Chemical evolution of dissolved inorganic carbon species flowing in thin water films and its implications for (rapid) degassing of CO_2 during speleothem growth

摘要

Rapid degassing of CO_2 from a thin film of drip water on the surface of stalagmites is often considered to have a large effect on both speleothem growth and stable isotope values and is offered as an explanation for higher δ~(13)C and δ~(18)O values than expected under conditions of stable isotope equilibrium. However, the time constant for degassing of CO_2 from the solution only depends on film thickness and the coefficient of molecular diffusion for CO_2. Thus, for thin films, the time for degassing of CO_2 is much shorter than the time for subsequent equilibration of the dissolved carbon species and precipitation of CaCO_3. In this context, degassing of CO_2 is always fast.Here we present three experiments that enable the determination of the time constants for degassing of CO_2, τ_(deg), subsequent equilibration to a new pCO_2, τ_(eq), and precipitation of CaCO_3, τ_(pr), in a thin film of an H2O-CO_2-CaCO_3 solution flowing on a calcite surface. The experiments are performed under cave-analogue conditions.At a temperature of 20°C and for a film thickness of δ≈0.01cm, τ_(deg)≈2s. τ_(eq)≈13s and, thus, one order of magnitude larger. Finally, τ_(pr)≈400s for δ≈0.01cm, again one order of magnitude larger. The experimentally determined values for τ_(deg), τ_(eq) τ_(pr) are in good agreement with the theoretical predictions.Our results confirm that the chemical evolution of the drip water proceeds in three subsequent major steps. During the first step of degassing of CO_2, pH and Ca~(2+) concentration remain almost constant. During equilibration to the lower pCO_2 of the solution, pH increases to about 8 whereas the Ca~(2+) concentration still remains constant. Finally, during precipitation of calcite, pCO_2 remains at its low level and pH decreases slightly.These results suggest that the drip rate may have an important influence on the stable isotope signals recorded in speleothems. Stalagmites growing beneath drip sites with stable, intermediate drip rates (i.e., in the range of τ_(eq)) may be best suited for palaeoclimate reconstruction.