Stable Isotope Profiles in Modern Desert Soils

从成壤碳酸盐的稳定同位素数据中获得的信息已被证明对我们理解陆地环境是非常宝贵的。土壤中C_3/C_4生物量之比影响成土碳酸盐的DeltaC_(13)值，这种比值随大气P(CO_2)和降水季节的变化而变化。DeltaO18值反映了当地的大气降水，这与气候直接相关。然而，可以从成壤碳酸盐中提取的信息量是有限的，因为关于它们形成的某些假设从未经过测试。问题包括：1)碳酸盐是在特定季节形成的，还是在风暴事件之后形成的？2)碳酸盐是在与土壤二氧化碳和土壤水的同位素平衡状态下形成的吗？3)不同的生物群(除了C3/C4比率)有没有不同的特征，可以用来更好地模拟古代成壤碳酸盐的碳同位素测量？我们通过测量土壤二氧化碳和水的同位素组成作为深度的函数来解决这些问题，并将这些数据与现代(晚全新世)土壤中新形态碳酸盐的测量值进行比较。控制DeltaC13土壤二氧化碳的其他几个参数也将在全年进行监测，以确定随温度和湿度变化而变化的情况。这项研究的结果将允许更详细和准确地利用土壤碳酸盐地球化学来重建古气候。我们已经确定了两个研究地点：1)阿尔伯克基以东的桑迪亚山脉的山前和2)位于阿尔伯克基以南约1小时车程的塞维莱塔国家野生动物保护区(SNWR)。SNWR是理想的，因为它包含了四个不同生物群的汇聚。这些生物群在过去100年里随着气候变化而迁移，使我们能够评估土壤成因碳酸盐中发生的溶解/再沉淀程度。这项研究所需的所有分析技术都经过了充分的审查。我们开发了一种简单的技术来提取和测量土壤二氧化碳的DeltaC13和DeltaO18值，以及另一种分析亚毫米土壤水分的DeltaO18值的技术。我们已经证明了我们有能力测量土壤剖面中新形态碳酸盐的DeltaC13和DeltaO18值。该项目将受益于两个PIs的专业知识，Sharp用于稳定同位素地球化学，McFadden用于土壤成因和土壤碳酸盐形成的建模。广泛的影响：这项工作涉及研究生、本科生和K-12水平的重要学生研究部分。它构成了丹·布雷克博士研究的基础，除了将土壤学与地球化学相结合的广泛实地工作外，还将使他接触到广泛的地球化学分析技术。北卡罗来纳大学被指定为少数族裔服务机构，是该国仅有的四个也被指定为西班牙裔服务机构的研究/博士生机构之一。像过去一样，我们将招收几名少数族裔本科生来承担高级荣誉学位论文，作为他们教育的一部分，并为该项目服务。最后，我们预计将有几名NASA Sharp学生(NASA资助的针对少数族裔/女性学生的暑期实习)在11/12年级水平上致力于该项目的一个方面。皮夏普已经有五名成功的NASA夏普学生参与了各种项目。

The information available from stable isotope data of pedogenic carbonates has proven invaluable in our understanding of the terrestrial environment. DeltaC13 values of pedogenic carbonate are influenced by the ratio of C3 to C4 biomass in soils, which varies with changes in atmospheric p(CO2) and the seasonality of precipitation. DeltaO18 values reflect local meteoric water, which is directly tied to climate. The amount of information that can be extracted from pedogenic carbonates is limited, however, because certain assumptions regarding their formation have never been tested. Questions include: 1) Do carbonates form during a particular season or following discreet storm events?; 2) Do carbonates form in isotopic equilibrium with soil CO2 and soil water?; 3) Are there distinct characteristics of different biomes (in addition to C3/C4 ratios) that can be used to better model carbon isotope measurements of ancient pedogenic carbonate? We address these questions by measuring the isotopic composition of soil CO2 and water as a function of depth and comparing these data with measured values of neoform carbonate in modern (late Holocene) soils. Several other parameters that control deltaC13soil CO2 will also be monitored throughout the year to identifying variability in response to changes in temperature and moisture. The results of this study will allow for more detailed and accurate reconstruction of paleoclimate using soil carbonate geochemistry.We have identified two sites for the study: 1) the piedmont of the Sandia mountains east of Albuquerque and 2) the Sevilleta National Wildlife Refuge (SNWR), located approximately 1 hour south of Albuquerque. The SNWR is ideal because it encompasses the convergence of four distinct biomes. These biomes have migrated over the last 100 years in response to climate change, allowing us to evaluate the degree of dissolution/reprecipitation that occurs in pedogenic carbonate. All analytical techniques required for this study have been fully vetted. We have developed a simple technique for extracting and measuring the deltaC13 and deltaO18 values of soil CO2, and another for analyzing the deltaO18 value of sub-millimeter amounts of soil water. We have demonstrated our ability to measure the deltaC13 and deltaO18 values of neoform carbonates in soil profiles. The project will benefit from the expertise of the two PIs, Sharp for stable isotope geochemistry, and McFadden for soil genesis and modeling of soil carbonate formation.Broader impacts: This work involves a significant student-research component at graduate, undergraduate and K-12 levels. It forms the basis of a Ph.D. study by Dan Breecker and will expose him to a wide range of geochemical analytical techniques in addition to the extensive field work combining pedology with geochemistry. UNM is designated as a Minority Serving Institution and is one of only four Research/Doctoral-Extensive institutions in the country to also be designated as a Hispanic Serving Institution. As in the past, we will recruit several minority undergraduate students to undertake senior honor's theses as part of their education and for the benefit of the project. Finally we expect to have several NASA SHARP students (NASA funded summer internships for minority/women students) at the 11th/12th grade level working on an aspect of this project. PI Sharp has already had five successful NASA SHARP students working on various projects.