DeepEarthShape - Weathering Geochemistry: Reaction fronts in dep regolith and their advance mechanism

Investigator Names and Contact Info:

• Friedhelm von Blanckenburg (Geochemistry, Cosmogenic Nuclides, Metal Stable Isotopes). German Research Centre for GeoSciences (GFZ) Potsdam, Germany

Chilean Collaborators Involved:

• Pablo Sánchez-Alfaro (Mineralogy, Rheology). Universidad Austral, Valdivia, Chile
• Alida Perez-Fodich (reactive transport models). Universidad de Chile, Santiago, Chile

PhD:

Using innovative isotope geochemical weathering to explore deep (up to 80 m) rock weathering

• Laura Krone GFZ Potsdam & Freie Universität Berlin, Germany

supervisor: F. von Blanckenburg

Project summary:

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. At the heart of all DeepEarthshape projects is a drilling campaign. At all four primary study sites we will extend previous soil excavations by drilling through soil and saprolite into the unweathered bedrock (up tp 80m depth), informed by geophysical imaging of the deep Critical Zone.

We will explore how the range in rainfall and plant cover along the Earthshape transect is reflected in weathering front advance. 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 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.