Revisiting the equable climate problem during the Late Cretaceous greenhouse using paleosol carbonate clumped isotope temperatures from the Campanian of the Western Interior Basin, USA

摘要

Greenhouse climates such as the Late Cretaceous period provide important reference frames for understanding modern anthropogenic climate change. Upper Cretaceous terrestrial climate proxies have been interpreted as evidence for "equable" climates with reduced seasonal variations in temperature. However, climate models have largely failed to reproduce these reconstructions unless parameters such as atmospheric CO2 concentrations are set to unreasonable values. To help resolve such model-proxy disagreements, we reconstruct mean annual range in temperature (MART) for the Campanian (similar to 75 Ma) Kaiparowits (south-central Utah) and Two Medicine (northwest Montana) Formations using warmest mean monthly temperature reconstructions from the clumped isotope composition of paleosol carbonate nodules, and reconstructions of local mean annual air temperatures from other methods. An evaluation of the applicability of bulk elemental soil geochemistry temperature proxies in these deposits supports the use of previous leaf physiognomy-based estimates of mean annual temperature for our MART reconstructions. We test the validity of several common assumptions made in reconstructing MART in two novel ways. First, MART is commonly calculated as twice the difference between local mean annual air temperature and warmest mean monthly temperature, and we validate this method by estimating modern MART for a range of environments using climate reanalysis data. Second, we constrain the effect of radiative soil heating on our soil carbonate temperature estimates by showing that for most environments likely to be preserved in the geologic record, summer soil temperatures are 3 degrees C higher than air temperatures. Our findings suggest that warmest mean monthly temperatures were 30 to 35 +/- 4 degrees C at the two study sites, and that MART was 21 to 29 degrees C for the Kaiparowits Formation, and 21 to 27 degrees C for the Two Medicine Formation. Mid-latitude Late Cretaceous MARTs w