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Pohl, Alexandre
Universite de Versailles-Saint-Quentin-En-Yvelines, Batiment Buffon, 55 Avenue de Paris, 78000 Versailles (France); Universite de Paris-Saclay, Espace Technologique Bat. Discovery - RD 128, 2e et, 91190 Saint-Aubin (France); CEA, DRF-LSCE (France)2016
Universite de Versailles-Saint-Quentin-En-Yvelines, Batiment Buffon, 55 Avenue de Paris, 78000 Versailles (France); Universite de Paris-Saclay, Espace Technologique Bat. Discovery - RD 128, 2e et, 91190 Saint-Aubin (France); CEA, DRF-LSCE (France)2016
AbstractAbstract
[en] The Ordovician (485-444 Ma) is a geological period characterized by the concomitance of a major glaciation and one of the 'Big Five' mass extinction events that punctuated the Earth's history. This dissertation aimed at developing a better understanding of the climatic evolution at that time through numerical modeling, in order to provide a consistent picture of the glaciation. First, it was shown that the Ordovician continental configuration leads to a particular ocean dynamics, which induces in turn the development of a climatic instability that allows global climate to cool suddenly in response to subtle changes in the atmospheric partial pressure of CO2 (pCO2). Secondly, an innovative climate-ice sheet coupled model produced the first simulation of the glaciation that is supported by geological data, in the context of a decrease in pCO2. Results show that glacial onset may have occurred as early as the Mid Ordovician (465 Ma), i.e., some 20 million years earlier than thought initially. In this scenario, the climatic instability is reached during the latest Ordovician and accounts for the onset of the Hirnantian glacial maximum (445-444 Ma). Experiments conducted with a non-vascular vegetation model reveal that the origination and expansion of their first land plants significantly intensified continental weathering during the Ordovician and potentially drove the drop in atmospheric CO2. Finally, the interactions between climate and the marine biosphere were investigated based on 2 complementary axes. (i) News constraints on the paleo-biogeography of marine living communities were brought through the publication of maps showing the ocean surface circulation modeled at various pCO2 levels during the Early, Middle and Late Ordovician. (ii) The relationships between climatic variations and the redox state of the ocean were studied using a recent ocean model with biogeochemical capabilities (MITgcm). The simulations suggest partial and global oceanic anoxic events during the Katian and the early Silurian respectively. They also show that anoxia is probably not responsible for the latest Ordovician mass extinction event. (author)
[fr]
L'Ordovicien (485-444 Ma) est une periode geologique caracterisee par la concomitance d'une glaciation majeure et de l'une des 5 plus grandes extinctions de masse de l'histoire de la Terre. Cette these avait pour objectif d'ameliorer la comprehension de l'evolution du climat a cette epoque a l'aide de la modelisation numerique, afin de fournir une image coherente de la glaciation. Nous avons d'abord demontre que la configuration continentale ordovicienne induit une dynamique oceanique particuliere, a l'origine d'une instabilite climatique permettant un refroidissement brutal du climat global sans variation importante de la concentration atmospherique en CO2 (pCO2). Dans un deuxieme temps, un modele innovant couple climat-calotte a permis de produire la premiere simulation de la mise en place de la glaciation supportee par les donnees geologiques, sous un scenario coherent de baisse de la pCO2. Les resultats indiquent que les premieres glaces continentales se seraient mises en place des l'Ordovicien Moyen (465 Ma), quelque 20 millions d'annees plus tot qu'initialement envisage. Dans ce scenario, le franchissement de l'instabilite climatique ordovicienne marque la mise en place du maximum glaciaire au cours de l'Ordovicien terminal Hirnantien (445-444 Ma). Des experiences realisees avec un modele de vegetation primitive montrent que le developpement des plantes non-vasculaires a pu constituer le mecanisme a l'origine de la chute de la pCO2, via une intensification de l'alteration des surfaces continentales. Enfin, les interactions entre climat et biosphere marine ont ete envisagees selon 2 axes complementaires. (i) De nouvelles contraintes ont ete fournies pour comprendre la paleobiogeographie des communautes marines, par la publication de cartes de la circulation oceanique de surface modelisee sous differentes pCO2 au cours de l'Ordovicien Inferieur, Moyen et Superieur. (ii) Les relations entre variations climatiques et etat redox de l'ocean ont ete etudiees avec un modele d'ocean recent beneficiant d'un module de biogeochimie marine (MITgcm). Les simulations suggerent des anoxies partielles (durant le Katien) ou globales (durant le Silurien inferieur) au cours de la transition Ordovicien-Silurien. Elles demontrent egalement que l'extinction de l'Ordovicien terminal ne serait pas liee a un evenement d'anoxie. (auteur)Original Title
Comprehension du climat de l'Ordovicien a l'aide de la modelisation numerique
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16 Nov 2016; 313 p; 377 refs.; Available from the INIS Liaison Officer for France, see the INIS website for current contact and E-mail addresses; Meteorologie, Oceanographie, Physique de l'Environnement
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