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2003-2004 USAP
Field Season
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Glaciology
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Dr. Julie Palais
Program Manager
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I-184-M
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NSF/OPP
02-30452
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Station:
McMurdo Station
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RPSC POC:
Jessie Crain
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Research Site(s):
McMurdo Station, Megadunes
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Dates in Antarctica:
Mid December to late January
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How thick is the convective zone?: A study of firn air in the megadunes near Vostok
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Photo not available.
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Deploying Team Members:
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Lou Albershardt
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Jeffrey P. Severinghaus
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Makoto Suwa
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Mark (Tony) A. Wumkes
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Research Objectives:
In the megadunes, extremely low snow accumulation rates lead to structural changes (large grains, pipes, and cracks) that make the permeability of firn-to-air movement orders of magnitude higher than normal. The unknown thickness of the convective zone has hampered the interpretation of ice-core nitrogen/argon isotope ratios as indicators of past firn thickness, which is a key constraint on the climatically important variables of temperature, accumulation rate, and gas age–ice age difference. We will therefore study the chemical composition of air in the snow layer (firn) in a region of megadunes near Vostok Station to test the hypothesis that a deep convective zone of vigorous wind-driven mixing can prevent gas fractionation in the upper third of the polar firn layer. Studying this extreme end-member example will better define the role of the convective zone in gas reconstructions.
We will pump air from a profile of about 20 depths in the firn to definitively test for the presence of a convective zone based on how well inert gas isotopes fit a molecular- and eddy-diffusion model. Permeability measurements on the core and two-dimensional air flow modeling will permit a more physically realistic interpretation of the isotope data and will relate mixing vigor to air velocities. We will also test a new proxy indicator of convective zone thickness on firn and ice-core bubble air; this indicator is based on the principle that isotopes of slow-diffusing heavy noble gases (krypton, xenon) should be more affected by convection than isotopes of fast-diffusing nitrogen.
Finally, we intend to test the hypothesis that the megadunes and a deep convective zone existed at the Vostok site during glacial periods; this would explain the anomalously low nitrogen/argon isotope ratios in the Vostok ice-core glacial periods. Our work will clarify phase relationships of greenhouse gases and temperature in ice-core records, with implications for understanding past and future climates.
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