WHAT DETERMINES SEDIMENTARY C-S RATIOS

The weight ratio of organic carbon to pyrite sulfur (C/S) has been observed to fall within a relatively narrow range (2.8 +/- 0.8) in most fine-grained normal marine siliciclastic sediments. Although an increasing number of exceptions to this typical C/S ratio have been observed, it is nonetheless remarkable that so many modern and ancient marine sediments fall within a range of about a factor of two in C/S ratios. Here we demonstrate that this demands that the three major factors that control this ratio must be quite closely coupled to each other. These factors are (1) the fraction of organic carbon that is metabolized, (2) the fraction of metabolized organic carbon that is metabolized via sulfate reduction, and (3) the fraction of reduced sulfide that is not oxidized and is buried in pyrite. A simple model is presented that has utility in demonstrating the relationships among these parameters that are necessary to produce typical normal marine sediment C/S ratios.