HR: 0800h
AN: PP11A-0236    [Abstracts]
TI: ε_Nd Across OAE 2 in the Late Cretaceous at Demerara Rise (ODP Leg 207) and Blake Nose Plateau (ODP Leg 171)
AU: * Bourbon, E
EM: elo05@ufl.edu
AF: Dept. of Geological Sciences, University of Florida, Gainesville, FL 32611, United States
AU: Matin, E E
EM: eemartin@ufl.edu
AF: Dept. of Geological Sciences, University of Florida, Gainesville, FL 32611, United States
AU: MacLeod, K G
EM: MacLeodK@missouri.edu
AF: Dept. of Geological Sciences, University of Missouri-Columbia, Columbia, MO 65211, United States
AU: Jiménez Berrocoso, A
EM: JimenezA@missouri.edu
AF: Dept. of Geological Sciences, University of Missouri-Columbia, Columbia, MO 65211, United States
AB: We analyzed Nd isotopes on Late Cretaceous fossil fish teeth and debris and extractions from Fe-Mn oxide coatings from a total of six sites along two paleodepth transects in the western North Atlantic: Demerara Rise Sites 1258, 1260 and 1261, and Blake Nose Sites 1049, 1050 and 1052. Both transects span a range of more than 1000 m and the sites were at middle to upper bathyal paleodepths during the Late Cretaceous. These samples were analyzed to test and better define vertical and horizontal variation in North Atlantic ε_Nd values with particular focus on low background values (-16 to -14) punctuated by a dramatic 8 unit positive excursion across Oceanic Anoxic Event 2 (OAE2) previously observed at Site 1258. Extractions of Nd from dispersed Fe-Mn oxide coatings on black shales and carbonate marls above, within and below the excursion yield isotopic ratios that are consistent with data from fish teeth, indicating that the oxide coatings are robust archives for Nd isotopes in a range of lithologies.
Despite the range of depths sampled, all three Demerara Rise sites record similar nonradiogenic background ε_Nd values and a similar magnitude excursion during OAE2, indicating that the water column was well mixed over these depths at this location. In contrast, there appears to be a depth gradient in ε_Nd values in the older samples from Blake Nose that disappears by Campanian-Maastrichtian times. During the Albian-early Cenomanian, the shallowest site (1052) records the least radiogenic ε_Nd values (-5 to -4) observed in our study, whereas the intermediate depth site (1050) records values of -6 to -5 and the deepest site (1049) records values of -6 to -7. All of these values are very radiogenic compared to Cenozoic data from Fe-Mn crusts from the North Atlantic, and seem to imply a component of Pacific water was incorporated into Albian-Cenomanian bottom waters on Blake Nose. Much of the Cenomanian-Campanian is missing in various Blake Nose sites, and we have yet to run samples from the portion of OAE2 recovered in Hole 1050C. However, after this event, ε_Nd values on Blake Nose are distinctly lower and similar at all three sites from the Late Campanian into the Eocene. Further, the ε_Nd values observed (-9 to -8) are consistent with Cenozoic data from Fe-Mn crusts located in the North Atlantic. Together, these data suggest a transition from a stratified water column dominated by Pacific waters at intermediate depths in the Albian- Cenomanian to a well mixed water column with a strong component of North Atlantic water following OAE2 in the Blake Nose region.
DE: 1040 Radiogenic isotope geochemistry
DE: 4283 Water masses
DE: 4532 General circulation (1218, 1222)
DE: 4900 PALEOCEANOGRAPHY (0473, 3344)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting