Quantifying Changes in Hydrologic Cycle Fluxes over the Americas During the Mid-Cretaceous (Albian) Greenhouse World

University Of Iowa, Iowa City IA

Investigators

Abstract

The mid Cretaceous (Albian) is recognized as a greenhouse world under which atmospheric carbon dioxide concentrations, mean annual global temperature, and global precipitation rates were all much higher than present day. Geologic proxy data as well as general circulation models (GCM's) have been utilized in attempts to quantify climate variables (e.g. temperature and precipitation rates). We have utilized the oxygen isotopic composition (18-O) of siderite in spherical nodules (sphaerosiderites found in wetland paleosols) and an isotope mass balance model to quantify changes in precipitation rates along the coastal plains of the Cretaceous Western Interior Basin (KWIB) of North America (Kansas to Alaska). Our calculations suggest that at mid-high latitude precipitation rates greatly exceeded modern rates (290-390% greater in mid latitudes and 450% greater at high latitudes), that the subtropical dry-belt region had a greater precipitation deficit (i.e. was drier), and that the precipitation rates in the tropical-equatorial regions did not differ significantly from the modern. While our precipitation rates at mid latitudes and the sub-tropical dry belts precipitation deficit estimates are much greater than those estimated by GCM's, our tropical-equatorial region precipitation rates are much lower than GCM's estimates. However, empirical data from the tropics and sub-tropics that are needed to further constrain our model are lacking. This project will expand the northern hemisphere paleolatitudinal 18-O proxy coverage for the mid-Cretaceous (Albian-Cenomanian) to cover the mid-Cretaceous Americas of the northern hemisphere. The expanded latitudinal coverage will allow 1) refinements of isotope mass-balance models and constrain precipitation-evaporation fluxes for the sub-tropical to equatorial regions; 2) minimize uncertainties on modeling the hydrologic cycle in the northern hemisphere; and 3) better quantification of vapor sources and moisture recycling for the KWIB during the Albian. Paleoprecipitation 18-O compositions in the dry-belt region (15 to 30 N paleolatitude) will be determined by sampling and analyzing calcretes in siliciclastic and carbonate coastal plain deposits, and limestones from isolated carbonate platforms in Texas and Mexico, and freshwater lacustrine carbonates in central and southern Mexico. Equatorial (~10 S to 10 N paleolatitude) paleoprecipitation 18-O values will be determined by sampling and analyzing sphaerosiderites from nonmarine and coastal plain deposits in Colombia.