Molecular and isotopic abundances of long-chain n-fatty acids in open marine sediments of the western North Pacific

Abstract

Molecular abundance and compound-specific carbon isotope ratios of long-chain n-fatty acids (LCFAs) ranging from C 20 to C 30 are reported for open marine sediments of the western North Pacific. A positive correlation ( r 2=0.94) is observed between total organic carbon (TOC) and LCFA concentrations, although the TOC concentrations varied from 0.34 to 2.62 (wt.% in dry sediment). The relatively uniform ratio of LCFAs to TOC (0.74±0.12 mg/gC) is similar to that of the central Pacific reported by Ohkouchi et al. [Ohkouchi, N., Kawamura, K., Kawahata, H., Taira, A., 1997. Latitudinal distributions of terrestrial biomarkers in the sediments from the Central Pacific. Geochim. Cosmochim. Acta 61, 1911–1918] and in a core sample from the tropical central Pacific by Prahl et al. [Prahl, F.G., Muehlhausen, L.A., Lyle, M., 1989. An organic geochemical assessment of oceanographic conditions at MANOP site C over the past 26,000 years. Paleoceanography 4, 495–510]. On the other hand, LCFA concentrations in riverine and estuarine sediments are apparently higher than in open marine sediments. Carbon isotopic compositions of individual LCFAs in open marine sediments are similar (−26±1‰ for n–C 24 and n–C 26), being more enriched in 13 C than bay and riverine sediments by up to 6‰. Terrestrial C3 higher plants commonly have a δ 13 C value of ∼−26‰ in bulk organic carbon, and because lipid components including fatty acids are known to be depleted in 13 C by several per mil compared to bulk organic matter, this study indicates that the open marine sedimentary LCFAs are not derived from normal terrestrial C3 higher plants. Rather, the isotopic composition is consistent with a mixed source of C4 and C3 terrestrial higher plants, and/or marine organisms. Low carbon preference index (CPI) values of the LCFAs, and the isotopic difference between the northern and southern samples suggest that the LCFAs of this study are likely to be related to marine primary productivity, even though LCFAs have often been used to infer a terrestrial higher plant input to marine sediments.