The mid-Cretaceous greenhouse climate and marine stable oxygen isotope: Insights from general circulation model experiments

The mid-Cretaceous (∼112-89 Ma) was the warmest period in the past 144 my. Understanding the mid-Cretaceous greenhouse climate is essential to the study of climate change in the past and future. Using state-of-the art general circulation models (GCMs), this dissertation investigates 1) mechanisms and consequences of mid-Cretaceous greenhouse climate; and 2) uncertainties of the marine carbonate δ18O_c climatic interpretation. More specifically, Part I explores the effects of vegetation and high atmospheric pCO₂ on mid-Cretaceous climate using a fully coupled ocean-atmosphere GCM with dynamic global vegetation (CCSM3). Results show that temperatures, hydrological cycle, atmospheric and oceanic circulations are sensitive to vegetation and atmospheric pCO₂. Furthermore, Arctic climate variability also varies with atmospheric pCO₂, promoting polar temperature amplification and reorganization of oceanic circulation. While both vegetation and high atmospheric pCO₂ lead to a substantial warming and a reduced meridional thermal gradient, they could not sustain an Arctic climate as warm as the proxy estimates. Therefore, other mechanisms in addition to high atmospheric pCO₂ and vegetation are required to explain the mid-Cretaceous polar warmth.;Part II employs a water isotope-enabled ocean-atmosphere GCM (GENMOM) to determine the responses of seawater δ18O_sw and marine carbonate δ18O_c, to paleogeography, atmospheric pCO₂, sea level and freshwater forcing. Results reveal that mid-Cretaceous surface seawater δ18O_sw is depleted by 1.3‰ SMOW at low-to-mid latitudes and up to 2.0‰ SMOW at high latitudes in comparison to modern values. Thus, conventional assumptions of seawater δ18O_sw may lead to an overestimation of sea-surface temperatures, especially at mid-to-high latitudes. Moreover, model results suggest that the variations of carbonate δ 18O_c preserved in the mid-Cretaceous marine record reflect regional oceanographic rather than global climate changes. As a result, the extremely high temperatures in the high-latitude South Atlantic cannot be used to indicate mid-Cretaceous meridional thermal gradient. We conclude that the mid-Cretaceous was neither a thermal maximum nor an ice age and that the mid-Cretaceous equator-to-pole thermal gradient was likely much higher than reported in previous studies.