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

该奖项由极地项目办公室的南极地质和地球物理项目提供，为研究南极洲麦克默多干谷地区土壤中的硫酸盐和硝酸盐提供资金。这个目标是量化大气中硫酸盐和硝酸盐的沉积，并产生第一个关于干谷土壤中大气硫酸盐和硝酸盐的起源、分布和沉积后变化的定量模型。除了验证干谷的地貌已经稳定了数百万年的假设之外，这项工作的结果将为地球上其他地方和火星上极度干旱沙漠的定量土壤发展提供有价值的参考。该方法包括对不同年龄和母质土壤中水溶性氧阴离子、硫酸盐和硝酸盐中硫、氧和氮的同位素分析。使这一建议成为可能的突破是最近在干谷土壤中发现的硫酸盐和硝酸盐的质量无关的氧同位素组成，这表明随着时间的推移，大气沉积为冷沙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谷土壤中硫酸盐氧同位素组成的空间模式，并要求存在两个硫酸盐末端成员，海盐硫酸盐和生物硫酸盐。在这里，生物源硫酸盐是指那些由还原的生物源硫气体（如二甲基硫化物）在大气中氧化形成的硫酸盐。与海盐硫酸盐相比，生物源硫酸盐由于其停留时间长，可以传播更远的距离。将在本项目中进行的氧和硫的同位素分析将区分干谷土壤中的生物硫酸盐和海盐硫酸盐。此外，来自这些土壤的硝酸盐氧同位素组成的初步数据显示，硝酸盐氧-17异常异常，空间格局反映了单一的硝酸盐来源，而不是像硫酸盐那样的两个来源。为了量化硫酸盐和硝酸盐的长期大气输入及其随后在这一独特环境中土壤中的流动性，该项目将(1)以厘米至亚厘米尺度分辨率采样垂直土壤剖面，(2)系统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)检查各种辐射年龄和母材的土壤，包括古火山灰，塌陷层，沉积物，沉积物，(4)构建硫酸盐和硝酸盐在垂直土壤剖面中的简单一维迁移模型。该项目代表了低温同位素地球化学家和在干谷地区拥有丰富经验的地貌学家之间的合作。当与过去15年来发展的干谷土壤的现有放射性同位素年代学相结合时，提出的稳定同位素分析将首次量化干谷地区土壤发育的速度和类型以及地面演化的模式。这些定量数据对于实现对干谷生态系统的全面了解以及建立基线数据至关重要，以便与火星样品返回任务后对火星风化层的预期分析进行比较。

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.