Biogeochemistry of dissolved hydrogen sulfide species and carbonyl sulfide in the western North Atlantic Ocean

Abstract

The biogeochemistry of total sulfide dissolved in the open ocean is a poorly understood component of the global sulfur cycle. Here, the cycling of total sulfide was examined in the western North Atlantic Ocean using specially developed sampling and analytical methods. Total sulfide (particulate + dissolved sulfide) concentrations ranged from <2–550 pmol/L; concentrations were highest in the mixed layer and decreased with depth. Significant levels (up to 19 pmol/L) of free sulfide (uncomplexed sulfide) were determined in the top 50 m of the water column. Sources of total sulfide were examined. In particular, the rate of carbonyl sulfide (OCS) hydrolysis was redetermined under oceanographic conditions, and the depth distribution of OCS was examined. The patterns of near-surface enrichment (up to 150 pmol/L) and depletion at depth observed in OCS depth profiles suggest in situ production of OCS. To quantify the sources and sinks of total sulfide in the mixed layer of the Sargasso Sea, a budget was constructed. The rate of total sulfide production was 5.5 pmol L−1 h−1 (OCS hydrolysis + atmospheric input), and total sulfide removal rate was 115 pmol L −1 h−1 (oxidation + particulate sinking). The significant difference between the known sources and sinks indicates that other processes are important for the cycling of sulfide. Similarities in the depth distribution of total sulfide and chlorophyll a, and results from recent laboratory experiments argue strongly in favor of biological involvement in the production of total sulfide in the open ocean.