An experimental study of the controls on incongruent dissolution of CaCO3 under analogue glacial conditions

This paper addresses the nature of incongruent dissolution of calcite in glacial settings using an experimental approach. Various CaCO3 samples were comminuted using two contrasting techniques (dry machine-milling, and hand-grinding of an ice-water mixture) and dissolved to calcite saturation in both high-pCO(2) (10(-2) atm) and low-pCO(2) (10(-3.5) = atmospheric) conditions. Ion yields of Ca, Mg and Sr were determined at the end of the experiments. Leachates displayed enhancement of Mg/Ca and Sr/Ca by a factor of 1.3-8.3 compared with bulk solid carbonate. Lower Mg/Ca and Sr/Ca ratios under higher-pCO(2) conditions reflect the percentage dissolution of the carbonate, not the pCO(2) per se. The experimental results imply that reported natural incongruent dissolution is readily reproducible in the laboratory and is primarily dependent on the water/rock ratio. Quantitative analysis of new and previous results suggests that the effect relates to the most reactive 0.15-0.25% of the sample, equating volumetrically to the outer few lattice layers, and it is interpreted as an anomalous leaching behaviour of calcite surfaces freshly exposed by crushing. This phenomenon could serve as an index of the relative efficiencies of fragmenting and dissolutional processes in glacial environments.