The Mid-Cretaceous Greenhouse Climate and Marine Stable Oxygen Isotope: Insights from General Circulation Model Experiments.

Abstract

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 oxygen isotope 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 oxygen isotope and marine carbonate oxygen isotope to paleogeography, atmospheric pCO2, sea level and freshwater forcing. Results reveal that mid-Cretaceous surface seawater oxygen isotope 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 oxygen isotope may lead to an overestimation of sea-surface temperatures, especially at mid-to-high latitudes. Moreover, model results suggest that the variations of carbonate oxygen isootpe 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.