THE ROLE OF SULFUR IN REGULATING THE EXOGENIC CARBON AND OXYGEN CYCLES

Over the past twenty years, advances in our understanding of sulfur isotope fractionations associated with a host of microbially-mediated reactions, coupled with an improved ability to evaluate the sulfur isotope composition of ancient seawater sulfate (via carbonate-associated sulfate) have improved our understanding of the Precambrian exogenic sulfur cycle. In particular, sulfur isotope studies have shown that Precambrian sulfate levels were very low (relative to the modern ocean) and that an increase in the oxidation state of the ocean-atmosphere system in the terminal Neoproterozoic led to elevated seawater sulfate concentrations. While it had long been held that sulfate concentrations remained high and approximated modern levels for much of the Phanerozoic, recent work has shown that significant portions of the Paleozoic and Mesozoic may have been characterized by very low sulfate levels.

Importantly, in low sulfate systems, sulfate concentrations strongly influence the efficiency of nutrient recycling and therefore primary production, by affecting phosphorus availability. For example, empirical evidence has shown that sulfate concentrations in modern lakes (<1 mM) are inversely correlated to Fe/P ratios in both oxic and anoxic bottom waters. This inverse relationship stems from the positive correlation between sulfate concentration and bacterial sulfate reduction. In other words, P release from sediments to bottom waters is enhanced in systems characterized by higher sulfate levels that support increased bacterial sulfate reduction and associated organic carbon (OC) remineralization. Furthermore, P released during organic carbon degradation may be retained in sediments via adsorption to Fe (oxyhydr)oxides. However, in the presence of hydrogen sulfide, Fe (oxyhydr)oxides will react to form Fe sulfide thereby inhibiting Fe-bound P removal. In this study, I will present evidence that supports the hypothesis that sulfur plays an important role in regulating the marine carbon cycle, particularly in the low sulfate oceans that apparently dominated the Paleozoic.