The influence of diagenesis on carbon and oxygen isotope values in shallow water carbonates from the Atlantic and Pacific: Implications for the interpretation of the global carbon cycle

Abstract

Global correlation of negative excursions in the δ13C values of shallow water carbonates have been used to interpret and identify major environmental changes through geologic time, such as the glaciations of the Neoproterozoic. As a result of their global reproducibility and similar geometry, these signals have predominantly been interpreted as being representative of the original δ13C values of dissolved inorganic carbon in the oceans, which in turn reflect major changes in the global carbon cycle. In this paper δ13C values from cores drilled in Enewetak Atoll (Pacific) and the Great Bahama Bank (Atlantic), are shown as examples of how similarly correlating variations in global δ13C values, can arise from diagenetic processes. It is further shown that while the profiles of the changes in δ13C values may appear synchronous, as a result of varying rates of deposition and subsidence, that the actual timing of the two changes can be quite different. By comparison of the geometry and global correlation of δ13C values measured in this study, it is proposed that the δ13C values of ancient sequences, may have resulted from non-synchronous diagenetic processes, rather than true changes in the global carbon system. This emphasizes the need for precise chronological control when interpreting sediment records, which is not present during many older geological time periods, in order to constrain the origin and correlation of carbon isotope variations.