The Sulfur Isotope Composition of the Pyrite Burial Flux in the Modern Ocean

Microbial sulfate reduction followed by sulfide mineral burial, typically as pyrite, represents the largest removal pathway for sulfur from the exogenic sulfur reservoir. During microbial sulfate reduction, sulfur isotopes are partitioned such that the lighter 32S isotope is preferentially reduced; the magnitude of this partitioning has a large range (0 to 72‰), and therefore the average sulfur isotope composition of the global pyrite burial flux remains enigmatic. Recent work has mapped the global spatial distribution of microbial sulfate reduction rates in the modern ocean, which allows conclusions to be drawn concerning the conditions and controls on where sulfate respiration occurs (Bowles et al., 2014). The local rate of sulfate reduction in a particular sediment column can be calculated by the change in sulfate concentrations in pore fluids below the surface, which yields the net flux of sulfate into marine sediments. A flux of sulfate into the sediment is assumed to be due to diffusion along a concentration gradient set up by the consumption of sulfate at depth. We have calculated the deep-sea rates of microbial sulfate reduction using over 700 drilling sites and arrive at a similar estimate of the global modern rate of sulfate respiration. Rates of sulfate reduction are not, however, the same as the rates of pyrite burial, which is likely limited to the uppermost sediments where reactive iron may be available, or in the most nearshore environments where the terrestrial supply of iron is high and rates of sulfate reduction are orders of magnitude higher than those in the deep-sea. Sulfur isotope fractionation during microbial sulfate reduction is a function of several environmental factors, including the rate of sulfate reduction. We use a new compilation of the link between sulfate reduction rate and sulfur isotope fractionation with a model of pyrite burial in a range of modern marine sediments to derive an estimate of the global pyrite burial flux and its sulfur isotope composition. We conclude that the deep ocean pyrite burial flux is isotopically lighter than the shallow ocean pyrite burial flux. This suggests that in past oceans the distribution of shallower environments may be the key control on the sulfur isotope composition of the global pyrite burial flux.