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Abstract Blue carbon ecosystems such as seagrass meadows, mangrove forests, and salt marshes are important carbon sinks that can store carbon for millennia. Recently, organic matter sulfurization and pyritization have been proposed as mechanisms of net carbon storage in blue carbon ecosystems. At our study site, organic sulfur that is resistant to acid hydrolysis (protokerogen) is an order of magnitude less abundant than pyrite sulfur, suggesting a dominance of pyritization over sulfurization. The C/N ratios and carbon isotope compositions suggest that nearly half of total organic carbon and ≥ 80% of protokerogen is composed of marsh plant material. Sediment protokerogen appears to be sulfurized based on its low δ³⁴S values (− 10‰), abundance of disulfides, and higher S/C ratio (~ 1.0%) relative to potential biogenic sulfur sources. However, the interpretation of protokerogen δ³⁴S values is complicated by the wide range in sulfur isotope compositions of marsh plants. Evidence for sulfurization occurs within the shallowest sediments across different vegetation zones, yielding consistent products, while pyritization appears to be more sensitive to alterations in sediment redox conditions. Based on organic sulfur and pyrite content, sulfurization may be a more spatially consistent process than pyritization, with implications for carbon storage. The relative abundance of pyrite and protokerogen organic sulfur indicates that pyritization is favored at our study site, but this is likely to vary across the spectrum of blue carbon ecosystems. 

Organic sulfur (OS) in the ocean is produced in vast quantities by primary producers that fix inorganic sulfate into proteins, metabolites, and other ubiquitous biomolecules. As biogenic OS is transported and transformed through the marine environment, it is joined by OS from two additional sources: abiogenic OS from sulfurization under anoxic conditions, and geological OS from the weathering of sediments and rocks. Important differences in the properties of the OS from these sources affect its fate in the environment and underlie the formation of recalcitrant dissolved organic matter and sedimentary kerogen. This review builds connections between the rapid OS cycle in the surface ocean and these longer-lived reservoirs, applying our growing knowledge of particle fluxes and organic matter dynamics at the sediment–water interface. Future studies on marine OS are poised to help us better understand the implications of these fluxes for the carbon cycle and climate across human and geological timescales.