Paleogene deep water circulation in the Atlantic sector of the Southern Ocean revealed from neodymium isotopes

Deep water circulation patterns for the middle Eocene to middle Miocene were reconstructed by using epsilonNd values of fossil fish teeth recovered from three deep sea drill cores in the Atlantic sector of the Southern Ocean, ODP Sites 689, 690, and 1090. epsilonNd values of Paleogene water masses were used as end-members to evaluate water mass mixing. Secular variability observed at the resolution of these records (50-300 kyr) is primarily attributed to changes in the relative contributions of different water mass end-members in response to tectonic and climatic conditions.;At ∼41 Ma all three of the sites demonstrate a shift toward radiogenic epsilonNd values that appears to be a regionally synchronous event in response to the initial opening of Drake Passage, which introduced radiogenic Pacific seawater into the Atlantic sector of the Southern Ocean. Subsequent shifts toward more radiogenic epsilonNd values at Site 689 (37 Ma) and 1090 (28.5 Ma and 22 Ma) reflect increased throughflow of Pacific seawater into the Atlantic sector, resulting from the progressive widening and deepening of Drake Passage. An intermediate depth connection between the Pacific and Atlantic Oceans in the late Eocene is inferred from the Nd isotope data. This conclusion is corroborated by evidence for increases in phytoplankton production in the Atlantic sector during the late Eocene.;The vertical water mass distribution in the Southern Ocean was examined with the Nd isotopic results from neighboring Sites 689 and 690. During the late middle Eocene Site 690 was bathed by seawater with less radiogenic epsilon Nd values (-9.3) compared to the water mass that bathed Site 689 (epsilon Nd = -8.5). Lower epsilonNd values at Site 690 coincide with a previously recognized delta18O inversion between the two sites, suggesting the incursion of a warm water mass with nonradiogneic epsilon Nd values. The Nd isotope data from Maud Rise are interpreted as evidence for the export of Warm Saline Deep Water (WSDW) from the Tethys Sea. Export of nonradiogenic seawater from the North Atlantic, the only other region characterized by nonradiogenic epsilonNd values, is not supported by epsilon Nd gradients between the North Atlantic and Southern Ocean.