Crystal-growth rates in firn and shallow ice at high-accumulation sites
AbstractCrystal growth in firn and shallow ice is studied by examining crystal size and ,c-axis orientation fabrics in two ice cores drilled at sites Dome Summit South and DE08, near the summit of Law Dome, East Antarctica. The snow-accumulation rates at the core sites are particularly high (640 and 1160 kg m−2 a−1, respectively) compared to other Antarctic sites. Crystal-growth rates above the firn/ice transition depth (at 70-80 m) are found to be in agreement with the generally used growth-rate-temperature relation (Stephenson, 1967; Gow, 1969), sometimes referred to as "normal grain growth". In the shallow ice layers below this depth and down to about 300 m, the observed crystal-growth rates are enhanced compared to normal grain growth. Also in this shallow ice, crystal ,c-axis orientation measurements show development of anisotropic fabrics indicative of ice flow at strains well above 1%.In earlier work, Jacka and Li (1994) described the development in clean ice of steady-state ice-crystal size (inversely proportional to the stress and largely independent of temperature) during the onset of flow-related crystal anisotropy, i.e. dynamic recrystallisation. It is concluded here that as a consequence of the high accumulation rates, relatively high deformation rates are generated in the shallow ice. The deformation rates are sufficiently high that "dynamic recrystallisation" takes over from "normal crystal growth" as the dominant crystal-growth mechanism. This leads to a rapid increase in crystal size from the slow-growing small firn crystals towards the larger size appropriate to the stress.
