How Thick Is the Convective Zone: A Study of Firn Air in the Megadunes Near Vostok, Antarctica

该奖项支持对南极Vostok站附近“megadunes”地区雪层（firn）中空气化学成分的研究。它将检验一个假说，即强烈的风驱动混合的深层“对流区”可以阻止极地固体层上部三分之一的气体分馏。在特大暴雪中，超低的积雪率导致结构变化（大颗粒、管道和裂缝），使雪对空气运动的渗透性比正常情况高几个数量级。对流带厚度的未知阻碍了冰芯15N/14N和40Ar/36Ar比值作为过去冰层厚度指标的解释，而冰层厚度是温度、积累速率和气-冰期差异等重要气候变量的关键制约因素。研究这个“极端端元”的例子将更好地定义对流区在气体重建中的作用。本研究将从公司约20深度的剖面中抽取空气，根据观察到的15n /14N和40Ar/36Ar与分子和涡流扩散模型的拟合，最终测试对流区的存在。岩心渗透率测量和二维气流模型（与M. Albert合作）将允许对同位素数据进行更实际的物理解释，并将混合活力与空气速度联系起来。基于慢扩散重惰性气体（Kr, Xe）的同位素比快扩散的N2的同位素受对流影响更大的原理，将在铁芯和冰芯气泡空气中测试一种新的对流区厚度代理指标。这些工具将应用于对Vostok冰芯冰期存在大涡和深对流带的假设的检验，这将解释Vostok冰芯冰期异常低的15N/14N和40Ar/36Ar。这项工作的更广泛的影响包括：1)澄清冰芯记录中温室气体和温度的相位关系，对理解过去和未来气候的影响；2)教育一名研究生；3)与五名研究人员建立合作关系。

This award supports a study of the chemical composition of air in the snow layer (firn) in a region of "megadunes" near Vostok station, Antarctica. It will test the hypothesis that a deep "convective zone" of vigorous wind-driven mixing can prevent gas fractionation in the upper one-third of the polar firn layer. In the megadunes, ultralow 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 15N/14N and 40Ar/36Ar 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. Studying this "extreme end-member" example will better define the role of the convective zone in gas reconstructions. This study will pump air from a profile of ~20 depths in the firn, to definitively test for the presence of a convective zone based on the fit of observed 15 N/14N and 40Ar/36Ar to a molecular- and eddy-diffusion model. Permeability measurements on the core and 2-D air flow modeling (in collaboration with M. Albert) will permit a more physically realistic interpretation of the isotope data and will relate mixing vigor to air velocities. A new proxy indicator of convective zone thickness will be tested on firn and ice core bubble air, based on the principle that isotopes of slow-diffusing heavy noble gases (Kr, Xe) should be more affected by convection than isotopes of fast-diffusing N2 . These tools will be applied to a test of the hypothesis that the megadunes and a deep convective zone existed at the Vostok site during glacial periods, which would explain the anomalously low 15N/14N and 40Ar/36Ar in the Vostok ice core glacial periods. The broader impacts of this work include 1) clarification of phase relationships of greenhouse gases and temperature in ice core records, with implications for understanding of past and future climates, 2) education of one graduate student, and 3) building of collaborative relationships with five investigators.