Growth Mechanisms of Speleothems in Castleguard Cave, Columbia Icefields, Alberta, Canada^∗

Abstract

In spite of its location beneath high alpine terrain and active glaciers, Castleguard Cave contains many actively growing calcite speleothems. Four hypotheses as to their growth mechanism were tested against field data on water chemistry, temperatures, CO₂ content of cave air, and evaporation rates. The hypotheses were (1) that a biogenic source of high PCO₂ is unlikely, so calcite deposition by CO₂ degassing would require a nonbiogenic source; (2) warming of water after entering the cave, or in passage through the rocks above it might cause calcite supersaturation; (3) calcite might be deposited because of evaporation of seepage; and (4) waters dissolving several Ca-bearing minerals might precipitate calcite as the least soluble by the common-ion effect. The field data indicate that hypothesis (4) accounts for most calcite deposition, with groundwaters evolving by dissolving dolomite incongruently and by oxidizing pyrite to sulfuric acid. The acid dissolves carbonate minerals and the CO₂ evolved slightly raises the PCO₂ of the solution. Considerable calcite supersaturation is achieved at almost constant PCO₂ and no biogenic CO₂ source need be invoked. Of the alternative mechanisms, evaporation contributes no more than a few percent of total calcite deposition, while temperature effects may make a smaller contribution. An important implication for paleoclimate studies is that, given suitable conditions of mixed gypsum-limestone or dolomite-limestone terrain or rocks containing abundant pyrite, calcite speleothems can form even under glaciers provided cave temperature is above 0°C. Thus, ancient speleothems generally indicate air-filled caves and lack of continuous permafrost, but do not necessarily imply vegetated soils.