Long-term carbon cycle trends : from the late Paleocene to the early Eocene climatic optimum

Abstract

A prominent decrease of δ13C as well as δ18O over the late Paleocene and early Eocene (∼57-52 Ma) has been observed in many sediment records. The δ18O paleorecords indicate a longterm warming trend (∼4◦C) of the Earth system over this time interval, while planktic and benthic stable carbon isotope ratios appear to gradually drop by about 2h, signifying a possible change in the carbon cycle. Concurrently, deep-sea carbonate records at several sites indicate a deepening of the calcite compensation depth (CCD). This study investigates possible causes (e.g. increased volcanic degassing and/or decreased organic carbon burial) for the observed climate shifts and unlike other studies, the evolution of the CCD is also considered in the model. The model employed here is a modified version of the GEOCARB III model, which uses more accurate input data (e.g. δ13C of carbonate records), coupled to the LOSCAR model. Besides the CCD, the coupled model separately simulates surface and deep ocean δ13C and it also includes full CO2 seawater chemistry. Several different scenarios are investigated with the goal of achieving a consistent scenario with respect to the observed temperature increase, the CCD change and the surface to deep ocean δ13C gradient. The results indicate that the most likely cause of the climate shift during the late Paleocene and early Eocene was mainly due to a decrease in net organic carbon burial, although an increase in metamorphic activity might have contributed to the overall trend. The model successfully recreates the temperature change, inferred from the δ18O record, caused by the radiative forcing of atmospheric CO2, as well as the drop in the CCD. At the moment, the model cannot recreate the surface to deep gradient in δ13C, which according to data remained constant during the studied time interval; the deep δ13C change predicted by the model is too small. The model also shows potential for explaining a 2 million year lag between the cessation of the carbon cycle perturbation (∼52 Ma) and the onset of the cooling of the Earth system (∼50 Ma) in the early Eocene.