Using sulfur isotopes to elucidate the origin of barite associated with high organic matter accumulation events in marine sediments

Events of widespread deposition of organic-carbon–rich marine sediments, identified as ocean anoxic events, occurred in the middle of the Cretaceous. Similar deposits termed sapropels occurred during the Pliocene and Pleistocene in the Mediterranean Basin. High biological productivity and/or anoxia have been invoked as possible causes for these widespread high organic carbon deposition events. We use the S isotopic composition of barite associated with these events to confirm that high barite accumulation rates are a result of elevated marine biological productivity and not a diagenetic artifact. The accumulation and good preservation of biogenic barite, which dissolves when pore-water sulfate concentrations are low, in association with high organic matter and authigenic pyrite, indicates that the rate of bacterial sulfate reduction was low enough for downward diffusion of seawater sulfate to replenish the pore water and prevent depletion of sulfate. The organic C to S burial ratio in samples with high barite accumulation is typically high (>5 wt ratio), supporting burial in high-productivity open-ocean regions, where pyrite formation is restricted.