The Ca and Sr (radiogenic and stable) isotope geochemistry of weathering in Iceland

This three-year project aims to elucidate the role of basalt weathering in the long-term carbon cycle. While basalt represents only ~5% of the crustal rocks exposed at the Earth's surface, basalt weathering may have a disproportionately large effect on long-term climate change because Ca-silicate minerals composing basalt appear to dissolve easily, thereby consuming atmospheric CO2 at a rapid rate. Several studies have examined basalt weathering in Iceland. A major assumption is that all of the riverine Ca and carbonate alkalinity originates from silicate weathering. However, hydrothermal calcite occurs throughout Iceland, and even trace levels are expected to impact river geochemistry owing to the mineral’s high solubility and fast dissolution rate. Importantly, because Icelandic hydrothermal calcite contains mantle carbon, its dissolution by carbonic acid has no impact on long-term CO2 levels. We recently traced sources of Ca in Icelandic rivers using a new, high-precision Ca isotope TIMS method (Jacobson et al., 2015). Rivers have higher δ44/40Ca values than basalt. A previous study attributed the pattern to isotope fractionation in the weathering zone, e.g., preferential adsorption of 40Ca onto suspended sediment surfaces. We, on the other hand, attributed the pattern to mixing of Ca from basalt and calcite, which has higher δ44/40Ca values than rivers. Up to 90% of the riverine Ca could originate from calcite weathering. Resolving whether fractionation or mixing controls the Ca isotope geochemistry of Icelandic rivers has critical implications. If fractionation dominates, then the basalt weathering paradigm remains valid. If mixing dominates, then basaltic terrains may not necessarily represent atmospheric CO2 consumption “hot spots.” Herein, we propose to test several hypotheses surrounding controls on the Ca isotope composition of Icelandic rivers. Our research will include analyses of radiogenic and stable Sr isotope ratios (87Sr/86Sr and δ88/86Sr), which should provide additional constraints on the sources and cycling of Ca. Fieldwork conducted during the spring, summer, and fall will mostly concentrate on the Skagafjördur Valley in northern Iceland. We will collect river water, spring water (thermal and non-thermal), rocks, minerals, soils, and plants. We will also collect rock and mineral specimens from the Berufjördur region in eastern Iceland. Calcite and the zeolite mineral heulandite form during hydrothermal alteration of basalt in the deep lava pile and often occur together in metabasalts now exposed at the surface. Jacobson et al. (2015) showed for the first time that heulandite δ44/40Ca values are ~1‰ lower than basalt. The Ca isotope composition of heulandite likely reflects the preferential incorporation of 40Ca during equilibrium isotope exchange at hydrothermal temperatures, implying that the residual 44Ca-enriched waters determine the high δ44/40Ca values of calcite. To test this hypothesis, the proposed study also includes bench-scale experiments designed to quantify the heulandite Ca isotope fractionation factor under a range of physicochemical conditions. Intellectual Merit: The project is potentially transformative because it challenges a long-standing paradigm surrounding the role of basalt weathering in the long-term carbon cycle. Zeolite facies metamorphism and hydrothermal fluid circulation are ubiquitous characteristics of basaltic eruptions. If hydrothermal calcite in other basaltic settings contains mantle carbon, then the contribution of basalt weathering to long-term CO2 drawdown may be less si