HR: 16:00h
AN: PP54A-01    [Abstracts]
TI: Evidence for an Open Drake Passage in the Late Middle Eocene
AU: * Scher, H D
EM: hscher@ufl.edu
AF: University of Florida, Department of Geological Sciences, 241 Williamson Hall, P.O. Box 112120, Gainesville, FL 32611-2120 United States
AU: Martin, E E
EM: emartin@geology.ufl.edu
AF: University of Florida, Department of Geological Sciences, 241 Williamson Hall, P.O. Box 112120, Gainesville, FL 32611-2120 United States
AB: Circulation of the Antarctic Circumpolar Current (ACC) appears to require an unimpeded circumpolar path, thus arguments for initiation of the ACC hinge on the timing of tectonic gateway development in the Southern Ocean. However, timing of the onset of seawater exchange between the Pacific and Atlantic oceans through the Drake Passage is widely debated, with estimates ranging from the late Eocene to the early Miocene. Hiatuses and productivity increases in South Atlantic sediment records imply that ocean currents accelerated during the Eocene and Oligocene, though there is not yet independent and unequivocal evidence for Pacific seawater entering the Atlantic Ocean during the Paleogene. Nd isotopes in fossil fish teeth reflect water mass mixing and are unaffected by changes in temperature, salinity, nutrients, and productivity. We present the first Nd isotope record from the Atlantic sector of the Southern Ocean that covers the entire range of estimates for the opening of the Drake Passage; from ODP site 1090. The record indicates that a water mass with a Nd isotopic composition typical of Pacific seawater appeared in the Atlantic sector at 39 Ma, as illustrated by a rapid transition to radiogenic $\epsilon$$_N$$_d$ values ($\epsilon$$_N$$_d$ = -5.8). Pacific seawater was the only volumetrically significant water mass with such a radiogenic Nd signature during the Paleogene. A second transition to radiogenic $\epsilon$$_N$$_d$ values occurs at 28.5 Ma. Pronounced peaks in productivity at site 1090 coincide with the $\epsilon$$_N$$_d$ transition at 39 Ma. The data support the opening of the Drake Passage to shallow depths at 39 Ma. Moreover, the association of productivity peaks with this event implies that a shallow opening may have enabled a precursor to the modern ACC. The data also support establishment of a deep-water connection at 28.5 Ma. The interpretations imply that tectonically induced changes in ocean circulation preceded large shifts in global climate during the Paleogene. $\epsilon$$_N$$_d$ values during the late Oligocene to early Miocene covary with benthic $\delta$$^1$$^3$C values at this site, confirming the capability of Nd isotopes as a tracer of past changes in ocean circulation.
DE: 4808 Chemical tracers
DE: 3344 Paleoclimatology
DE: 4267 Paleoceanography
DE: 4283 Water masses
DE: 4512 Currents
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting