Extreme snow metamorphism in the Allan Hills, Antarctica, as

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TitleExtreme snow metamorphism in the Allan Hills, Antarctica, as an analogue for glacial conditions with implications for stable isotope compositionAbstractUnderstanding physical processes in near-zero accumulation areas can help us to better understand polar ice-core records, particularly during periods when accumulation rates were lower than today. We report measurements from a 5 m firn core from the Allan Hills, Antarctica, which include physical properties using computer tomography, stable isotope ratios ÃŽv ZD and ÃŽv Z18O, and 210Pb activity. The core shows a highly metamorphosed firn with homogeneous and stable structure, but with discrete layers near the surface. The observed firn structure is caused by a combination of unique depositional and post-depositional processes. The irregular dD and d18O signal does not follow the stratigraphic sequence and implies post-depositional modification caused by microscopic pressure gradients in the firn that can result from either forced ventilation over rough surfaces in the presence of wind or alternating temperature-gradients between the firn and the atmosphere. Our results also indicate impact snow deposition under high winds and with a high initial density and air exchange between the atmosphere and the snowpack. 210Pb activity below 0.3 m falls below the detection limit, implying that most of the core is more than 100 years old. We conclude that the Allan Hills record provides a unique opportunity to investigate important processes that would have affected ice-core records from glacial periods.AcknowledgementsThe fieldwork was supported by US National Science Foundation grant ANT-07-39779 to the University of Washington (principal investigator Stephen Warren). Melanie Fitzpatrick organized the logistics, and both she and Regina Carns participated in the fieldwork and discussions about the measurements. Logistical support by the US Antarctic Program was excellent. We thank Matthias Jaggi for help in the cold laboratory, Huw Horgan for numerous discussions, and two reviewers whose comments greatly improved the manuscript. Ruzica Dadic was supported by a postdoctoral fellowship from the Swiss National Science Foundation (Project 124273) and by the Marsden Fund of the Royal Society of New Zealand (VUW1314). Isotope measurements were supported through GNS Science funded from the New Zealand Ministry for Business, Innovation and Employment (grant 540GCT32).

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1st AuthorDadic, R.AuthorDadic, R.Schneebeli, M.Bertler, N.Schwikowski, M.Matzl, M.Year2015JournalJournal of GlaciologyVolume61Number230Pages1171-1182DOI10.3189/2015JoG15J027Keywordsglacial depositisotopic compositionmetamorphismpressure gradientrecrystallizationsnow accumulationsnow coversnowpackstable isotope, Allan HillsAntarcticaEast Antarctica, rank1Author KeywordsAtmosphere/ice/ocean interactionsPolar firnRecrystallizationSnow metamorphosisSnow/ice surface processes

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TypeArticleCitationDadic, R., Schneebeli, M., Bertler, N., Schwikowski, M. and Matzl, M. (2015). Extreme snow metamorphism in the Allan Hills, Antarctica, as an analogue for glacial conditions with implications for stable isotope composition. Journal of Glaciology, 61(230): 1171-1182 IdentifierDadic2015Relevancerank1

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Bertler, N.