18O/ 16O and D/ H ratios of pedogenic kaolinite in a North American Cenomanian laterite: Paleoclimatic implications

Abstract

Kaolinite, gibbsite and quartz are the dominant minerals in samples collected from two outcrops of a Cenomanian (∼95 Ma) laterite in southwestern Minnesota. A combination of measured yields and isotope ratios permitted mass balance calculations of the δ D and δ 18O values of the kaolinite in these samples. These calculations yielded kaolinite δ D values of about −73‰ and δ 18O values of about +18.7‰. The δ D and δ 18O values appear to preserve information on the ancient weathering system. If formed in hydrogen and oxygen isotope equilibrium with water characterized by the global meteoric water line (GMWL), the kaolinite δ D and δ 18O values indicate a crystallization temperature of 22 (±5) °C. A nominal paleotemperature of 22 °C implies a δ 18O value for the corresponding water of −6.3‰. The combination of temperature and meteoric water δ 18O values is consistent with relatively intense rainfall at that mid-paleolatitude location (∼40°N) on the eastern shore of the North American Western Interior Seaway. The inferred Cenomanian paleosol temperature of ∼22 °C is in general accord with published mid-Cretaceous continental mean annual temperatures (MAT) estimated from leaf margin analyses of fossil plants. When compared with results from a published GCM-based Cenomanian climate simulation which specifies a latitudinal sea surface temperature (SST) gradient that was either near modern or smaller-than-modern, the kaolinite paleotemperature of 22 °C is closer to the GCM-predicted MAT for a smaller equator-to-pole temperature difference in the mid-Cretaceous. Moreover, the warm, kaolinite-derived, mid-paleolatitude temperature of 22 °C is associated with proxy estimates of high concentrations of atmospheric CO 2 in the Cenomanian. The overall similarity of proxy and model results suggests that the general features of Cenomanian continental climate in that North American locale are probably being revealed.