Preprints
https://doi.org/10.5194/cp-2021-184
https://doi.org/10.5194/cp-2021-184
 
25 Jan 2022
25 Jan 2022
Status: this preprint is currently under review for the journal CP.

Sea surface temperature evolution of the North Atlantic Ocean across the Eocene-Oligocene Transition

Kasia K. Śliwińska1,2, Helen K. Coxall3, David K. Hutchinson3,4, Diederik Liebrand5, Stefan Schouten2,6, and Agatha M. de Boer3 Kasia K. Śliwińska et al.
  • 1Department of Stratigraphy, Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, 1350 Copenhagen, Denmark
  • 2NIOZ Royal Netherlands Institute for Sea Research, Department of Marine Microbiology and Biogeochemistry, Landsdiep 4, 1797 SZ 't Horntje, Texel, the Netherland
  • 3Department of Geological Sciences, Stockholm University, Svante Arrhenius väg 8, 114 18 Stockholm, Sweden
  • 4Climate Change Research Centre, University of New South Wales, Sydney NSW 2052, Australia
  • 5National Oceanography Centre, European Way, SO14 3ZH, Southampton, United Kingdom
  • 6Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Vening Meinesz building A, Princetonlaan 8a, 3584 CB Utrecht, the Netherlands

Abstract. A major step in the long-term Cenozoic evolution toward a glacially-driven climate occurred at the Eocene Oligocene Transition (EOT), ~34.44 to 33.65 million years ago (Ma). Evidence for high latitude cooling and increased latitudinal temperature gradients across the EOT has been found in a range of marine and terrestrial environments. However, the timing and magnitude of temperature change in the North Atlantic remains highly unconstrained. Here, we use two independent organic geochemical paleo-thermometers to reconstruct sea surface temperatures (SSTs) from the southern Labrador Sea (Ocean Drilling Program – ODP Site 647) across the EOT. We find a permanent cooling step of ~3 °C (from 27 to 24 °C), between 34.9 Ma and 34.3 Ma, which is ~500 kyr prior to Antarctic glaciation. This step in SST values is asynchronous across Atlantic sites, signifiying considerable spatiotemporal variability in SST evolution. However, it is part of an overall cooling observed across sites in the North Atlantic (NA) in the 5 million years bracketing the EOT. Such cooling is unexpected in light of proxy and modelling studies suggesting the startup or strengething of the Atlantic Meridional Overturning Circulation (AMOC) before the EOT, which would warm the NA, although parallel Eocene CO2 decline on the decent into the Oligocene icehouse might counter this feedback. Here we show, using a published modelling study, that a reduction in atmospheric CO2 from 800 to 400 ppm is not sufficient to produce the observed cooling, if combined with NA warming from an AMOC startup, simulated here through Arctic closure from the Atlantic. Possible explanations of the apparent discrepancy are discussed and include uncertainty in the SST data, paleogeography and atmospheric CO2 boundary conditions, model weaknesses, and an earlier AMOC startup that just strengthened at the EOT. The results highlight the remaining uncertainty in many aspects of data and modelling results which need to be improved before we can draw robust conclusions of the processes acting before and across the EOT.

Kasia K. Śliwińska et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on cp-2021-184', Michiel Baatsen, 15 Mar 2022
    • AC1: 'Reply on RC1', Kasia K. Sliwinska, 30 Apr 2022
  • RC2: 'Comment on cp-2021-184', Anonymous Referee #2, 22 Mar 2022
    • AC3: 'Reply on RC2', Kasia K. Sliwinska, 04 May 2022

Kasia K. Śliwińska et al.

Kasia K. Śliwińska et al.

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Short summary
We provide a sea surface temperature record from the Labrador Sea (ODP Site 647) based on organic geochemical proxies across the late Eocene and early Oligocene. Our study reveals heterogenic cooling of the Atlantic. The cooling of the North Atlantic is difficult to reconcile with the active Atlantic Meridional Overturning Circulation. We discuss possible explanations such as uncertainty in the data, paleogeography and atmospheric CO2 boundary conditions, model weaknesses, and AMOC activity.