ISOTOPE ANALYSES OF MOLECULAR AND TOTAL ORGANIC CARBON FROM MIOCENE SEDIMENTS - WAX ESTERS, STERYL ESTERS, TRIACYLGLYCEROLS AND ALKYLDIACYLGLYCEROLS

Abstract

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.δTOC values lack a coherent relationship to %TOC and δ13C37:2. Differential alteration and mixing of diverse isotopic signals most likely contribute to temporal variation and spatial differences in δTOC. Therefore, when working with sediments from oligotrophic settings, we do not recommend δTOC as an indicator of phytoplankton δ13C values.