Investigating Basalt Chemical Weathering in Iceland and its Role in Climate Regulation with Stable Calcium Isotopes

Nelson, Claire

doi:https://doi.org/10.21985/n2-2jjp-wy42

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Investigating Basalt Chemical Weathering in Iceland and its Role in Climate Regulation with Stable Calcium Isotopes

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This dissertation investigates processes controlling the Ca isotope geochemistry (δ44/40Ca) of Icelandic samples, from an atomic to island-wide scale. These studies embody the range of applications for which the Ca isotope proxy can apply in surficial environments, and I offer novel δ44/40Ca data for Icelandic rivers, rocks, minerals, soil, and vegetation. By characterizing the δ44/40Ca values of mineralogical end members (Chapter 2) and identifying mechanisms that control the transport, distribution, and fate of Ca isotopes (Chapter 3), I use Ca isotopes of Icelandic rivers to evaluate the chemistry of basalt-draining rivers, which has previously been taken to reflect rapid basalt weathering rates and disproportionate atmospheric CO2 drawdown (Chapter 4). The studies combined provide key new insights into processes that control Ca isotope cycling, thereby advancing the tracer and broadening its potential applications. Only one study has reported Ca isotope compositions of zeolite minerals, and this dissertation is the first to specifically investigate controls on the Ca isotope geochemistry of the minerals. Icelandic zeolites show a δ44/40Ca range of 1.4‰, which is on the order of the range observed for all igneous rocks thus far measured. Zeolite δ44/40Ca values strongly correlate with average mineral Ca-O bond lengths, which appears most consistent with equilibrium isotope partitioning. Equilibrium controlled fractionation of Ca isotopes by zeolites elevates the δ44/40Ca of coexisting groundwaters, from which calcite then precipitates at or near equilibrium. This study reports novel Ca isotope data for zeolites and offers a new perspective on the mechanisms controlling Ca isotope fractionation during mineral precipitation in general, which has implications for the carbonate δ44/40Ca paleoclimate proxy. The results also have significance for using Ca isotopes to trace basalt weathering and its role in long-term climate regulation, as zeolites pervasively form in basaltic settings. Through a high-resolution field study of the Skagafjörður region in North Iceland, I characterize key reservoirs hypothesized to control the Ca isotope composition of surface and groundwaters including vegetation, soil, clays, and colloidal and suspended particulates. While these reservoirs display some δ44/40Ca variability, the primary control on river δ44/40Ca appears to be mixing with isotopically heavy Ca sourced from groundwater and calcite weathering. Groundwaters show trends between δ44/40Ca and pH, temperature, distance from the coast, and Sr/Ca ratios, indicating that geochemical evolution controls groundwater δ44/40Ca values due to fractionation by zeolites. Only direct-runoff tributaries draining minimally altered, crystalline basalt show Ca isotope ratios indicative of basalt weathering by atmospheric CO2, and solute fluxes in these rivers are much lower than those employed in previous attempts to estimate basalt weathering rates. Finally, I studied the Ca isotope geochemistry of rivers draining the Icelandic highlands to investigate the relative mineral stability hypothesis, which predicts that mafic minerals should weather faster than felsic minerals at the Earth’s surface. In the highlands, subglacial basaltic eruptions generate breccias bearing crystalline clasts and highly reactive basaltic glass. The highlands also support vigorous hydrothermal activity that results in the precipitation of secondary minerals, including calcite. Glacial highland rivers display the highest δ44/40Ca values of all rivers measured in Iceland, clearly indicating that basalt weathering by atmospheric CO2 does not control riverine solute fluxes. High solute fluxes in basalt-draining rivers globally have been interpreted as evidence for rapid basalt weathering, and for decades, Earth scientists have hypothesized that basalt weathering disproportionately consumes atmospheric CO2. Results from this study challenge the underlying assumptions regarding the role of basalt weathering on long-term climate regulation and indicate that solute fluxes used to assess basalt weathering rates have likely resulted in significant overestimates.

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