On Some Aspects of the Chemical Evolution of Cave Waters

Abstract

The evolution of cave waters can be divided naturally into three stages: a stage of carbonation in the soil zone, a stage of solution of calcite and/or dolomite, and a stage of equilibration with cave air. Much of the necessary physical-chemical data for the interpretation of the chemistry of cave waters is available: the solubility of $$CO_{2}$$ in water is well known, as is the solubility of calcite and aragonite in pure water and in solutions containing dissolved $$CO_{2}$$. Only the solubility of dolomite is still in doubt. Water samples from Indian Echo Cave and Carpenter Cave in Pennsylvania and from the Luray Caverns in Virginia were analyzed, and their composition was used to interpret their chemical evolution. The waters in each cave must have absorbed a large amount of $$CO_{2}$$ on their passage through the soil zone. Even so, the calcium and magnesium content of some of the water at Luray is so large that impossibly high $$CO_{2}$$ pressures would have had to prevail in the soil zone if the solubility data of Yanat'eva (1954) and of Garrels, Thompson, and Siever (1960) for dolomite were correct. We suggest that the solubility product of dolomite is probably near $$10^{17}$$, about two orders of magnitude higher than the value proposed by Garrels, Thompson, and Siever (1960). During the precipitation of $$O_{3}$$ from cave waters, the activity product $$a_{Ca^{+2}}\cdot a_{CO_{3}^{-}}$$ has been found to exceed the solubility product of calcite by as much as a factor of seven. In bodies of standing water from which no $$CaCO_{3}$$ is precipitating the activity product is very nearly equal to the accepted value of the solubility product of calcite. Precipitation of $$O_{3}$$ takes place essentially entirely by escape of $$CO_{2}$$ from cave water to the cave air. This is demonstrated convincingly by the observed constancy of the magnesium concentration in these waters during $$O_{3}$$ precipitation. At Luray nearly all of the water is supersaturated with respect to dolomite. However, no dolomite has been observed to precipitate in the cave even where the activity product $$a_{Ca^{+2}}\cdot a_{Mg^{+2}}\cdot a_{CO_{3}^{-}}^{2}$$ is in excess of $$2 \times 10^{-15}$$. The strontium content of $$O_{3}$$ precipitated from cave water depends on the mineralogy of the precipitate and on the ratio of the concentration of strontium to the concentration of calcium in the parent water. The strontium content of our Luray aragonites is always greater than the strontium content of the Luray calcites, and the absolute value of the strontium concentration can be related satisfactorily to the strontium-calcium ratio in the parent water via the distribution coefficients $$k_{Sr}^{C}$$ and $$k_{Sr}^{A}$$ determined in the laboratory. The strontium content of the cave waters at Luray is determined by the amount of dolomite dissolved by the carbonated rain water, by the strontium content of the dissolved dolomite, and by the amount and mineralogy of the $$O_{3}$$ deposited during equilibration with cave air.