Iron, sulfur, and carbon diagenesis in sediments of Tomales Bay, California

Analysis of 3-m sediment cores revealed that profiles of carbon (C), sulfur (S), and iron (Fe) varied with relative distance from marine and terrestrial sediment sources in Tomales Bay, California. Despite relatively high sedimentation rates throughout the bay (historically 3-30 mm yr(-1)), sulfate reduction of deposited organic matter led to free-sulfide accumulation in sediments only at the location farthest from terrestrial runoff, the source of reactive iron Acid-volatile sulfide concentrations in all sediments (<10 mu mol g(-1)) were low relative to concentrations of chromium-reducible sulfide (up to 400 mu mol g(-1) farthest from the reactive iron source). A calculated index of iron availability, used to describe sediment resistance to build-up of free sulfide, was lowest at this location. Recent, upward shifts in reactive Fe concentration and in the relative contribution of terrestrial organic carbon (measured as a shift in delta(13)C of bulk sediment organic matter) in all cores indicated that erosion and transport of sediments from the watershed surrounding Tomales Bay increased after European settlement in the 1850s.