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 at

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 pCO2 = 500 ppmV 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. 7000 s 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.