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

Molecular abundance and compound-specific carbon isotope ratios of long-cha in n-fatty acids (LCFAs) ranging from C-20 to C-30 are reported for open ma rine sediments of the western North Pacific. A positive correlation (r(2) = 0.94) is observed between total organic carbon (TOC) and LCFA concentratio ns, although the TOC concentrations varied from 0.34 to 1.62. (wt.% in dry sediment). The relatively uniform ratio of LCFAs to TOC (0.74 +/- 0.12 mg/g C) is similar to that of the central Pacific reported by Ohkouchi et al. [O hkouchi, N., Kawamura, K., Kawahata, H., Taira, A., 1997. Latitudinal distr ibutions of terrestrial biomarkers in the sediments from the Central Pacifi c. Geochim. Cosmochim. Acta 61, 1911-1918] and in a core sample from the tr opical central Pacific by Prahl et al. [Prahl, F.G., Muehlhausen, L.A., Lyl e, hi., 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 compos itions of individual LCFAs in open marine sediments an similar (-26 +/- 1 p arts per thousand for n-C-24 and n-C-26), being more enriched in C-13 than bay and riverine sediments by up to 6 parts per thousand. Terrestrial C3 hi gher plants commonly have a delta(13)C value of similar to -26 parts per th ousand in bulk organic carbon, and because lipid components including fatty acids are known to be depleted in C-13 by several per mil compared to bulk organic matter. this study indicates that the open marine sedimentary LCFA s are not derived from normal terrestrial C3 higher plants. Rather, the iso topic composition is consistent with a mixed source of C4 and C3 terrestria l 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 rela ted to marine primary productivity, even though LCFAs have often been used to infer a terrestrial higher plant input to marine sediments. (C) 2000 Els evier Science B.V. All rights reserved.