PII S0016-7037(99)00306-8 Addition of sulfur to organic matter during early diagenesis of lake sediments N. R. URBAN,* ,†,1 K. ERNST, 2 and S. BERNASCONI 3 1 Lake Research Laboratory, Swiss Federal Institute for Environmental Science & Technology (EAWAG), CH-6047 Kastanienbaum, Switzerland 2 Section Metallurgy and Surface Analysis, Swiss Federal Laboratories for Materials Testing and Research (EMPA), CH-8600 Du ¨bendorf, Switzerland 3 Dept. Geology, Swiss Federal Technical Institute—Zu ¨rich (ETHZ), CH-8092 Zu ¨rich, Switzerland (Received April 30, 1998; accepted in revised form September 24, 1998) Abstract—Enrichment of organic matter with sulfur during early diagenesis has been amply documented in marine sediments. The importance of such reactions in lacustrine sediments is not as well appreciated. In this study the organic sulfur contents of sediments from seven lakes were compared, and the speciation of the sulfur in the humic acid fraction of the sedimentary organic matter was examined with X-ray photoelectron spectroscopy (XPS). Diagenetic enrichment of organic matter with sulfur occurred in five of seven lakes as evidenced by C:S ratios and stable isotope ratios. The availability of sulfide, organic matter, and reactive iron were not important determinants of the extent of organic S enrichment. The only environmental parameter that appeared to be related to S enrichment was lake trophic state. Together with data from the literature, our results suggest that sediments in most eutrophic lakes are enriched in organic S, while organic matter is enriched in only few oligotrophic lakes. Organic sulfides or thiols are the dominant forms of reduced organic S in the humic acids, and sulfur enrichment occurs primarily by formation of organic sulfides or thiols. Addition of S to polyunsaturated molecules could account for 5–10% of the S enrichment. Di- and polysulfides did not comprise a significant fraction of organic S in any of the sediment humic acids. Thiophenes could be identified tentatively only in the oldest (60 yr) sediment sample analyzed. Sulfoxides were observed in several samples. Lake trophic state and exposure to oxygen appear to be major factors influencing the extent and pathways of S addition to organic matter. Copyright © 1999 Elsevier Science Ltd 1. INTRODUCTION Considerable evidence indicates that sulfur addition to organic matter occurs very early in diagenesis of marine sediments. Thiophenic compounds not found in biomass commonly are found in surface marine sediments and are attributed to the addition of sulfide to chlorophyll derivatives (Fukushima et al., 1992; Rowland et al., 1993). Numerous organic sulfur com- pounds were found in surface sediments of the Black Sea (Wakeham et al., 1995); diagenesis at the sediment surface or in the water column was implicated. Ratios of S:C and con- centrations of S-containing pyrolysis products increase rapidly within the first meter of sediments below the Peru upwelling (Eglinton et al., 1994). The timescale for these reactions re- mains to be clarified. It also is pertinent to study sulfur enrichment processes in lake sediments. Important petroleum source rocks were depos- ited under lacustrine conditions, and modern analogs of these conditions exist (e.g., Kelts et al., 1988). Sinninghe Damste ´ et al. (1993) reported extensive sulfur enrichment in organic mat- ter from three lacustrine oil shales. Putschew et al. (1995) observed extensive sulfurization of phytane even in surface sediments of a meromictic lake. There is a widespread percep- tion that sulfur addition to organic matter will be unimportant because of the low sulfate concentrations in most lakes (e.g., Fukushima et al., 1992; Sinninghe Damste ´ et al., 1993). Abun- dant evidence exists to the contrary. Sulfate reduction occurs in sediments of most lakes (e.g., Sorokin, 1975; Smith and Klug, 1981; Rudd et al., 1986b); rates often are comparable to those in marine sediments even in lakes with low sulfate concentra- tions (Urban et al., 1994). Losher and Kelts (1989) and Davison et al. (1985) reported that C:S ratios in some modern lake sediments are as low as those in marine sediments (cf. Berner et al., 1979; Berner and Raiswell, 1984). Incorporation of 35 S into sediment organic matter in lakes occurs over a timescale of hours to months (Rudd et al., 1986a; Landers and Mitchell, 1988; Baker et al., 1989). Stable sulfur isotope ratios also clearly show that microbially reduced S is incorporated into the organic matter of numerous lake sediments (e.g., Nriagu and Soon, 1985; Fry, 1986; Thode, 1991). Because of the better time resolution afforded by lake sediment profiles, it should be easier to determine the rates of diagenetic sulfur addition to organic matter in lakes than in marine sediments. Three analytical approaches are used for the analysis of organic S in sediments. The first consists of classical methods for extraction and identification of specific compounds. Solvent extraction, column or thin layer chromatography, and gas chro- matography mass spectrometry (GCMS) analysis have been used to identify numerous organic sulfur compounds in the past decade (e.g., review in Sinninghe Damste ´ and de Leeuw, 1990). Knowledge of specific compound structures and simulation experiments have indicated the probable mechanisms for their formation. However, even as compound-specific analyses can identify only a small fraction of the constituents of bulk organic matter in sediments, so also the compound-specific analyses must be presumed to have identified only a minute fraction of the existing organic sulfur compounds in sediments. A second *Author to whom correspondence should be addressed (nurban@mtu; edu). † Present address: Dr. N. R. Urban, Dept. Civil and Environmental Engineering, Michigan Technological University, Dow Environ. Sci. & Engineering Bldg., 1400 Townsend Drive, Houghton, MI 49931-1295. Pergamon Geochimica et Cosmochimica Acta, Vol. 63, No. 6, pp. 837– 853, 1999 Copyright © 1999 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/99 $20.00 + .00 837