Biogeochemical controls on reaction of sedimentary organic matter and aqueous sulfides in holocene sediments of Mud Lake, Florida

The distribution and quantity of organic sulfur and iron sulfur species were determined in the Holocene sediments from Mud Lake, Florida. The sediments of this shallow, sinkhole lake are characterized by high sulfur and organic carbon contents as well as active sulfate reduction. They record a shift from a basal peat (below 2 m) comprised of water lily-dominated organic matter to the present cyanobacterial/algal-dominated lake deposit (upper 1 m). This shift in depositional environment and subsequent organic matter source was accompanied by variation in the amount of reactive iron delivered to the sediments, which in turn influenced the type and extent of organic matter sulfurization. Extractable intramolecular organic sulfur is principally found as C₂₅ highly branched isoprenoid (HBI) thiolanes. Extractable polysulfide-linked lipids, determined by selective chemical cleavage with MeLi/MeI and analyzed as methylthioethers (MTE), are dominated by n-alkanes with sulfur attachments at position 1 and 2, as well as lower amounts of C₂₅ HBI-MTE. The δ¹³C values and carbon-chain length distribution of both series of n-alkylMTE indicate that they are derived from distinct biological precursors. Among the n-alkylMTE with sulfur attachment at position 1 there are three homologous series: one saturated and two with both cis and trans enethiol isomers. The identification of the enethiol in the sulfur-linked macromolecules indicates that n-alkylaldehydes are precursors lipids. The intervals of high concentration of bulk organic sulfur and sulfurized lipids coincide with the intervals of high mineral sulfur content (acid volatile sulfide and chromium reducible sulfur). We suggest that the main control on the enhanced addition of sulfur to the organic matter in Mud Lake was the increased formation of polysulfides during the reduction of iron hydroxides and the subsequent reaction of those polysulfides with mildly oxidized sedimentary organic matter.