Dynamics of gas transport in pore waters of lacustrine sediments
This work focuses on the dynamics of gases in lacustrine sediments. It emphazises the effects of transport processes on noble gases dissolved in the sediment pore water. In general, the noble gas concentrations in lacustrine sediments agree with the concentrations in the overlying lake water column. In turn, the noble gas concentrations in the overlying water column, i.e. air saturated water (ASW) concentrations, are determined by the partitioning between atmosphere and lake water. Due to their biogeochemical inertness, any deviation of noble gas concentrations in sediment pore water from ASW concentration is attributed to physical processes. Therefore, noble gases have been established as excellent tracers for the characterization of gas dynamics and transport processes in lacustrine sediments.
However, there are still open questions regarding the mechanism of the noble gas transport in the sediment pore water: (1) The behaviour of noble gas isotopes during molecular diffusion in water lacks of a sound physical basis. (2) The gas transport in lacustrine sediments varies extremely depending on the sediment properties. In three case studies noble gases were used to investigate on one hand the gas dynamics in supersaturated sediments (Lake Rotsee and Lake Lungern) and on the other hand the suppressed noble gas transport in highly compacted sediments (Lake Zurich).
The details, on how these questions were addressed, are explained in the following: (1) Molecular diffusion is the key transport process for noble gases dissolved in the sediment pore water. Noble gas diffusion in water is commonly expected to cause a fractionation of noble gas isotopes in accordance with the square root relation, which describes the isotope fractionation to be inversely proportional to the square root of the ratio of their atomic mass. However recent studies challenge the applicability of the square root relation, derived from the kinetic theory of gases, to predict the fractionation of noble gas isotope by reason of molecular diffusion in water. Therefore the fractionation of the noble gas isotopes (Ne, Ar, Kr Xe) due to molecular diffusion in water was determined in a laboratory experiment. Surprisingly, only the fractionation of Ar isotopes can be adequately described by the square root relation. The fractionation of Ne, Kr and Xe isotopes was found to be much lower and even negligible. The results from the laboratory experiments provide a robust basis for studies that use the fractionation of the noble gas isotope ratios to assess molecular diffusion in water bodies, e.g. to investigate diffusive transport of noble gases dissolved in the pore water to gas bubbles formed in the sediment pore space.
(2) In sediments characterised by an active microbial gas production in their pore waters and hence supersaturated with reactive gases (e.g. CH4), the transport of noble gases through the pore space is enhanced relative to the diffusion of noble gases in the pore water. The enhanced transport of noble gases results from their interaction with gas bubbles that are formed in the sediment pore space and which are eventually released. Methods that are commonly used to determine CH4 concentrations in the sediment pore water do not prevent gas exchange with the atmosphere during sampling and are therefore not suitable to quantify CH4 in supersaturated sediments. To improve the accuracy of CH4 measurements a new method for the analysis of CH4 concentrations in sediment pore waters has been developed in the framework of this thesis.
This newly developed method was combined with noble gas analysis and applied to the sediments of two lakes, Lake Rotsee and Lake Lungern, which are characterised by an active bubble ebullition (mostly CH4) from their sediments. The continuous CH4 flux from the sediments of Lake Rotsee was determined using the noble gas concentration gradient, showing an increasing depletion relative to air saturated water (ASW) concentration with increasing sediment depth. The zone of active bubble ebullition from the sediment was indicated by the CH4 concentration peak determined in the pore water, which exceeds the local saturation concentration by one order of magnitude.
Lake Lungern is subject to extreme water level variations, which induce bubble formation in the sediment pore water and emission of CH4 together with the bubbles leaving the sediment. Furthermore, parts of the sediment fall dry during low water level. The noble gas excess relative to ASW determined in this zone was used to estimate the oxygen supply to the "dry falling" sediment.
In a pilot study on a glacial clay sediment of Lake Zurich a noble gas excess relative to ASW was determined as well. This excess might originate from in owing glacial meltwater as a strong 4He enrichment and the noble gas pattern in the pore water of the sediment are interpreted to indicate that noble gases have been quantitatively trapped in the clay for a period of several thousand years.
Overall the results of the laboratory experiments on noble gas isotope diffusion in water represent an important step towards establishing the ratio of 36Ar/40Ar as isotopic tracer for diffusive transport in unconsolidated sediments. Furthermore, this thesis gives a specific insight into the interaction of dissolved noble gases and reactive gas bubbles in the sediment pore water of gas supersaturated sediments, and the method developed for the determination of real in situ CH4 concentrations contributes to the further investigation of such sediments. To summarize, this thesis illustrates that noble gases are ideal tracers for the gas dynamics in a wide range of lacustrine sediments, from gas supersaturated sediments to highly compacted clay sediments.