SUBTROPICAL TURONIAN STABLE ISOTOPE RATIOS FROM “GLASSY” FORAMINIFERA : NO EVIDENCE FOR GREENHOUSE ICE SHEETS

Sediments in southern coastal Tanzania yield exceptionally well preserved microfossils and have been targeted in recent years by the Tanzania Drilling Project (TDP), an informal collaboration of researchers studying Cretaceous-Neogene paleoclimate. Drilling during the 2007-2009 field seasons recovered a stratigraphically complete Turonian sequence of claystones and clayey siltsones, much of which yields diverse benthic and planktonic foraminiferal assemblages exhibiting `glassy` preservation (no apparent test recrystallization at the submicron scale). During the Turonian the drill sites were located at about 30 degrees south paleolatitude and the sediments were deposited at an upper bathyal paleodepth. The extraordinarily good microfossil preservation enables reliable estimates of surface and bottom water temperatures spanning a 2.5 m.y. period from the earliest Turonian Whiteinella archaeocretacea Zone through the late middle Turonian Helvetoglobotruncana helvetica Zone. This provides an opportunity to test the greenhouse ice sheet hypothesis that has been proposed based on sequence stratigraphic interpretations from the lower Turonian of the New Jersey Coastal Plain and in the Western Interior.

At least two benthic and three planktic species were analyzed throughout a 120 m sequence at TDP Site 22 and a 70 m sequence at TDP Site 31. Results show surprisingly coherent trends with high stability in both d¹⁸O and d¹³C values. Using standard assumptions for the oxygen isotopic composition of the Cretaceous ocean, we calculate that sea surface temperatures averaged 32-34C and the bottom water averaged 18-21C. Consistency in benthic and planktic d18O and d13C values and lack of evidence for changes in sea level on the East African passive margin indicate that the intermittent polar ice sheets that have been proposed for this time period either did not exist or were too small to be detectable in foraminiferal oxygen isotope records.