18 Dec 2020

18 Dec 2020

Review status: this preprint is currently under review for the journal CP.

Southern Ocean bottom water cooling and ice sheet expansion during the middle Miocene climate transition

Thomas J. Leutert1,2,a, Sevasti Modestou1,2, Stefano M. Bernasconi3, and A. Nele Meckler1,2 Thomas J. Leutert et al.
  • 1Bjerknes Centre for Climate Research, Bergen, 5007, Norway
  • 2Department of Earth Science, University of Bergen, Bergen, 5007, Norway
  • 3Geological Institute, ETH Zurich, Zurich, 8092, Switzerland
  • apresent address: Max Planck Institute for Chemistry, Mainz, 55128, Germany

Abstract. The middle Miocene climate transition (MMCT, ~14.5–13.0 Ma) was associated with a significant expansion of Antarctic ice, but the mechanisms triggering the event remain enigmatic. We present a new clumped isotope (∆47) bottom water temperature (BWT) record from 16.0 Ma to 12.2 Ma from Ocean Drilling Program (ODP) Site 747 in the Southern Ocean, and compare it to existing BWT records. We show that BWTs in the Southern Ocean were ~8–10 °C during the middle Miocene greenhouse, and thus considerably warmer than today. Nonetheless, bottom water δ18O (calculated from foraminiferal δ18O and ∆47) suggests substantial amounts of land ice throughout the interval of the study. Our dataset demonstrates that BWTs at Site 747 decreased by ~3–5 °C across the MMCT. This cooling preceded the stepped main increase in global ice volume, and appears to have been followed by a transient bottom water warming starting during or slightly after the main ice volume increase. We speculate that a regional freshening of the upper water column at this time may have increased stratification and reduced bottom water heat loss to the atmosphere, counteracting global cooling in the bottom waters of the Southern Ocean and possibly even at larger scales. Additional processes and feedbacks required for substantial ice growth may have contributed to the observed decoupling of Southern Ocean BWT and global ice volume.

Thomas J. Leutert et al.

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Thomas J. Leutert et al.

Data sets

Middle Miocene bottom water carbonate clumped isotope temperatures, ODP Hole 120-747A, Kerguelen Plateau Thomas Jan Leutert, Sevasti Modestou, Stefano M. Bernasconi, and A. Nele Meckler

Thomas J. Leutert et al.


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Short summary
The Miocene climatic optimum associated with high atmospheric CO2 levels (~17–14 million years ago) was followed by a period of dramatic ice sheet growth on Antarctica. We present a clumped isotope-based bottom water temperature record from the Southern Ocean covering this key climate transition. Our record reveals warm conditions and a substantial cooling preceding the main ice volume increase, possibly caused by thresholds involved in ice growth and/or regional effects at our study site.