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Advancing the use of noble gases as paleoclimate indicators

Final Activity Report Summary - ADNOGAPALIN (Advancing the use of noble gases as paleoclimate indicators)

The main goal of this project was to advance the use and understanding of noble gases in palaeoclimatic studies, in particular with respect to excess air formation in groundwater and the study of fluid inclusions in speleothems.

A prerequisite to reach this goal was the availability of a dedicated, highly sensitive and precise system for mass spectrometric measurements of noble gas isotopes dissolved in different types of water samples. The researcher contributed substantially to the establishment of a new noble gas laboratory which was able to handle all five noble gases dissolved in groundwater, a task that only a few labs around the world were able to perform.

Along with analytical developments, several field and laboratory projects were carried out, mainly with the intention to investigate the excess air formation under laboratory and natural conditions and the potential of noble gas concentrations in fluid inclusions of stalagmites as a palaeotemperature proxy. In order to improve the understanding of the excess air formation, the physical processes that affected the amount and the gas composition of the excess air were studied in detail. In laboratory experiments, plexiglas columns filled with different porous media were filled with water under varying hydrostatic pressure. A series of experiments revealed a quasi-linear positive relationship between excess air amount and hydrostatic pressure. In field experiments, which took place at a site where groundwater level was regularly fluctuating because of artificial recharge, the same link between hydrostatic pressure and excess air pattern could be demonstrated. It was therefore concluded that the excess air formation was driven solely by solubility and not by any processes related to diffusion.

This solubility controlled gas partitioning could be mostly described by the closed system equilibration model (CE). Additionally, some hints appeared in the field experiment on the role of oxygen depletion to the excess air amount dissolved in groundwater. Based on noble gas measurements, the researchers determined excess air amount and found that it was higher than expected from the water level increase. This latter finding required to take the effect of oxygen depletion into account; however this was necessary only in the entrapped air and not in the free soil air as recently proposed in literature (OD-model).

In the course of the project a special case of gas partitioning was investigated. The dissolved noble gas content of the groundwater could be lower than expected in solubility equilibrium. This so-called degassing effect could be explained by the formation of gas bubbles free of noble gases in the saturated zone, where a re-partitioning of the noble gases, as well as other gases, occurred between the liquid and the gas phase. Laboratory experiments were carried out to study this process. It was shown that a generalisation of the CE model could describe the dissolved gases in this latter case.

A great effort was also made to investigate whether it was possible to determine equilibration temperatures through measuring noble gas concentrations in very small water samples extracted from fluid inclusions in stalagmites. The researcher contributed to achieve a good measurement precision of very small water samples that, in principle, allowed for determination of noble gas temperatures on fluid inclusions of stalagmites. Even though the first results showed that many stalagmite samples included a significant fraction of air-filled inclusions that made the sample preparation and evaluation more complicated, it was found that appropriate stalagmite samples that contained very little air inclusions did exist. In this latter case an error of approximately 1 degree Celsius for noble gas temperature determination could be achieved, providing the possibility to apply this method as a palaeoclimate proxy.