Silicate versus carbonate weathering in Iceland: New insights from Ca isotopes

Several studies have measured riverine fluxes of Ca and carbonate alkalinity in Iceland with the aim of quantifying the role of basalt weathering in the long-term carbon cycle. A major assumption is that all of the Ca and alkalinity originates from the dissolution of Ca-bearing silicate minerals, such as plagioclase and clinopyroxene. 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. To test this hypothesis, we used a new, high-precision Ca isotope MC-TIMS method (δ^44/40Ca; 2σ_SD=±0.04‰) to trace sources of Ca in Icelandic rivers. We report elemental and Ca isotope data for rivers, high- and low-temperature groundwater, basalt, hydrothermal calcite (including Iceland Spar), and stilbite and heulandite, which are two types of zeolites commonly formed during low-grade metamorphism of basalt. In agreement with previous research, we find that rivers have higher δ^44/40Ca values than basalt, with a maximum difference of ~0.40‰. This difference may reflect isotope fractionation in the weathering zone, i.e., preferential uptake of ⁴⁰Ca during clay mineral formation, adsorption, and other geochemical processes that cycle Ca. However, calcite δ^44/40Ca values are also up to ~0.40‰ higher than bedrock values, and on a diagram of δ^44/40Ca versus Sr/Ca, nearly all waters plot within a plausible mixing domain bounded by the measured compositions of basalt and calcite, with glacial rivers plotting closer to calcite than non-glacial rivers. Calcite and heulandite form during hydrothermal alteration of basalt in the deep lava pile and often occur together in metabasalts now exposed at the surface. Because heulandite δ^44/40Ca values are ~1-2‰ lower than basalt, we suggest that ⁴⁰Ca uptake by heudlandite explains the relatively high δ^44/40Ca values of calcite and that calcite weathering in turn elevates riverine δ^44/40Ca values. High mechanical erosion rates are known to facilitate the exposure and weathering of calcite, which explains the isotopic contrast between glacial and non-glacial watersheds. Using a mixing model, we find that calcite weathering provides ~0-65% of the Ca in non-glacial rivers and ~25-90% of the Ca in glacial rivers, with silicate weathering providing the remainder. Icelandic hydrothermal calcite contains mantle carbon. Noting that zeolite facies metamorphism and hydrothermal fluid circulation are ubiquitous characteristics of basaltic eruptions and assuming that hydrothermal calcite in other basaltic settings also contains mantle carbon, we suggest that the contribution of basalt weathering to long-term CO₂ drawdown and climate regulation may be less significant than previously realized.