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Crystal-growth rates in firn and shallow ice at high-accumulation sites

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Abstract:

Crystal 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.

Document Type: Research Article

DOI: http://dx.doi.org/10.3189/172756499781821508

Publication date: June 1, 1999

More about this publication?
  • The Annals of Glaciology is a peer-reviewed, thematic journal published 2 to 4 times a year by the International Glaciological Society (IGS). Publication frequency is determined and volume/issue numbers assigned by the IGS Council on a year-to-year basis and with a lead time of 3 to 4 years. The Annals of Glaciology is included in the ISI Science Citation Index from volume 50, number 50 onwards.

    Themes can be on any aspect of the study of snow and ice. Individual members can make a suggestion for a theme for an Annals issue to the Secretary General, who will forward it to the IGS Publications Committee. The IGS Publication Committee will make a recommendation for an individual themed Annals issue, together with a potential Annals Chief Editor for that issue, to IGS Council. The IGS Council will make the decision whether to proceed, taking into account the spread of topics and the overall capacity for publication of pages in Annals.

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