Both tectonic activity and CO2 variability affect temperature gradients in the Cretaceous

Abstract The Cretaceous ‘greenhouse’ period (~145 to ~65 million years ago, Ma) in Earth’s history is relatively well documented by multiple paleo proxy records, which indicate that the meridional sea surface temperature (SST) gradient increased (non-monotonically) from the Valanginian (~135 Ma) to the Maastrichtian (~68 Ma). Changes in atmospheric CO2 concentration, solar constant, and paleogeography are the primary drivers of variations in the spatiotemporal distribution of SST, which plays a key role in the evolution of the rich biodiversity that persisted during the Cretaceous. However, the particular contribution of each of these drivers (and their underlying mechanisms) to changes in the SST distribution remains poorly understood. Here we use data from a suite of paleoclimate simulations to show that atmospheric CO2 fluctuations and paleogeographic changes have a comparable effect on mid-latitudinal SST gradient through the Cretaceous. Further, we use a fundamental model of wind-driven ocean gyres to quantify how changes in the northern hemisphere paleogeography drive a reduction in the meridional oceanic heat transport, leading to an increase in extratropical SSTs. This reduction in heat transport results from a weakening of the subtropical ocean gyres, caused by a decrease in the ratio of meridional to zonal extent of the ‘gyral basin’.