Equator To Pole in the Cretaceous: A Comparison of Clumped Isotope Data and CESM Model Runs

Abstract:

An outstanding issue in the field of paleoclimate is the inability of models to reproduce the shallower equator-to-pole temperature gradients suggested by proxies for past greenhouse periods. Here, we focus on the Late Cretaceous (Maastrichtian, 72-66 Ma), when estimated CO₂ levels were ~400-1000ppm. New clumped isotope temperature data from more than 10 sites spanning 65°S to 48°N are used to reconstruct the Maastrichtian equator-to-pole temperature gradient. This data is compared to CESM model simulations of the Maastrichtian, run using relevant paleogeography and atmospheric CO₂ levels of 560 and 1120 ppm. Due to a reduced “proxy toolkit” this far in the past, much of our knowledge of Cretaceous climate comes from the oxygen isotope paleothermometer, which incorporates an assumption about the oxygen isotopic composition of seawater (δ¹⁸O_sw), a quantity often related to salinity. With the clumped isotope paleothermometer, we can directly calculate δ¹⁸O_sw. This will be used to test commonly applied assumptions about water composition, and will be compared to modeled ocean salinity. We also discuss basin-to-basin differences and their implications for paleo-circulation patterns.