Processes affecting the stable isotope composition of calcite during precipitation on the surface of stalagmites: Laboratory experiments investigating the isotope exchange between DIC in the solution layer on top of a speleothem and the CO2 of the cave atmosphere

Abstract

We present a theoretical derivation of the exchange time, τex, needed to establish isotopic equilibrium between atmospheric CO2 in a cave and HCO3− dissolved in a thin water film covering the surface of a speleothem. The result is τex=τredex·[HCO3-]KH·pCO2cave, where τredex depends on the depth, a, of the water film and on temperature. [HCO3-] is the concentration of bicarbonate, pCO2cave the partial pressure of CO2, and KH is Henry’s constant. To test the theory we prepared stagnant or flowing thin films of a NaHCO3 solution and exposed them at 20°C to an CO2 containing atmosphere of pCO2 500, 12,500, or 25,000ppmV and defined isotope composition. The δ13C and δ18O values of the DIC in the solution were measured as a function of the exposure time. For stagnant films with depths between 0.06 and 0.2cm the δ13C values exhibit an exponential approach towards isotope equilibrium with the atmospheric CO2 with exchange time, τex. The δ18O values first evolve towards isotopic equilibrium with atmospheric CO2, reach a minimum value and then drift away from the isotopic equilibrium with atmospheric CO2 approaching a steady state caused by isotopic exchange of oxygen with water. The experimental findings are in satisfactory agreement with the theoretical predictions.To further investigate isotope evolution in cave analogue conditions, a water film containing 5mmol/L of NaHCO3 with a depth of 0.013cm flowing down an inclined borosilicate glass plate was exposed to an atmosphere with pCO2=500ppmV at a temperature of 20°C. The δ13C and δ18O values were measured as a function of flow (exposure) time, t. The isotope compositions in the DIC of the water film decrease linear in time by δDIC(t)=δDIC(0)-(δDIC(0)-δDIC(∞))·t/τex where δDIC(0) is the initial isotope composition of dissolved inorganic carbon (DIC) in the water film and δDIC(∞) its final value. From these data an exchange time τex of ca. 7000s was obtained, in satisfactory agreement with the theoretical predictions. The exchange times can be calculated by τex=τredex·[HCO3-]KH·pCO2cave, where τredex is given by the theory as function of temperature and the depth, a, of the water film. This way it is possible to obtain exchange times for various conditions of stalagmite growth as they occur in caves.