Constraining the greenhouse-icehouse transition during the Paleogene using the geochemistry of the coccoliths

• Main Author: Tremblin, Maxime
• Other Authors: Institut des Sciences de la Terre de Paris (iSTeP), Institut national des sciences de l'Univers (INSU - CNRS)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS), Sorbonne Université, Fabrice Minoletti, Michaël Hermoso, Delphine Desmares
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2018
• Subjects: Paleoclimates; Coccoliths; Geochemistry; Paleogene; Temperatures; PCO2; Paléoclimats; Coccolithes; Géochimie; Paléogène; Températures; [SDU.STU.CL]Sciences of the Universe [physics]/Earth Sciences/Climatology; [SDU.STU.GL]Sciences of the Universe [physics]/Earth Sciences/Glaciology; Antarctic; Drake Passage; Antarc*; Ice Sheet
• Online Access: https://tel.archives-ouvertes.fr/tel-02924848; https://tel.archives-ouvertes.fr/tel-02924848/document; https://tel.archives-ouvertes.fr/tel-02924848/file/these_tremblin_maxime_2018.pdf

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. Un obstacle majeur à la caractérisation des paléoclimats en période de type « greenhouse » tient à la mauvaise préservation des archives sédimentaires et à des limites dans les calibrations des marqueurs géochimiques utilisés. Ces limitations entrainent de grandes incertitudes sur les valeurs et la répartition des températures des eaux de ...