DeepEarthShape - Geophysical Imaging: Imaging weathering fronts in deep regolithwith seismic and electromagnetic methods [GIDES]

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This project is part of the interdisciplinary DeepEarthshape linkage to investigate the weathering or critical zone (CZ) with drilling and a combination of geophysical, geochemical and microbiological approaches. The CZ is the uppermost part of the Earth’s crust where rocks and soils experience breakdown either mechanically or chemically through the impact of air/gases, water and/or biological organisms. Its thickness depends on the balance between erosion and weathering processes that deepen the interface between weathered and fresh bedrock. Existing geochemical characterisation of the CZ has shown that it is much deeper than expected (possibly >30m). As a result, the weathering front has hardly been studied in the excavated soil pits. Although at shallow depths (1-2m) appreciable amounts of microbial biomass and DNA counts were observed that might be related to weathering, our insight into the entire CZ and its processes is still limited. We do not know for instance the depth of weathering, the process advancing it or the particular perpetrators. Since properties and characteristics of the CZ seem to be linked to climate, a set of four study sites is proposed within the framework of the DFG SPP 1803 belonging to different climate zones with different vegetation, precipitation and erosion. However, the long-stretched coast of Chile represents a prime location to examine climatic dependencies while staying in a similar geological complex - the Coastal Cordillera. Therefore by comparing the obtained results from these four study sites we finally want to test hypotheses for the CZ, like a possible linkage of the advance of the weathering front at depth with recent climate-driven processes and erosion at the surface through a biogeochemical feedback or microbial activity in the deep regolith fueled by young organic matter that advances weathering. Shallow geophysics is now emerging as an essential component of CZ investigations to test hydro-geomorphological and weathering front models. Here, we propose combined geophysical experiments using P- and S-wave seismics and shallow electromagnetic (Radiomagnetotelluric) measurements along ~500m long profiles at all four study sites. The major objectives to be addressed by these geophysical experiments are a) imaging the depth of the CZ and its variations; b) correlating changes in physical properties seen by seismic and EM methods with those found in cores drilled at the same site; c) assessing if borehole results are representative on a larger spatial scale; d) comparing geophysical images of the CZ with those predicted by hydro-geomorphological models; e) determining the depth of the water table and how it is linked to fractures providing pathways for meteoric water; f) linking seismic velocities with electrical conductivities to obtain reliable estimates of porosity and g) to derive a consistent geological interpretation of different geophysical, geochemical and microbiological observations.