THE PHANEROZOIC SULFUR CYCLE AND ATMOSPHERIC OXYGEN

The long term Phanerozoic sulfur cycle involves the exchange of sulfur between rocks and the oceans. Principlal processes are the weathering of pyrite, organic sulfur and CaSO4, the formation and burial of these substances in sediments, and their thermal decomposition, via metamorphism and volcanism after deep burial, with the release of sulfur-containing gases to the surface. Those processes involving reduced sulfur play an important part in controlling the level of atmospheric oxygen.

A model (based on the earlier work of Holser and Kaplan and Garrels and Lerman) for calculating levels of atmospheric oxygen over Phanerozoic time, combines the long term cycles of sulfur and carbon with the sulfur and carbon isotopic composition of the oceans as recorded in CaSO4 and CaCO3 minerals, respectively. Reasonable results for O2 over time are obtained only if the fractionation of C and S isotopes during photosynthesis and bacterial sulfate reduction ,are assumed to be a function of the level of atmospheric O2, an idea bolstered by laboratory photosynthesis experiments and by diagenetic reasoning. The model results show a rise of O2 during the Carboniferous to concentrations greater than 30% with a subsequent fall during the Permian. This agrees with independent calculations based on the abundance of pyrite and organic matter in Phanerozoic shales and with the presence of giant insects at this time.

Another result of the modeling is that the average global burial ratio of organic carbon to pyrite(plus organic) sulfur varied appreciably over the Phanerozoic. High values during the Permo-Carboniferous reflect the high proportional burial of organic matter in fresh water (that is characteristically depeted in dissolved sulfate) as would be expected for a major coal-forming epoch. The model indicates that about 20% of global organic matter burial occurred in fresh water at that time. Low C/S ratios for the early Paleozoic are in agreement with suggestions that (1) appreciable portions of the oceans were anoxic; (2) the organic matter available for bacterial sulfate reduction was more reactive because of the absence of land plant-derived debris, and (3) bioturbational re-oxidation of organic matter and pyrite was globally less extensive.