Sulfur, iron-, and calcium cycling associated with natural electric currents running through marine sediment

Abstract

Natural electric currents running through marine sediments have recently been found to couple oxygen reduction at the surface to sulfide oxidation in deeper anoxic layers. Here we show that such spatial separation of oxidation and reduction processes causes non-conventional sulfur, iron, and calcium mobilization and reallocation. Reduced marine sediment was incubated with overlying oxic water and the vertical distribution of solutes and solids was analyzed after 45–150days. As much as 44% of sediment oxygen consumption was driven by electric currents, and electrogenic oxidation of sulfide to sulfate with concurrent proton generation resulted in significant dissolution of iron sulfides and calcium carbonates in the anoxic layers of the sediment. Most of the mobilized iron diffused to the oxic zone where it formed oxidized iron minerals. Calcium precipitated in the oxic zone as magnesium-calcite. The electric coupling of biogeochemical processes in distant regions thus generates unique chemical conditions in marine sediments whereby key elements are mobilized and relocated, probably along with trace elements and nutrients. We suggest that such electrically coupled biogeochemistry flourishes in marine sediments after transient oxygen depletion, leaving distinct signatures of such events in the geological record.