What Controls Carbonate Formation in Fine-grained and Clay-rich Soils? Gaining Perspective from Serial Soil Water Stable Isotope Datasets

长期以来，研究人员一直使用碳酸钙（CaCO3）的小而不规则的致密生长（“结节”）中的氧和碳的稳定同位素来重建过去的气候。然而，为了更好地解释这些结节中保存的信息，有必要更好地了解结节如何在现代环境中形成。在该项目中，研究人员将研究土壤温度、土壤化学和土壤水分蒸发如何促进细粒和富含粘土的土壤中碳酸钙结核的形成，以及如何通过结核的稳定同位素记录土壤温度和化学信号。这项研究之所以重要和及时，有三个原因。首先，更全面地了解过去的气候状态可以提供有关地球气候系统如何基本运作的背景和信息。其次，土壤柱中碳酸钙的广泛发展会限制植物的生产力和水分通过土壤柱的流动，给农业带来挑战。更好地了解在某些土壤中碳酸盐形成的原因可以帮助土地管理者做出选择。第三，本研究还将进一步测试新开发的工具-土壤水同位素存储系统（SWISS）。这种新设备能够自动采样土壤水蒸气，然后将蒸汽存储在实验室环境中供以后分析。研究人员特别感兴趣的是在土壤柱中创建稳定的水同位素值的次周记录。土壤中不同深度的水的稳定同位素组成的微小差异可能是水如何通过土壤以及水如何在土壤柱中蒸发的细节的结果，因此测量这些值可以提供有关这些过程的信息。因此，该工具对农业科学问题也有广泛的用途，包括提高对土壤如何吸收水分，根系如何吸收水分以及水分如何通过蒸发离开土壤的理解。此外，这个工具可以用于大气科学，以更好地估计大气中来自陆地的水蒸气量。该项目将通过SMART倡议和resress本科研究项目为一名博士生和两名来自代表性不足群体的本科生提供研究培训。此外，中学生将通过参观实验室和根据研究结果开发的课程来了解这项研究。成土碳酸盐岩的稳定同位素值反映了土壤温度、蒸发、水文、大气CO2和植被等广泛的环境参数，是重要的古环境信息档案。为了更好地了解今天测量的成土碳酸盐的稳定同位素组成如何反映过去的环境，研究人员需要更好地了解现代环境中成土碳酸盐形成的时间和机制。特别是，最近的研究利用了新的碳酸盐团块同位素温度计来更好地了解碳酸盐形成的时间和驱动因素；这些研究主要集中在粗粒土壤中发现的碳酸盐如何记录周围环境的信息。然而，更深层的古气候记录主要建立在细粒含粘土古土壤中发现的成土碳酸盐结核上。目前尚不清楚，对于这些不同的土壤质地，驱动成土碳酸盐形成的机制是否相同，因此也不清楚这两种形式的古气候数据是否可以用相同的方式解释。为了解决这个问题，研究人员将限制三种现代、细粒、含粘土土壤中成土碳酸盐结核形成的时间和风格。研究者将监测地表和地下环境参数（即温度、降水量、土壤湿度、土壤CO2浓度和土壤pH值）和稳定同位素参数（即降水、土壤CO2和土壤水分的稳定同位素值）。这项研究的一个重要而独特的贡献将是为每个土壤站点创建~周分辨率的土壤水同位素数据集，这是现代环境的一个方面，在以前的现代成土碳酸盐岩块状同位素研究中很少测量到。这一贡献是可能的，因为研究者已经克服了与制作这些数据集相关的技术挑战，通过开发设备来自动采样和存储土壤水蒸气以进行稳定同位素分析。研究人员将比较现代环境参数范围的观测结果与来自同一土壤的全新世碳酸盐结核的稳定同位素组成，以限制碳酸盐形成的可能时间和驱动因素，以便科学界可以从陆地古气候信息的重要地质档案中改进古气候解释。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The stable isotopes of oxygen and carbon from small, irregularly-shaped, compact growths (“nodules”) of calcium carbonate (CaCO3) have long been used by researchers to reconstruct past climate. To better interpret the information preserved in these nodules, however, it is necessary to have a better understanding of how the nodules form in modern environments. In this project, researchers will study how soil temperature, soil chemistry, and evaporation of soil water, all contribute to the formation of calcium carbonate nodules in fine-grained and clay-rich soils, and how signals of soil temperature and chemistry are recorded by the stable isotopes of the nodules. This research is important and timely for three reasons. First, understanding past climate states more fully can provide context and information about how Earth's climate system fundamentally functions. Second, widespread calcium carbonate development in the soil column can limit plant productivity and water movement through the soil column, creating challenges for agriculture. Better understanding what drives carbonate formation in certain soils could help to inform choices that land-managers make. Third, this study will also serve to further test a newly developed tool, the Soil Water Isotope Storage System (SWISS). This new device is able to automatically sample soil water vapor, and then store that vapor for later analysis in a laboratory setting. In particular, researchers are interested in creating a sub-weekly record of stable water isotope values in the soil column. Small differences in the stable isotope composition of water at different depths in the soil may be the result of the details of how water passes through the soil and how water evaporates in the soil column, so measuring these values can provide information about these processes. Therefore, this tool also has broad utility for agricultural science questions, including improving understanding of how soils absorb water, how roots take-up water, and how water leaves a soil through evaporation. Additionally, this tool could be used in atmospheric science to better estimate the amount of water vapor in the atmosphere that comes from land. This project will provide training in research for a PhD student, as well as two undergraduate students from underrepresented groups via the SMART initiative and RRESESS undergraduate research programs. In addition, middle school students will learn about this research via tours of the laboratories and curriculum developed based on research's results. Stable isotope values of pedogenic carbonate are important archives of paleoenvironmental information because they reflect a wide range of environmental parameters like soil temperature, evaporation and hydrology, as well as atmospheric CO2 and vegetation. To better understand how the stable isotope composition of pedogenic carbonates that is measured today reflect the environments of the past, researchers need to better understand the timing and mechanisms of pedogenic carbonate formation in modern environments. In particular, recent research has utilized the new carbonate clumped isotope thermometer to better understand the timing and drivers of carbonate formation; these studies have primarily focused on how carbonate found in coarse-grained soils records information about the surrounding environment. However, the deeper-time paleoclimate record is primarily built on pedogenic carbonate nodules found in finer-grained, clay-containing paleosols. It is unclear, at present, if the mechanisms that drive the formation of pedogenic carbonate are the same for these different soil textures, and therefore it is unclear if paleoclimate data from these two forms can be interpreted in the same way. To address this question, the investigator will constrain the timing and style of pedogenic carbonate nodule formation in three modern, fine-grained, clay-bearing soils. The investigator will monitor both surface and subsurface environmental parameters (i.e. temperature, precipitation amounts, soil moisture, soil CO2 concentration, and soil pH) and stable isotope parameters (i.e. stable isotope values of precipitation, soil CO2 and soil water). An important and unique contribution of this study will be the creation of ~weekly resolution soil water isotope datasets for each soil site, which is an aspect of the modern environment rarely measured for previous clumped isotope studies of modern pedogenic carbonate. This contribution is possible because the investigator has overcome technical challenges associated with making these datasets by developing equipment to automatically sample and store soil water vapor for stable isotope analysis. The investigator will compare observations of the range of modern environmental parameters with the stable isotope composition of Holocene carbonate nodules from the same soils to constrain the likely timing and drivers of carbonate formation, so that the scientific community can improve paleoclimate interpretations from this important geologic archive of terrestrial paleoclimate information.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.