Deposits formed by subglacial precipitation of CaCO3

Abstract

Limestone and other calcareous outcrops recently exposed by retreating temperate glaciers in the Rocky Mountains are widely covered with patchy carbonate coatings that formed as a result of chemical precipitation under the moving ice. They form because CaCO 3 dissolved from the bedrock on the stoss sides of bed obstacles is concentrated at the lee sides by the freezing of the meltwater in the regelation-slip process, which is an essential part of the mechanism by which temperate glaciers slide over their beds. Freezing experiments show that ice grown from CaCO 3 solutions at rates similar to those expected in connection with regelation sliding has calcium ion concentrations 50 to 100 times smaller than the solutions; hence the freezing process strongly concentrates solutes in the melt. The experimental results are interpreted in terms of the pertinent phase relations, which were derived numerically on the basis of the most recent solubility data for calcite and an estimate of the freezing point lowering due to the presence of solutes. The subglacial chemical deposits indicate clearly that chemical exchange is active under temperate glaciers and that glaciochemical processes do at times play a dominant role in modifying the glacial bed. Moreover, they are of particular interest because the chemical and physical processes by which they form may affect the behavior of entire glacial masses by impeding basal sliding.