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Contraindre la transition greenhouse-icehouse du Paléogène par la géochimie des coccolithes

Abstract : Among the major obstacles in constraining paleoclimates for periods of “greenhouse” type is the poor preservation state of the sedimentary archive, and the existence of limits for each of the different applied geochemical markers. These obstacles lead to great uncertainties on the values and distribution of sea surface temperatures (SSTs) and atmospheric CO2 concentrations. Therefore, the mechanisms that brought the Earth’s system from a “greenhouse” to an “icehouse” regime throughout the Paleogene are not well constrained. In this thesis, we reconstructed the thermal evolution of the superficial ocean and the evolution of atmospheric pCO2 during the Paleogene from the isotopic signal (δ18O and ∆13C) of the fossilized carbonate exoskeleton of coccolithophores: coccoliths. Our data reveal a global cooling of surface waters from the Early to the Middle Eocene. During the Late Eocene and across the Eocene-Oligocene transition, values for pCO2 decrease, and cooling at high latitudes continues, allowing for the set up of an ice sheet in Antarctic, while the tropical belt warms. This change in the distribution of heat fluxes at the Earth’s surface during this period is synchronous to the deepening of the Drake Passage and to the set up of a vigorous Antarctic Circumpolar Current. These results thus highlight the driving role of changing oceanic circulations for climate dynamics during this transition. This thesis also proves the existence of important latitudinal thermal gradients throughout the Paleogene. The hypothesis of a homogeneous distribution of SSTs between the Equator and the poles, which prevails in the literature but remains hard to modelise, can thus be rejected.
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Submitted on : Friday, August 28, 2020 - 2:23:08 PM
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  • HAL Id : tel-02924848, version 1

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Maxime Tremblin. Contraindre la transition greenhouse-icehouse du Paléogène par la géochimie des coccolithes. Climatologie. Sorbonne Université, 2018. Français. ⟨NNT : 2018SORUS450⟩. ⟨tel-02924848⟩

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