Isotope analyses of molecular and total organic carbon from Miocene sediments

Carbon-isotope compositions of n-alkanes, pristane and phytane, and total organic carbon were measured and compared against isotopic trends of coeval alkadienones from Miocene sediments containing very low organic-carbon contents. Compound-specific isotope analysis of n-alkanes and isoprenoid lipids, in conjunction with abundance distributions of n-alkanes reveal the influence of terrestrially derived organic carbon at all sites analyzed. In general, n-alkanes are derived from allochthonous sources with the exception of n-C₃₇ from site 516, which appears genetically related to coeval alkadienones. Further, pristane and phytane from pelagic sites 608 and 516 apparently derive from terrestrial sources as well, although a marine origin cannot be excluded. δ(TOC) values lack a coherent relationship to %TOC and δ¹³C(37:2). Differential alteration and mixing of diverse isotopic signals most likely contribute to temporal variation and spatial differences in δ(TOC). Therefore, when working with sediments from oligotrophic settings, we do not recommend δ(TOC) as an indicator of phytoplankton δ¹³C values. Copyright (C) 1999 Elsevier Science Ltd.