Modelling firn thickness evolution during the last deglaciation: constraints on sensitivity to temperature and impurities
The transformation of snow into ice is a complex phenomenon that is difficult to model. Depending on surface temperature and accumulation rate, it may take several decades to millennia for air to be entrapped in ice. The air is thus always younger than the surrounding ice. The resulting gas–ice age...
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ftcopernicus:oai:publications.copernicus.org:cp54741 2023-05-15T13:54:27+02:00 Modelling firn thickness evolution during the last deglaciation: constraints on sensitivity to temperature and impurities Bréant, Camille Martinerie, Patricia Orsi, Anaïs Arnaud, Laurent Landais, Amaëlle 2018-09-27 application/pdf https://doi.org/10.5194/cp-13-833-2017 https://cp.copernicus.org/articles/13/833/2017/ eng eng doi:10.5194/cp-13-833-2017 https://cp.copernicus.org/articles/13/833/2017/ eISSN: 1814-9332 Text 2018 ftcopernicus https://doi.org/10.5194/cp-13-833-2017 2020-07-20T16:23:40Z The transformation of snow into ice is a complex phenomenon that is difficult to model. Depending on surface temperature and accumulation rate, it may take several decades to millennia for air to be entrapped in ice. The air is thus always younger than the surrounding ice. The resulting gas–ice age difference is essential to documenting the phasing between CO 2 and temperature changes, especially during deglaciations. The air trapping depth can be inferred in the past using a firn densification model, or using δ 15 N of air measured in ice cores. All firn densification models applied to deglaciations show a large disagreement with δ 15 N measurements at several sites in East Antarctica, predicting larger firn thickness during the Last Glacial Maximum, whereas δ 15 N suggests a reduced firn thickness compared to the Holocene. Here we present modifications of the LGGE firn densification model, which significantly reduce the model–data mismatch for the gas trapping depth evolution over the last deglaciation at the coldest sites in East Antarctica (Vostok, Dome C), while preserving the good agreement between measured and modelled modern firn density profiles. In particular, we introduce a dependency of the creep factor on temperature and impurities in the firn densification rate calculation. The temperature influence intends to reflect the dominance of different mechanisms for firn compaction at different temperatures. We show that both the new temperature parameterization and the influence of impurities contribute to the increased agreement between modelled and measured δ 15 N evolution during the last deglaciation at sites with low temperature and low accumulation rate, such as Dome C or Vostok. We find that a very low sensitivity of the densification rate to temperature has to be used in the coldest conditions. The inclusion of impurity effects improves the agreement between modelled and measured δ 15 N at cold East Antarctic sites during the last deglaciation, but deteriorates the agreement between modelled and measured δ 15 N evolution at Greenland and Antarctic sites with high accumulation unless threshold effects are taken into account. We thus do not provide a definite solution to the firnification at very cold Antarctic sites but propose potential pathways for future studies. Text Antarc* Antarctic Antarctica East Antarctica Greenland Copernicus Publications: E-Journals Antarctic East Antarctica Greenland Climate of the Past 13 7 833 853 |
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Open Polar |
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Copernicus Publications: E-Journals |
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ftcopernicus |
language |
English |
description |
The transformation of snow into ice is a complex phenomenon that is difficult to model. Depending on surface temperature and accumulation rate, it may take several decades to millennia for air to be entrapped in ice. The air is thus always younger than the surrounding ice. The resulting gas–ice age difference is essential to documenting the phasing between CO 2 and temperature changes, especially during deglaciations. The air trapping depth can be inferred in the past using a firn densification model, or using δ 15 N of air measured in ice cores. All firn densification models applied to deglaciations show a large disagreement with δ 15 N measurements at several sites in East Antarctica, predicting larger firn thickness during the Last Glacial Maximum, whereas δ 15 N suggests a reduced firn thickness compared to the Holocene. Here we present modifications of the LGGE firn densification model, which significantly reduce the model–data mismatch for the gas trapping depth evolution over the last deglaciation at the coldest sites in East Antarctica (Vostok, Dome C), while preserving the good agreement between measured and modelled modern firn density profiles. In particular, we introduce a dependency of the creep factor on temperature and impurities in the firn densification rate calculation. The temperature influence intends to reflect the dominance of different mechanisms for firn compaction at different temperatures. We show that both the new temperature parameterization and the influence of impurities contribute to the increased agreement between modelled and measured δ 15 N evolution during the last deglaciation at sites with low temperature and low accumulation rate, such as Dome C or Vostok. We find that a very low sensitivity of the densification rate to temperature has to be used in the coldest conditions. The inclusion of impurity effects improves the agreement between modelled and measured δ 15 N at cold East Antarctic sites during the last deglaciation, but deteriorates the agreement between modelled and measured δ 15 N evolution at Greenland and Antarctic sites with high accumulation unless threshold effects are taken into account. We thus do not provide a definite solution to the firnification at very cold Antarctic sites but propose potential pathways for future studies. |
format |
Text |
author |
Bréant, Camille Martinerie, Patricia Orsi, Anaïs Arnaud, Laurent Landais, Amaëlle |
spellingShingle |
Bréant, Camille Martinerie, Patricia Orsi, Anaïs Arnaud, Laurent Landais, Amaëlle Modelling firn thickness evolution during the last deglaciation: constraints on sensitivity to temperature and impurities |
author_facet |
Bréant, Camille Martinerie, Patricia Orsi, Anaïs Arnaud, Laurent Landais, Amaëlle |
author_sort |
Bréant, Camille |
title |
Modelling firn thickness evolution during the last deglaciation: constraints on sensitivity to temperature and impurities |
title_short |
Modelling firn thickness evolution during the last deglaciation: constraints on sensitivity to temperature and impurities |
title_full |
Modelling firn thickness evolution during the last deglaciation: constraints on sensitivity to temperature and impurities |
title_fullStr |
Modelling firn thickness evolution during the last deglaciation: constraints on sensitivity to temperature and impurities |
title_full_unstemmed |
Modelling firn thickness evolution during the last deglaciation: constraints on sensitivity to temperature and impurities |
title_sort |
modelling firn thickness evolution during the last deglaciation: constraints on sensitivity to temperature and impurities |
publishDate |
2018 |
url |
https://doi.org/10.5194/cp-13-833-2017 https://cp.copernicus.org/articles/13/833/2017/ |
geographic |
Antarctic East Antarctica Greenland |
geographic_facet |
Antarctic East Antarctica Greenland |
genre |
Antarc* Antarctic Antarctica East Antarctica Greenland |
genre_facet |
Antarc* Antarctic Antarctica East Antarctica Greenland |
op_source |
eISSN: 1814-9332 |
op_relation |
doi:10.5194/cp-13-833-2017 https://cp.copernicus.org/articles/13/833/2017/ |
op_doi |
https://doi.org/10.5194/cp-13-833-2017 |
container_title |
Climate of the Past |
container_volume |
13 |
container_issue |
7 |
container_start_page |
833 |
op_container_end_page |
853 |
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1766260322653437952 |