Multi-scale imaging of the weathering front with geophysical and geochemical techniques

Investigator Names and Contact Info:

• Mirjam Schaller (Geology/Geochemistry). Department of Geosciences, University of Tuebingen, Germany
• Jan van der Kruk (Geophysics). Forschungszentrum Jülich, Germany

Chilean Collaborators Involved:

• Juan-Pablo Fuentes. Forestales & Conservacion de la Naturaleza, Universidad de Chile, Santiago, Chile

Geophysical characterization (GPR and EMI) of the weathering front on hillslopes in combination with geochemical proxies.

PhD-Student:

• Igor Dal Bo. Forschungszentrum Jülich, Germany

Supervisor: Prof. Jan van der Kruk, Co-supervisor: Dr. Mirjam Schaller

Project Summary:

Soil is an important carrier of life and is produced by chemical weathering and physical erosion of rock. The extent of soil is governed by tectonics, climate, and vegetation. However, little is known about the extent of weathering fronts in different climatic and biotic settings. In this interdisciplinary proposal the weathering front will be investigated on three different hillslope sites along coastal Chile with different climatic settings and different vegetation by combining geophysical and geochemical techniques. In this way, the labor intensive and spatially restricted geochemical imaging of the weathering front will be used and combined with the geophysical methods that are able to noninvasively map the structural organization of soil and soil depth at larger scales. A top-down multi-scale approach will be used where a large-scale multi-configuration EMI mapping will be performed to characterize the dominant large-scale features in electrical conductivity. From these images, several transects will be selected where a more detailed geophysical imaging (EMI and GPR) will be employed. Two state-of-the-art multi-configuration EMI devices will be employed having a depth of investigation ranging from 0.20 m up to 6m. In addition, several GPR antennas will be employed ranging from 100 up to 1000 MHz. Based on these geophysical images, augered soil samples will be made at several locations and analyzed for texture, pH, Corg, ECeff and soil water content. Dug soil pits on selected transects will enable the analysis of major and trace elements to decipher weathering fronts. Cosmogenic nuclides to decipher soil denudation rates as well as soil mixing depths will be carried out in the key soil pits. All these results will be regressed with the inverted EMI and GPR electrical parameters at the soil pit locations to investigate whether the obtained EMI and/or GPR layer thicknesses correspond to the weathering thickness and whether changes in the obtained electrical conductivity for low (EMI) and high frequencies (GPR) and permittivity are correlated to the determined parameters. We plan to include also other information obtained by other projects performed at the key soil pits in this analysis. In this way, we will upscale the spatially restricted knowledge of weathering fronts from several depth profiles using geophysical imaging methods to quantify weathering fronts on hillslope scales for different climatic settings and different vegetation.