Hydrocarbon source rocks and ocean anoxia events

Much of the world’s oil resources formed by maturation and migration of hydrocarbons from organic-rich, marine mudrocks, which seem to have formed episodically during Earth history.  A widely accepted view is that such source rocks’ content of organic matter fell to the ocean floor as the remains of tiny organism.  That they were not oxidized by bacterial action seems to suggest that the periods when source rocks accumulated were characterized by low oxygen levels in bottom waters.  Each major source rock has been linked to such ocean-anoxia events, and to periods when deep-ocean circulation effectively stopped, so cutting off oxygen supplies to deep levels.  However, studies of modern deposition of organic matter in marine sediments at continental margins reveals that discrete particles of organic matter are far outweighed by biological molecules that coat the surfaces of minerals, particularly those of clay minerals.  The amount of organic carbon in a modern sediment depends largely on its content of clay minerals derived from intense chemical weathering of continental rocks.  Such coatings are protected from normal processes of decay, so that the adsorbed organic carbon compounds can be buried, more or less intact

It should be possible to check whether ancient source rocks are similar to modern carbon-rich sediments by seeking a strong correlation between clay content and organic content – mudrocks also contain fine silt particles made of non absorbent quartz. It seems that in at least one Cretaceous source rock in the US Mid-West such a correlation is clear (Kennedy, M.J., Pevear, D.R. and Hill, R.J. 2002.  Mineral surface control of organic carbon in black shale.  Science, v. 295, p. 657-660).  This suggests that oil-shale deposition is as much related to the intensity of continental weathering of silicates as it is to ocean-water chemistry.  Since clays, especially the sponge-like smectites, adsorb organic molecules from solution in seawater, they draw on a vast pool of material, so that enhanced biological productivity need not be linked to oil-shale formation either.  The fact that most organic material in such rocks is structureless kerogen, rather than identifiable particles, also supports this alternative hypothesis.

Both petroleum geologists and palaeoclimatologists have assumed a source rock – ocean anoxia connection in both exploration strategies and assessment of past climate shifts.  So Martin Kennedy et al.’s painstaking findings are sure to cause a major stir.  However, what cannot be avoided is that increased chemical weathering of the continents is likely to accompany globally warm conditions, and they in turn sponsor growth in planktonic productivity.  Likewise, global warmth does not favour the formation of dense, cold and therefore oxygenated sea-surface water, which sinks to aerate deep oceans when the planet is cool.

Measuring erosion rates.

So many landscapes show evidence of changes in the rate of erosion, such as terraces, waterfalls and signs of changing rates of sediment deposition, that a means of accurately measuring rates opens up an important new phase in geomorphological research.  Precise dating of modern surfaces is not possible using stratigraphic or radio-carbon methods, and this has hidden much of landform history.  Once a surface is exhumed, it becomes exposed to cosmic ray bombardment.  These particles travel at near-relativistic speeds, and so have sufficient energy to transmute common element nuclei to unstable isotopes.  The longer the exposure of a surface material, the more radioactive it becomes, albeit very weakly so  Since erosion and sedimentary processes move and quickly bury particles dislodged from a surface, material has a finite time during which it can be irradiated.  The particles themselves carry the isotopic signature of their surface residence time, the slower erosion is the more radioactive are particles derived from the surface.

Cosmogenic dating uses sedimentary grains from sands deposited in a drainage basin, particularly those of quartz that are common and stable.  Oxygen and silicon in silica can become 10Be and 26Al when struck by cosmic rays.  Although sampling is fraught with pitfalls, essentially it amounts to scooping up a handful of sand that represents the past erosion of the entire catchment above a sample point.  Measuring the minute concentrations of new isotopes  costs of the order of $1000 per sample, using a high-energy accelerator mass spectrometer.  Since dozens of samples provide sufficient data for meaningful interpretation, this is not a method that will spread widely to places that come anywhere near fully reflecting the intricacies of erosional shifts over the large age range that cosmogenic dating can address.  Nonetheless, its early results are astonishing.  Work in Idaho suggests that through the period of the last glacial maximum into the early Holocene the average rate of erosion was 17 times faster than it is at present.  That possibly signifies either continual high erosion, that has petered out, or, more likely, that erosion has had episodic, catastrophic pulses.  As might be expected, anthropogenic disturbance of the surface enhances erosion rates, but a cosmogenic study of river sediments in Sri Lanka indicates that 200 years of intensive farming in rugged highland areas have resulted in a 20- to 100-fold acceleration.  Most awkward of all, another study of long-term erosion in California’s Sierra Nevada showed no relation between weathering and erosion rates and climate change.  Geochemists contributing to the debate over climate controls by weathering take note.  It seems that the primary control of erosion rates in western California was purely tectonic, which could tally with the notion that newly rising mountains have a major influence over sequestering of CO2 by silicate breakdown. 

The obvious next step is blending cosmogenic sediment dating with that of crustal exhumation from Ar-Ar and U-Th/He dating of cooling due to uplift and erosion.

Source:  Greensfelder, L. 2002.  Subtleties of sand reveal how mountains crumble.  Science, v. 295, p. 256-258.

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