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

该奖项支持对南极洲沃斯托克站附近的大沙丘地区的雪层(FIRN)中空气化学成分的研究。它将检验这样一种假设，即一个由强劲的风驱动的混合形成的深对流区可以防止极地降雪层上部三分之一的气体分馏。在大沙丘，超低的积雪速率会导致结构变化(大颗粒、管道和裂缝)，使降雪对空气运动的渗透率比正常情况下高几个数量级。对流层厚度的未知阻碍了将冰芯15N/14N和40Ar/36Ar比值解释为过去降雪厚度的指标，这是温度、积累率和气体年龄-冰期差异等气候重要变量的关键制约因素。研究这个“极端端元”的例子将更好地确定对流区在气体重建中的作用。这项研究将从降雪中约20层的剖面中抽出空气，根据观测到的15N/14N和40Ar/36Ar与分子和涡旋扩散模型的拟合来确定对流区的存在。岩心渗透率测量和二维气流建模(与M.Albert合作)将允许对同位素数据进行更现实的解释，并将混合强度与空气速度联系起来。一种新的对流层厚度的替代指标将在降雪和冰芯气泡空气中进行测试，其原理是，缓慢扩散的重惰性气体(Kr，Xe)的同位素应该比快速扩散的氮气的同位素更受对流的影响。这些工具将被用来检验在冰川时期在沃斯托克遗址存在大沙丘和深对流带的假设，这将解释沃斯托克冰芯冰期15N/14N和40Ar/36Ar异常低的原因。这项工作的更广泛影响包括1)澄清冰芯记录中温室气体和温度的相关系，这对理解过去和未来的气候有影响，2)对一名研究生进行教育，以及3)与5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.