanic crust.  The implication of the osmium-isotopic shift is odd; it requires an increase in the rate of continental weathering by 4 to 8 times that in the preceding period.  That is a vast change, even if it only lasted for a short period, but it tallies with what is known about the temperature dependence of the dissolved loads of rivers in more recent times.  If the osmium isotope excursion truly reflects massive continental weathering, then it is possible to calculate the drawdown of the excess CO2 in the atmosphere from a commensurate flux of calcium and magnesium ions from the continents, that would eventually form marine carbonates.  The authors estimate a mere 37-123 ka to get rid of it.  Yet continent-derived radiogenic 87Sr remained high for much longer, and the authors’ arguments become tricky.  One interesting aside is that, unlike today, more groundwater found its way to the oceans than surface run-off during the Jurassic, perhaps 6 times more.  It is easy to look on weathering as what happens at the interface between rocks and the weather; the land surface.  Not so.  A great deal of chemistry that releases soluble ions goes on in the subsurface, above and below the water table.  It is by no means as simple as reactions between carbonic acid in rainwater and silicate minerals.  Weathering is the product of hydrogen ions’ (whatever their source) effects on silicates.  Bacteria are extremely important actors in modifying pH below the surface, for example the sulphate-sulphide reducers, and the oxidative dissolution of sulphides produces sulphuric acid.  Even more interesting for the chemistry of groundwater is the curious role of iron hydroxide.  Under oxidising conditions it adsorbs many elements from solution, including platinum-group elements, such as osmium.  Should conditions become reducing, dissolution of goethite skins on sedimentary grains releases the accumulated elements.  A warming trend almost inevitably results in increased precipitation, and rising water tables.  It also should boost biological productivity on land and an increase in the amount of buried organic matter, which create reducing conditions in groundwater.


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