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Publication Title | Deep water temperature carbonate ion and ice volume changes across the Eocene‐Oligocene climate transition

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PALEOCEANOGRAPHY, VOL. 26, PA2205, doi:10.1029/2010PA001950, 2011

Deep water temperature, carbonate ion, and ice volume changes across the Eocene‐Oligocene climate transition

A. E. Pusz,1 R. C. Thunell,1 and K. G. Miller2

Received 16 February 2010; revised 11 January 2011; accepted 28 January 2011; published 21 April 2011.

[1] Paired benthic foraminiferal stable isotope and Mg/Ca data are used to estimate bottom water temperature (BWT) and ice volume changes associated with the Eocene‐Oligocene Transition (EOT), the largest global climate event of the past 50 Myr. We utilized ODP Sites 1090 and 1265 in the South Atlantic to assess seawater d18O (dw), Antarctic ice volume, and sea level changes across the EOT (∼33.8–33.54 Ma). We also use benthic d13C data to reconstruct the sources of the deep water masses in this region during the EOT. Our data, together with previously published records, indicate that a pulse of Northern Component Water influenced the South Atlantic immediately prior to and following the EOT. Benthic d18O records show a 0.5‰ increase at ∼33.8 Ma (EOT‐1) that represents a ∼2°C cooling and a small (∼10 m) eustatic fall that is followed by a 1.0‰ increase associated with Oi‐1. The expected cooling of deep waters at Oi‐1 (∼33.54 Ma) is not apparent in our Mg/Ca records. We suggest the cooling is masked by coeval changes in the carbonate saturation state (D[CO23−]) which affect the Mg/Ca data. To account for this, the BWT, ice volume, and dw estimates are corrected for a change in the D[CO23−] of deep waters on the basis of recently published work. Corrected BWT at Sites 1090 and 1265 show a ∼1.5°C cooling coincident with Oi‐1 and an average dw increase of ∼0.75‰. The increase in ice volume during Oi‐1 resulted in a ∼70 m drop in global sea level and the development of an Antarctic ice sheet that was near modern size or slightly larger.

Citation: Pusz, A. E., R. C. Thunell, and K. G. Miller (2011), Deep water temperature, carbonate ion, and ice volume changes across the Eocene‐Oligocene climate transition, Paleoceanography, 26, PA2205, doi:10.1029/2010PA001950.

1. Introduction

[2] Earth’s climate has experienced a long‐term cooling over the past 50 million years as evidenced by a 5‰

18

increase in deep sea benthic foraminiferal d O during this

period [Savin et al., 1975; Shackleton and Kennett, 1975; Miller et al., 1987, 2005; Zachos et al., 2001]. Resultant oxygen isotope temperature estimates [Miller et al., 1987, 2005; Zachos et al., 2001], along with benthic foraminiferal Mg/Ca paleotemperatures, suggest as much as a ∼12°C cooling of deep waters over the past 50 Myr [Lear et al., 2000; Billups and Schrag, 2002, 2003]. The Cenozoic cooling trend has been attributed to changes in ocean‐ atmosphere circulation patterns associated with opening and closing of ocean gateways [Kennett, 1977; Schnitker, 1980; Woodruff and Savin, 1989; Wright et al., 1992; Raymo, 1994] and changes in atmospheric CO2 levels [Vincent and Berger, 1985; Flower and Kennett, 1993; DeConto and Pollard, 2003; Pagani et al., 2005; Pearson et al., 2009].

1Department of Earth and Ocean Sciences, University of South Carolina, Columbia, South Carolina, USA.

2Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey, USA.

Copyright 2011 by the American Geophysical Union. 0883‐8305/11/2010PA001950

[3] The Eocene‐Oligocene transition (EOT) was the largest of several abrupt events that punctuated the overall Cenozoic cooling trend [Berger, 1982; Miller et al., 1987; Zachos et al., 2001]. In well‐resolved records, the ∼1.5‰ benthic foraminiferal d18O increase at the EOT appears to be a two‐step event with the first phase beginning at 33.8 Ma (0.5‰) followed by a second increase of 1.0‰, Oi‐1 (“Oligocene isotope event 1”), at 33.54 Ma [Miller et al., 1991, 2008; Zachos et al., 1996; Coxall et al., 2005]. The d18O shift across the EOT reflects both deep water cooling and the development of continental‐size ice sheets on Ant- arctica [Miller et al., 1991, 2005; Zachos et al., 1996; Coxall et al., 2005; Lear et al., 2008]. Independent evidence for the onset of Antarctic glaciation associated with the EOT in- cludes the presence of ice rafted detritus at high southern latitudes and changes in clay mineralogy in the Southern Ocean [Ehrmann, 1991; Ehrmann and Mackensen, 1992; Zachos et al., 1992; Robert et al., 2002].

[4] Mg/Ca ratios of benthic foraminiferal calcite can

serve as an independent proxy for bottom water temper-

ature [Nurnberg et al., 1996; Rosenthal et al., 1997; Lea

et al., 1999; Elderfield and Ganssen, 2000]. However, the

primary BWT component can be masked by changes in

2−

the carbonate ion concentration ([CO3 ]) and saturation

state (D[CO23−]) of deep waters [Elderfield et al., 2006; Rosenthal et al., 2006; Yu and Elderfield, 2008]. The [CO23−] ion effect is based on core top Mg/Ca calibrations that show

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