Oxygen isotopes in calcite grown under cave-analogue conditions

Speleothem oxygen isotopes and growth rates are valuable proxies for reconstructing climate history. There is debate, however, about the conditions that allow speleothems to grow in oxygen isotope equilibrium, and about the correct equilibrium fractionation factors. We report results from a series of carbonate growth experiments in karst-analogue conditions in the laboratory. The setup closely mimics natural processes (e.g. precipitation driven by CO₂-degassing, low ionic strength solution, thin solution film) but with a tight control on growth conditions (temperature, pCO₂, drip rate, calcite saturation index and the composition of the intial solution). Calcite is dissolved in water in a 20,000 ppmV pCO₂ environment. This solution is dripped onto glass plates (coated with seed-carbonate) in a lower pCO₂ environment (<2500 ppmV), where degassing leads to calcite growth. Experiments were performed at 7, 15, 23 and 35°C. At each temperature, calcite was grown at three drip rates (2, 6 and 10 drips per minute) on separate plates. The mass of calcite grown in these experiments varies with temperature (T in K) and drip rate (d_r in drips min⁻¹) according to the relationship daily growth mass = 1.254 + 1.478 * 10⁻⁹ *e0.00679T +(e0.00248T-2)*(-0.779d_r² + 10.05d_r + 11.69). This relationship indicates a substantial increase of growth mass with temperature, a smaller influence of drip rate on growth mass at low temperature and a non-linear relationship between drip rate and growth mass at higher temperatures. Low temperature, fast dripping conditions are found to be most favourable for reducing effects associated with evaporation and rapid depletion of the dissolved inorganic carbon reservoir (rapid DIC-depletion). The impact of evaporation can be so large so caves with high relative humidity are also preferable for palaeoclimate reconstruction. Even allowing for the maximum offsets that may have been induced by evaporation and rapid DIC-depletion, δ18O measured in some of our experiments remained higher than those predicted by Kim and O'Neil (1997). Our new results are well explained by equilibrium at a significantly higher αcalcite-water, with a kinetic-isotope effect that favours 16O incorporation as growth rate increases. This scenario agrees with recent studies by Coplen (2007) and Dietzel et al. (2009). Overall, our results suggest that three separate processes cause δ18O to deviate from true isotope equilibrium in the cave environment. Two of these drive δ18O to higher values (evaporation and rapid DIC-depletion) while one drives δ18O to lower values (preferential incorporation of 16O in the solid carbonate at faster growth rates). While evaporation and DIC-depletion can be avoided in some settings, the third may be inescapable in the cave environment and means that any temperature to δ18O relationship is an approximation. The controlled conditions of the present experiments also display limitations in the use of the Hendy test to identifying equilibrium growth