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 terrestri al sediment sources in Tomales Bay, California. Despite relatively high sed imentation rates throughout the bay (historically 3-30 mm yr(-1)), sulfate reduction of deposited organic matter led to free-sulfide accumulation in s ediments 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 su lfide (up to 400 mu mol g(-1) farthest from the reactive iron source). A ca lculated index of iron availability, used to describe sediment resistance t o build-up of free sulfide, was lowest at this location. Recent, upward shi fts in reactive Fe concentration and in the relative contribution of terres trial organic carbon (measured as a shift in delta(13)C of bulk sediment or ganic matter) in all cores indicated that erosion and transport of sediment s from the watershed surrounding Tomales Bay increased after European settl ement in the 1850s.