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Gislason et al. (2006) make a claim that would significantly change our understanding of the long-term carbon cycle. They argue that reaction of basalt glass in seawater (especially in what they call the “fluidized bed reactors” in the delta environment) constitutes a significant, heretofore unrecognized, carbon sink, because dissolved Ca from this reaction ends up precipitating in the ocean as calcium carbonate. This conclusion is derived from experimental results given in their cited paper (Stefánsdóttir and Gislason, 2005). However, these same experiments demonstrate that the uptake of Mg from seawater in this reaction exceeds the supply of Ca by a factor of ~1.3 (molar ratio), with a large uncertainty. The Mg is reported to probably end up in calcite or a clay mineral (Stefánsdóttir and Gislason, 2005). The authors acknowledge that dissolved Mg supply to the ocean contributes to the carbon sink. The change in amount of divalent cations available to precipitate as carbonate minerals as a result of this process would be the amount of Ca released minus the amount of Mg removed. Because the amount of Mg removed may be greater than the amount of Ca released, this process may represent a reduction in cations available to the marine sedimentary carbonate sink, which effectively would be a carbon source with respect to the ocean/atmosphere pool. It is unclear whether the possible movement of seawater Mg into calcite in these experiments is by a precipitation reaction with dissolved bicarbonate. What fraction of this Mg goes into clay is also uncertain. Assuming such Mg goes into clay, if a sufficient amount is subsequently replaced by the exchange of seawater Na and/or K, then there may be a net carbon sink from the authors' proposed reaction [neglecting any change in dissolved inorganic compound (DIC) in the proposed reaction; DIC concentrations are not reported in their results]. Given these uncertainties, it is not possible to estimate the primary alkalinity flux to the ocean for the proposed reaction. Therefore, the authors' claim of a new carbon sink is not robustly supported by the reported experimental results from their cited paper (Stefánsdóttir and Gislason, 2005).