HR: 0800h
AN: PP51D-1352    [Abstracts]
TI: Evolution of North Atlantic Thermohaline Circulation: From the Greenhouse to the Icehouse
AU: Via, R K
EM: rvia@ocean.tamu.edu
AF: Department of Oceanography, Texas A&M University, College Station, TX 77843-3146 United States
AU: * Thomas, D J
EM: dthomas@ocean.tamu.edu
AF: Department of Oceanography, Texas A&M University, College Station, TX 77843-3146 United States
AB: A growing body of data indicates that the pattern of oceanic thermohaline circulation has varied through the geologic past. At any given time, tectonic and climatic boundary conditions will tend to dictate the operating pattern of thermohaline circulation. For example, in the Early Cenozoic prior to the opening of the northern North Atlantic basins, the Southern Ocean was the dominant source region for deep water formation. As global climate cooled and tectonic gateways changed through the Cenozoic, this mode gradually evolved into the Late Cenozoic mode, characterized by Southern Ocean and North Atlantic sources of deep water formation. To better understand how the evolution of Cenozoic thermohaline circulation related to changes in global climate and ocean basin configuration, we generated Nd isotope records from a depth transect of Ocean Drilling Program sites in the southeastern Atlantic to track deep water mass composition through time. We used fossil fish debris from ODP sites 1262-1264 (Leg 208), which span present-day water depths of 2500 m to 4750 m, to reconstruct the isotopic signature of deep waters over the past 55 Ma. The data indicate an initial transition from relatively non-radiogenic values ($\sim$-10 $\epsilon$$_{Nd}$ units) at 55 Ma to more radiogenic values ($\sim$-8.5) at 32 Ma. From 32 Ma to 3.85 Ma, the Nd signal becomes more non-radiogenic, approximately -12.3 at the top of our record. Comparison of our data with Nd isotopic records derived from a North Atlantic Fe-Mn crust show similar non-radiogenic values ($\sim$-10.5) in the 50 - 32 Ma interval and a trend toward more non-radiogenic values beginning at approximately 20 Ma. The data likely reflect an overall shift from a Southern Ocean deep water source to the ultimate incursion of deep waters from the North Atlantic. The non-radiogenic values at the base of the record reflect a Southern Ocean source of deep water. The shift toward more radiogenic values indicates an increased contribution of Pacific waters to the Southern Ocean source as the Drake Passage began to open to deep flow at $\sim$33 Ma. Though the subsequent trend toward more non-radiogenic Nd isotope values does not directly correlate to any major tectonic event, it is approximately coincident with the increase of benthic foraminiferal $\delta$$^{18}$O values, based on comparison with the Zachos et al (2001) global compilation. Thus the build-up of continental ice on Antarctica and global cooling may have altered the character of Southern Ocean deep waters during the early Oligocene.
DE: 4267 Paleoceanography
DE: 3099 General or miscellaneous
DE: 1045 Low-temperature geochemistry
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1635 Oceans (4203)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting