Impact of Physical Properties and Accumulation Rate on Pore Close-Off in Layered Firn
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 infl...
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ftdartmouthcoll:oai:digitalcommons.dartmouth.edu:facoa-3320 2023-07-16T03:54:28+02:00 Impact of Physical Properties and Accumulation Rate on Pore Close-Off in Layered Firn Gregory, S. A Albert, M. R Baker, I. 2014-01-13T08:00:00Z application/pdf https://digitalcommons.dartmouth.edu/facoa/2316 https://doi.org/10.5194/tc-8-91-2014 https://digitalcommons.dartmouth.edu/context/facoa/article/3320/viewcontent/tc_8_91_2014.pdf unknown Dartmouth Digital Commons https://digitalcommons.dartmouth.edu/facoa/2316 doi:10.5194/tc-8-91-2014 https://digitalcommons.dartmouth.edu/context/facoa/article/3320/viewcontent/tc_8_91_2014.pdf Dartmouth Scholarship Earth Sciences Glaciology Physical Sciences and Mathematics text 2014 ftdartmouthcoll https://doi.org/10.5194/tc-8-91-2014 2023-06-28T10:38:26Z 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 ... Text Antarc* Antarctic Antarctica East Antarctica ice core Ice Sheet Dartmouth Digital Commons (Dartmouth College) Antarctic East Antarctica West Antarctic Ice Sheet The Cryosphere 8 1 91 105 |
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Open Polar |
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Dartmouth Digital Commons (Dartmouth College) |
op_collection_id |
ftdartmouthcoll |
language |
unknown |
topic |
Earth Sciences Glaciology Physical Sciences and Mathematics |
spellingShingle |
Earth Sciences Glaciology Physical Sciences and Mathematics Gregory, S. A Albert, M. R Baker, I. Impact of Physical Properties and Accumulation Rate on Pore Close-Off in Layered Firn |
topic_facet |
Earth Sciences Glaciology Physical Sciences and Mathematics |
description |
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 ... |
format |
Text |
author |
Gregory, S. A Albert, M. R Baker, I. |
author_facet |
Gregory, S. A Albert, M. R Baker, I. |
author_sort |
Gregory, S. A |
title |
Impact of Physical Properties and Accumulation Rate on Pore Close-Off in Layered Firn |
title_short |
Impact of Physical Properties and Accumulation Rate on Pore Close-Off in Layered Firn |
title_full |
Impact of Physical Properties and Accumulation Rate on Pore Close-Off in Layered Firn |
title_fullStr |
Impact of Physical Properties and Accumulation Rate on Pore Close-Off in Layered Firn |
title_full_unstemmed |
Impact of Physical Properties and Accumulation Rate on Pore Close-Off in Layered Firn |
title_sort |
impact of physical properties and accumulation rate on pore close-off in layered firn |
publisher |
Dartmouth Digital Commons |
publishDate |
2014 |
url |
https://digitalcommons.dartmouth.edu/facoa/2316 https://doi.org/10.5194/tc-8-91-2014 https://digitalcommons.dartmouth.edu/context/facoa/article/3320/viewcontent/tc_8_91_2014.pdf |
geographic |
Antarctic East Antarctica West Antarctic Ice Sheet |
geographic_facet |
Antarctic East Antarctica West Antarctic Ice Sheet |
genre |
Antarc* Antarctic Antarctica East Antarctica ice core Ice Sheet |
genre_facet |
Antarc* Antarctic Antarctica East Antarctica ice core Ice Sheet |
op_source |
Dartmouth Scholarship |
op_relation |
https://digitalcommons.dartmouth.edu/facoa/2316 doi:10.5194/tc-8-91-2014 https://digitalcommons.dartmouth.edu/context/facoa/article/3320/viewcontent/tc_8_91_2014.pdf |
op_doi |
https://doi.org/10.5194/tc-8-91-2014 |
container_title |
The Cryosphere |
container_volume |
8 |
container_issue |
1 |
container_start_page |
91 |
op_container_end_page |
105 |
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1771551710896979968 |