Rapid organic matter sulfurization in sinking particles from the Cariaco Basin water column

Organic matter (OM) burial in marine sediments is a potentially important control on global climate and the long-term redox state of the earth’s surface. Still, we have only a limited understanding of the processes that stabilize OM and facilitate its preservation in the geologic record. Abiotic reactions with (poly)sulfides can enhance the preservation potential of OM, but for this process to be significant it needs to compete with OM remineralization, the majority of which occurs before sinking particles reach the sea floor. Here we investigate whether OM sulfurization occurs within sinking particles in the Cariaco Basin, a modern sulfidic marine environment with high rates of OM burial. Proto-kerogen in sinking particles is frequently more sulfur-rich and 34S-depleted than expectations for biomass, with a composition that is difficult to explain by mixing with resuspended or terrigenous material. Instead, it appears that sulfur is being incorporated into OM on a timescale of days in sinking particles. The flux of this abiogenic organic S from particles is equivalent to approximately two-thirds of the total amount of proto-kerogen S at 10 cm depth in underlying sediments (ODP Core 1002B); after 6000 years of more gradual sulfurization reactions, potential water column sources are still equivalent to nearly half of the total proto-kerogen S in Cariaco sediments. Water column sulfurization is most extensive during periods of upwelling and high primary productivity and appears to involve elemental S, possibly via polysulfides. This process has the potential to deliver large amounts of OM to the sediments by making it less available for remineralization, generating OM-rich deposits. It represents a potentially dynamic sink in the global carbon cycle that can respond to changes in environmental conditions, including the size and intensity of O2-depleted environments. Water column OM sulfurization could also have played a more significant role in the carbon cycle during ocean anoxic events, for example during the Cretaceous.