Pyrolytic Character of Organic Matter in Cenozoic Sediments on the Oman Shelf

Abstract

Analysis of the molecular composition and quantity of pyrolytic hydrocarbons in 41 samples from Owen Ridge and the Oman margin enabled us to identify chemical differences in the organic matter from Owen Ridge and the Oman margin. The differences may be attributed to regional variability in organic matter composition between margin and ridge, effects of kerogen formation and condensation with age, and effects of changes in the depositional environment on the Oman margin. Pyrolytic hydrocarbons from ridge sediments are relatively more enriched in heterocompounds, aromatic molecules, and «-alkanes and n-monoalkenes in the range n-C9 to «-C14 when compared to margin sediments. This may be indicative of input of degraded organic material on Owen Ridge and less degraded material on the Oman margin. Increases of long-chain n-alkanes and rt-monoalkenes with depth in sediments from the Oman margin are a result of the concentration of precursor moieties in the kerogen during low-temperature diagenesis. Differences in the depositional environment during deposition of sediments on the Oman margin (changes in the oxygen content of bottom waters and changing benthic activity in a variable oxygen minimum zone) appear to be mirrored in the distribution of monounsaturated isoprenoid hydrocarbons prist-1-ene and prist-2-ene and alkylbenzene. BACKGROUND AND OBJECTIVES We examine here chemical differences in organic matter from sediments deposited underneath the monsoonal upwelling system of the northwestern Arabian Sea by pyrolysis and pyrolysisgas-chromatography/mass spectroscopy. Our first objective was to find molecular criteria that would allow us to distinguish organic matter deposited under anaerobic conditions on the Oman margin, which are laminated, from sediments that are bioturbated and indicate aerobic bottom water conditions. Sediments from the Oman shelf and upper slope are uniquely suited to address the question of differences in organic matter preservation (e.g., Demaison and Moore, 1980; Henrichs and Reeburgh, 1987), because sections recovered at Site 723 on the Oman continental margin (Prell, Niitsuma, et al., 1989) appear to have been deposited under oxic and dysoxic to anoxic conditions in an oxygen minimum zone that changed intensity and/or depth range in the geological past. Analyzing the organic matter composition in sediment intervals deposited under aerobic (bioturbated facies) or anaerobic (laminated facies) conditions may thus provide insight into organic matter composition and rates of decay of compound classes as functions of aerobic and anaerobic bacterial metabolism. We can further calibrate our results using recent surface sediments, for which oxygen levels are known. A second objective was to test whether organic matter character is uniform on the shelf, i.e., underneath the most productive coastal upwelling centers, and the distal Owen Ridge, which has lower accumulation rates of organic carbon and where sedimentation may be strongly influenced by lateral transport from the Oman margin and eolian input.