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doi:10.2204/iodp.proc.308.208.2008

Introduction

Pleistocene and Holocene sediments within the Brazos-Trinity and Ursa Basins (northwestern Gulf of Mexico) were largely deposited by turbidity currents and have been deformed by a number of mass transport events (Winker and Booth, 2000). Sediment sequences recovered from Brazos-Trinity Basin IV are typified by turbidites deposited over the last 112 k.y. (see the “Expedition 308 summary” chapter). The Ursa Basin sediments are levee-channel remnants of the ancient Mississippi River that were deposited within the last 68 k.y. (see the “Expedition 308 summary” chapter). In this study, the isotope compositions of sedimentary organic carbon and total nitrogen (TN), coupled with carbon to nitrogen molar ratios, were determined in order to assess the provenance of organic matter within these respective basins and to determine if the organic matter signature would allow for observation of a record of turbidite events in these basins.

Carbon to nitrogen molar ratios (C/N) are an index with which to determine the relative contributions of marine or terrigenous organic matter to the sedimentary record. Typically, marine organic matter C/N values range between 5 and 8 and terrestrial ratios are generally >20 (Emerson and Hedges, 1988; Meyers, 1997; Bouloubassi et al., 1999). Caveats to the use of C/N values as organic geochemical proxies include preferential nitrogen enrichment from clay sorption or nitrogen loss during microbial decomposition (Pimmel and Claypool, 2001). Organic geochemical results indicate the potential for preferential preservation of organic matter, whether terrestrial or marine, within the sediment as a response to sea level change. Organic carbon δ13C depletions of up to 6‰ are attributed to enhanced terrigenous supply during the Last Glacial Maximum (20 ka) and glacial periods throughout the Pleistocene (Newman et al., 1973; Jasper and Gagosian, 1990). Initial shipboard carbon and nitrogen elemental analyses suggested sedimentary organic matter is primarily derived from algal sources in Brazos-Trinity Basin IV, whereas organic precursors in Ursa Basin are hemipelagic to terrestrial. Proportionation of the inputs to these sediments, based on carbon isotope end-members (δ13Cmarine ≈ –20‰; δ13Cterrestrial ≈ –27‰), should elucidate potential sources and relative contributions of organic matter to these Gulf of Mexico sediments.