Circulation in the Southern Ocean during the Paleogene inferred from neodymium isotopes

Long-term records of neodymium (Nd) isotopes from sedimentary archives can be influenced by both changes, in water mass mixing and continental weathering. Results of Nd isotopic analyses of fossil fish teeth from ODP Site 689 (Maud Rise, Southern Ocean) provide a long, continuous, high-resolution marine sediment Nd isotope record (expressed in epsilon(Nd) units). Correlation of down core secular variations between the epsilon(Nd) record, delta(13)C values from benthic foraminifera, and clay mineral assemblages demonstrates that long-term variability of Nd isotope ratios reflect changes in ocean circulation. and that only minor fluctuations in epsilon(Nd) values are associated with changes in continental weathering on Antarctica. Nonradiogenic epsilon(Nd) values at Site 689 during the middle Eocene require the contribution of an end member with epsilon(Nd)<-9.5. Southern Ocean deep water may have been too radiogenic in the middle Eocene (epsilon(Nd)=-8.5), though this end member may not be fully characterized. A possible source of deep water outside of the Southern Ocean in the middle Eocene is the Tethys Sea (epsilon(Nd)=-9.3 to -9.8). The presence of Warm Saline Deep Water (WSDW) on Maud Rise is consistent with the Nd isotope. results. The onset of more radiogenic epsilon(Nd) values at similar to40.8 Ma coincides with other changes at Site 689 which are consistent with a switch from a warm bottom water mass in the middle Eocene to a colder bottom water mass in the late middle Eocene. A rapid shift to radiogenic epsilon(Nd) values beginning at 37 Ma is best explained by the opening of Drake Passage. The shift coincides with increases in phytoplankton production throughout the Atlantic sector of the Southern Ocean. that document the development of upwelling cells presumably related to more effective latitudinal circulation. After the Eocene/Oligocene boundary when large-scale ice sheets developed on Antarctica, Southern Ocean sourced water masses, such as Antarctic Intermediate Water (AAIW) and Antarctic Bottom Water (AABW), had a greater influence on the hydrography of the study area. An early Oligocene trend to nonradiogenic compositions resulted in similar values to the modern epsilonNd values of these water masses. The modem epsilon(Nd) values of AAIW and AABW reflect a significant contribution of North Atlantic Deep Water (NADW), thus decreasing epsilon(Nd) values in the early Oligocene may have resulted from the export of Northern Component Water (NCW, similar to modem NADW). During the late Oligocene and early Miocene, the long-term trends of the record follow benthie delta(13)C values. Variability in the Nd isotope record most likely reflects fluctuations in ocean circulation arising from changes in the relative contributions of different end member water masses to the Southern Ocean. An interval where epsilon(Nd) values and delta(13)C values are not correlated may reflect the influence of a short-lived weathering event on the epsilon(Nd) record. Early Miocene epsilon(Nd) values resemble those of modem Southern Ocean water masses, indicating a shift toward present-day patterns of ocean circulation. (C) 2004 Elsevier B.V. All rights reserved.