Systematic investigation of stable isotope fractionation in speleothems with novel laboratory experiments
Final Report Abstract
We developed a worldwide unique experimental setup to study the basic isotope fractionation processes during precipitation of speleothem calcite. The setup allows to precisely adjust the experimental parameters (temperature, pCO2, δ13C and δ18O of the gaseous CO2, relative humidity, thickness of the solution film, etc.) to those of natural cave systems and to control them during the experiment. In addition, the temporal evolution of solution chemistry (pH, electrical conductivity) and, thereby, the rate of calcite precipitation can be monitored during the experiments. Within the experiments, thin solution films (0.1 mm in thickness, as they occur on the surface of speleothems) of a CaCO3-CO2-H2O solution flow down an inclined marble or glass plate, progressively precipitating CaCO3 along the flow path. The solution is collected after different distances of flow, and pH, electrical conductivity as well as the δ13C and δ18O values of the DIC are determined. In addition, we determine the δ13C and δ18O values of the calcite precipitated along the flow path. The combination of both experiments enables us, for the first time, to directly determine carbon and oxygen isotope fractionation between the DIC and the CaCO3 for different temperatures, precipitation rates and pCO2 values of the box atmosphere. First, we studied carbon and oxygen isotope exchange between the gaseous CO2 in the box and the DIC in the thin solution film, which may have a significant effect on the evolution of the δ13C and δ18O values of the DIC and the CaCO3 precipitated along the flow path. In addition, two models describing isotope exchange were developed. In general, both models reproduce the experimental results. However, for low pCO2 (i.e., between 500 and 1000 ppmV), as observed in many cave systems, the subsequently developed complete diffusion reaction model provides more realistic results. For the interpretation of speleothem paleoclimate records, isotope exchange may have an influence in case of very long drip intervals and slow calcite precipitation rates. The calcite precipitation experiments were conducted at different temperatures, pCO2 values and different concentrations of dissolved CaCO3. Stable isotope fractionation between calcite and DIC, εCaCO3/DIC, is strongly negative for both carbon and oxygen isotopes. This is particularly surprising for carbon isotope fractionation, where equilibrium fractionation between CaCO3 and DIC is small, but positive. In addition, the absolute value of isotope fractionation decreases along the plates and, thus, with decreasing supersaturation and precipitation rate. This rate dependence of the fractionation factors is the first experimental evidence that isotope fractionation during precipitation of speleothem calcite is kinetic. Furthermore, our experimental data enable us to determine oxygen isotope fractionation between water and speleothem calcite, which is a requirement for palaeotemperature reconstructions. Our experimentally determined values as well as their temperature dependence agree best with the fractionation factors determined in natural cave systems. We also determined clumped isotope (∆47) values of the DIC and calcite samples. The ∆47 values observed during our experiments are lower than the equilibrium ∆47 value expected for the experimental temperature and suggest apparently higher temperatures. This confirms the observations from natural cave systems, which show a systematic offset to higher temperatures. Finally, experiments to study trace element partitioning into speleothem calcite were conducted. For several elements, the element-to-Ca ratios of both the solution and the precipitated calcite show the expected evolution along the channel. For instance, the Mg/Ca ratios of both the solution and the calcite increase along the channel. This is in agreement with a Mg distribution coefficient much lower than 1. By comparison of the trace element-to-calcite ratios of the solution and the precipitated calcite, we could, in principle, determine the distribution coefficients for various elements as well as their dependence on temperature and precipitation rate. However, this would require more detailed experiments as well as a larger dataset.
Publications
- (2019) Simulating speleothem growth in the laboratory: Determination of the stable isotope fractionation (δ13C and δ18O) between H2O, DIC and CaCO3. Chemical Geology 509 20–44
Hansen, Maximilian; Scholz, Denis; Schöne, Bernd R.; Spötl, Christoph
(See online at https://doi.org/10.1016/j.chemgeo.2018.12.012) - (2019) Simulating speleothem growth in the laboratory: Determination of the stable isotope fractionation (δ13C and δ18O) between H2O, DIC and CaCO3. Chemical Geology 509 20–44
Hansen, M., Scholz, D., Schöne, B.R. and Spötl, C.
(See online at https://doi.org/10.1016/j.chemgeo.2018.12.012) - (2016) Problems in using the approach of Rayleigh distillation to interpret the 13C and 18O isotope compositions in stalagmite calcite. Acta Carsologica 45, 85-93
Dreybrodt, W.
(See online at https://dx.doi.org/10.3986/ac.v45i3.4698) - (2016) 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. Geochimica et Cosmochimica Acta 174, 247-262
Dreybrodt, W., Hansen, M. and Scholz, D.
(See online at https://doi.org/10.1016/j.gca.2015.11.012) - (2016) The evolution of 13C and 18O isotope composition of DIC in a calcite depositing film of water with isotope exchange between the DIC and a CO2 containing atmosphere, and simultaneous evaporation of the water. Implication to climate proxies from stalagmites: A theoretical model. Geochimica et Cosmochimica Acta 195, 323-338
Dreybrodt, W. and Romanov, D.
(See online at https://doi.org/10.1016/j.gca.2016.07.034) - (2017) Carbon isotope exchange between gaseous CO2 and thin solution films: Artificial cave experiments and a complete diffusion-reaction model. Geochimica et Cosmochimica Acta 211, 28-47
Hansen, M., Scholz, D., Froeschmann, M.-L., Schöne, B.R. and Spötl, C.
(See online at https://doi.org/10.1016/j.gca.2017.05.005) - (2017) Isotope exchange between DIC in a calcite depositing water layer and the CO2 in the surrounding atmosphere: Resolving a recent controversy. Acta Carsologica 46, 339-345
Dreybrodt, W.
(See online at https://doi.org/10.3986/ac.v46i2-3.6597)
