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

Abstract

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 ɛ Nd units). Correlation of down core secular variations between the ɛ Nd record, δ 13C 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 ɛ Nd values are associated with changes in continental weathering on Antarctica. Nonradiogenic ɛ Nd values at Site 689 during the middle Eocene require the contribution of an end member with ɛ Nd<−9.5. Southern Ocean deep water may have been too radiogenic in the middle Eocene ( ɛ 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 ( ɛ 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 ɛ Nd values at ∼40.8 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 ɛ 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 ɛ Nd values of these water masses. The modern ɛ Nd values of AAIW and AABW reflect a significant contribution of North Atlantic Deep Water (NADW), thus decreasing ɛ Nd values in the early Oligocene may have resulted from the export of Northern Component Water (NCW, similar to modern NADW). During the late Oligocene and early Miocene, the long-term trends of the record follow benthic δ 13C 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 ɛ Nd values and δ 13C values are not correlated may reflect the influence of a short-lived weathering event on the ɛ Nd record. Early Miocene ɛ Nd values resemble those of modern Southern Ocean water masses, indicating a shift toward present-day patterns of ocean circulation.