Kinetically controlled variations of major components and carbon and oxygen isotopes in a calcite-precipitating spring

Abstract

A calcite-depositing stream in the Pyrenees, south France, enabled us to study the equilibration of major components and carbon and oxygen isotopes during the formation of calcite and the degassing of CO 2, and to examine the possible influence of aquatic flora on these processes. Calcite is not precipitated from the solution until a supersaturation of ∼ 10 is attained. In order to start the reaction the activity of CO 2− 3 has to be increased from ∼ 5 · 10 −6 to 22 · 10 −6. This is caused by an increase of pH due to the loss of CO 2 to the atmosphere. The excess free energy provided by CO 2 degassing is ∼ −5 kJ mol −1. If formed under equilibrium conditions, the calcite should differ isotopically by Δ 13 C = + 2.3‰ from the dissolved carbonate. However, the observed difference is virtually zero and demonstrates substantial disequilibrium with respect to the stable carbon isotopes. Isotopic disequilibrium is also indicated by oxygen. 13O-temperatures differ on the average by ∼ + 5°C from the observed temperatures of calcite precipitation. In contrast to calcite formation carbon-isotope equilibrium is attained between the dissolved carbonate and the atmosphere on the degassing of CO 2. The observed changes of δ 13C are in agreement with theoretical expectations, unless the surface area of the water is large. The variations of the major components and the carbon and oxygen isotopes observed during diurnal cycles are small and demonstrate that metabolic effects are negligible in a system with a high supply rate of ∼ 25 g HCO − 3 s −1.