Steady state fractionation of heavy noble gas isotopes in a deep unsaturated zone: UZ NOBLE GAS ISOTOPIC FRACTIONATION
Steady state fractionation of heavy noble gas isotopes in a deep unsaturated zone: UZ NOBLE GAS ISOTOPIC FRACTIONATION
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深层不饱和区重惰性气体同位素的稳态分馏：UZ 稀有气体同位素分馏
DOI：
10.1002/2016wr019655
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发表时间：
2017
影响因子：
5.4
通讯作者：
Stonestrom, David A.
中科院分区：
文献类型：
--
作者：
Seltzer, Alan M.;Severinghaus, Jeffrey P.;Andraski, Brian J.;Stonestrom, David A.
为了探索不饱和带（UZ）的稳态分选过程，我们在美国内华达州美国地质调查局（USGS）阿玛戈萨沙漠研究基地（ADRS）的一个~ 110 m深的UZ中测量了氩、氪和氙的同位素比率。先前的研究表明，在多孔介质中，重力沉降应该会在停滞气柱的底部产生近线性的重-轻同位素比值增加。我们的高精度测量揭示了(1)预期的稳态同位素组成和(2)未分馏的大气空气之间的二元混合物。我们假设，在我们第一次（2015年）测量之前，由于广泛的UZ气体采样，连接地表和地下水位的非密封管道的存在允许地表空气直接流入。在密封管道一年后收集的深UZ样品中观察到的同位素重新沉淀支持了这一解释。数据和模型都表明，重力沉降的强影响和热扩散以及co2和水蒸气通量的弱影响准确地描述了UZ内氩、氪和氙的稳态同位素分馏。数据证实，重惰性气体同位素是UZ深度的敏感指标。基于这一发现，我们概述了一种潜在的反向方法，在考虑UZ空气和气泡溶解过程中的分馏后，通过古地下水中稀有气体同位素的测量来量化过去的地下水位深度。
To explore steady state fractionation processes in the unsaturated zone (UZ), we measured argon, krypton, and xenon isotope ratios throughout a ∼110 m deep UZ at the United States Geological Survey (USGS) Amargosa Desert Research Site (ADRS) in Nevada, USA. Prior work has suggested that gravitational settling should create a nearly linear increase in heavy‐to‐light isotope ratios toward the bottom of stagnant air columns in porous media. Our high‐precision measurements revealed a binary mixture between (1) expected steady state isotopic compositions and (2) unfractionated atmospheric air. We hypothesize that the presence of an unsealed pipe connecting the surface to the water table allowed for direct inflow of surface air in response to extensive UZ gas sampling prior to our first (2015) measurements. Observed isotopic resettling in deep UZ samples collected a year later, after sealing the pipe, supports this interpretation. Data and modeling each suggest that the strong influence of gravitational settling and weaker influences of thermal diffusion and fluxes of CO2and water vapor accurately describe steady state isotopic fractionation of argon, krypton, and xenon within the UZ. The data confirm that heavy noble gas isotopes are sensitive indicators of UZ depth. Based on this finding, we outline a potential inverse approach to quantify past water table depths from noble gas isotope measurements in paleogroundwater, after accounting for fractionation during dissolution of UZ air and bubbles.