Did late cretaceous cooling trigger the Campanian-Maastrichtian Boundary Event?

The Campanian-Maastrichtian (83-66 Ma) was a period of global climate cooling, featuring significant negative carbon-isotope (d13C) anomalies, such as the Late Campanian Event (LCE) and the Campanian-Maastrichtian Boundary Event (CMBE). A variety of factors, including changes in temperature, oceanic circulation and gateway opening, have been invoked to explain these d13C perturbations, but no precise mechanism has yet been well constrained. In order to improve our understanding of these events, we measured stable carbon and oxygen isotopes of hemipelagic sediments from the Shuqualak-Evans cored borehole (Mississippi, USA) and compared the data with previously published sea-surface temperature (SST) estimates from the same core. We found that the CMBE can be recognised, unambiguously, in the Shuqualak- Evans core, and that it is associated with an interval of cooler SSTs suggesting a possible mechanistic link between palaeotemperature change and this event. Determining the precise position of the LCE in the Shuqualak-Evans core is more problematic, but it may also be associated with cooler SSTs. Our combined records of carbon cycling and SSTs compare well with other studies and provide evidence that cooling during the CMBE (and possibly LCE) was global in nature and affected surface waters, in addition to the deep-ocean. We suggest that short-term cooling drove intensification of high-latitude deep-water formation, which in turn led to changes in the ratio of carbonate to organic carbon burial that led to a negative d13C excursion. Critically, the absence of warming during these intervals implies that the Late Cretaceous events must not have been associated with an appreciable increase in atmospheric pCO2, and was likely associated with decreased pCO2.