2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 9
Presentation Time: 3:30 PM

Turonian Subtropical Paleotemperatures from “Glassy” Foraminifera in East Africa and Their Implications for Modeling Greenhouse Climates


MACLEOD, Kenneth G.1, HUBER, Brian2, BICE, Karen L.3 and JIMÉNEZ-BERROCOSO, Álvaro1, (1)Geological Sciences, University of Missouri, Columbia, MO 65211, (2)Smithsonian Institution, P.O. Box 37012 NHB MRC 121, Washington, DC 20013, (3)Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, macleodk@missouri.edu

Specimens of “glassy” foraminifera from southern coastal Tanzania indicate sea surface temperatures up to 34°C for subtropical shelfal waters (assuming a seawater δ18O value of -1‰SMOW) and a consistent planktic temperature gradient of 2-3°C through most of the Turonian. Using “glassy” preservation (preservation where there is no apparent test recrystallization at the submicron scale) as a criterion for evaluating δ18O results has greatly improved confidence in absolute paleotemperature estimates. However, data remain sparse for the Cretaceous. Further, existing data come largely from short intervals with unusual lithologic and faunal characteristics. These unusual characteristics raise the possibility that the results reflect temperatures or seawater compositions that were not typical for the time. Our new Tanzanian samples address these concerns. They contain a diverse assemblage of “glassy” planktic and benthic foraminifera preserved in otherwise unremarkable outer shelf claystones and silty claystones, and they span most of the Turonian (Whitinella archaeocretacea through Helvetoglobotruncana helvetica Zones). Additional samples from the Cenomanian, Coniacian, and Campanian are being processed.

Our calculated paleotemperatures are several degrees lower than some Cretaceous tropical estimates but are still ~6°C higher than summer seawater temperatures in comparable modern settings. Reproducing such temperatures in global climate models using Cretaceous boundary conditions requires atmospheric CO2 concentrations of 2500 ppm or more (almost 10x pre-industrial CO2 concentrations), a 10- to 20-fold increase in atmospheric methane combinations, or some combination of the two. These CO2 levels are higher than most estimates for the Cretaceous atmosphere supporting the conclusion that sensitivity to greenhouse gas forcing, at least on long time scales, is too low in the current climate models.