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| Titel |
Impact of physical properties and accumulation rate on pore close-off in layered firn |
| VerfasserIn |
S. A. Gregory, M. R. Albert, I. Baker |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1994-0416
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| Digitales Dokument |
URL |
| Erschienen |
In: The Cryosphere ; 8, no. 1 ; Nr. 8, no. 1 (2014-01-13), S.91-105 |
| Datensatznummer |
250116008
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| Publikation (Nr.) |
 copernicus.org/tc-8-91-2014.pdf |
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| Zusammenfassung |
| Investigations into the physical characteristics of deep firn near the
lock-in zone through pore close-off are needed to improve understanding of
ice core records of past atmospheric composition. Specifically, the
permeability and microstructure profiles of the firn through the diffusive
column influence the entrapment of air into bubbles and thus the ice age–gas
age difference. The purpose of this study is to examine the nature of pore
closure processes at two polar sites with very different local temperatures
and accumulation rates. Density, permeability, and microstructure
measurements were made on firn cores from the West Antarctic Ice Sheet (WAIS) Divide, a
site that has moderate accumulation rates with a seasonal climate archive,
and Megadunes in East Antarctica, a site that is a natural laboratory for
accumulation rate effects in the cold low-accumulation desert. We found that
the open pore structure plays a more important role than density in
predicting gas transport properties, throughout the porous firn matrix. For
firn below 50 m depth at both WAIS Divide and Megadunes, finer-grained
layers experience close-off shallower in the firn column than do
coarser-grained layers, regardless of which grain size layer is the denser
layer at depth. Pore close-off occurs at a critical open porosity that is
accumulation rate dependent. Defining pore close-off at a critical open
porosity for a given accumulation rate as opposed to a critical total
porosity accounts for the pore space available for gas transport. Below the
critical open porosity, the firn becomes impermeable despite having small
amounts of interconnected pore space. The low-accumulation sites, with
generally coarse grains, close off at lower open porosities (~<10%)
than the open porosity (~>10%) of high-accumulation sites that have generally finer grains. The
microstructure and permeability even near the bottom of the firn column are
relic indicators of the nature of accumulation when that firn was at the
surface. The physical structure and layering are the primary controlling
factors on pore close-off. In contrast to current assumptions for polar
firn, the depth and length of the lock-in zone is primarily dependent upon
accumulation rate and microstructural variability due to differences in
grain size and pore structure, rather than the density variability of the
layers. |
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