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

大气二氧化碳(CO2)水平的上升构成了一个全球性的挑战。理解和准备应对二氧化碳上升的未来影响，需要我们回顾地球历史上大气二氧化碳水平高于今天的过去。这种观察性方法对于帮助测试和改进用于预测未来场景的模型至关重要。该项目旨在提高量化过去大气二氧化碳水平的能力。拟议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。公认的理论预测，土壤碳酸盐中的B对地下土壤孔隙中二氧化碳的丰度很敏感，这是一个已确立的、长期存在的古二氧化碳“替代品”或指标中最不确定的输入。这个项目将通过实验室实验和对天然现代土壤的研究来验证理论预测。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既定计划得到指导，其中包括德克萨斯大学奥斯汀分校和赖斯大学的研究培训体验和NSF本科生研究体验项目。还将启动多项招生努力，以帮助提高本科生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Onramps计划以及与学生中种族多样性较高和/或贫困学生比例较高的地区性磁石学校合作。化石土壤或古土壤的化学成分可以记录关于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被认为是分析的目标，因为它们被认为可以记录土壤水和气体的组成，从而能够确定古代大气中的二氧化碳等变量。然而，基于碳酸盐的“传统”土壤替代物(例如，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了解，并推动了新的替代物的发展-例如这里提出的B同位素比值(增量11B)--它对土壤化学和潜在的大气二氧化碳提供了补充的、但又是相互垂直的约束。B在水中的形态与pH有关，在其他条件不变的情况下，土壤的pH是土壤PCO2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率无关的土壤气体信息，因此，它们一起强烈制约着古代大气成分和生态系统对碳循环扰动的反应。作为概念验证，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和增量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。为了提高对这些数据的准确和定量的解释，他们建议开发土壤中B循环的新理论，并通过实验和田间观测来验证它。关键的是，他们的方法将解决土壤碳酸盐三角洲11B的(土壤二氧化碳)替代控制，例如风化和生物循环，这可能会混淆对二氧化碳变化的解释。拟议的工作包括对土壤碳酸盐进行微量分析成像和分析，制定和测试土壤碳酸盐的B同位素分析方案，土壤吸附实验，降水实验，以及研究自然界土壤中二氧化碳的垂直梯度、水平梯度(气候序列)和时间梯度(季节变化)中的B化学。他们将使用表面络合模型来帮助解释实验和经验结果。拟议的工作还包括开发反应运移模型，以调查生物群和风化对滩涂土壤中B化学的影响，包括将表层络合模型与现有的泛滥平原景观演变模型合并。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.