Interpreting carbonate and organic carbon isotope covariance in the sedimentary record

Many negative delta C-13 excursions in marine carbonates from the geological record are interpreted to record significant biogeochemical events in early Earth history. The assumption that no post-depositional processes can simultaneously alter carbonate and organic delta C-13 values towards more negative values is the cornerstone of this approach. However, the effects of post-depositional alteration on the relationship between carbonate and organic delta C-13 values have not been directly evaluated. Here we present paired carbonate and organic delta C-13 records that exhibit a coupled negative excursion resulting from multiple periods of meteoric alteration of the carbonate delta C-13 record, and consequent contributions of isotopically negative terrestrial organic matter to the sedimentary record. The possibility that carbonate and organic delta C-13 records can be simultaneously shifted towards lower delta C-13 values during periods of subaerial exposure may necessitate the reappraisal of some of the delta C-13 anomalies associated with noteworthy biogeochemical events throughout Earth history.