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

该奖项由极地方案办公室南极地质和地球物理方案提供，为研究南极洲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。这一目标是量化硫酸盐和硝酸盐的大气沉积，并生成第一个定量模型的起源，分布和沉积后的大气硫酸盐和硝酸盐在干谷土壤中的变化。除了测试干旱山谷的地貌数百万年来一直稳定的假设外，这项工作的结果将为地球上其他地方和火星上超干旱沙漠的定量土壤发育提供有价值的参考。该方法包括对不同年龄和母质土壤中水溶性含氧阴离子硫酸盐和硝酸盐中的硫、氧和氮进行同位素分析。使这一提议成为可能的突破是最近发现干谷土壤中硫酸盐和硝酸盐的氧同位素组成与质量无关，这表明随着时间的推移，大气沉降为寒冷沙漠土壤提供了大量的硫酸盐和硝酸盐。干河谷土壤中硫酸盐的同位素组成，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。这里，生物源硫酸盐是指通过还原的生物源硫气体（例如，二甲基硫醚）在大气中。生物硫酸盐，由于其长的停留时间可以移动更远的距离相对于海盐硫酸盐。在本项目中进行的氧和硫同位素分析将区分干谷土壤中的生物源硫酸盐和海盐硫酸盐。此外，从这些土壤硝酸盐的氧同位素组成的初步数据显示异常大的硝酸盐氧-17异常和空间格局，反映了一个单一的硝酸盐来源，而不是两个来源的硫酸盐。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的土壤中的流动性，该项目将（1）以厘米至亚厘米尺度的分辨率对垂直土壤剖面进行采样，（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的土壤和母质，包括古火山灰、崩积物、堆积物和冰升华物;（4）建立一个简单的一维土壤剖面中硫酸盐和硝酸盐迁移模型。当结合现有的放射性同位素年代学的干谷土壤开发在过去的15年中，拟议的稳定同位素分析，将首次量化的速度和风格的土壤发育和图案地面演变的干谷地区。这些定量数据对于彻底了解干谷生态系统和建立基线数据以与预期的火星风化层分析进行比较至关重要，这些分析是在火星取样返回使命之后进行的。

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.