HR: 0830h
AN: B21D-0747    [PDF]
TI: Quaternary Branched Alkanes: A Geologic Proxy for Biological Sulfur Oxidation in Pleistocene Coastal Sediments of California
AU: * Zinniker, D A
EM: zinniker@pangea.stanford.edu
AF: Department of Geological & Environmental Sciences, Stanford University, Bldg. 320, Rm. 118, Stanford, CA 94305-2115 United States
AU: Green Nylen, N
EM: nellgree@pangea.stanford.edu
AF: Department of Geological & Environmental Sciences, Stanford University, Bldg. 320, Rm. 118, Stanford, CA 94305-2115 United States
AU: Moldowan, J M
EM: moldowan@pangea.stanford.edu
AF: Department of Geological & Environmental Sciences, Stanford University, Bldg. 320, Rm. 118, Stanford, CA 94305-2115 United States
AU: Denisevich, P
EM: pdenis@pangea.stanford.edu
AF: Department of Geological & Environmental Sciences, Stanford University, Bldg. 320, Rm. 118, Stanford, CA 94305-2115 United States
AU: Ingle, J C
EM: ingle@pangea.stanford.edu
AF: Department of Geological & Environmental Sciences, Stanford University, Bldg. 320, Rm. 118, Stanford, CA 94305-2115 United States
AB: Branched alkanes with quaternary substituted carbon atoms (QBAs) have been identified in two Pleistocene sedimentary sequences exposed along the California coastline. While these compounds have yet to be isolated from cultured microorganisms, a growing body of evidence points towards their origin as membrane components of colorless sulfur oxidizing bacteria (Kenig et al., 2002). This evidence includes an association with sulfide minerals, an association with environments or paleoenvironments with sharp sulfide:oxygen gradients, a positive correlation with the bacterial lipid diploptene, and (in this study) a positive correlation between the abundance of QBAs and the abundance of elemental sulfur in organic extracts. The wide distribution of QBAs in modern environments (deep sea hydrothermal systems, Arctic shelf sediments, and sediment traps from an oligotrophic marine site) may implicate chemoautotrophic and/or chemoorganotrophic members of the epsilon Proteobacteria as the source of QBAs. Sediments of the neritic to nonmarine Merced Formation near San Francisco and the bathyal to neritic Rio Dell Formation north of Cape Mendocino have been found to contain QBAs - predominantly 5,5-diethylalkanes and 2,2-dimethylalkanes. 5,5-diethylalkanes were identified by comparison of their spectra and elution time with published reports. 2,2-dimethylalkanes were identified through the chemical synthesis of authentic standards. In both sedimentary sequences, QBAs vary in abundance by more than two orders of magnitude over glacial-interglacial cycles. This variation appears to be crudely correlated with the abundance of elemental sulfur. In the Rio Dell Formation QBAs show an inverse correlation with the neritic benthic foraminifer biofacies as well as trace and macrofossil assemblages indicative of higher benthic oxygen concentrations. The occurrence of QBAs in shelf deposits of the lower Merced Formation supports previous interpretations that suggest a restricted basin setting with limited circulation. This is consistent with shoreface deposits indicative of low wave energy (on a currently high energy coast), a low diversity foraminifer assemblage, and the preservation of fine laminae and physical sedimentary structures in deeper water shelf facies. Variation in the relative abundance of different QBAs and related monomethyl alkanes may reveal finer taxonomic or environmental detail. Reference: Kenig, F., D. -J.H. Simons, D. Crich, J.P. Cowen, G.T. Ventura, T.C. Brown and T. Rehbein., (2002), Goldschmidt Conference Abstracts, p. A393.
DE: 0400 Biogeosciences
DE: 1055 Organic geochemistry
DE: 4267 Paleoceanography
DE: 4850 Organic marine chemistry
SC: Biogeosciences [B]
MN: 2003 Fall Meeting