Thermal metamorphism and ocean anoxia

Now and again in the geological record, evidence turns up that suggests that the deep oceans were devoid of oxygen.  Ocean anoxia encourages burial of dead organic remains that gives rise to carbon-isotope “excursions”: signals of the anoxia itself.  A likely mechanism that starves the deep oceans of oxygen is the shut down of that part of the ocean “conveyor” driven by sinking of cold, dense brines, as happens today in the North Atlantic and around Antarctica.  Gases dissolve more efficiently in cold water than in warm.  Quite probably most oceanic anoxia events are related to global warming and increases in the “greenhouse” effect due to CO2 rises in the atmosphere.  A group of US and British geoscientists have examined one such anoxia event in the Lower Jurassic (~183 Ma) of Denmark using both carbon isotopes and the density of pores (stomata) on fossil leaves (McElwain, J.C. et al. 2005.  Changes in carbon dioxide during an oceanic anoxia event linked to intrusion into Gondwana coals.  Nature, v. 435, p. 479-482).  Stomatal density is inversely related to the amount of CO2 in the atmosphere, so is very useful in seeking evidence for an anoxia-climate link.

This particular anoxia event has been linked either to release of methane, which quickly causes warming and then oxidises to CO2, from gas hydrate or to massive release of carbon dioxide itself.  McElwain et al. neatly show that the event first experienced drawdown of ”greenhouse” gas and cooling of around 2.5 °C, then sudden quadrupling of CO2 and warming of around 6.5°C.  Such an odd pattern cannot be ascribed to methane release, but coincides with the formation of the Karroo-Ferrar continental flood-basalt igneous activity in southern Africa and Antarctica.  That involved massive intrusion into coal-bearing strata, whose thermal metamorphism would have released huge amounts of “greenhouse” gases.  Calculations of the amount of carbon mobilised to cause the shifts in CO2 suggest between 2.5 and 4.4 trillion metric tons, vastly more than the probable amount of methane hydrate beneath the Jurassic sea floor.


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