Alternative Explanations for Variations in the Patterns of Carbon Isotopes in Organic and Inorganic Records

Recent work has shown that variations in the patterns of stable carbon isotopes obtained from platform derived carbonates at locations in the Atlantic Ocean, Indian Ocean, and the Pacific Ocean all show synchronous patterns over the past 10 to 25 Myrs. These patterns however are not related to the global patterns of carbon isotopic variations seen in open oceanic sediments over the same period. This absence of agreement between these two records is believed to be caused by simple changes in the input of materials with differing carbon isotopic compositions originating on shallow-water carbonate platforms. Such a phenomenon is a well established for controlling the carbon isotopic composition of bulk organic material (i.e. terrestrial vs. marine), but has been largely ignored for carbonate materials. In this presentation alternative explanations for variations in the carbon isotopic composition (other than changes in the amount of buried organic carbon) of carbonates are explored. These include differences in the origin of the sediments, diagenesis, changes in the predominant mineralogy precipitated in the oceans throughout geological time, and variations in the pCO2 of the atmosphere.