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

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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.