Carbonate Formation Induced by CO2 Degassing: Controls on the Isotopic Compositions and Implications for Clumped Isotope Thermometry
Calcium carbonate formation induced by carbon dioxide (CO2) degassing is ubiquitous in nature and produces some of the most important paleoclimate archives available, most notably speleothems. These cave carbonate deposits hold huge potential for reconstructing terrestrial paleoclimate, because their isotopic compositions reflect mean annual surface temperatures and their formation age can be precisely dated. However, various kinetic processes are involved in this type of carbonate formation, complicating the interpretation of their isotopic compositions. This impedes the realization of the full potential of these paleoclimate archives. Recent development of the carbonate "clumped isotope" thermometer offers new opportunities for reconstructing paleotemperatures based on these carbonates. Unlike the conventional carbonate-water oxygen isotope thermometer, this novel clumped isotope thermometer can determine carbonate formation temperatures without assumptions about the isotopic compositions of the water from which carbonate precipitates. But this thermometer has not escaped the influence of kinetic effects. Direct application of this thermometer to natural speleothems has resulted in systematic overestimations of their formation temperatures. This proposal seeks to systematically determine the isotope effects, especially clumped isotope effects, associated with carbonate formation induced by CO2 degassing, through series of well-characterized laboratory experiments. The overarching goal of this project is to provide a quantitative framework for interpreting the isotopic compositions of this type of carbonates in nature and thus enable robust paleotemperature reconstructions based on their isotope compositions. Undergraduate students will be exposed to the state-of-the-art technologies for aqueous carbonate chemistry measurements and stable isotope analysis during the course of this project. Specifically, the investigators propose to determine (1) the kinetics of the clumped isotope exchange between different dissolved inorganic carbon species and water, and (2) the kinetic isotope fractionation factors associated with bicarbonate (HCO3-) dehydration and dehydroxylation reactions - the key pathways for converting dissolved HCO3- to CO2. This proposed work will bring together the respective expertise of two investigators in clumped isotope geochemistry and aquatic carbonate chemistry. The use of state-of-the-art technologies for solution carbonate chemistry measurements will make it feasible to pair the changes in solution carbonate chemistry with isotope measurements, and thus allow detailed investigation of the kinetic controls on the isotopic compositions of carbonate precipitates. The parameters to be determined in this project are fundamental for understanding the isotope systematics of aqueous carbonate system, and will be applicable in many future studies.