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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - DryMIN (Mineral weathering in the unsaturated zone from the molecular to macro scale)

Teaser

Summary

\"\"\"Mineral weathering in the unsaturated zone from the molecular to macro scale (DryMIN)â€
Earthâ€™s shallow subsurface, or â€œcritical zone,â€ is of fundamental importance for supporting terrestrial life and maintaining water quality. A vital part of the critical zone is the unsaturated region located nearest to the surface. This zone is also most likely to be affected by future global change due to altered rainfall patterns and evapotranspiration rates across the globe. In order to predict how long-term variations in the water budget will impact nutrient and carbon cycling it is necessary to evaluate the influence of water saturation on mineral weathering reactions. The objective of the research was to elucidate the fundamental mechanisms governing reaction rates as a function of water availability from the molecular to macro scale. This was achieved through a combination of experimental studies, geochemical modelling, and analytical techniques. Experimental results indicated that mineral weathering reactions are sensitive to the availability of water below a threshold of water availability. Mineral dissolution-precipitation reactions were observed to proceed in the presence of an adsorbed water film only, but the extent of reaction was limited below 100% humidity. At 100% humidity, reactions proceeded at a rate similar to water-unlimited conditions. Column experiments also revealed that element release during weathering is sensitive to wetting and drying cycles. This implies that weathering rates are likely to be impacted by changes in water availability and evaporation-recharge patterns. Our experimental results will help to develop a mechanistic reactive transport model that can better predict reaction rates as a function of water saturation. This research has significant implications for the maintenance of a sustainable nutrient supply for natural and agricultural vegetation, the carbon cycle, and remediation of groundwater resources, issues critical to the long-term sustainable growth of our society.\"

Work performed

\"Element and nutrient fluxes are commonly considered to depend on the wetted surface area of mineral phases. Yet, the definition of â€œwetted surface areaâ€ 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 Earthâ€™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, SorÃ¨ze, 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.\"

Final results

The objective of DryMIN was to elucidate the fundamental mechanisms governing mineral dissolution/precipitation rates in the unsaturated zone from the molecular to macro-scale. Experimental results will facilitate the development of mechanistic reactive transport models to better quantify the response of mineral weathering reactions to changes in water saturation. This research helps answer the fundamental question of why mineral dissolution-precipitation reactions behave differently in natural compared to laboratory settings, and has implications for the carbon cycle, the maintenance of a sustainable nutrient supply for natural and agricultural vegetation, and the remediation of groundwater resources, issues critical to the long-term sustainable growth of our society. Research outcomes include: 1) Determination of a water-limit required to allow water-limited reactions to proceed; 2) An improved fundamental understanding of the limits on mineral reactivity in low-water environments; 3) Evaluation of the extent to which wetting-drying cycles impact macro-scale mineral reaction rates; and 4) An improved understanding of isotopic fractionation during carbonate mineral dissolution and precipitation that can inform interpretation of isotopic signatures measured in natural weathering environments.