DeepEarthshape - Reaction fronts in deep regolith and their advance mechanism

• Applicant: Professor Dr. Friedhelm von Blanckenburg
• Subject Area: Palaeontology ; Mineralogy, Petrology and Geochemistry
• Term: from 2015 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 280508270

The majority of Earth’s ecosystems exist in the “deep biosphere”—habitats located deep beneath the Earth’s surface in permanent darkness. The weathering zone - the subsurface part of the Earths “Critical Zone”, is an active part of this habitat. We will use innovative geochemical and isotope methods to explore the geochemical transformations shaping this zone. We do so within the DeepEarthshape package, that links projects in Geochemistry, Microbiology, Geophysics, Geology, and Biogeochemistry. The DeepEarthshape concept arose from findings in Earthshape phase 1. In all four primary study sites the weathering zone was so deep that the weathering front was never encountered in deeply excavated soil pits. Strikingly also, appreciable amounts of microbial biomass was found throughout the saprolite.Here we will explore how the range in rainfall and plant cover along the Earthshape transect is reflected in weathering front advance. Specifically we will evaluate the hypotheses that 1) weathering fronts at the Earthshape sites are recent features still evolving today; 2) mass removal by erosion and elemental dissolution is roughly balanced by the advance rate of the weathering front; and 3) the weathering zone comprises a series of discrete, nested fronts that reflect different drivers of chemical weathering (e.g. water infiltration, iron oxidation, mineralogical transformations, microbial activity, and cycling of organic carbon). At the heart of all DeepEarthshape projects is a drilling campaign, informed by geophysical imaging of the deep critical zone. At all four primary study sites we will extend previous soil excavations by drilling through soil and saprolite into the unweathered bedrock. We will assess the balance between production of weathered material at depth and loss at the surface through an innovative combination of Uranium-decay series analyses (to determine the rate of weathering front advance) and in situ cosmogenic Beryllium-10 (10Be) analyses (to determine surface denudation rates). In addition, we will use the depth distribution of meteoric cosmogenic 10Be as a proxy for water infiltration, and the depth distribution of stable 9Be as a proxy for silicate weathering at depth. We will characterise the cores for mineralogical and chemical composition, and will measure elemental depletion, density, porosity, surface area, and iron redox state to detect nested weathering fronts. We will synthesise these results to evaluate how the arrangement and advance rate of the nested weathering fronts depend on climate and vegetation along the Earthshape transect. The relative importance of these two factors will be evaluated through a mass balance model that links weathering kinetics with the chemical and nutrient demands of plant biomass growth. Ultimately, these results will inform as to the feedbacks through which the deep biosphere and critical zone modulate CO2 consumption and thus Earth’s climate.

• DFG Programme: Priority Programmes
• Subproject of: SPP 1803: EarthShape: Earth Surface Shaping by Biota
• International Connection: Chile
• Cooperation Partner: Dr. Pablo Sanchez-Alfaro