Isotope analyses of molecular and total organic carbon from Miocene sediments

Carbon-isotope compositions of n-alkanes, pristane and phytane, and total o rganic carbon were measured and compared against isotopic trends of coeval alkadienones from Miocene sediments containing very low organic-carbon cont ents. Compound-specific isotope analysis of n-alkanes and isoprenoid lipids , in conjunction with abundance distributions of n-alkanes reveal the influ ence of terrestrially derived organic carbon at all sites analyzed. In gene ral, n-alkanes are derived from allochthonous sources with the exception of n-C-37 from site 516, which appears genetically related to coeval alkadien ones. Further, pristane and phytane from pelagic sites 608 and 516 apparent ly derive from terrestrial sources as well, although a marine origin cannot be excluded. delta(TOC) values lack a coherent relationship to %TOC and delta(13)C(37:2) . Differential alteration and mixing of diverse isotopic signals most likel y contribute to temporal variation and spatial differences in delta(TOC). T herefore, when working with sediments from oligotrophic settings, we do not recommend delta(TOC) as an indicator of phytoplankton delta(13)C values. C opyright (C) 1999 Elsevier Science Ltd.