Geophysical Borehole logging at Santa Gracia, Chile

The DFG funded DeepEarthshape project within the SPP1803 EarthShape (second phase) combines several geoscientific methods and approaches to study the weathering zone in detail in dependence of climate conditions. Projects of the first phase have shown that the weathering zone is much deeper than expected, so that the weathering front was never encountered in the excavated soil pits. At depth of 1 – 2 m appreciable amounts of microbial biomass and DNA counts were encountered. It was further found that bacteria and archaea colonizing rock surfaces are close relatives to those from deeper soil zones. Because we do not know a) the depth of weathering; b) the process advancing it; c) whether this advance is driven by water, gases, and/or biological activity and concentrated along faults; d) whether this zone presents a habitat and interacts with the surface biosphere, we have designed a drilling campaign at all four study sites for joint geochemical, biogeochemical and microbiological exploration and a geophysical campaign for imaging the depth and physical properties of the critical zone. The principle hypotheses of the DeepEarthshape projects are: 1) The advance of the weathering front at depth is a recent process that is linked to climate and coupled with erosion at the surface through a biogeochemical feedback 2) Microbial activity in the deep regolith that advances weathering is fuelled by young organic matter. The four study sites are distributed along the coast of Chile to have a similar geological setting at one hand but different climatic conditions. Here we present the logging data of the first geophysical borehole survey which took place at Santa Gracia, 40 km NE of La Serena (Coquimbo Region, Chile). The data were acquired on the 2nd of April 2019 between . The borehole logging was conducted by COMPROBE. The vertical borehole reached down to 87.2 m depth and had a diameter (PQ) of 83.5 mm.