PII S0016-7037(98)00032-5 The early diagenetic formation of organic sulfur in the sediments of Mangrove Lake, Bermuda DONALD E. CANFIELD, 1, *BERNARD P. BOUDREAU, 2 ALFONSO MUCCI, 3 and JENS K. GUNDERSEN 4 1 Institute of Biological Sciences, Odense University, Campusvej 55, 5230 Odense M, Denmark 2 Department of Oceanography, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada 3 Department of Earth and Planetary Sciences, McGill University, Montre ´al, Quebec H3A 2A7, Canada 4 Department of Microbial Ecology, Institute of Biological Sciences, University of Aarhus, Ny Munkegade, DK 8000 Aarhus C, Denmark (Received June 2, 1997; accepted in revised form November 26, 1997) Abstract—Due to a low mineral content, the sapropelic sediments depositing in Mangrove Lake, Bermuda, provide an excellent opportunity to explore for possible additions of sulfur to organic matter during the early stages of diagenesis. We evaluated early diagenetic organic sulfur transformations by monitoring the concentrations and stable isotopic compositions of a number of inorganic and organic sulfur pools, thereby accounting for all of the sulfur in the sediments. We have identified and quantified the following sulfur pools: porewater sulfate, porewater sulfide, elemental sulfur, pyrite sulfur, hydrolyzable organic sulfur (HYOS), chromium-reducible organic sulfur (CROS), and nonchromium-reducible organic sulfur (Non-CROS). Of the organic sulfur pools, the Non-CROS pool is by far the largest, followed by CROS, and finally HYOS. By 60 cm depth these pools contribute, respectively, to 85, 7.9, and 3.6% of the total solid phase sulfur. The HYOS pool is probably of biological origin and shows no interaction with the sulfur compounds produced during diagenesis. By contrast, CROS is produced, most likely, from the diagenetic addition of polysulfides to functionalized lipids in the upper, H 2 S-poor, elemental sulfur-rich, region of the sediment. A portion of this sulfur pool is unstable and decomposes on contact with the H 2 S-rich porewaters. The portion of CROS that remains in the sulfidic waters appears to readily exchange sulfur isotopes with H 2 S. While some of the Non-CROS pool is of biological origin, some is also formed by the diagenetic addition of sulfur to organic compounds in the upper H 2 S-poor region of the sediment. By contrast with CROS, Non-CROS is not diagenetically active in the H 2 S-rich porewaters. Overall, somewhere between 27 and 53 % of the organic sulfur buried in Mangrove Lake sediments is of diagenetic origin, with the remaining organic sulfur derived from biosynthesis. We extrapolate our Mangrove Lake results and calculate that in typical coastal marine sediments between 11 and 29 mol g -1 of organic sulfur will form during early diagenesis, of which 2–5 mol g -1 will be chromium reducible. Copyright © 1998 Elsevier Science Ltd 1. INTRODUCTION Organic sulfur may, in some cases, constitute the largest pool of sulfur in organic-rich sediments, particularly those relatively poor in continental clastics (e.g., Altschuler et al., 1983; Mitch- ell et al., 1984; Nriagu and Soon, 1985; Aizenshtat et al., 1983; Kenig and Huc, 1990; Ferdelman et al., 1991). The remains of dead organisms provide a ready source of organic sulfur to sediments, particularly in the form of the sulfur-containing amino acids, cysteine, and methionine, along with numerous other oxidized and reduced organic sulfur forms (e.g., Balzer, 1981; Vairavamurthy et al., 1994, 1995). By contrast, the organic sulfur that accumulates in sediments is believed to be partly, if not dominantly, of diagenetic origin (e.g., Raiswell et al., 1993; Andersen and Pratt, 1995). For example, organic sulfur contained in humic acids, fulvic acids, bitumens, and kerogen is depleted, often by considerable amounts, in 34 S relative to the isotopic composition of the original plant- and animal-derived organic sulfur that deposits onto the sediment (Nissenbaum and Kaplan, 1972; Francois, 1987; Mossmann et al., 1991; Zaback and Pratt, 1992; Raiswell et al., 1993; Ander- son and Pratt, 1995; Bru ¨chert and Pratt, 1996). The isotopic composition of the latter, normally near to water-column sul- fate, reflects the assimilatory reduction of sulfate by organisms which proceeds with only a small isotope fractionation (Kaplan and Rittenberg, 1964). The 34 S depletion in organic sulfur is thus accounted for by the addition to sedimentary organic matter of isotopically depleted sulfides and/or polysulfides pro- duced by bacterial sulfate reduction. There is both experimental and direct analytical evidence that such addition occurs. The geochemical analysis of sedi- ments, oil, and oil shales have documented the formation of organic sulfur compounds by the incorporation of sulfide and/or polysulfides into functionalized lipids, in particular those unsaturated with at least two pairs of double bonds (Sinninghe Damste ´ et al., 1989a,b; Kohnen et al., 1990, 1991; Wakeham et al., 1995). This reaction may occur during very early diagenesis (over several decades, Wakeham et al., 1995) and is of considerable potential importance as a precursor in the condensation of lower weight organic compounds into higher molecular weight substances and subsequent stabilization of these compounds towards further degradation (Sinninghe Damste ´ et al., 1989a). These principles have been substantiated in simple laboratory experiments where polysulfides were quickly added to the double bonds of various organic com- pounds, producing in some cases high molecular weight poly- sulfide-linked oligomers (Vairavamurthy and Mopper, 1989; de Graaf et al., 1992). * Author to whom correspondence should be addressed (dec@ biology.ou.dk). Pergamon Geochimica et Cosmochimica Acta, Vol. 62, No. 5, pp. 767–781, 1998 Copyright © 1998 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/98 $19.00 + .00 767