ELI - Soutenance publique de thèse - Carlos Francisco ARAGON TOBAR

Weathering of powdered volcanic rocks : laboratory and modelling studies for assessing rates and environmental impacts

Chemical weathering of subaerial volcanic rocks influences the chemical composition of rivers, lakes and soils where they have been deposited and/or transported. In the case of basaltic rocks, it also contributes significantly to the consumption of atmospheric CO2. However, the rates at which these rocks, which are characterised by various degrees of crystallinity and silica contents, dissolve upon contact with water are still not well described. The principal aim of this PhD study was to advance our understanding of this issue. Through laboratory experiments carried out with a home-built fluidized bed reactor, we provide new insights into the dissolution rates of volcanic rocks differing in composition and crystallinity. The main study findings are: (a) polycrystalline rocks dissolve incongruently, leaving a residual silica/aluminium-rich layer at the surface of the weathered material; (b) polycrystalline rocks dissolve at faster rates than their synthetic glassy counterparts, and the difference is more pronounced for rocks with higher silica contents; and (c) the dissolution rates of intermediate and silicic volcanic rocks can be approximated based on the dissolution kinetics of their major constituents, i.e. silicate glass and plagioclases ± pyroxenes. In addition to the experimental work, we modelled the weathering of a continent-size silicic tephra deposit over multimillennial timescales. Such deposits are typically emplaced by so-called supereruptions, for example, Yellowstone 0.64 Ma ago. The impact that these deposits may have on the geochemical fluxes of silicon and cations and atmospheric CO2 consumption over millennial timescales is not known. Using a mechanistic model (WITCH), we predict that the weathering of supereruption’s silicic tephra deposit can strongly enhance the continental flux of dissolved silicon to the ocean for several thousands of years. However, its effect on the concentration of atmospheric CO2 is minor, which is in contrast with the proven influence of flood basalt weathering on the carbon cycle. Overall, our study contributes to a better understanding of the volcanic influence on the carbon and silica cycle.

Jury members :

• Prof. Pierre Delmelle (UCLouvain), supervisor
• Prof. Bruno Henry de Frahan (UCLouvain), chairperson
• Prof. Sophie Opfergelt (UCLouvain), secretary
• Prof. Yves Goddéris (Géosciences Environnement Toulouse, OMP, France)
• Prof. Lei Chou (ULB)

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