The Sulfur Cycle and Atmospheric Oxygen

摘要

The isotopic composition of carbon and sulfur in rocks and sediments provides an important record of global change over geologic time. Changes in the ratios of ~(13)C/~(12)C and ~(34)S/~(32)S of the oceans largely reflect changes in global biological processes that cause fractionation of these isotopes. The biological processes include photosynthesis, which brings about the depletion of ~(13)C (compared with its abundance in seawater) in sedimentary organic matter, and bacterial sulfate reduction, which brings about the depletion of ~(34)S in sedimentary pyrite (FeS_2). Because organic matter and pyrite burial in sediments, along with their weathering on the continents, constitute the four major processes affecting atmospheric oxygen, study of the rates of these processes is important, not only for the evolution of the carbon and sulfur cycles but also for the history of atmospheric oxygen (see figure).