ALBERT

Description: We investigate the evolution of Cenozoic climate and ice volume as evidenced by the oxygen isotopic composition of seawater (delta18Osw) derived from benthic foraminiferal Mg/Ca ratios to constrain the temperature effect contained in foraminiferal delta18O values. We have constructed two benthic foraminiferal Mg/Ca records from intermediate water depth sites (Ocean Drilling Program sites 757 and 689 from the subtropical Indian Ocean and the Weddell Sea, respectively). Together with the previously published composite record of Lear et al. (2002, doi:10.1126/science.287.5451.269) and the Neogene record from the Southern Ocean of Billups and Schrag (2002, doi:10.1029/2000PA000567), we obtain three, almost complete representations of the delta18Osw for the past 52 Myr. We discuss the sensitivity of early Cenozoic Mg/Ca-derived paleotemperatures (and hence the delta18Osw) to assumptions about seawater Mg/Ca ratios. We find that during the middle Eocene (~ 49-40 Ma), modern seawater ratios yield Mg/Ca-derived temperatures that are in good agreement with the oxygen isotope paleothermometer assuming ice-free conditions. Intermediate waters cooled during the middle Eocene reaching minimum temperatures by 40 Ma. The corresponding delta18Osw reconstructions support ice growth on Antarctica beginning by at least 40 Ma. At the Eocene/Oligocene boundary, Mg/Ca ratios (and hence temperatures) from Weddell Sea site 689 display a well-defined maximum. We caution against a paleoclimatic significance of this result and put forth that the partitioning coefficient of Mg in benthic foraminifera may be sensitive to factors other than temperature. Throughout the remainder of the Cenozoic, the temporal variability among delta18Osw records is similar and similar to longer-term trends in the benthic foraminiferal delta18O record. An exception occurs during the Pliocene when delta18Osw minima in two of the three records suggest reductions in global ice volume that are not apparent in foraminiferal delta18O records, which provides a new perspective to the ongoing debate about the stability of the Antarctic ice sheet. Maximum delta18Osw values recorded during the Pleistocene at Southern Ocean site 747 agree well with values derived from the geochemistry of pore waters (Schrag et al., 1996, doi:10.1126/science.272.5270.1930) further highlighting the value of the new Mg/Ca calibrations of Martin et al. (2002, doi:10.1016/S0012-821X(02)00472-7) and Lear et al. (2002, doi:10.1016/S0016-7037(02)00941-9) applied in this study. We conclude that the application of foraminiferal Mg/Ca ratios allows a refined view of Cenozoic ice volume history despite uncertainties related to the geochemical cycling of Mg and Ca on long time scales.

Description: We explore the applicability of paired Mg/Ca and 18O/16O measurements on benthic foraminifera from Southern Ocean site 747 to paleoceanographic reconstructions on pre-Pleistocene timescales. We focus on the late Oligocene through Pleistocene (27–0 Ma) history of paleotemperatures and the evolution of the d18O values of seawater (d18Osw) at a temporal resolution of ~100–200 kyr. Absolute paleotemperature estimates depend on assumptions of how Mg/Ca ratios of seawater have changed over the past 27 Myr, but relative changes that occur on geologically brief timescales are robust. Results indicate that at the Oligocene to Miocene boundary (23.8 Ma), temperatures lag the increase in global ice-volume deduced from benthic foraminiferal d18O values, but the smaller-scale Miocene glaciations are accompanied by ocean cooling of -1°C. During the mid-Miocene phase of Antarctic ice sheet growth (~15-13 Ma), water temperatures cool by ~3°C. Unlike the benthic foraminiferal d18O values, which remain relatively constant thereafter, temperatures vary (by 3°C) and reach maxima at ~12 and ~8.5 Ma. The onset of significant Northern Hemisphere glaciation during the late Pliocene is synchronous with an ~4°C cooling at site 747. A comparison of our d18Osw curve to the Haq et al. (1987, doi:10.1126/science.235.4793.1156 ) sea level curve yields excellent agreement between sequence boundaries and times of increasing seawater 18O/16O ratios. At ~12-11 Ma in particular, when benthic foraminiferal d18O values do not support a further increase in ice volume, the d18Osw curve comes to a maximum that corresponds to a major mid-Miocene sea level regression. The agreement between the character of our Mg/Ca-based d18Osw curve and sequence stratigraphy demonstrates that benthic foramaniferal Mg/Ca ratios can be used to trace the d18Osw on pre-Pleistocene timescales despite a number of uncertainties related to poorly constrained temperature calibrations and paleoseawater Mg/Ca ratios. The Mg/Ca record also highlights that deep ocean temperatures can vary independently and unexpectedly from ice volume changes, which can lead to misinterpretations of the d18O record.