Foraminiferal Mg/Ca evidence for Southern Ocean cooling across the Eocene-Oligocene transition

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doi: 10.1016/j.epsl.2011.11.037
Author(s): Bohaty, Steven M.; Zachos, James C.; Delaney, Margaret L.
Author Affiliation(s): Primary:
University of California, Santa Cruz, Department of Earth and Planetary Sciences, Santa Cruz, CA, United States
Volume Title: Earth and Planetary Science Letters
Source: Earth and Planetary Science Letters, Vol.317-318, p.251-261. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0012-821X CODEN: EPSLA2
Note: In English. Supplementary data available in online version. 89 refs.; illus., incl. 2 tables
Summary: Constraining the magnitude of high-latitude temperature change across the Eocene-Oligocene transition (EOT) is essential for quantifying the magnitude of Antarctic ice-sheet expansion and understanding regional climate response to this event. To this end, we constructed high-resolution stable oxygen isotope (δ18O) and magnesium/calcium (Mg/Ca) records from planktic and benthic foraminifera at four Ocean Drilling Program (ODP) sites in the Southern Ocean. Planktic foraminiferal Mg/Ca records from the Kerguelen Plateau (ODP Sites 738, 744, and 748) show a consistent pattern of temperature change, indicating 2-3°C cooling in direct conjunction with the first step of a two-step increase in benthic and planktic foraminiferal δ18O values across the EOT. In contrast, benthic Mg/Ca records from Maud Rise (ODP Site 689) and the Kerguelen Plateau (ODP Site 748) do not exhibit significant temperature change. The contrasting temperature histories derived from the planktic and benthic Mg/Ca records are not reconcilable, since vertical δ18O gradients remained nearly constant at all sites between 35.0 and 32.5 Ma. Based on the coherency of the planktic Mg/Ca records from the Kerguelen Plateau sites and complications with benthic Mg/Ca paleothermometry at low temperatures, the planktic Mg/Ca records are deemed the most reliable measure of Southern Ocean temperature change. We therefore interpret a uniform cooling of 2-3°C in both deep surface (thermocline) waters and intermediate deep waters of the Southern Ocean across the EOT. Cooling of Southern Ocean surface waters across the EOT was likely propagated to the deep ocean, since deep waters were primarily sourced on the Antarctic margin throughout this time interval. Removal of the temperature component from the observed foraminiferal δ18O shift indicates that seawater δ18O values increased by 0.6±0.15 ppm across the EOT interval, corresponding to an increase in global ice volume to a level equivalent with 60-130% modern East Antarctic ice sheet volume. Abstract Copyright (2012) Elsevier, B.V.
Year of Publication: 2012
Research Program: ODP Ocean Drilling Program
Key Words: 12 Stratigraphy, Historical Geology and Paleoecology; Alkaline earth metals; Antarctic ice sheet; Antarctica; Benthic taxa; Calcium; Cenozoic; Eocene; Foraminifera; Glaciation; Indian Ocean; Invertebrata; Isotope ratios; Isotopes; Kerguelen Plateau; Leg 113; Leg 119; Leg 120; Magnesium; Maud Rise; Metals; Mg/Ca; Microfossils; O-18/O-16; ODP Site 689; ODP Site 738; ODP Site 744; ODP Site 748; Ocean Drilling Program; Oligocene; Oxygen; Paleogene; Paleotemperature; Planktonic taxa; Protista; Southern Ocean; Stable isotopes; Tertiary; Volume; Weddell Sea
Coordinates: S643101 S643100 E0030600 E0030559
S624233 S624232 E0824715 E0824714
S613440 S613439 E0803528 E0803527
S582627 S582627 E0785854 E0785853
Record ID: 2012051983
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands