Timing of early diagenetic sulfurization of organic matter:: A precursor-product relationship in Holocene sediments of the anoxic Cariaco Basin, Venezuela

The incorporation of reduced inorganic sulfur into organic matter is a significant mechanism for the preservation of functionalized organic compounds in the sedimentary environment, but the timing of this process is not currently known. Analysis of organic matter in the Holocene and latest Pleistocene sediments of the Cariaco Basin indicates conversion of a triunsaturated tricyclic triterpenoid, (17E)-13 beta(H)-malabarica-14(27),17,21-triene, identified by isolation and subsequent one- and two-dimensional H-1 and C-13 NMR spectroscopy, to a monounsaturated triterpenoid thiane through incorporation of reduced inorganic sulfur species into the isoprenoid side chain within the upper 6 m of sediments. Time control provided by AMS C-14 dates was used in conjunction with measured relative abundances of the two compounds to calculate the reaction rate for the sulfurization of this triterpenoid, and empirically determine the first order rate constant (2 x 10(-4) yr(-1)) for the reaction. This is the first time that such a precursor-product relationship has been positively identified for a sulfurization reaction in sediments. Copyright (C) 2000 Elsevier Science Ltd.