Recently experimental techniques for the sampling and analysis of dissolved noble gases in the pore water of lake sediments were developed. First applications of these methods indicate that dissolved noble gases in sediment pore water are promising proxies for (palaeo) environmental conditions in lakes and in the ocean, and for the transport of solutes in the sediment and their release into the overlying water body and into the atmosphere.
The key conclusion of all these studies is that in some sediments the diffusivity of noble gases in the sediment pore water is similar to their molecular diffusivity in bulk water. In other sediments, however, noble gases are quantitatively trapped in the sediment and diffusion is therefore strongly suppressed. This trapping results in a stratigraphically controlled noble-gas record in the sediment, which allows a time scale to be associated with the noble gas record in the pore water. However, the mechanisms resulting in this noble-gas trapping remain to be identified. A mechanistic understanding of the diffusion suppression is required to establish the conceptual basis needed for future applications of noble gases as proxies for environmental conditions and transport of pore fluids.
The aims of the PhD research project proposed here are (i) to improve the mechanistic understanding of how and why noble gases are trapped in some sediments, but not in others (work packages A and B) and (ii) to apply the methods and concepts established in our previous projects in a study targeted at specific geological and environmental questions in a Swiss lake (work package C):
A. Study of microscopic pore-space geometry in relation to diffusion suppression: We aim to study the microscopic pore-space geometry of different sediments to identify the mechanisms that result in noble-gas trapping in the sediment.
B. Quantification of the effective diffusivities of noble-gas isotopes: We aim to measure the effective noble-gas diffusivities in different sediments to quantify the extent of the diffusion suppression.
C. Investigation of the dynamics and origin of CH4-rich fluids in the sediment of Lake Lungern (Switzerland): the sediments of Lake Lungern contain large amounts of CH4-rich fluids. We aim to use noble-gas isotopes as proxies for the transport and origin of these fluids, which may also be associated with the formation of distinct dome-like elevations (mounds) on the generally flat lake floor.
Based on these approaches, we aim to consolidate and further expand the uses of noble gas geochemistry as a widely applicable tool to study the transport of fluids and solutes in unconsolidated sediments, and to extend the area of application of noble-gas geochemistry in the environmental sciences.
Furthermore, knowledge of the transport properties of noble gases in relation to the characteristics of the sediment will allow targeted choices of the most suitable study sites for future research projects. Also, this work will be of direct benefit to other research not covered by the current project. For instance, interpretation of the noble-gas data that will be obtained from the deep-drilling sediment cores of Lake Van (Turkey) within the framework of the ICDP-PaeloVan project will benefit directly from the improved knowledge on noble-gas trans- port mechanisms in lake.