ALBERT

Description: 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, delta13C 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 ~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 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 epsilon-Nd values of these water masses. The modern 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 modern NADW). During the late Oligocene and early Miocene, the long-term trends of the record follow benthic delta13C 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 delta13C 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 modern Southern Ocean water masses, indicating a shift toward present-day patterns of ocean circulation.

Description: We analyzed 87Sr/86Sr ratios in foraminifera, pore fluids, and fish teeth for samples ranging in age from Eocene to Pleistocene from four Ocean Drilling Program sites distributed around the globe: Site 1090 in the Cape Basin of the Southern Ocean, Site 757 on the Ninetyeast Ridge in the Indian Ocean, Site 807 on the Ontong-Java Plateau in the western equatorial Pacific, and Site 689 on the Maud Rise in the Southern Ocean. Sr isotopic ratios for dated foraminifera consistently plot on the global seawater Sr isotope curve. For Sites 1090, 757, and 807 Sr isotopic values of the pore fluids are generally less radiogenic than contemporaneous seawater values, as are values for fossil fish teeth. In contrast, pore fluid 87Sr/86Sr values at Site 689 are more radiogenic than contemporaneous seawater, and the corresponding fish teeth also record more radiogenic values. Thus, Sr isotopic values preserved in fossil fish teeth are consistently altered in the direction of the pore fluid values; furthermore, there is a correlation between the magnitude of the offset between the pore fluids and the seawater curve, and the associated offset between the fish teeth and the seawater curve. These data suggest that the hydroxyfluorapatite of the fossil fish teeth continues to recrystallize and exchange Sr with its surroundings during burial and diagenesis. Therefore, Sr chemostratigraphy can be used to determine rough ages for fossil fish teeth in these cores, but cannot be used to fine-tune age models. In contrast to the Sr isotopic system, our Nd concentration data, combined with published isotopic and rare earth element data, suggest that fish teeth acquire Nd during early diagenesis while they are still in direct contact with seawater. The concentrations of Nd acquired at this stage are extremely high relative to the concentrations in surrounding pore fluids. As a result, Nd isotopes are not altered during burial and later diagenesis. Therefore, fossil fish teeth from a variety of marine environments preserve a reliable and robust record of deep seawater Nd isotopic compositions from the time of deposition.

Description: At Ocean Drilling Program (ODP) Site 1090 (lat 42°54.8'S, long 8°54.0'E) located in a water depth of 3702 m on the Agulhas Ridge in the sub-Antarctic South Atlantic, ~300 m of middle Eocene to middle Miocene sediments were recovered with the advanced piston corer (APC) and the extended core barrel (XCB). U-channel samples from the 70-230 meters composite depth (mcd) interval provide a magnetic polarity stratigraphy that is extended to 380 mcd by shipboard whole-core and discrete sample data. The magnetostratigraphy can be interpreted by the fit of the polarity-zone pattern to the geomagnetic polarity time scale (GPTS) augmented by isotope data and bioevents with documented correlation to the GPTS. Three normal-polarity subchrons (C5Dr.1n, C7Ar.1n, and C13r.1n), not included in the standard GPTS, are recorded at Site 1090. The base of the sampled section is correlated to C19n (middle Eocene), although the interpretation is unclear beyond C17r. The top of the sampled section is correlated to C5Cn (late early Miocene), although, in the uppermost 10 m of the sampled section, a foraminifer (Globorotalia sphericomiozea) usually associated with the Messinian and early Pliocene has been identified. 87Sr/86Sr, d13C, and d18O values measured on foraminifera, including the d18O and d13C shifts close to the Eocene/Oligocene boundary, support the correlation to the GPTS. For the interval spanning the Oligocene/Miocene boundary, benthic d13C, d18O, and 87Sr/86Sr records from Site 1090 can be correlated to isotope records from ODP Site 929 (Ceara Rise), providing support for the recently-published Oligocene/Miocene boundary age (22.92 Ma) of Shackleton et al.