Organic Matter Sulfurization in the Cariaco Water Column Revealed by High-Sensitivity and Compound-Specific d34S Analyses.

Abstract:

Organic matter burial in marine sediments is a major process in the global carbon cycle, and enhanced organic matter burial is often associated with periods of global climatic and ecological change. Still, we have only a limited understanding of the processes that drive enhanced OM burial during oxygen-deficient conditions. Abiotic OM sulfurization has the potential to enhance the preservation of OM, but for this process to be significant it must compete with heterotrophic remineralization, most of which occurs before sinking particles reach the sea floor. We investigate the sources of sulfur to sinking particles in a modern marine basin using samples from the CARIACO fixed sediment trap time-series, applying recently developed methods for d³⁴S analysis of small (≥20 nmol) sulfur pools and individual volatile organosulfur compounds. Relative to expectations for planktonic biomass, we find that sinking particles are both sulfur-rich and ³⁴S-depleted. Higher apparent fluxes of ³⁴S-depleted organic sulfur are associated with high OM export from the surface ocean, low terrestrial inputs, and high concentrations of both elemental S and the dominant non-polar organosulfur compound, C₂₀ thiophene. We conclude that OM sulfurization is occurring in particles sinking through the Cariaco water column on timescales of days or less. Depending on the frequency of high OM export events, we estimate that this rapid sulfurization delivers roughly half of the total organic S present at 5 cm depth in underlying sediments. Accordingly, many OM-rich deposits in the geologic record may represent the products of water column sulfurization. This process provides a strong mechanistic feedback between oxygen deficiency and OM preservation.