The Nature of Carbonate and Organic δ13C Covariance Through Geological Time

Abstract:

Significant evolutionary, climatic, and oceanographic events in Earth history are often accompanied by excursions in the carbon isotope composition (δ¹³C) of marine carbonates and co-occurring sedimentary organic material. The observation of synchronous excursions in the δ¹³C values of marine carbonates and coeval organic matter is commonly thought to prove that the deposit has not been altered by diagenesis, and that the variations in the δ¹³C records are the result of a significant change in global carbon cycling. Furthermore, this model suggests that the covariance of carbonate and organic δ¹³C records is driven only by changes in the δ¹³C value of the dissolved inorganic carbon in the surface waters of the ocean. However, recent work suggests that there may be at least two alternate models for generating covariance between carbonate and organic δ¹³C values in the geologic record. One of the models invokes sea-level driven syndepositional mixing between isotopically distinct sources of carbonate and organic material to produce positive covariance between carbonate and organic δ¹³C values. The second model suggests that post-depositional alteration to the carbonate δ¹³C values during meteoric diagenesis, in concert with concurrent contributions of terrestrial organic material during subaerial exposure, can also produce co-occurring negative excursions with tightly covariant δ¹³C records. In contrast to earlier interpretations of covariant δ¹³C values, these models suggest that both syndepositional and post-depositional factors can significantly influence the relationship between carbonate and organic δ¹³C values in a variety of depositional environments. The implications for reconstructions of ancient global carbon cycle events will be explored within the context of these three models, and their relative importance throughout geologic time will be discussed.