"Element and nutrient fluxes are commonly considered to depend on the wetted surface area of mineral phases. Yet, the definition of “wetted surface area” is not straightforward. Numerous experiments were conducted in the absence of ""liquid"" water, but with humidity-controlled atmospheres to assess the amount of water required to facilitate reaction of the Mg-hydroxide mineral brucite [Mg(OH)2] with CO2 to form Mg-carbonate minerals. This reaction is of interest for engineered CO2 sequestration via a process known as ""mineral carbonation"" but also serves as a model system to study mineral-fluid-gas reaction mechanisms more generally. Our experiments reveal that brucite dissolves and Mg-carbonate minerals precipitate on day to month timescales depending on the CO2 content of the atmosphere. Fourier transform infrared spectroscopy, scanning electron microscopy, and X-ray computed micro-tomography analyses were used to assess the extent of reaction and identify mineral precipitates. Results of this study were presented at the Goldschmidt conference in Paris, August 13-18, 2017, as well as the Gordon Research Seminar in June, 2017.
Wetting and drying cycles physically alter the structure of sediments and soils. Although physical impacts of wetting and drying cycles have been recognized, their potential influence on the chemical composition of the pore fluid and mineral dissolution-precipitation reactions are not well understood. In this project, we explored the impact of changing mineral-water-gas interfaces during periodic evaporation-recharge cycles on element release from clay minerals common at the Earth’s surface. A series of flow-through column experiments was conducted that contained common clay minerals. Evaporation was promoted in one set of experiments, while a second set of experiments received the same water volume and frequency of artificial rainfall but evaporation was suppressed. The release of elements was found to differ significantly between columns subjected to repeated wetting and drying compared to wetting alone. Results suggest that the physico-chemical changes influence reaction rates or pathways, and therefore may be an important control on element fluxes and nutrient cycles in the unsaturated zone. We are in the process of finalizing these results and writing a manuscript for submission to a peer-reviewed international journal. The results were presented at the Goldschmidt conference in Boston, August 12-17, 2018.
An additional study conducted as part of this project investigated the fractionation of stable isotopes during mineral weathering reactions. We conducted two experimental studies. The first examined Mg and C isotope fractionation in Mg-carbonate minerals, like those precipitated in our humidity-controlled experiments. The second examined Ca isotope fractionation between dissolved Ca and carbonate minerals. These studies help identify the mechanisms controlling isotope fractionation in weathering environments, and define our ability to use isotopes as tracers of mineral weathering processes. Some of the results of the Mg-carbonate experiments were published in 2019 in the international journal, Chemical Geology. Additional results of these experiments were presented at the Metal Stable Isotope Geochemistry Conference and Final IsoNose Workshop, Sorèze, France, Jan 8-11, 2018, and the European Geosciences Union General Assembly, Vienna, Austria, Apr 8-13, 2018.
The results of the project have also been disseminated locally with graduate students, and during seminars for at the host institution UCL, as well as the University of Calgary. Dissemination to the general public occurred at the UCL open day in September 2017. A website has been created that is available to all members of the public."
