Evidence for molecular size dependent gas fractionation in firn air derived from noble gases, oxygen, and nitrogen measurements
We present elemental and isotopic measurements of noble gases (He, Ne, Ar, Kr, and Xe), oxygen and nitrogen of firn air from two sites. The first set of samples was taken in 1998 at the summit of the Devon Ice Cap in the eastern part of Devon Island. The second set was taken in 2001 at NGRIP locatio...
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ftdatacite:10.48350/20741 2023-05-15T16:00:47+02:00 Evidence for molecular size dependent gas fractionation in firn air derived from noble gases, oxygen, and nitrogen measurements Huber, Christof Beyerle, Urs Leuenberger, Markus Schwander, Jakob Kipfer, R Spahni, Renato Severinghaus, J.P. Weiler, Karin 2006 https://dx.doi.org/10.48350/20741 https://boris.unibe.ch/20741/ unknown Elsevier https://dx.doi.org/10.1016/j.epsl.2005.12.036 restricted access publisher holds copyright http://purl.org/coar/access_right/c_16ec journal article article-journal Text ScholarlyArticle 2006 ftdatacite https://doi.org/10.48350/20741 https://doi.org/10.1016/j.epsl.2005.12.036 2022-02-08T16:39:48Z We present elemental and isotopic measurements of noble gases (He, Ne, Ar, Kr, and Xe), oxygen and nitrogen of firn air from two sites. The first set of samples was taken in 1998 at the summit of the Devon Ice Cap in the eastern part of Devon Island. The second set was taken in 2001 at NGRIP location (North Greenland). He and Ne are heavily enriched relative to Ar with respect to the atmosphere in the air near the close-off depth at around 50–70 m. The enrichment increases with depth and reaches the maximum value in the deepest samples just above the zone of impermeable ice where no free air could be extracted anymore. Similarly, elemental ratios of O2 / N2, O2 / Ar and Ar / N2 are increasing with depth. In contrast but in line with expectations, isotopic ratios of 15N / 14N, 18O / 16O, and 36Ar / 40Ar show no significant enrichment near the close-off depth. The observed isotopic ratios in the firn air column can be explained within the uncertainty ranges by the well-known processes of gravitational enrichment and thermal diffusion. To explain the elemental ratios, however, an additional fractionation process during bubble inclusion has to be considered. We implemented this additional process into our firn air model. The fractionation factors were found by fitting model profiles to the data. We found a very similar close-off fractionation behavior for the different molecules at both sites. For smaller gas species (mainly He and Ne) the fractionation factors are linearly correlated to the molecule size, whereas for diameters greater than about 3.6 Å the fractionation seems to be significantly smaller or even negligible. An explanation for this size dependent fractionation process could be gas diffusion through the ice lattice. At Devon Island the enrichment at the bottom of the firn air column is about four times higher compared to NGRIP. We explain this by lower firn diffusivity at Devon Island, most probably due to melt layers, resulting in significantly reduced back diffusion of the excess gas near the close-off depth. The results of this study considerably increase the understanding of the processes occurring during air bubble inclusion near the close-off depth in firn and can help to improve the interpretation of direct firn air measurements, as well as air bubble measurements in ice cores, which are used in numerous studies as paleo proxies. Text Devon Island Greenland Ice cap NGRIP North Greenland DataCite Metadata Store (German National Library of Science and Technology) Greenland Devon Island ENVELOPE(-88.000,-88.000,75.252,75.252) Devon Ice Cap ENVELOPE(-82.499,-82.499,75.335,75.335) |
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DataCite Metadata Store (German National Library of Science and Technology) |
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description |
