Collaborative Research: Boron in soil carbonates: development of a quantitative soil CO2 proxy

合作研究：土壤碳酸盐中的硼：开发定量土壤二氧化碳代理

• 批准号: 2050323
• 负责人: Daniel Breecker
• 金额: $ 38.82万
• 依托单位: University of Texas at Austin
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2021
• 资助国家: 美国
• 起止时间: 2021-09-01 至 2024-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2050323&HistoricalAwards=false
• 关键词: Collaborative; Research; Boron; soil; carbonates

Rising atmospheric carbon dioxide (CO2) levels pose a global challenge. Understanding and preparing for the future impacts of rising CO2 requires us to look into the past at times periods during Earth's history when atmospheric CO2 levels were higher than they are today. This observational approach is crucial to help test and improve models that are used to project scenarios for the future. This project is aimed at improving the ability to quantify atmospheric CO2 levels of the past. The proposed improvement involves measurements of boron (B) levels and isotopic abundances in calcium carbonate minerals formed in ancient soils. Established theory predicts that B in soil carbonates is sensitive to the abundance of CO2 belowground in soil pore spaces, which is the most uncertain input into an established and longstanding paleo-CO2 ‘proxy’ or indicator. This project will test that theoretical predication with laboratory experiments and studies of natural, modern soils. During the course of the project, undergraduate students from groups underrepresented in the sciences will be mentored through a series of established programs including the Research Traineeship Experience and an NSF Research Experiences for Undergraduates project at UT Austin and Rice University. Multiple recruiting efforts will also be initiated to help improve diversity in undergraduate geoscience programs, including cooperation with the OnRamps program at UT Austin and with regional magnet schools that have a high ethnic diversity within the student population and/or high percentage of underprivileged students. The chemistry of fossilized soils, or paleosols, can record quantitative information about ancient climates and ecosystems. In particular, the carbonate minerals that form within some modern and ancient soils have been targeted for analysis as they are thought to record the composition of soil water and gas in ways that permit the determination of ancient atmospheric pCO2 among other variables. However, critical uncertainties in the "traditional" soil carbonate based proxies (e.g., 13C/12C ratios) fundamentally limit understanding of past environments and motivates the development of new proxies --- such as the work on B isotopic ratios (delta 11B) proposed here -- that provide complementary, but orthogonal constraints on soil chemistry and, potentially, atmospheric CO2. The aqueous speciation of B is pH-dependent and, all else held constant, the pH of soils is a function of soil pCO2. So, the delta 11B of soil carbonates may record information about soil gas that is independent of C isotopic ratios such that, together, they strongly constrain ancient atmospheric compositions and the ecosystem response to C cycle perturbations. As a proof-of-concept, investigators' new measurements of Eocene paleosol carbonates show a decrease in B/Ca and delta 11B values during the hyperthermal event ETM2. The directionality of these changes are entirely consistent with an increase in soil (and atmospheric) CO2. To advance an accurate and quantitative interpretation of these data, they propose to develop new theory for B cycling in soils as well as validate it using experiments and field observations. Critically, their approach will address alternative (to soil pCO2) controls on soil carbonate delta 11B, such as weathering and biotic cycling, that might confound interpretations of CO2 change. The proposed work involves microanalytical imaging and analysis of soil carbonates, development and testing of protocols for B isotopic analysis of soil carbonates, soil sorption experiments, precipitation experiments, and the study of B chemistry across soil CO2 gradients in nature: vertical within individual soils, horizontal across landscapes (climosequence), and temporal (seasonal variation). They will use surface complexation modeling to help interpret experimental and empirical results. The proposed work also involves development of reactive transport models to investigate the effects of biota and weathering on B chemistry in floodplain soils, including the merging of surface complexation models with existing floodplain landscape evolution models.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.

大气二氧化碳（CO2）水平上升构成全球挑战。理解和准备对上升的二氧化碳的未来影响需要我们在地球历史上的时期进行大气二氧化碳水平高于今天的时期的过去。这种观察方法对于帮助测试和改进用于未来计划方案的模型至关重要。该项目旨在提高量化过去大气二氧化碳水平的能力。提出的改进涉及在古代土壤中形成的碳酸钙矿物质中硼（B）水平和同位素丰度的测量。建立的理论预测，土壤碳酸盐中的B对土壤孔隙空间中的二氧化碳的丰度敏感，这是对已建立且长期存在的古CO2“代理”或指示的最不确定的输入。该项目将通过实验室实验和自然现代土壤的研究来测试理论上的鉴定。在项目的过程中，将通过一系列既定计划来考虑来自科学领域不足的小组的本科生，包括研究培训经验和UT Austin和Rice University的本科生项目的NSF研究经验。还将开始进行多种招聘工作，以帮助改善本科地球科学计划的多样性，包括与UT Austin的Onramps计划以及与学生人数中种族多样性和/或高度弱势阶层学生百分比的区域磁铁学校合作。化石土壤或古化学的化学可以记录有关古代气候和生态系统的定量信息。特别是，在某些现代和古老的土壤中形成的碳酸盐矿物已被旨在进行分析，因为它们被认为以允许确定古代大气PCO2的方式记录土壤水和天然气的组成。但是，“传统”基于碳酸盐的代理（例如13c/12c比率）的关键不确定性从根本上限制了对过去环境的了解，并激发了新代理的发展 - 例如，在这里提出的B同位素比（Delta 11b）的工作 - 提供了完整性，但可提供完整性，但对正质的构成了对土壤化学化学和潜在的co2 co2，又有氛围。 B的水体规格是pH依赖性的，其他所有恒定，土壤的pH是土壤PCO2的函数。因此，土壤碳酸盐的三角洲11b可能记录有关土壤气体的信息，该信息与C同位素比率无关，从而强烈限制了古代大气组成和生态系统对C周期扰动的反应。作为概念验证，研究人员对始新世古碳酸盐的新测量结果显示，在热热事件ETM2期间，B/CA和Delta 11b值的降低。这些变化的方向性与土壤（和大气）CO2的增加完全一致。为了促进对这些数据的准确和定量解释，他们建议开发用于土壤中B循环的新理论，并使用实验和现场观测来验证它。至关重要的是，他们的方法将解决土壤碳酸盐三角洲11B（例如风化和生物循环）上的替代方法（插座PCO2）的控制，这可能会混淆二氧化碳变化的解释。提出的工作涉及对土壤碳酸盐的微分析成像和分析，对土壤碳酸盐的B同位素分析的开发和测试，用于土壤碳酸盐，土壤吸附实验，降水实验以及本质上的土壤CO2跨B化学的研究：在各个土壤中垂直的土壤二氧化碳梯度，跨景观，跨越跨度（攀登）和季节性（季节性变异）。他们将使用表面络合模型来解释实验和经验结果。拟议的工作还涉及开发反应性传输模型，以研究Biota和风化对洪泛区土壤中B化学的影响，包括与现有的洪泛区景观演化模型的表面络合模型合并。该奖项反映了NSF的法定任务，并通过使用该基金会的知识优点和广泛影响来评估NSF的法定任务。