Workshop on Cretaceous Climate and Ocean Dynamics

The Cretaceous Carbon Cycle: Interpreting the Carbon Isotope Record

A quantitative approach to carbon cycle modeling provides some surprising insights, some of them counterintuitive. For this purpose, I employ a simple model of the global carbon cycle which is subjected to a number of different perturbations on scales and magnitudes appropriate to the Cretaceous. In addition to standard considerations of carbon mass and isotopic fluxes to the ocean-atmosphere system from weathering and volcanism and fluxes of organic carbon and carbonate-carbon to sediments, the model incorporates sensitivity of the photosynthetic carbon isotope effect to changes in pCO₂ . The inclusion of this parameter leads to unexpected carbon isotope responses to forcing that causes increased rates of organic carbon burial. I will compare the model simulated carbon isotopic responses for several experiments to paired carbonate- and organic-carbon records to illustrate how these records might be interpreted in light of the model response. In particular, I will focus on characteristics of the Aptian OAE 1a event and demonstrate the diffences in system response to various volcanic forcing scenarios as opposed to that produced by a large release of methane from gas hydrates. The results of carbon cycle modeling suggest that gas hydrate release cannot be the primary cause of the OAE and carbon isotope variations, whereas a "superplume" event appears to best produce the responses seen in the sedimentary and geochemical record. Carbon cycle modeling suggests that the Cenomanian-Turonian (OAE 2) event is not clearly related to either increased volcanic carbon dioxide degassing or to methane clathrate release.