We present elemental and isotopic measurements of noble gases (He, Ne, Ar, Kr, and Xe), oxygen and nitrogen of firn air from two sites. The first set of samples was taken in 1998 at the summit of the Devon Ice Cap in the eastern part of Devon Island. The second set was taken in 2001 at NGRIP location (North Greenland). He and Ne are heavily enriched relative to Ar with respect to the atmosphere in the air near the close-off depth at around 50–70 m. The enrichment increases with depth and reaches the maximum value in the deepest samples just above the zone of impermeable ice where no free air could be extracted anymore. Similarly, elemental ratios of O2 / N2, O2 / Ar and Ar / N2 are increasing with depth. In contrast but in line with expectations, isotopic ratios of 15N / 14N, 18O / 16O, and 36Ar / 40Ar show no significant enrichment near the close-off depth. The observed isotopic ratios in the firn air column can be explained within the uncertainty ranges by the well-known processes of gravitational enrichment and thermal diffusion. To explain the elemental ratios, however, an additional fractionation process during bubble inclusion has to be considered. We implemented this additional process into our firn air model. The fractionation factors were found by fitting model profiles to the data. We found a very similar close-off fractionation behavior for the different molecules at both sites. For smaller gas species (mainly He and Ne) the fractionation factors are linearly correlated to the molecule size, whereas for diameters greater than about 3.6 Å the fractionation seems to be significantly smaller or even negligible. An explanation for this size dependent fractionation process could be gas diffusion through the ice lattice. At Devon Island the enrichment at the bottom of the firn air column is about four times higher compared to NGRIP. We explain this by lower firn diffusivity at Devon Island, most probably due to melt layers, resulting in significantly reduced back diffusion of the excess gas near the close-off depth. The results of this study considerably increase the understanding of the processes occurring during air bubble inclusion near the close-off depth in firn and can help to improve the interpretation of direct firn air measurements, as well as air bubble measurements in ice cores, which are used in numerous studies as paleo proxies. |
format |
Text |
author |
Huber, Christof Beyerle, Urs Leuenberger, Markus Schwander, Jakob Kipfer, R Spahni, Renato Severinghaus, J.P. Weiler, Karin |
spellingShingle |
Huber, Christof Beyerle, Urs Leuenberger, Markus Schwander, Jakob Kipfer, R Spahni, Renato Severinghaus, J.P. Weiler, Karin Evidence for molecular size dependent gas fractionation in firn air derived from noble gases, oxygen, and nitrogen measurements |
author_facet |
Huber, Christof Beyerle, Urs Leuenberger, Markus Schwander, Jakob Kipfer, R Spahni, Renato Severinghaus, J.P. Weiler, Karin |
author_sort |
Huber, Christof |
title |
Evidence for molecular size dependent gas fractionation in firn air derived from noble gases, oxygen, and nitrogen measurements |
title_short |
Evidence for molecular size dependent gas fractionation in firn air derived from noble gases, oxygen, and nitrogen measurements |
title_full |
Evidence for molecular size dependent gas fractionation in firn air derived from noble gases, oxygen, and nitrogen measurements |
title_fullStr |
Evidence for molecular size dependent gas fractionation in firn air derived from noble gases, oxygen, and nitrogen measurements |
title_full_unstemmed |
Evidence for molecular size dependent gas fractionation in firn air derived from noble gases, oxygen, and nitrogen measurements |
title_sort |
evidence for molecular size dependent gas fractionation in firn air derived from noble gases, oxygen, and nitrogen measurements |
publisher |
Elsevier |
publishDate |
2006 |
url |
https://dx.doi.org/10.48350/20741 https://boris.unibe.ch/20741/ |
long_lat |
ENVELOPE(-88.000,-88.000,75.252,75.252) ENVELOPE(-82.499,-82.499,75.335,75.335) |
geographic |
Greenland Devon Island Devon Ice Cap |
geographic_facet |
Greenland Devon Island Devon Ice Cap |
genre |
Devon Island Greenland Ice cap NGRIP North Greenland |
genre_facet |
Devon Island Greenland Ice cap NGRIP North Greenland |
op_relation |
https://dx.doi.org/10.1016/j.epsl.2005.12.036 |
op_rights |
restricted access publisher holds copyright http://purl.org/coar/access_right/c_16ec |
op_doi |
https://doi.org/10.48350/20741 https://doi.org/10.1016/j.epsl.2005.12.036 |
_version_ |
1766396794740146176 |