20. Isotopic Chemistry of Organic Carbon in Sediments from Leg 184

Carbon isotopes were determined on the organic matter (OM) in sediments recovered from four sites (1143, 1144, 1146, and 1148) in the South China Sea during Ocean Drilling Program Leg 184. Concentrations of finely disseminated OM in the sediments were low (generally 0.1 to 0.5 wt%) and show organic δ13C isotope values (generally –22‰ to –24‰) indicative of marine to mixed marine and land-derived OM. The shipboard sedimentologists noted and the author collected seven samples of discrete centimeter-size chunks of organic carbon–rich material, such as fragments of wood, from cores from the five sites. The δ13C values in these samples ranged from –24‰ to –30‰. Humic material extracted from two wood/lignite samples and one plant fragment sample has nearly the same δ13C value as the bulk material. The δ13C values of humic extracts from the woody materials are different from those of marine materials, indicating that the extractable material is original, land-derived OM rather than absorbed/adsorbed OM from the marine environment. The δ13C values (–28‰ to –30‰) for the two plant fragment samples appear unusually light but are probably reasonable for nonwoody land-derived OM, which is rarely observed, preserved, or analyzed in deep-sea cores. Assuming carbon isotope values of –20‰ for “pure” marine and –27‰ for “pure” terrigenous OM, samples from Leg 184 generally preserve 40%–60% marine and 40%–60% terrigenous OM. However, because marine OM is more metabolizable than terrigenous OM and because these samples contain pyrite sulfur, the actual concentration of marine OM originally deposited with the sediments was probably in the range of 60%–90%. 1Leventhal, J.S., 2004. Isotopic chemistry of organic carbon in sediments from Leg 184. In Prell, W.L., Wang, P., Blum, P., Rea, D.K., and Clemens, S.C. (Eds.), Proc. ODP, Sci. Results, 184, 1–13 [Online]. Available from World Wide Web: . [Cited YYYY-MM-DD] 2Diversified Geochemistry, 8944 West Warren Drive, Lakewood CO 80227, USA. [email protected] Initial receipt: 4 December 2001 Acceptance: 6 January 2004 Web publication: 4 March 2004 Ms 184SR-215 J.S. LEVENTHAL ISOTOPIC CHEMISTRY OF ORGANIC CARBON 2