HR: 0800h
AN: PP31C-0539    [Abstracts]
TI: A Cretaceous Benthic Foraminiferal Stable Isotope Compilation
AU: * Friedrich, O
EM: ofriedrich@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093, United States
AU: Norris, R D
EM: rnorris@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093, United States
AU: Erbacher, J
EM: erbacher@bgr.de
AF: Bundesanstalt fuer Geowissenschaften und Rohstoffe, Stilleweg 2, Hannover, 30655, Germany
AB: We produced new stable isotope data sets of Cenomanian to Santonian benthic foraminifera from the western equatorial Atlantic (ODP Leg 207) and from the tropical Pacific Ocean (DSDP Sites 305 and 463). Together with literature data our results are compiled into a global isotope compilation, resulting in a continuous benthic d18O record from 115-65 Ma. This compilation shows four main intervals: (1) increasing temperatures before 97 Ma and (2) a subsequent super-greenhouse which are both paralleled by increasing d13C values, (3) a long-lasting cooling and decrease in carbon isotopes (90-78 Ma), and (4) globally similar d13C and d18O values after 78 Ma. Increasing sea-surface temperatures sometimes exceeding 35°C are well-known for Intervals 1 and 2. But our compilation shows, that deep-ocean temperatures were significantly warmer than today, especially in the proto- North Atlantic (20-28°C). These high temperatures are explained by a lack of cold bottom-water formation, the restricted nature of the North Atlantic, and the formation of warm saline bottom waters that sporadically were formed within epicontinental seas. The parallel positive trend in d13C is believed to reflect massive storage of Corg during Cretaceous black shale formation. Interestingly, however, d13C values of the tropical Atlantic show a similar trend but more negative values. We propose that this reflects a combination of extensive remineralization of 12C and a long residence time due to the sporadic formation of warm and saline waters. During the following interval 3, benthic d18O values of all ocean basins show similar values. This trend is interpreted to be the result of the beginning opening of the Equatorial Atlantic Gateway. This deepening and a parallel reorganization of the oceanic circulation with longitudinal water-mass and heat exchange may have favoured the observed cooling trend of interval 3. This explanation is supported by the global decrease in d13C, proposed to reflect a better connection of the former restricted North Atlantic that allows the oxidization of the organic-rich sediments formed in this basin. In contrast to the former intervals, the last 13 Ma of the Cretaceous are, on a global scale, characterized by similar values for both, oxygen and carbon isotopes. This is proposed to indicate a full connection between all ocean basins.
DE: 4806 Carbon cycling (0428)
DE: 4870 Stable isotopes (0454, 1041)
DE: 4900 PALEOCEANOGRAPHY (0473, 3344)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting