PRELUDE TO THE TURONIAN THERMAL MAXIMUM: CENOMANIAN FORAMINIFERAL OXYGEN ISOTOPE TEMPERATURE RECORDS FROM SOUTHERN HIGH LATITUDES

The evolution of Cenomanian marine temperatures leading up to the Turonian Thermal Maximum has not been adequately characterized because reliable δ¹⁸O data are few especially at high latitudes. Ocean drilling at IODP Sites U1513, U1514, and U1516 in the Mentelle Basin (SE Indian Ocean) recovered a nearly complete late Albian-Cenomanian sediment record deposited at 60°S paleolatitude. Analyses of samples from these cores will improve knowledge of the pre-Turonian austral marine paleotemperature history. Mid- to upper Cenomanian cores from all three sites yield moderately to well-preserved foraminifera with planktic:benthic ratios varying from 70-90%. Benthic foraminifera within this time interval include a relatively diverse assemblage of calcareous and agglutinated species that indicate deposition at bathyal paleodepths. Planktonic assemblages in this interval are small in size and low in diversity, and species used to identify tropical biozones are absent from most samples. Down-core from the lower Cenomanian to upper Albian, foraminiferal assemblages are increasingly dominated by agglutinated species and planktic:benthic ratios vary between 0-50%. Using standard assumptions for the isotopic composition of Cretaceous seawater, preliminary oxygen isotope records from mid- late Cenomanian calcareous benthic taxa indicate warm high latitude temperatures with bottom waters mostly ranging between 14-16°C and surface waters mostly between 19-22°C. Although these oxygen isotope temperature estimates indicate hot greenhouse conditions at southern high latitudes during the Cenomanian, the planktic δ¹⁸O values are still more than 1‰ higher (>4°C cooler) than planktic values obtained from Turonian samples from DSDP Site 258, a site that was drilled previously at the IODP Site U1513 location. Additional foraminiferal stable isotope analyses of mid-Cretaceous foraminifera from these sites will provide new insight and better resolution on the paleoclimatic and paleoceanographic conditions prior to widespread organic carbon burial during Oceanic Anoxic Event 2 and the subsequent Turonian Thermal Maximum.