Thesis defense of Ivan Osorio-Leon

Oxic-Anoxic Dynamics in the Continental Subsurface: Prediction and Control on Rock Weathering and Deep Biomass

Date and time: October 16th, 2023, at 2:00 PM (Paris time, GMT+2).

Abstract:

Redox reactions involving dissolved oxygen (DO) are the most energetic and provide a major source of energy for the deep biosphere. The ability of fractured rocks to rapidly transport oxygenated waters from the surface to depth allows for the existence of a deep oxic hydrosphere (DOH), which has been historically neglected. This thesis focuses on the origin, dynamics, and consequences of this DOH.

First, we establish a conceptual framework to identify the factors controlling the reactive transport of DO at the watershed scale. We develop a water-rock interaction model to predict the depth of the DOH. This model is used to explain the occurrence of a DOH within the first 300 meters of the aquifer in the Critical Zone Observatory of Ploemeur. We investigate the δ18O of DO within the DOH to identify the distribution of biotic and abiotic processes that control the reactivity of DO in the watershed.

Second, we conduct two in-situ experiments to explore the consequences of the DOH on the biogeochemical functioning of the subsurface. A reactive DO tracer test and a mineral incubation experiment are developed to study the response of, respectively, the planktonic and mineral-attached biomass to oxic and anoxic dynamics in the subsurface.This work highlights a DOH in which subsurface biogeochemical processes are sensitive to surface hydrological dynamics involving the transport of DO, thus challenging the paradigm of the deep biosphere as an inert and anoxic system.

Keywords: Critical Zone, Deep biosphere, Reactive transport, Groundwater, Rock weathering, Dissolved oxygen.

Location: OSUR Room, Building 14B – University of Rennes, Beaulieu campus, Rennes.

The presentation will be given in English.