GLOBAL HYDRLOGIC PERSPECTIVES ON THE MID-CRETACEOUS GREENHOUSE CLIMATE (APTIAN-ALBIAN)

Abstract

This dissertation examines the mid-Cretaceous greenhouse climate in the Aptian-Albian through the perspective of the global hydrologic cycle. Stable isotopic compositions of pedogenic and exposure surface carbonates presented herein provides low latitude constraints on the oxygen isotopic composition meteoric water during the Cretaceous. Petrographic and isotopic evidence from the Tlayua Formation of Mexico (18.5°N paleolatitude) establishes a shallow-water coastal environment that experienced supratidal conditions in which early meteoric diagenesis occurred. Trends in &delta18O vs. &delta13C space define meteoric calcite lines that are characterized by average &delta18O values of -7.78 / VPDB (Vienna Pee Dee Belemnite), and are used to calculate meteoric water compositions of -5.46 ± 0.56 / VSMOW (Vienna Standard Mean Ocean Water). Positive linear covariant trends are defined and used to estimate evaporative losses of vadose groundwater ranging from 8% to 12%. Meteoric sphaerosiderite lines from coastal wetland soils of the Caballos Formation in Colombia (2°N paleolatitude) have an average oxygen isotopic composition of -4.41 ± 0.37 / VPDB, and are used to calculate meteoric water compositions of -4.56 ± 0.38 / VSMOW. This new data is incorporated into a revised mass balance model that simulates components of the hydrologic cycle (precipitation , evaporation, oxygen isotopic composition of seawater, relative humidity and continental feedback). The model simulates the oxygen isotopic composition of water vapor and resulting precipitation. Results suggest precipitation flux and rates were greater in the mid-Cretaceous than modern precipitation flux and rates. Global average precipitation rates for the mid-Cretaceous range between 371 mm/year to 1154 mm/year greater than modern precipitation rates. Sensitivity testing of the model indicates that temperature, air mass origin and pathways, the oxygen isotopic composition of seawater in the low latitudes, and relative humidity are variables that have the potential to greatly affect the oxygen isotopic composition of precipitation, and need greater constraints. These results can be used to further constrain and improve more complex general circulation models to more accurately model the hydrologic cycle during the Aptian-Albian and for future greenhouse climates.