High magnesium isotope ratios in subglacial icelandic waters: impacts of carbonate precipitation and implications for CO2 sequestration

Magnesium isotopes are increasingly used to trace silicate weathering, one of the key removal processes of atmospheric CO2. Generally, silicate weathering processes are thought to drive Mg isotopes in river waters to isotopically lighter compositions than silicate rocks. An anomaly in this behaviour has been glacially-sourced rivers from Iceland, which are isotopically heavier. This study examines this phenomenon further, by analysing more high-pH glacial and groundwaters from Iceland, which tend to have high δ26Mg values. Mineral Mg/Si stoichiometry shows that the cause of this isotopic change is unlikely to be associated with Mg-silicate secondary minerals, but rather with the sub-glacial and groundwater precipitation of calcite. The riverine δ26Mg of specifically sub-glacial and groundwaters also co-vary with pH and the calcite saturation index. This likely dominance of the Mg isotope fractionation by a single phase allows the calculation of how much of that phase is forming, given “known” fractionation factors. This suggests that calcite formation fluxes are on average ∼0.7 t/km2/yr for the Langjökull glacier, and 25 t/km2/yr for the considerably larger Vatnajökull icecap. Overall, this study apparently answers the enigma of isotopically heavy surface waters in Iceland, and also demonstrates the potential use of Mg isotopes in determining carbonate precipitation rates, and their effects on atmospheric pCO2.