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124 CHAPTER 4. AQUATIC SYSTEMS 4.1.4 Noble gas measurements on fluid inclusions in speleothems Tobias Kluge (participating scientists: Werner Aeschbach-Hertig) Abstract Dissolved atmospheric noble gases in groundwater constitute a reliable paleothermometer, based on their precisely known temperature-dependent solubilities. This archive is limited due to the dispersive mixing and dating problems. In contrast, speleothems can be well dated using uraniumthorium dating. However there is no paleothermometer comparable to the noble gas thermometer so far. The idea of this project is to combine the advantages of both archives. Xe (ccSTP/g) 1.2x10 -7 8.0x10 -8 4.0x10 -8 aew 0.0 1.0x10 -5 0.0 2.0x10 -5 Ne (ccSTP/g) Kr (ccSTP/g) 3.0x10 -5 1.6x10 -6 1.2x10 -6 8.0x10 -7 4.0x10 -7 aew 0.0 4.0x10 -3 0.0 8.0x10 -3 Ar (ccsTP/g) Figure 4.5: Noble gas concentrations for the elements Ne, Ar, Kr and Xe, calculated for stalagmite samples in a first measurement series. The points in the blue circle are values of air-equilibrated water at temperatures between 0 o C and 30 o C. The black lines indicate addition of ”excess-air” (upper line: water at 0 o C, lower line: water at 30 o C). Most of the samples show ratios in the expected area, i.e. they are situated on the mixing line of equilibrated water with unfractionated air. However the uncertainties are quite large and typically in the range of 10%. Only the best sample has an uncertainty of 2-3%. Background Noble gases in groundwaters can be used to reconstruct paleotemperature due to the temperature-dependent solubility. Additionally, the noble gas concentrations in groundwater are affected by dissolution of trapped air bubbles. The effect of this so-called excess-air can be corrected. If noble gases are extracted from fluid inclusions in stalagmites, similar patterns can be detected (see figure). Compared to groundwater, stalagmite samples have much less water (4 orders of magnitude), but a 100 times higher excess-air to water ratio. Therefore the calculation of noble gas temperatures becomes difficult. Methods and results Noble gases have been exctraced from the stalagmites by crushing, respectively heating the sample in evacuated copper tubes. In the next step the amount of released water was determined using the water vapour pressure. The extracted water amount is strongly dependent on the stalagmite (H12 (Oman) 0.25 wt%, MA1 and MA2 (Chile) 0.01 wt%, OBI 5 (Austria) 0.004 wt%) and on the techniques (crushing, heating, microwave treatment). The noble gas measurements are performed by 1.2x10 -2 mass spectrometry . For the calculation of absolute noble gas amounts a diluted air standard is used. The reproducibility of standard measurement is in the range of 1–2%. Sample uncertainties have been much higher, because of a high and variable background and additionaly in some cases low noble gas signals. Nevertheless the background corrected Xe-Ne and Ar-Kr plot are indicating that the values are not far away from the expected mixing between noble gases from water filled inclusions and some unfractionated ”excessair”. Calculation of noble gas temperatures in case of the best samples (from a cave with a recent cave temperature of 26 o C) leads to 26, respectively 30 o C (although with very large uncertainties) using the inverse modelling technique of Aeschbach- Hertig et al. [1999]. This may be a hint that noble gases from fluid inclusions can be used to determine noble gas temperatures if the problems (high uncertainties, high water/air volume ratios) can be managed. Funding This work is financially supported by the DFG (grant AE 93/3) as part of the DFG- Forschergruppe DAPHNE.
4.1. GROUNDWATER AND PALEOCLIMATE 125 4.1.5 Gas partitioning in groundwater Werner Aeschbach-Hertig Abstract Using dissolved gas concentrations in groundwater to derive residence times and recharge temperatures requires an adequate understanding of observed excesses and sometimes deficits of atmospheric gases. A comprehensive theory has been developed that explains both excess air and degassing with a single model equation describing equilibrium gas partitioning. concentration relative to equilibrium 1 0.8 0.6 0.4 0.2 LPA17 LPA20 LPA22 LPA23 LPA23b Ne Ar Kr Xe Noble Gas LPA32 LPA32b LPA33 LPA38 LPA39 Figure 4.6: Measured (symbols) and modeled (lines) concentrations of the atmospheric noble gases relative to solubility equilibrium in 10 degassed samples from the Ledo-Paniselian Aquifer in Belgium (unpublished data). The generalised CE-model describes most samples reasonably well, even strongly degassed ones. Background The partitioning of gases between groundwater and soil air is important for a variety of issues, such as the availability of oxygen, the fate of volatile contaminants, and the distribution of gaseous tracers. Several widely used environmental tracer methods are based on gases, e.g. CFCs, SF6, He isotopes, or radon. The atmospheric noble gases are usually used to derive recharge temperatures, but are also powerful tools to trace and study gas partitioning. Methods and results Many noble gas studies have clearly shown the widespread presence of a gas excess above atmospheric solubility equilibrium (so-called ”excess air”) in groundwater. An increasing number of studies, however, also reports undersaturations, which are attributed to degassing. A quantitative understanding of the patterns of dissolved noble gases in groundwater is required for a reliable determination of noble gas recharge temperatures and gas tracer ages. Models assuming a diffusion-controlled degassing process can often be ruled out because of the absence of an indicative isotope fractionation. Instead, both excesses and deficits of dissolved gases in groundwater can be explained by equilibrium partitioning of gases between the water and trapped gas bubbles. The concept of closed- system equilibration (CE-model) proved very successful in modeling excess air [Aeschbach-Hertig et al. , 2000]. It can also be used to model the loss of dissolved gases that occurs if gas bubbles form in the subsurface. Excess air appears to be closely related to water table fluctuations and related pressure variations in the recharge zone. Degassing could be due to the accumulation of geogenic or biogenic gases and/or a pressure decrease in the groundwater discharge area. Currently only few complete noble gas data sets of degassed groundwaters exist, which could be used to test the applicability of the generalised CE-model to describe degassing. In the available cases, the model performs reasonably well. Outlook/Future work Simple laboratory experiments are currently performed to verify the model concept. Further field data sets are required in order to test the model’s usefulness in deriving recharge temperatures and tracer ages from degassed samples Main publication A paper describing the recent advancements in model formulation is in preparation, an invited conference presentation has been given [Aeschbach-Hertig, 2006d].
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4 CONTENTS 2.4.10 Satellite monitor
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184 CHAPTER 7. BIBLIOGRAPHY Wang, P
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7.5. INVITED TALKS 193 Invited Talk
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