Speciation and distribution of arsenic in cold seep sediments of the South China Sea

Arsenic (As) is an abundant metalloid in marine environments, while the biogeochemical cycling of As in cold seeps remains poorly understood. We characterized the speciation of As and investigated controls of As distribution in cold seeps of South China Sea. High methane concentrations (0.2-5.5 mmol/L) and rapid sulfate depletion were observed in the seepage. Dissolved inorganic arsenic (DIAs) was enriched in the porewater ranging from 7.5 to 23.5 μg/L. As in the solid phase ranged from 2.9 to 22.6 μg/g, and sulfide mineral-bound As dominated the total arsenic (TAs) pool, followed by iron (manganese, aluminum) oxide-bound As. The significant correlations between porewater Fe2+ and DIAs reflect the controls of iron on DIAs release. Incubation experiments showed that adsorption to the solid phase and sulfate reduction activity affected the bioavailability and removal of DIAs, suggesting that multiple processes regulate the speciation and transformation of As in seep sediments.