Large fractionation of calcium isotopes during cave-analogue calcium carbonate growth

Abstract

We have measured d 44/42 Ca of laboratory-precipitated calcite grown in an experimental setup that closely replicates stalagmite formation. Calcium solutions were dripped onto two different substrates in tightly-controlled conditions and calcite precipitated due to rapid CO 2 degassing. With seeded glass slides as the substrate, we observe a Ca isotope ratio in the calcite which is $0.5& per amu lower than that in the growth solution. This fractionation is generally almost twice that observed in previously published calcite growth experiments and indicates a large kinetic effect on Ca isotopes in the stalagmite growth environment. The precipitate forming near the spot where the drip lands shows slightly greater solution-to-precipitate fractionation than calcite further from the drip reflecting a decrease in this kinetic fractionation as precipitation continues. We interpret these results in the context of the model of Fantle and DePaolo (2007) which involves surface entrapment of light Ca isotopes to decrease calcite d 44/42 Ca, and depletion of Ca from the solution in the direct vicinity of the growing calcite to increase calcite d 44/42 Ca. In the stalagmite setting, the second of these effects is minimized so that calcite Ca isotope ratios are unusually light. This interpretation suggests that stalagmite Ca isotope ratios should decrease with the saturation state of the drip water (i.e. with the growth rate of calcite). Ca isotopes might therefore allow reconstruction of surface entrapment of trace metals and isotopes more generally and might, for instance, allow an assessment of the appropriate relationship between oxygen isotope fractionation and temperature for periods of past growth in stalagmites.

Strontium isotope fractionation of planktic foraminifera and inorganic calcite

“…x value of Reynard et al (2011) the maximum D 44/40 Ca DBL would increase to 1.57& or 0.07&, respectively. In any case, the heavy isotope enrichment effect of a 44 X dominates over the diffusive depletion, a 44 d .…”

Comment on “Oxygen isotopes in calcite grown under cave-analogue conditions” by C.C. Day and G.M. Henderson

“…If the residence time of the water on the glass plate (or stalagmite) is small compared to the exchange time the 18 O enrichment in the oxygen atoms of the water cannot be transferred into the carbonates. This is the case in the experimental set up, where residence times are about 462 s for 1.6 drips/min, 107 s for 5.8 drips/min and 70 s for 10.4 drips/min respectively (Reynard et al, 2011). If one assumes instantaneous evaporation of 10% of the water from the film Rayleigh distillation causes an increase in the initial 18 in O H 2 O in the water by O 18 H 2 O of about 1& in the temperature range from 5 to 25°C (Mook, 2000 (Beck et al, 2005;Dreybrodt, 2008, Halas andWolacewicz, 1982) where 18 in O car is the isotope composition in the carbonate system prior to evaporation of the water and t the time after instantaneous evaporation.…”

“…When the water flows down the plate SI c increases and consequently precipitation of calcite is enhanced as seen by the white regions. As the drip rates increase the residence time of a water parcel flowing down the plate decreases and consequently its velocity increases (Reynard et al, 2011). Therefore a sufficiently high SI c to support precipitation is attained more distant from the drip point.…”

Technical Note: A simple method for vaterite precipitation for isotopic studies: implications for bulk and clumped isotope analysis

“…Nebel and Epple, 2008). As rapid mineral growth may induce disequilibrium fractionation related to a mineral surface effect (Watson, 2004;Dietzel et al, 2009;Watson and Müller, 2009;DePaolo, 2011;Reynard et al, 2011;Gabitov et al, 2012;Gabitov, 2013), experiments with quasi-instantaneous mineral growth are not suitable for isotope studies. Consequently, we neither rapidly grew vaterite nor conducted isotope measurements on potentially fast growth phases.…”