HR: 1340h
AN: PP43B-0684    [Abstracts]
TI: Using Oxygen and Neodymium Isotopes to Track Ocean Circulation in the North Atlantic During the Late Cretaceous
AU: * Blair, S W
EM: swblair@ufl.edu
AF: Dept. of Geological Sciences, University of Florida, Gainesville, FL 36211 United States
AU: Martin, E E
EM: emartin@geology.ufl.edu
AF: Dept. of Geological Sciences, University of Florida, Gainesville, FL 36211 United States
AU: MacLeod, K G
EM: MacLeodK@missouri.edu
AF: Dept. of Geological Sciences, University of Missouri, Columbia, Columbia, MO 65211 United States
AB: We have undertaken parallel analyses of δ¹⁸O and ε_Nd in Late Cretaceous fish debris in an effort to determine the relative importance of tectonics and regional ocean circulation compared to global climate in controlling the nature of deposition on Demerara Rise. Demerara Rise is a submarine plateau along the NE margin of South America and was one of the last points of contact between Africa and South America as the Atlantic opened. Paleotemperatures estimated from the δ¹⁸O values suggest thermocline water temperatures declined by >10°C from the early Cenomanian to the Maastrichtian. In contrast, initial ε_Nd(T) values show relatively little variation and are surprisingly nonradiogenic across this interval. Values for mid-Cenomanian fossil fish debris are ~-13.7, those in the lower Turonian are ~-16.7, and those from the Maastrichtian are ~-16. These values are less radiogenic than any reported from Fe-Mn crusts and fish teeth throughout the Cretaceous to Neogene. For comparison, Late Cretaceous values from the Pacific (Site 886) are ~-4.5. The Atlantic data suggest that North Atlantic deep waters may have been isolated from other ocean basins into the Maastrichtian. Alternatively, the nonradiogenic values may reflect local weathering inputs or diagenesis. Assuming the Nd isotopic data represent bottom water values, the continuity of the Nd data do not support the idea that lithologic and temperature shifts were associated with a change in deep water circulation and ventilation. In this case, we attribute the apparent temperature decrease from the early Cenomanian into the Turonian to subsidence from shelfal to bathyal depths post-rifting. The continued temperature decline into the Maastrichtian parallels global climate trends. With additional Nd isotopic analyses we hope to document end member values for the Late Cretaceous North Atlantic and track the timing of the deep connections between the North Atlantic and other ocean basins.
DE: 1040 Radiogenic isotope geochemistry
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 4900 PALEOCEANOGRAPHY (0473, 3344)
DE: 4962 Thermohaline
DE: 9610 Cretaceous
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005