The role of sulfur in the preservation of isoprenoid hydrocarbons in sedimentary materials of the Washington continental margin Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/2n49t520p

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  • A systematic study of highly branched isoprenoids (HBI) was carried out in suspended particulate material (SPM) and Washington coastal sediments to determine their origin and fate. SPM collected at 10 m depth was filtered through Nitex membranes. C₂₅ HBI were found only in the 1.2-40 μm range over the shelf. The particle size fractionation of SPM shows different enrichment for HEH, a common hydrocarbon in phytoplankton, and the sum of C₂₅ HBI in the finer fractions suggesting these hydrocarbons do not share a common source. The distribution of C₂₅ and C₃₀ HBI correlates with the chlorophyll maxima suggesting an upper-water microbial source associated with phytoplankton biomass. It has been hypothesized that sulfur addition into specific biomarkers occurs during the early stages of diagenesis. Incorporation of the HBI into a refractory geomacromolecule via a sulfur linkage or formation of HBI-thiophenes are not evident in the sedimentary lipids. HBI show a rapid decrease in concentration with depth in both midshelf and slope sediments suggesting that biodegradation is the major pathway for their disappearance in Washington coastal sediments. Sediment cores from a midshelf and slope locations show the existence of suboxic/anaerobic conditions within the first 5 cm in the sediments. Elemental sulfur distribution in the midshelf appears to be controlled by bioturbation. On the slope, its profile indicates a quasi steady state regime. Phytane and phytenes (∑Phy) are the major products of Raney nickel desulfurization in both midshelf and slope sediments. Spinach and a strain of Emiliana huxleyi treated with Raney nickel showed strikingly similar patterns to the desulfurization products of sedimentary lipids. The amount of ∑Phy in the slope decreases abruptly by a factor of 6 in the top 2 cm and gradually increases with depth. These results are interpreted as phytyl coming from two sources: (1) chlorophyll-a and (2) S-bound to geomacromolecules. Partial released of phytyl moieties from chlorophyll-a warrants a reevaluation of Raney nickel as a selective desulfurizing agent before its application for paleoenvironmental reconstruction. Reducing micro-environments appear to exist within the bioturbated zone in shelf sediments. However, there is no clear evidence for phytyl moieties S-linked to macromolecules within the mixed layer. Results obtained during this study indicate that sulfur incorporation to biomarkers, although present, does not represent a significant mechanism for the preservation of organic carbon in normal marine sediments.
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