ORGANIC-CARBON DELTA-C-13 VARIATIONS IN SEDIMENTARY-ROCKS AS CHEMOSTRATIGRAPHIC AND PALEOENVIRONMENTAL TOOLS

Carbon isotopic variations in marine carbonate rocks and organic matte r can provide important information on stratigraphic correlations and paleoenvironments. In particular, carbon isotopic variations in marine organic materials may yield information about the concentration of oc eanic dissolved CO2 and/or the physiology of the microalgal community. This paper reviews evidence from laboratory experiments and oceanogra phic studies aimed at understanding controls on carbon isotopic fracti onation by marine phytoplankton. We also review factors affecting carb on isotopic compositions of bulk sedimentary organic matter and indivi dual biomarker compounds. Guidelines for analysis of isotopic composit ions of organic materials and interpretations of these results for use as chemostratigraphic and paleoenvironmental tools are presented. We recommend caution in interpreting carbon isotopic variations of sedime ntary organic matter because no environmental factor explains adequate ly variations in the carbon isotopic composition of modern phytoplankt on. Studies of ancient materials should include compound-specific isot opic analyses of known phytoplankton biomarkers of organism-specific i sotopic analyses with bulk isotopic results to assess the proportions of allochthonous organic materials, the degree of reworking and the ex tent of thermal degradation to confirm any trends present in delta-val ues of bulk sedimentary organic matter. (C) 1997 Elsevier Science B.V.