Abstract
The role of near-surface snow processes for the formation of climate signals through densification into deep polar firn is still barely understood. To this end we have analyzed a shallow snow pit (0-3 meters) from EastGRIP (Greenland) and derived high-resolution profiles of different types of mechanically relevant fabric tensors. The structural fabric, which characterizes the anisotropic geometry of ice matrix and pore space, was obtained by X-ray tomography. The crystallographic fabric, which characterizes the anisotropic distribution of the c-axis (or optical axis) orientations of snow crystals, was obtained from automatic analysis of thin sections. The structural fabric profile unambiguously reveals the seasonal cycles at EastGRIP, as a consequence of temperature gradient metamorphism, and in contrast to featureless signals of parameters like density or specific surface area. The crystallographic fabric profile unambiguously reveals a signal of cluster-type texture already at shallow depth. We make use of order of magnitude estimates for the formation time of both fabric signals and discuss potential coupling effects in the context of snow and firn densification.
Highlights
The mechanical behavior of porous ice is controlled by different types of anisotropy
A large part of the snowpack is characterized by medium to strong structural fabrics, as it can be detected on some of the micro-computed (contrast) tomography (microCT) images showing a chain-like structure of the ice matrix, and with ǫ values larger than 1.2
We analyzed the microstructural characteristic of snow along a three-meter deep pit from the EastGRIP site, Greenland
Summary
The mechanical behavior of porous ice is controlled by different types of anisotropy. The anisotropic geometry of the ice matrix and pore space, characterized by the structural fabric, affects the macroscopic mechanical properties of snow, e.g., the elasticity tensor (Hagenmuller et al, 2015; Srivastava et al, 2016; Gerling et al, 2017). As snow is made of individual crystals of ice, another type of anisotropy comes into play that is the anisotropic distribution of the c-axis (or optical axis) orientations, characterized by the crystallographic fabric. The latter has been widely investigated for dense ice and its impact on the elastic, viscoplastic and large scale flow behaviors was shown (Nakaya and Marshall, 1954; Duval et al, 1983). In Antarctica and Greenland, snow transforms into firn and ice by compaction of subsequent seasonal layers whose mechanical properties may affect densification mechanisms at depth
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