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| Titel |
Extreme metamorphism in a firn core from the Allan Hills, Antarctica, as an analogue for glacial conditions |
| VerfasserIn |
Ruzica Dadic, Martin Schneebeli, Nancy Bertler, Margit Schwikowski, Margret Matzl |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250108166
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| Publikation (Nr.) |
 EGU/EGU2015-7907.pdf |
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| Zusammenfassung |
| Understanding processes in near-zero accumulation areas can help to better understand the
ranges of isotopic composition in ice cores, particularly during ice ages, when accumulation
rates were lower than today. Snow metamorphism is a primary driver of the transition from
snow to ice and can be accompanied by altered isotopic compositions and chemical species
concentration. High degree snow metamorphism, which results in major structural changes, is
little-studied but has been identified in certain places in Antarctica. Here we report on a
5–m firn core collected adjacent to a blue-ice field in the Allan Hills, Antarctica.
We determined the physical properties of the snow using computer tomography
(microCT) and measured the isotopic composition of δD and δ18O, as well as 210Pb
activity. The core shows a high degree of snow metamorphism and an exponential
decrease in specific surface area (SSA), but no clear densification, with depth. The
micro-CT measurements show a homogenous and stable structure throughout the
entire core, with obvious erosion features in the near-surface, where high-resolution
data is available. The observed firn structure is likely caused by a combination of
unique depositional and post-depositional processes. The defining depositional
process is the impact deposition under high winds and with a high initial density.
The defining post-depositional processes are a) increased moisture transport due to
forced ventilation and high winds and b) decades of temperature-gradient driven
metamorphic growth in the near surface due to prolonged exposure to seasonal temperature
cycling. Both post–processes are enhanced in low accumulation regions where
snow stays close to surface for a long time. We observe an irregular signal in δD
and δ18O that does not follow the stratigraphic sequence. The isotopic signal is
likely caused by the same post-depositional processes that are responsible for the
firn structure, and that are driven by local climate. Mechanical processes such as
scouring and spatial distribution of snow by wind are also likely to affect the isotope
content. We use 210Pb activity to date the core, but find no signal below 0.3 m. The
lack of any 210Pb activity implies that most of the snow is older than 100 years. |
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