Authors:	Hansen, Maximilian
Title:	Determination of the stable isotope fractionation (d18O and d13C) during precipitation of speleothem calcite: Novel cave analogue laboratory experiments
Online publication date:	15-Mar-2022
Year of first publication:	2022
Language:	english
Abstract:	Speleothems are unique paleoclimate archives, often providing continuous long term stable isotope records from all parts of the world. Quantitative interpretation of these isotope records in terms of past climate and/or environmental variability remains challenging, because various processes occurring in the atmosphere, the soil, the epikarst and inside the cave, may obscure these climate signals. In particular, the basic fractionation processes which influence the incorporation of stable isotope signals into speleothems are not fully understood yet and are still a matter of controversial discussion in the speleothem community. In the framework of this thesis experiments were developed and conducted in order to test the most established fractionation models. The experiments were performed under completely controlled, cave analogue conditions aiming to simulate all processes occurring during the formation of speleothems. The different experimental setups are placed inside a climate box, which enables to control all surrounding conditions, such as temperature, relative humidity and pCO2 as well as isotope composition of the CO2 and adjust them to natural conditions. For the first time it is possible to observe the temporal evolution of all chemical parameters (pH, precipitation rate, supersaturation with respect to calcite) and the isotope composition of the DIC, as well as of the directly precipitated CaCO3 at the same time. This enables to directly investigate the stable isotope fractionation of all involved species in the system (CO2, DIC, CaCO3 and H2O) as a function of the experimental parameters (e.g., temperature and pCO2). One process, which potentially obscures the isotope signal of a calcite precipitating solution, is the isotope exchange between the thin solution film on top of a speleothem and the gaseous CO2 of the (cave) atmosphere. In order to quantify this process in a first study a theory was developed, describing the isotope exchange between thin solution films and gaseous CO2. To test this experiments were conducted exposing NaHCO3-solution films of two concentrations to CO2 containing atmospheres at a temperature of 20 °C. The experimental results can be well explained by the previously derived theory. In a second study the carbon isotope exchange processes was further investigated by performing new experiments closer to natural conditions at a wider range of NaHCO3-concentrations and temperatures, as well as significantly lower pCO2 and with film thicknesses in the range of natural observations. Additionally, a complete diffusion-reaction model was introduced to calculate the time constants for the carbon isotope exchange. The model explains the experimentally observed values very well and suggests that carbon isotope exchange might get significant at an earlier stage than expected by the model of the first study. In a second type of experiments, the temporal evolution of a CaCO3 precipitating solution was investigated by closely mimicking all processes potentially affecting the isotope composition of (speleothem) calcite. During the experiments, thin solution films of CaCO3-solutions of different concentrations were caused to flow down inclined marble or sand blasted borosilicate glass plates. After different distances of flow and, thus, residence times the stable isotope composition (d13C and d18O) of the dissolved inorganic carbon and the directly precipitated CaCO3 were analyzed. Additionally, the pH and electrical conductivity values were determined. For the d13C values, a strong negative fractionation between CaCO3 and DIC was observed, indicating “kinetic” or disequilibrium isotope fractionation. The initial fractionation of d18O between calcite and water corresponds well to previously reported values from natural systems, proving that the climate box closely simulates a natural (cave) system. With increasing residence time on the plates, however, the calcite progressively shows increasing d18O and d13C values, away from previously reported values. The fractionation for both d13C and d18O-values additionally seems to depend on precipitation rate, showing stronger fractionations for higher precipitation rates. The temporal evolution of d18O and d13C values for both, the DIC and the CaCO3 experiments, can be well explained using a Rayleigh distillation model. All these results show, that there is great potential for future work with the experimental setup which was developed within this thesis. Currently samples from the precipitation experiments are analyzed for their clumped isotope composition, which has not been possible at this resolution for speleothem analogue generated samples so far. In principle almost every relevant climate proxy used in speleothem science could be investigated using the here presented setup under very well constrained conditions (e.g., incorporation of trace elements or isotope).
DDC:	500 Naturwissenschaften 500 Natural sciences and mathematics
Institution:	Johannes Gutenberg-Universität Mainz
Department:	FB 09 Chemie, Pharmazie u. Geowissensch.
Place:	Mainz
ROR:	https://ror.org/023b0x485
DOI:	http://doi.org/10.25358/openscience-6803
URN:	urn:nbn:de:hebis:77-openscience-dfbbe38e-b92f-44de-9a92-69bdc1c174952
Version:	Original work
Publication type:	Dissertation
License:	In Copyright
Information on rights of use:	http://rightsstatements.org/vocab/InC/1.0/
Extent:	XXI, 204 Seiten (Ilustrationen, Diagramme)
Appears in collections:	JGU-Publikationen