Collaborative Research: Calibrating the paleosol carbonate CO2 barometer for vertic paleosols by monitoring soil CO2 in modern Vertisols

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).Intellectual Merit: A calibration of the paleosol carbonate CO2 barometer is proposed in order to improve estimates of the concentration of CO2 in Earth?s ancient atmosphere. The concentration of CO2 in the soil during soil carbonate formation, S(z), is required in order to calculate atmospheric CO2 concentrations using the barometer. Recent research has shown that the value of S(z) in desert soils is significantly lower than values typically used in the barometer because carbonate does not form under mean growing season conditions as previously assumed but instead forms during times of year when the soil is sufficiently dry to limit respiration (i.e. soil CO2 production). Atmospheric CO2 levels that are significantly lower than previously reported (~1000 ppmV versus ~3000 ppmV) result if the seasonally low S(z) value determined for desert soils is used to calculate atmospheric CO2 concentrations from previously compiled paleosol data. The persistence of greenhouse climates (warm, ice-free Earth) under these relatively low CO2 concentrations suggests that the Earth?s surface temperature is much more sensitive to atmospheric CO2 than currently thought. A thorough evaluation of this possibility requires further study because desert soils are not the most appropriate modern analog for most paleosols used in paleoatmospheric CO2 studies. The proposed study is intended to constrain S(z) in modern Vertisols, which are high clay-content, high shrink-swell potential soils. Vertisols are chosen for this study because their ancient equivalents have been widely used to reconstruct ancient atmospheric CO2. The following three-part hypothesis will be tested: 1) drying of the soil reduces respiration rates, 2) cracking of the soil (when it is dry) allows respired CO2 to escape from the soil more rapidly and 3) the decrease in soil CO2 concentration resulting from both drying and cracking causes the formation of pedogenic carbonate in Vertisols. A suite of measurements will be carried out in Texas Vertisols to determine what portion of seasonal variability pedogenic carbonate records. The concentration of soil CO2, the stable isotope composition of soil CO2, soil temperature and soil moisture will be monitored and the stable isotope composition of pedogenic carbonate in the soil will be measured. The timing of pedogenic carbonate formation, and by association the most appropriate value(s) for S(z), will be determined based on when isotope equilibrium between soil CO2 and carbonate occurs and when minimum calcite solubility occurs. If the value(s) determined for S(z) are significantly different than those used in previous studies, the new values will be used to recalculate paleoatmospheric CO2 concentrations and to estimate climate sensitivity from Paleozoic and Mesozoic paleo-Vertisols. Broader Impacts: Predicting the severity of the impending climate crisis is one of the most important scientific challenges facing humanity today. The proposed research directly addresses this challenge by reevaluating the sensitivity of Earth?s climate to elevated atmospheric CO2 concentrations. Therefore, the broadest impacts of the proposed research will be to help predict future climate change and to help inform policy makers about the potential consequences of continued, unmitigated anthropogenic CO2 emissions. Personal edification, ranging from the mutual intellectual benefits of collaborative research to graduate and undergraduate education to public outreach in various venues, will have a more immediate and individual impact.