HR: 0800h
AN: PP51C-0619    [Abstracts]
TI: Southern Ocean Paleoceanographic Change Across the Eocene-Oligocene Boundary From Foraminiferal Stable Isotope and Mg/Ca Records
AU: * Bohaty, S M
EM: sbohaty@pmc.ucsc.edu
AF: Earth Sciences Department/ Institute of Marine Sciences, 1156 High Street, University of California, Santa Cruz , Santa Cruz, CA 95064 United States
AU: Zachos, J C
EM: jzachos@pmc.ucsc.edu
AF: Earth Sciences Department/ Institute of Marine Sciences, 1156 High Street, University of California, Santa Cruz , Santa Cruz, CA 95064 United States
AU: Delaney, M L
EM: delaney@ucsc.edu
AF: Ocean Sciences Department/ Institute of Marine Sciences, 1156 High Street, University of California, Santa Cruz , Santa Cruz, CA 95064 United States
AB: The Oi-1 event at the Eocene-Oligocene boundary (~34 Ma) represents a well-characterized shift in foraminiferal oxygen isotope values at numerous deep-sea sites. A ~1.3‰ increase in benthic δ¹⁸O values has been primarily attributed to the growth of an Antarctic ice sheet, but the potential contribution of cooling to this signal remains unconstrained. Previous work has indicated no significant temperature change in deep waters; this interpretation, however, requires an Antarctic sheet with at least 20% greater volume than present day or substantial storage of ice outside of East Antarctica. We investigated the latest Eocene-to-earliest Oligocene interval in several drillcores from the Southern Ocean. High-resolution stable isotope records were generated for both planktonic and benthic foraminifera, and minor element ratios (Mg/Ca, Sr/Ca, Mn/Ca) were determined for the planktonic genus Subbotina. The goal of this work was to determine the nature of the oxygen isotope shift, as well as to document regional paleoceanographic changes associated with Oi-1 event. Determining the precise timing of these changes in the high latitudes is critical to evaluating proposed mechanisms of climatic change across Eocene-Oligocene boundary. High-resolution stable isotope records from the Southern Ocean reveal a complex series of changes that closely mirror the benthic record recently constructed from cores in the tropical Pacific [ Coxall et al., 2005]. The initial stage of Oi-1 transition is characterized by a negative δ¹³C excursion in benthic foraminiferal records (~0.5 ‰) at the Eocene-Oligocene boundary. This excursion is immediately followed by a small positive step (~0.4 ‰) in both planktonic and benthic δ¹⁸O values. After a ~300 k.y. period of relative stability, a second, more rapid, step is characterized by a ~0.9‰ increase in δ¹⁸O values. Planktonic Mg/Ca-based temperature records indicate that the initial positive δ¹⁸O step of Oi-1 event represents a 2°C cooling of high-latitude surface waters. A consistent δ¹⁸O gradient between benthic and planktonic foraminifera through the Oi-1 transition further suggests that this cooling also affected deep waters that were formed in the southern high latitudes. The second phase of the δ¹⁸O shift is interpreted to primarily represent an increase in seawater δ¹⁸O due to the rapid glaciation of Antarctica to perhaps 80% modern ice volume. The magnitude and timing of the "glacial" component of the Oi-1 event is supported by several lines of reasoning, including 1) the synchroneity and amplitude of relative δ¹⁸O change at all sites, 2) the timing of ice rafting on the southern Kerguelen Plateau, and 3) a diminished amplitude of the δ¹⁸O shift on the Campbell Plateau, where no significant temperature change is indicated by the planktonic Mg/Ca record. The Oi-1 transition at Southern Ocean sites is also associated with a dramatic decrease in vertical δ¹³C gradients that begins during the initial cooling phase. Minimum values are reached after the final step of the Oi-1 transition. In the Southern Ocean, this change may be related to increased vertical mixing, but a similar trend is also observed at DSDP Site 522 (mid-latitude Atlantic).
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1065 Major and trace element geochemistry
DE: 4999 General or miscellaneous
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005