Using Soils to Constrain Past and Future Terrestrial Climate Change.

Abstract

Future ecosystem and climatic changes due to anthropogenic emissions of CO2 are of increasing importance to society. Soils are the largest terrestrial reservoir of carbon, have the potential to become new sources or sinks of carbon to the atmosphere, and also can record information about climate and environment. In this dissertation, I use modern and ancient soil to constrain past and future climate and environmental change. To better determine the sensitivity of ecosystems to changes in atmospheric CO2 and climate, we must first increase the precision of past atmospheric CO2 reconstructions. I have developed a new proxy for soil-respired CO2 based on its relationship with precipitation to reduce the uncertainties in the soil carbonate paleobarometer, the most widely applicable method of atmospheric CO2 reconstructions. Using this new method of paleobarometry, I have refined previous estimates of atmospheric CO2 and find estimates now support CO2-temperature coupling throughout the Phanerozoic. This new respired CO2 proxy is also useful for predictions of carbon cycle changes. Silicate weathering consumes atmospheric CO2, and the concentration of soil CO2 is an important factor in weathering rates. Here, I couple the newly derived relationship to precipitation simulations from regional climate models to predict future changes to soil CO2 and the concentration of dissolved CO2. I find large increases in dissolved CO2 for the central Great Plains region and moderate decreases for the Southwestern United States for the decade of 2050-2060, indicating that the Great Plains may become a new sink for atmospheric CO2 in the future. While soils may act to buffer anthropogenic CO2 emissions through chemical weathering on short time scales, on longer time scales warming may destabilize the carbon pool stored in soils. Here, I use paleosols from the Paleocene-Eocene Thermal Maximum to determine how soils respond to rapid climate warming on thousand year time scales. Carbon isotopic compositions of pedogenic carbonates and soil organic matter indicate changes to soil respiration that likely reduced carbon burial and increased fluxes of carbon to the atmosphere. Results from this dissertation have important implications for changes to climate, environment and carbon cycling in the next century and beyond.