Abstract
Interpretation of greenhouse gas records in polar ice cores requires a good understanding of the mechanisms controlling gas trapping in polar ice, and therefore of the processes of densification and pore closure in firn (compacted snow). Current firn densification models are based on a macroscopic description of the firn and rely on empirical laws and/or idealized geometries to obtain the equations governing the densification and pore closure. Here, we propose a physically-based methodology explicitly representing the porous structure and its evolution over time. In order to handle the complex geometry and topological changes that occur during firn densification, we rely on a Level-Set representation of the interface between the ice and the pores. Two mechanisms are considered for the displacement of the interface: (i) mass surface diffusion driven by local pore curvature and (ii) ice dislocation creep. For the latter, ice is modeled as a viscous material and the flow velocities are solutions of the Stokes equations. First applications show that the model is able to densify firn and split pores. Using the model in cold and arid conditions of the Antarctic plateau, we show that gas trapping models do not have to consider the reduced compressibility of closed pores compared to open pores in the deepest part of firns. Our results also suggest that the mechanism of curvature-driven surface diffusion does not result in pore splitting, and that ice creep has to be taken into account for pores to close. Future applications of this type of model could help quantify the evolution and closure of firn porous networks for various accumulation and temperature conditions.
Highlights
Polar ice cores are important natural archives to study past climate dynamics
The emphasis of this paper is to demonstrate the capability of the developed micro-mechanical model to handle the densification of deep firn, including pore closure, rather than address a specific scientific question
Combining the Finite Element and Level-Set methods is a promising modeling approach to simulate the topological evolution of firn under compression
Summary
Polar ice cores are important natural archives to study past climate dynamics. Ice cores are the only archive to encapsulate bubbles of air from past atmospheres, enabling direct measurements of past concentrations of greenhouse gases. The parallel study of ice core bubbles and isotopic composition of the ice matrix (which is used to reconstruct past temperatures: Dansgaard, 1953; Johnsen et al, 1989) has highlighted the strong link between greenhouse gases and climate (Barnola et al, 1987; Lüthi et al, 2008). In order to properly interpret the gas records of ice cores in terms of atmospheric concentrations, it is necessary to understand how atmospheric air gets embedded in ice. Gas trapping is the result of the slow compaction of snow near the surface of polar ice sheets. Due to the continuous deposition of new snow at the surface, snow strata
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