Early diagenesis of sulfur in estuarine sediments: The role of sedimentary humic and fulvic acids

Sediments were collected from anoxic, organic-rich estuarine muds of St. Andrew Bay, Florida to assess the early diagenetic pathways leading to the formation of organic and inorganic sedimentary sulfide species. Abundance and sulfur isotopic composition were determined for dissolved sulfide, dissolved sulfate, elemental sulfur, acid-volatile sulfide, chromium-reducible sulfur, fulvic acid-sulfur, humic acid-sulfur, and residual organic (protokerogen) sulfur. Fulvic acid-sulfur, elemental sulfur, and acid-volatile sulfides are the initial sinks for dissolved sulfide and form before pyrite. Subsequently, two-thirds of fulvic acid-sulfur are lost. A fraction of fulvic acid-sulfur is transferred to higher molecular weight compounds such as humic acids and protokerogens, but most of the sulfides an recycled back to the porewaters. Enrichments in the isotopic composition of organic sulfur fractions, relative to pyrite and acid-volatile sulfides, suggest a contribution of detrital biosynthetic sulfur to the organic sulfur pool. Accounting for the biosynthetic component in fulvic acid-sulfur, the isotopic similarities between elemental sulfur, acid-volatile sulfides, and fulvic acid-sulfur suggest isotopic exchange in the uppermost 3 cm of sediment. Isotopic mass calculations suggest an additional isotopic fractionation takes place during recycling of sulfide released from fulvic acids and elemental sulfur by bacterial disproportionation of polysulfides and elemental sulfur. Subsequent precipitation of recycled, S-34-depleted sulfide as pyrite reconciles the consistent depletion in S-34 of pyrite-S relative to all other sedimentary sulfide species. Copyright (C) 1998 Elsevier Science Ltd.