Collaborative Research: Multiple Isotope Analyses of Soil Sulfate and Nitrate in the Antarctic Dry Valleys

该奖项由极地计划办公室南极地质和地球物理计划提供，为研究南极麦克默多干旱山谷地区土壤中的硫酸盐和硝酸盐提供资金。这一目标是为了量化大气中硫酸盐和硝酸盐的沉积，并建立第一个关于干旱河谷土壤中大气硫酸盐和硝酸盐的来源、分布和沉积后变化的定量模型。除了验证干旱山谷地貌几百万年来一直稳定的假设外，这项工作的结果还将为地球其他地方和火星上极度干旱的沙漠的土壤定量开发提供有价值的参考。该方法包括对不同年龄和母质的土壤中的硫酸盐和硝酸盐中的水溶性氧离子、硫酸盐和硝酸盐中的硫、氧和氮进行同位素分析。使这一建议成为可能的突破性进展是最近在干旱河谷土壤中发现了硫酸盐和硝酸盐的质量独立氧同位素组成，这表明随着时间的推移，大气沉降为寒冷荒漠土壤贡献了大量的硫酸盐和硝酸盐。PI最近的工作确定了干燥河谷土壤中硫酸盐氧同位素组成的空间格局，并要求存在两个硫酸盐端元，即海盐硫酸盐和生物硫酸盐。这里，生物硫酸盐是指大气中还原的生物硫磺气体(如二甲基硫化物)氧化形成的硫酸盐。生物硫酸盐由于其较长的停留时间，相对于海盐硫酸盐可以移动更远的距离。将在该项目中进行的氧和硫的同位素分析将区分干旱河谷土壤中的生物硫酸盐和海盐硫酸盐。此外，关于这些土壤硝酸盐氧同位素组成的初步数据显示，硝态氧-17异常异常巨大，其空间格局反映了硝酸盐的单一来源，而不是硫酸盐的两个来源。为了量化长期大气中硫酸盐和硝酸盐的输入及其在这种独特环境下土壤中的迁移性，该项目将(1)在厘米到亚厘米尺度的垂直土壤剖面上采样，(2)系统地分析硫酸盐中的氧和硫同位素以及硝酸盐中的氧和氮同位素，(3)检查各种放射性年龄和母质的土壤，包括古火山灰、冰川、倒伏和冰升华土壤，和(4)建立一个简单的垂直土壤剖面中硫酸盐和硝酸盐的一维运移模型。该项目由一位低温同位素地球化学家和一位在干旱河谷地区有丰富经验的地貌学家合作完成。当与过去15年发展起来的干旱河谷土壤的现有放射性同位素年代学相结合时，拟议的稳定同位素分析将首次量化干旱河谷地区土壤发育的速度和类型以及地面模式演变。这些定量数据对于彻底了解干旱河谷生态系统，以及建立基线数据，以便与火星样品返回火星任务后预期的火星风化分析进行比较，是至关重要的。

This award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, provides funds for a study sulfate and nitrate in soils of the McMurdo Dry Valleys region of Antarctica. This goal is to quantify atmospheric deposition of sulfate and nitrate and to generate the first quantitative model for the origin, distribution, and post-depositional alteration of atmospheric sulfate and nitrate in Dry Valley soils. In addition to testing the hypothesis that landforms in the Dry Valleys have been stable for millions of years, results from this work will provide a valuable reference for quantitative soil development in hyper-arid deserts elsewhere on Earth and on Mars. The approach involves isotopic analyses of sulfur, oxygen, and nitrogen in the water-soluble oxy-anions, sulfate and nitrate, in soils of differing ages and parent materials. The breakthrough that makes this proposal possible is the recent discovery of mass-independent oxygen-isotopic composition for sulfate and nitrate in Dry Valley soils that shows that atmospheric deposition has contributed a significant amount of sulfate and nitrate to the cold-desert soils over time.Recent work by the PI's identified spatial patterns in the oxygen-isotopic composition of sulfate in Dry Valleys soils and requires the existence of two sulfate end members, sea-salt sulfate and biogenic sulfate. Here, the biogenic sulfate refers to those sulfates formed by the oxidation of reduced biogenic sulfur gases (e.g., dimethylsulfide) in the atmosphere. Biogenic sulfate, due to its long residence time can travel greater distances relative to sea-salt sulfate. The isotopic analyses of oxygen and sulfur that will be done within this project will differentiate between biogenic sulfate and sea-salt sulfate in Dry Valley soils. In addition, preliminary data on the oxygen-isotopic composition of nitrate from these soils reveal exceptionally large nitrate oxygen-17 anomalies and a spatial pattern that reflects a single nitrate source rather than two sources as for sulfate. To quantify long-term atmospheric input of sulfate and nitrate and their subsequent mobility in soils of this unique environment, this project will (1) sample vertical soil profiles at centimeter-to-sub-centimeter-scale resolution, (2) systematically analyze oxygen and sulfur isotopes in sulfate and oxygen and nitrogen isotopes in nitrate, (3) examine soils of a wide range of radiometric ages and parent materials, including ancient volcanic ashes, colluvium, lodgment tills, and ice-sublimation tills, and (4) construct a simple one-dimensional transport model for sulfate and nitrate in vertical soil profiles.This project represents collaboration between a low temperature isotope geochemist and a geomorphologist who has extensive experience in the Dry Valleys region. When combined with existing radiometric isotope chronology of Dry Valley soils developed over the last 15 years, the proposed stable isotopic analyses will, for the first time, quantify the rate and style of soil development and patterned ground evolution in the Dry Valleys region. These quantitative data are of paramount importance for achieving a thorough understanding of the Dry Valleys ecosystem and for establishing baseline data for comparison with anticipated analyses on Martian regolith following a sample return mission to Mars.