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Abstract

Most of the sulfide produced in surface marine sediments is eventually oxidized back to sulfate via sulfur compounds of intermediate oxidation state in a complex web of competing chemical and biological reactions. Improved handling, derivatization, and chromatographic techniques allowed us to more closely examine the occurrence and fate of the sulfur intermediates elemental sulfur (S0), thiosulfate (S2O32−), tetrathionate (S4O62−), and sulfite (SO32−) in Black Sea and North Sea sediments. Elemental sulfur was the most abundant sulfur intermediate with concentrations ~3 orders of magnitude higher than the dissolved species, which were typically in the low micromolar range or below. Turnover times of the intermediate sulfur compounds were inversely correlated with concentration and followed the order: SO3 2− ≈ S4O62− > S2O32− > S0. Experiments with anoxic but non-sulfi dic surface sediments from the Black Sea revealed that added sulfide and sulfite disappeared most rapidly, followed by thiosulfate. Competing chemical reactions, including the reaction of sulfite with sedimentary S0 that led to temporarily increased thiosulfate concentrations, resulted in the rapid disappearance of SO32−. Conversely, low thiosulfate concentrations in the Black Sea sediments (<3μM) were attributed to the activity of thiosulfate-consuming bacteria. Experiments with anoxic but non-sulfi dic sediments revealed that 1 mol of tetrathionate was rapidly converted to 2 moles of thiosulfate. This tetrathionate reduction was bacterially mediated and occurred generally much faster than thiosulfate consumption. The rapid reduction of tetrathionate back to thiosulfate creates a cul-de-sac in the sulfur cycle, with thiosulfate acting as a bottleneck for the oxidation pathways between sulfide and sulfate.