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

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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.