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

We present a theoretical derivation of the exchange time, sex, 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 tau(ex) = tau(ex)(red) . [HCO3-]/(K-H . p(CO2)(cave)), where tau(ex)(red) depends on the depth, a, of the water film and on temperature. [HCO3-] is the concentration of bicarbonate, p(CO2)(cave) the partial pressure of CO2, and K-H is Henry's constant. To test the theory we prepared stagnant or flowing thin films of a NaHCO3 solution and exposed them at 20 degrees C to an CO2 containing atmosphere of p(CO2) 500, 12,500, or 25,000 ppmV and defined isotope composition. The delta C-13 and delta O-18 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 0.2 cm the delta C-13 values exhibit an exponential approach towards isotope equilibrium with the atmospheric CO2 with exchange time, tau(ex). The delta O-18 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 5 mmol/L of NaHCO3 with a depth of 0.013 cm flowing down an inclined borosilicate glass plate was exposed to an atmosphere with p(CO2) = 500 ppmV at a temperature of 20 degrees C. The delta C-13 and delta O-18 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 delta(DIC)(t) = delta(DIC)(0) - (delta(DIC)(0) - delta(DIC)(infinity) . t/tau(ex) where delta(DIC)(0) is the initial isotope composition of dissolved inorganic carbon (DIC) in the water film and delta(DIC)(infinity) its final value. From these data an exchange time tau(ex) of ca. 7000 s was obtained, in satisfactory agreement with the theoretical predictions. The exchange times can be calculated by tau(ex) = tau(ex)(red) . [HCO3-]/(K-H . p(CO2)(cave)), where tau(ex)(red) 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. (c) 2015 Elsevier Ltd. All rights reserved.