HR: 11:20h
AN: PP52B-05    [Abstracts]
TI: Early Oligocene Onset of Deep-Water Production in the North Atlantic
AU: * Thomas, D J
EM: dthomas@ocean.tamu.edu
AF: Department of Oceanography Texas A&M University, O&M Building, TAMU 3146, College Station, TX 77843-3146 United States
AU: Via, R K
EM: rvia@neo.tamu.edu
AF: Department of Oceanography Texas A&M University, O&M Building, TAMU 3146, College Station, TX 77843-3146 United States
AB: The present mode of meridional overturning circulation is characterized by deep-water formation in both the North Atlantic and the Southern Ocean. However, a growing body of evidence indicates that the Southern Ocean was the dominant region of deep-water formation during the extreme warmth of the early Cenozoic. Establishment of a bipolar mode of deep-water circulation occurred during the tectonic evolution of the Atlantic basins and overall cooling of the Cenozoic. However, neither the timing nor the cause of this transition is known. Formation of the Panama Isthmus is assumed to represent the final step in this transition, yet it is unclear when deep waters first began forming in the North Atlantic prior to the development of true NADW. To reconstruct the evolution of Atlantic deep-sea circulation from the early Cenozoic mode to the bipolar mode, we generated neodymium isotope records from fossil fish teeth and bones. These records come from a depth transect of three Walvis Ridge sites cored during Ocean Drilling Program Leg 208. The depth transect, spanning more than 2200m water depth, enables us to resolve the vertical water mass structure through time to reconstruct changes in the relative deep-water contributions from the Southern Ocean and North Atlantic. Nd isotope data from the three sites indicates that a single water mass encompassed the entire depth range from ~55 to at least 17 million years ago. Published data from ODP Site 689 (2080m present water depth; ~1500m paleodepth in the Paleogene) in the Weddell Sea indicate similar values and the same trend with the Walvis Ridge data from ~46 to 33 Ma, suggesting a single Southern Ocean intermediate/deep-water mass until ~33 Ma. Approximately 33 Ma, Weddell Sea ε_Nd(t) values diverge from Walvis Ridge values, and remain consistently more radiogenic for the duration of the Site 689 record. Walvis Ridge ε_Nd(t) values become increasingly unradiogenic to the top of the investigated section. The most plausible explanation for decrease in Walvis Ridge ε_Nd(t) values as well as the divergence in Nd isotopic composition from Weddell Sea values is the onset of and gradual increase in a North Atlantic deep-water contribution to the waters in the southeastern Atlantic. Thus, we suggest that significant deep-water production began in the Atlantic 33Ma.
DE: 1040 Radiogenic isotope geochemistry
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 1635 Oceans (1616, 3305, 4215, 4513)
DE: 4294 Instruments and techniques
DE: 4962 Thermohaline
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005