A reinterpretation of Mid‐Cretaceous shallow marine temperatures through model‐data comparison

Mid‐Cretaceous δ 18 O paleotemperatures are compared with temperatures predicted by an ocean general circulation model. The δ 18 O paleotemperatures, calculated assuming a δ w of −1.0‰ (SMOW), are 5.6°C lower at low latitudes and 2.4°C higher at middle and high latitudes than the model‐predicted, mixed‐layer temperatures. The model‐data comparison is improved after accounting for the spatial variability of δ w and after considering the paleohabitats of the marine organisms from which δ 18 O values were measured. Paleotemperatures adjusted using the Broecker [1989] δ w ‐ S relationship and the model‐predicted salinity are 3.4°C higher at low‐latitude sites and 1.9°C lower at higher‐latitude sites. Moreover, seven of nine low‐latitude paleotemperatures intersect the model‐predicted temperature within the upper 100 m of the water column, a depth consistent with the interpreted paleohabitat of the analyzed organisms. However, adjusted paleotemperatures for three of five Southern Hemisphere high‐latitude locations are at least 9°C higher than the model‐predicted temperatures for these locations. The model‐data comparison suggests that mid‐Cretaceous δ 18 O values are compatible with sea surface temperatures as high or higher than those simulated by an ocean general circulation model of the mid‐Cretaceous with 4 × present‐day atmospheric pCO 2 .