Collaborative Research: Boron in soil carbonates: developing a quantitative soil CO2 proxy

大气中二氧化碳（CO2）含量的上升对全球构成了挑战。要了解和准备二氧化碳上升对未来的影响，我们需要研究地球历史上大气二氧化碳水平高于今天的时期。这种观察方法对于帮助测试和改进用于预测未来情景的模型至关重要。这个项目旨在提高过去大气二氧化碳水平的量化能力。提出的改进包括测量古代土壤中形成的碳酸钙矿物中的硼(B)水平和同位素丰度。已有的理论预测，土壤碳酸盐中的B对土壤孔隙空间中地下CO2的丰度很敏感，这是建立和长期存在的古CO2“代理”或指标中最不确定的输入。该项目将通过实验室实验和对自然、现代土壤的研究来验证这一理论预测。在项目过程中，来自科学领域代表性不足群体的本科生将通过一系列已建立的项目得到指导，包括研究实习经历和德克萨斯大学奥斯汀分校和莱斯大学的NSF本科生研究经历项目。还将启动多项招聘工作，以帮助提高本科地球科学项目的多样性，包括与德克萨斯大学奥斯汀分校的onramp项目合作，以及与学生群体中种族多样性高和/或贫困学生比例高的地区吸引力学校合作。土壤化石或古土壤的化学成分可以记录有关古代气候和生态系统的定量信息。特别是，在一些现代和古代土壤中形成的碳酸盐矿物已经成为分析的目标，因为它们被认为可以记录土壤水和气体的组成，从而可以确定古代大气中的二氧化碳分压和其他变量。然而，“传统”基于土壤碳酸盐的代用指标（如13C/12C比值）中的关键不确定性从根本上限制了对过去环境的理解，并推动了新代用指标的发展——例如本文提出的B同位素比值（δ 11B）的研究——这些代用指标为土壤化学和潜在的大气CO2提供了互补但正交的约束。B在水中的形态依赖于pH值，在其他条件不变的情况下，土壤的pH值是土壤二氧化碳分压的函数。因此，土壤碳酸盐三角洲11B可能记录了与碳同位素比例无关的土壤气体信息，这样，它们共同强烈地约束了古代大气成分和生态系统对碳循环扰动的响应。作为概念的证明，研究人员对始新世古土壤碳酸盐的新测量表明，在高温事件ETM2期间，B/Ca和δ 11B值下降。这些变化的方向性与土壤（和大气）二氧化碳的增加完全一致。为了对这些数据进行准确和定量的解释，他们建议发展土壤中B循环的新理论，并通过实验和实地观察来验证它。关键的是，他们的方法将解决土壤碳酸盐三角洲11B的替代（土壤二氧化碳分压）控制，如风化和生物循环，这可能会混淆对二氧化碳变化的解释。建议的工作包括土壤碳酸盐的微分析成像和分析，土壤碳酸盐B同位素分析方案的开发和测试，土壤吸收实验，降水实验，以及自然界中土壤CO2梯度B化学的研究：垂直于单个土壤，水平跨越景观（气候序列）和时间（季节变化）。他们将使用表面络合模型来帮助解释实验和经验结果。建议的工作还包括开发反应性输运模型，以研究生物群和风化对河漫滩土壤中B化学的影响，包括将表面络合模型与现有的河漫滩景观演化模型合并。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。

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.