13 - Reactive carbonate in glacial systems: a preliminary synthesis of its creation, dissolution and reincarnation

Summary

Abstract

Subglacial transport of carbonate rock debris produces abundant fine reactive particles (rock flour) which are susceptible to dissolution. Longer transport distance and mixing with quartz increases rock flour formation, but the role of primary grain size and the quantity of the most reactive submicron-sized material is unclear. Considerations of equilibrium solubility indicate enhanced dissolution will occur in systems open to atmospheric CO₂, or where acid production by pyrite oxidation is important, or where submicron-sized crystals are abundant. Kinetic considerations emphasize the increased reactivity of fine particles with freshly exposed surfaces and lattice defects, but dissolution is often limited by the sloth of reactions involving CO₂. A number of processes can allow reprecipitation of calcite in the glacial system: ripening, warming, freezing, the common ion effect, removal of CO₂ by organic or inorganic means, input of alkalinity from organic decomposition by bacteria, evaporation, transpiration and skeletal biomineralization. The relative importance of these mechanisms has yet to be established.

Examples of contemporary processes are discussed from ongoing work in Europe and North America. Meltwaters from carbonaterich glaciers are shown to have a wide variety of partial pressures of CO₂ reflecting their complex processes of evolution; waters become supersaturated in response to evaporation, and to some extent degassing. Regelation crusts occur on clasts and are morphologically distinct from those in vadose proglacial areas. Evidence of Holocene modification of Pleistocene glacial sediments by calcrete formation and vadose cementation of gravels is presented from other sites.