THE EARLY DIAGENETIC FORMATION OF ORGANIC SULFUR IN THE SEDIMENTS OF MANGROVE LAKE, BERMUDA

Due to a low mineral content, the sapropelic sediments depositing in M angrove Lake, Bermuda, provide an excellent opportunity to explore for possible additions of sulfur to organic matter during the early stage s of diagenesis. We evaluated early diagenetic organic sulfur transfor mations by monitoring the concentrations and stable isotopic compositi ons of a number of inorganic and organic sulfur pools, thereby account ing for all of the sulfur in the sediments. We have identified and qua ntified the following sulfur pools: porewater sulfate, porewater sulfi de, 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 du ring diagenesis. By contrast, CROS is produced, most likely, from the diagenetic addition of polysulfides to functionalized lipids in the up per, H2S-poor, elemental sulfur-rich, region of the sediment. A portio n of this sulfur pool is unstable and decomposes on contact with the H 2S-rich porewaters. The portion of CROS that remains in the sulfidic w aters appears to readily exchange sulfur isotopes with H2S. 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 H2 S-poor region of the sediment. By contrast with CROS, Non-CROS is not diagenetically active in the H2S rich porewaters. Overall, somewhere b etween 27 and 53% of the organic sulfur buried in Mangrove Lake sedime nts is of diagenetic origin, with the remaining organic sulfur derived from biosynthesis. We extrapolate our Mangrove Lake results and calcu late that in typical coastal marine sediments between 11 and 29 mu mol g(-1) of organic sulfur will form during early diagenesis, of which 2 -5 mu mol g(-1) will be chromium reducible. Copyright (C) 1998 Elsevie r Science Ltd.