Abstract
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). Cyclostratigraphic analysis of natural gamma radiation (NGR) log data yields duration estimates of Chrons C12r through C16n.1n, providing additional constraints on the Eocene timescale. Correlations between the orbital eccentricity curve and the 405 kyr tuned NGR time series indicate that 33.71 and 34.10 Ma are the most likely proposed ages of the EO boundary. Additionally, the 405 kyr tuning calibrates the most pronounced NGR cyclicity to a period of ∼1 Myr, matching the g1–g5 eccentricity term, supporting its significant expression in continental depositional environments, and hypothesizing that the paleolake level may have behaved as a low-pass 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. 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. Finally, the interval spanning the EOT precursor glacial event through EOT-1 is remarkably dominated by obliquity. This suggests preconditioning of the major Antarctic glaciation, either from obliquity directly affecting the formation/(in)stability of the incipient Antarctic Ice Sheet (AIS), or through obliquity modulation of the North Atlantic Deep Water production.
Highlights
The Eocene–Oligocene climate transition (EOT) is one of the most drastic climate changes of the Cenozoic era and the final stage of the switch from greenhouse to icehouse climates
Data from deep-sea carbon and oxygen stable isotopes suggest that the EOT occurred in successive steps (Coxall et al, 2005; Katz et al, 2008; Coxall and Wilson, 2011), and that the Antarctic glaciation was initiated, after climatic preconditioning by a decline in atmospheric CO2 enhanced by Published by Copernicus Publications on behalf of the European Geosciences Union
Lower natural remanent magnetization (NRM) values were generally recorded in the lower part of the core in particular below ca. 300 m depth where most samples yielded uninterpretable erratic demagnetization paths that can be attributed to their low NRM values, in the order of instrumental detection
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 switch from greenhouse to icehouse climates It is characterized by the onset of large and perennial ice sheets on Antarctica (e.g., Miller et al, 1991; Zachos et al, 2001b) inducing global cooling and significant sealevel lowering (Lear et al, 2008; Miller et al, 2008; Hren et al, 2013; Goldner et al, 2014); a deepening of the calcite compensation depth (Coxall et al, 2005; Kaminski and Ortiz, 2014); and severe and widespread disturbances in biotic, carbon, and hydrological cycles (e.g., Dupont-Nivet et al, 2007; Pearson et al, 2009; Coxall and Wilson, 2011; Xiao et al, 2010; Coxall et al, 2018; Hutchinson et al, 2021). Despite important efforts to fill this gap, discrepancies among published age models still persist (Speijer et al, 2020), in particular the age of the Eocene–Oligocene boundary (EOB), which is a fundamental tie point for the Cenozoic timescale, remains controversial (see Hilgen and Kuiper, 2009, for a review, and a later study by Sahy et al, 2017; Berggren et al, 2018)
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