Marine carbon burial flux and the carbon isotope record of Late Cretaceous (Coniacian–Santonian) Oceanic Anoxic Event III

Abstract

Episodes of widespread distribution of organic-rich marine facies within relatively narrow stratigraphic intervals have been interpreted as Oceanic Anoxic Events (OAEs). There are a number of such intervals in the Upper Cretaceous, some of which also exhibit significant positive excursions in carbon isotope records. As a result, a general model of the relationship between enhanced organic carbon burial and positive C-isotope shifts has emerged. Carbon isotope stratigraphy has, therefore, developed the dual utility of a chronostratigraphic tool and a means to help identify and define intervals of exceptional organic matter burial and preservation. However, variations in regional carbon isotope records of globally documented events suggest that the relationship between organic carbon burial flux and magnitude of carbon isotope excursion may not be entirely straightforward. For example, strata of Coniacian–Santonian age have long been recognized for enhanced organic content in many areas, but the designation of Oceanic Anoxic Event III for this interval has been rather equivocal in the literature, mostly due to the weak positive δ 13C excursion (≤ 0.5‰). Yet, in some areas organic carbon accumulation rates for Coniacian–Santonian strata are roughly double that of the older OAE II, which is marked by a 2–4‰ shift in δ 13C. Interestingly, the burial flux of carbonate, which leads to depletion rather than enrichment of the C-isotope signal, is also significantly elevated during Coniacian–Santonian time. In this study, the role of variations in organic carbon and carbonate accumulation rates on carbon isotopic expression are explored through application of data on the composition, accumulation rate, and areal distribution of end member facies types to the development of a global carbon isotopic mass balance model. The results are used to explore the hypothesis that net organic carbon burial during OAE III was not accurately represented in the carbon isotopic record. These results have important implications for interpretation of C-isotope records related to past perturbations of the global carbon cycle.