Organic carbon δ13C variations in sedimentary rocks as chemostratigraphic and paleoenvironmental tools

Carbon isotopic variations in marine carbonate rocks and organic matter 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 oceanic dissolved CO2 and/or the physiology of the microalgal community. This paper reviews evidence from laboratory experiments and oceanographic studies aimed at understanding controls on carbon isotopic fractionation by marine phytoplankton. We also review factors affecting carbon isotopic compositions of bulk sedimentary organic matter and individual biomarker compounds. Guidelines for analysis of isotopic compositions 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 sedimentary organic matter because no environmental factor explains adequately variations in the carbon isotopic composition of modern phytoplankton. Studies of ancient materials should include compound-specific isotopic analyses of known phytoplankton biomarkers or organism-specific isotopic analyses with bulk isotopic results to assess the proportions of allochthonous organic materials, the degree of reworking and the extent of thermal degradation to confirm any trends present in δ-values of bulk sedimentary organic matter.