Regional variability of polar snow densification during large climatic transitions

• Main Author: Bréant, Camille
• Other Authors: Laboratoire des Sciences du Climat et de l'Environnement Gif-sur-Yvette (LSCE), Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris-Saclay-Centre National de la Recherche Scientifique (CNRS), Université Paris Saclay (COmUE), Amaëlle Landais, Patricia Martinerie
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2017
• Subjects: Densification; Firn; Thermo-Mechanical model; Isotopes; Antarctic; Névé; Modèle thermo-Dynamique; Antarctique; [SDU.STU.CL]Sciences of the Universe [physics]/Earth Sciences/Climatology; Antarc*; Antarctica; Antarctique*; Ice Sheet
• Online Access: https://theses.hal.science/tel-01726272; https://theses.hal.science/tel-01726272/document; https://theses.hal.science/tel-01726272/file/67665_BREANT_2017_archivage.pdf

The phasing between increases in temperature and greenhouse gas concentrations during large climatic variations in the past is classically estimated using analyses in polar ice cores, in the ice phase for the temperature and in the gas phase (trapped air bubbles) for the concentration of greenhouse gases. This phasing is still insufficiently constrained and solving this problem requires a better understanding of the mechanical process of snow to ice metamorphism near to the top of the ice sheet (i.e. the firn, about 100 m deep). In the absence of melting, the transformation of snow (a material with open porosity in contact with the atmosphere) into ice (a material containing isolated bubbles) occurs progressively as a response to temperature gradients near the surface, and the weight of overlying snow in deeper layers. Depending on temperature and precipitation conditions, this process occurs in a few decades to several millennia and a ~100 meters depth range. It controls the age difference between the ice and the entrapped gases. Predicting the gas trapping depth is a major issue in paleoclimatology, especially in order to understand the phasing between temperature changes and changes in greenhouse gas concentrations.A thermo-mechanical model of snow densification has been developed at LGGE, it includes the main mechanical processes, the thermal properties of ice, and gas trapping criteria. The model performances can be tested and improved using experimental studies of modern firns (density, open/closed porosity ratio, etc). For firnification under ancient climates, measurements of isotopes of inert gases (d15N et d40Ar) in the air trapped in ice cores provide direct informations about past variations of firn structure (e.g. diffusive zone thickness). Large differences between firn densification model outputs and gas isotopic data are obtained in Antarctica, and imply a large uncertainty on past climatic reconstructions. Understanding this discrepancy is a major issue in paleoclimatology.As part of this thesis ...