SULFUR-BINDING IN RECENT ENVIRONMENTS - II - SPECIATION OF SULFUR ANDIRON AND IMPLICATIONS FOR THE OCCURRENCE OF ORGANOSULFUR COMPOUNDS

Speciation of iron and sulfur species was determined for two recent se diments (La Trinitat and Lake Ciso) which were deposited in environmen ts with a high biological productivity and sulfate-reducing activity. In sediments from calcite ponds of La Trinitat an excess of reactive i ron species (iron monosulfides, iron hydroxides) results in a depletio n of reactive sulfur which is accompanied by a virtual absence of orga no-sulfur compounds, both in low (LMW) and high molecular-weight (HMW) fractions. Small amounts of phytanyl and highly branched isoprenoid ( HBI) thiophenes in the extract demonstrate that these molecules exhibi t a higher reactivity towards reduced sulfur species as compared to de trital iron. Euxinic sediments from Lake Ciso are characterised by an excess of reduced sulfur species which can rapidly trap reactive iron. High concentrations of H2S results in the formation of organo-sulfur compounds which were encountered in both LMW and HMW fractions. The ma jor part of the organic sulfur is bound to the carbohydrate portion of woody tissues, whose presence was revealed by a specific alkylthiophe ne distribution in the flash pyrolysate and by Li/EtNH2 desulfurisatio n of the kerogen which resulted in the solubilisation of the sulfur-en riched hemicellulose fraction. Relatively high amounts of sulfurised C -25 HBI compounds in the sediment extract of Lake Ciso reflect the inc orporation of sulfur into algal derived organic matter upon early diag enesis. The combined approach of the speciation of iron and sulfur spe cies and the molecular analysis of sedimentary fractions demonstrates that abiotic sulfur binding to organic matter occurs at the earliest s tages of diagenesis under specific depositional conditions (anoxic, st ratified water column) in which an excess of reduced sulfur species re lative to the amount of reactive iron is a controlling factor. Copyrig ht (C) 1997 Elsevier Science Ltd.