Preprints
https://doi.org/10.5194/cp-2021-46
https://doi.org/10.5194/cp-2021-46

  17 May 2021

17 May 2021

Review status: a revised version of this preprint is currently under review for the journal CP.

Age and driving mechanisms of the Eocene-Oligocene Transition from astronomical tuning of a lacustrine record (Rennes Basin, France)

Slah Boulila1,2, Guillaume Dupont-Nivet3,4, Bruno Galbrun1, Hugues Bauer5, and Jean-Jacques Châteauneuf5 Slah Boulila et al.
  • 1Sorbonne Université, CNRS , Institut des Sciences de la Terre-Paris, ISTeP, F-75005 Paris, France
  • 2ASD/IMCCE, CNRS-UMR 8028, Observatoire de Paris, PSL University, Sorbonne Université, Paris, France
  • 3Geosciences Rennes UMR 6118, Université Rennes 1, CNRS, Rennes, France
  • 4Potsdam University, Department of Earth and Environmental Sciences, Potsdam-Golm, Germany
  • 5BRGM, Bureau de Recherches Géologiques et Minières, Orléans, France

Abstract. The Eocene-Oligocene Transition (EOT) marks the onset of the Antarctic glaciation and the switch from greenhouse to icehouse climates. However, the driving mechanisms and the precise timing of the EOT remain controversial mostly due to the lack of well-dated stratigraphic records, especially in continental environments. Here we present a cyclo-magnetostratigraphic and sedimentological study of a ∼7.6 Myr-long lacustrine record spanning the late Eocene to the earliest Oligocene, from a drill-core in the Rennes Basin (France). Time-series analysis of natural gamma-ray (NGR) log data shows evidence of Milankovitch cycle bands. In particular, the 405 kyr stable eccentricity is expressed with strong amplitudes. Astronomical calibration to this 405 kyr periodicity yields duration estimates of Chrons C12r through C16n.1n, providing additional constraints on the middle–early Eocene timescale. Correlations between the orbital eccentricity curve and the 405 kyr tuned NGR time series and assumptions on their phase relationships, enable to test previously proposed ages for the EO boundary, indicating that 33.71 and 34.10 Ma are the most likely. Additionally, the 405 kyr tuning calibrates the most pronounced NGR cyclicity to a period of ∼1 Myr matching the g1-g5 eccentricity term. Such cyclicity has been recorded in other continental records, pointing to its significant expression in continental depositional environments. The record of g1-g5 and sometimes g2-g5 eccentricity terms in previously acquired sedimentary facies proxies in CDB1 core led us to hypothesize that the paleolake level may have behaved as a lowpass filter for orbital forcing. Two prominent changes in the sedimentary facies were detected across the EOT, which are temporally equivalent to the two main climatic steps, EOT-1 and Oi-1. Combined with previously acquired geochemical (δ15Norg, TOC), mineralogical (Quartz, clays) and pollen assemblage proxies from CDB1, we suggest that these two facies changes reflect the two major Antarctic cooling/glacial phases via the hydrological cycle, as significant shifts to drier and cooler climate conditions, thus supporting the stepwise nature of the EOT. Remarkably, a strongly dominant obliquity expressed in the latest Eocene corresponds in time to the interval from the EOT precursor glacial event till the EOT-1. We interpret the obliquity dominance as reflecting preconditioning phases for the onset of the major Antarctic glaciation, either from its direct impact on the formation/(in)stability of the incipient Antactic Ice Sheet (AIS), or through its modulation of the North Atlantic Deep Water production given the North Atlantic coastal location of the CDB1 site.

Slah Boulila 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-2021-46', Anonymous Referee #1, 28 Jun 2021
    • AC1: 'Reply on RC1', Slah Boulila, 08 Jul 2021
  • RC2: 'Comment on cp-2021-46', Anonymous Referee #2, 04 Jul 2021
    • AC2: 'Reply on RC2', Slah Boulila, 08 Jul 2021

Slah Boulila et al.

Slah Boulila et al.

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Short summary
The Eocene–Oligocene climate transition (EOT) is one of the most drastic climate changes of the Cenozoic era and the final stage of the shift from ice-free to icehouce Earth. Here we present high-resolution records (geophysical, geochemical and sedimentological proxy data) of the EOT from lake deposits to detect the atmospheric expression of the EOT via the hydrological cycle. Such records provide strong constraints on climate modeling and on our comprehension of the forcing mechanisms of EOT.