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Accueil > Recherche > Thèses / HDR > Thèses en cours


Doctorat terre solide : Géodynamique des enveloppes supérieures, paléobiosphère
Université Paris-Sud
Ecole doctorale : Sciences Mécaniques et Energétiques, Matériaux, Géosciences (SMEMaG)
Financement : Beca para doctorado en el extranjero, Becas Chile, CONICYT 2017
Sujet : Estimation des âges 14C du réservoir océanique de surface et profond dans le secteur sud-est Pacifique de l’océan Austral depuis la dernière période glaciaire.
Directeur de thèse : Giuseppe Siani, GEOPS
Co-directrice de thèse  : Elisabeth Michel, LSCE
Unité de recherche : GEOPS – Géosciences de Paris Sud UMR 8148 - Orsay
Équipe : Paléoclimats & Dynamique Sédimentaires

Résumé :
The ocean is thought to have an important role in variations of the carbon budget during glacial-interglacial cycles. In particular, during the last deglaciation a change in oceanic circulation is proposed to be especially relevant by modulating the transfer of carbon between the atmosphere and the deep ocean. The transition from the Last Glacial Maximum (LGM) to the Holocene is characterized by abrupt increases in atmospheric CO2 concentration, amounting to 75 ppm (Barnola et al., 1987, Monnin et al. 2001, Marcott et al., 2014), which were paralleled by drops in atmospheric Δ14C adding to 190±10 ‰ (Reimer et al, 2013). Due to its large content of mobile carbon and the time scale of its mixing, the deep ocean has been pointed out as the most likely origin for this 14C-depleted CO2 release to the atmosphere (Broecker and Barker, 2007). In adittion, the Southern Ocean has been indicated as an area modulating the exchange of carbon between the deep ocean and the atmosphere. For the latter is important to produce well dated records in the deep ocean in order to characterize the variation between interglacial-glacial cycles in the search for understanding the mechanisms controlling the carbon exchange between reservoirs.
The first objective of this thesis is to produce new well dated deep ocean ( 3000 m) records in the South East Pacific and East Pacific sector of the Southern Ocean since the last glaciation, which to this time don’t exist. In order to characterize the changes in circulations a multi proxy approach is going to be used, combining radiocarbon dates in benthic foraminifera, along with stable isotope measurements and the oxygenation proxy δ13CCibicides- δ13CGlobobulimina. This will allow to have a more complete idea of what the observed changes represent, i.e. changes in water mass distribution or water mass characteristics.
In order to have a good interpretation of the data and comparison with other records, is essential to produce robust chronologies, especially when using radiocarbon as a ventilation proxy. Correcting planktic foraminifera 14C ages for a constant marine surface radiocarbon reservoir ages (Rs) has been broadly utilized as a method to produce marine sediment core chronologies. However, the assessment of precise Rs has demonstrated that these can vary greatly in time, especially during the late glacial and last deglaciation, when Siani et al. (2013) demonstrate the varying nature of the Rs in the SEP between 1.7–15 ka cal BP, ranging between 790 ± 160 years and 1320 ± 95 years. Thus, this thesis focus is also in obtaining Rs using criptotephrochronologie in the cores in order to obtain precise calendar ages in each core.
Tephrochronologie has a great potential of use in the South East Pacific and Southern South América because of the explosive character of the Southern Volcanique Zone which registers numerous explosions during the Pliocene. The aim of this thesis is to recognize tephra or criptotephra layers in the cores and identifying them on land using geochemistry of major and minor elements measured in individual glass shards, along with stratigraphy. Ideally, this will result in well dated cores on the sea and a direct correlation with cores on land, allowing a more complete interpretation of the paleoceanographic, paleoclimatic, variations between glacial-interglacial cycles.