Physics of pore closure in polar firn, and its implications for the understanding of past feedbacks between climate and carbon cycle

• Main Author: Fourteau, Kévin
• Other Authors: Institut des Géosciences de l’Environnement (IGE), Institut de Recherche pour le Développement (IRD)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes 2016-2019 (UGA 2016-2019 ), Université Grenoble Alpes, Patricia Martinerie
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: HAL CCSD 2019
• Subjects: Firn densification; Physics; Porosity; Climate; Glaciology; Glaciologie; Climatologie; Densification du névé; Physique; Porosité; [SDU.STU.CL]Sciences of the Universe [physics]/Earth Sciences/Climatology; [SDU.STU.GL]Sciences of the Universe [physics]/Earth Sciences/Glaciology; Antarctic; East Antarctica; Antarc*; Antarctica; ice core; Ice Sheet
• Online Access: https://tel.archives-ouvertes.fr/tel-02338731; https://tel.archives-ouvertes.fr/tel-02338731/document; https://tel.archives-ouvertes.fr/tel-02338731/file/FOURTEAU_2019_archivage.pdf

As they contain air from past atmospheres, ice cores are unparalleled climate paleo-archives. The study of the gases enclosed in ice cores from the arid region of East Antarctica allows to infer the past compositions of the atmosphere back to 800,000 years ago. Gases are trapped during the compaction of the snow deposited on top of the ice sheet. In the near-surface snow, also referred to as firn, the interstitial porous network shrinks until it eventually pinches and traps gases in the ice. However, the very process of gas trapping has impacts on the gas signals recorded in ice cores. The interpretation of gas records requires to characterize how ice core and atmospheric signals differ. The aim of this PhD is to study two effects altering ice core gas records, namely gas layered trapping that creates stratigraphic irregularities and firn smoothing that removes fast variability from the record. A specific focus is put on low-accumulation East Antarctic ice cores.This inquiry starts with the multi-tracer study of a firn core drilled at the Lock-In site, East Antarctica. The results show that the bottom of the firn can be seen as a stack of heterogeneous strata that densify following the same porous network evolution with density. In this vision, the stratification simply reflects the fact that some strata are in advance (or late) in their densification, but that pore closure happens in a similar fashion in all strata. This notably means that all strata contain nearly similar amounts of gases, as supported by direct measurements. High-resolution chemistry data indicate that denser strata are characterized by a high liquid conductivity, suggesting that deep firn stratification is due the impurity-induced preferential densification.This knowledge is then used to explain abrupt spikes observed in ice core methane records. For this PhD we rely on 6 new high-resolution methane records, measured in several East Antarctic ice cores at IGE. We show that the abrupt variations are layering artifacts due to stratigraphic ...