EVALUATING THE EXISTENCE OF SUPER-HEATED MID-CRETACEOUS TROPICS USING A COMBINED MODEL-DATA APPROACH

Lateritic and bauxitic paleosols are recognized as having been widespread on a global scale during the mid-Cretaceous warm episode. We evaluated the relationship between the paleogeographic distribution of laterites/bauxites and paleoclimate at the Albian-Cenomanian boundary using the GENESIS v. 2 general circulation model and temperature-precipitation relationships established for “modern” bauxites. Our study suggests that: 1) oft-cited levels of 4x present-day atmospheric carbon dioxide (PAL) and 4x present-day oceanic heat transport (Q) for the middle Cretaceous are insufficient to describe the distribution of the Albian-Cenomanian paleosols; 2) the paleosol distributions can be explained by 10x PAL and 6x Q; and, 3) depending on which bauxite formation criteria are applied, the paleosol distributions can be explained under conditions of 15x PAL and modern Q. Considering the somewhat ill-defined physics of an elevated Q, these results suggest that based on continental records of paleoclimate, a super-heated mid-Cretaceous tropical realm may have existed.

We further evaluated the possibility of super-heated mid-Cretaceous tropics using an oxygen isotopic database for Albian-Cenomanian marine strata (focused on foraminifera). Rather than calculating paleotemperatures from the data, we assumed that one of the paleotemperature profiles from our paleosol-distribution simulations best represents the mid-Cretaceous Earth. We derived mid-Cretaceous seawater oxygen isotope values using the model-derived paleotemperatures from each simulation and found a scattered distribution in our calculated values. However, our reevaluation of age, organism, and diagenetic state of the samples from which the raw data was obtained led to the removal of much of the scatter in the plot, and the creation of a “smooth” paleolatitudinal profile. Our results demonstrate that paleolatitudinal profiles of mid-Cretaceous seawater oxygen isotope values, under conditions of 4x PAL and 4x Q, closely mimic a profile obtained by uniformly subtracting 1.2 per mil from an average modern profile. At 15x PAL the mid-Cretaceous seawater oxygen isotopes profile more closely matches modern low-latitude profiles, and is similar to the “uniform-1.2 per mil-subtraction” profile in the mid to high latitudes.