Early Cretaceous Ocean Dynamics from Clumped Isotope Thermometry

Abstract:

Existing records suggest that Early Cretaceous temperatures were warmer than modern conditions, but were moderate compared to the mid-Cretaceous super-greenhouse. High-latitude marine oxygen isotopes indicate near-freezing polar temperatures, while glendonites and dropstones suggest periodic existence of polar ice. Conventional oxygen isotope paleothermometry is problematic due to its control by both temperature and seawater composition, which is dependent on both global ice volume and more regional hydrologic variations. In order to overcome this limitation, we performed clumped isotope analysis on belemnite rostra of Valanginian age (~140-134 Ma) from localities across a range of paleolatitudes. We find temperatures well above modern values for these sites, but still compatible with sub-freezing poles. Additionally, the oxygen isotopic composition of seawater calculated from these same analyses is remarkably similar to modern values, and differs significantly from modeled Cretaceous seawater. These high oxygen isotope ratios could be the product of isotopic disequilibrium in belemnites, but are more likely to be the result of previously unrecognized basin- or global-scale hydrological factors, or ice sheet growth. Our results therefore argue for a generally warm Valanginian climate but with the potential for significant polar ice.