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Proposition de Sujet de thèse 2015
Nom du laboratoire (et n° de l’unité) dans lequel se déroulera la thèse :
GET UMR 5563
Titre du sujet proposé :
Zone critique et changements climatiques d’origine anthropique
Financement :
 acquis (préciser nom de l’organisme)
X mis au concours (contrat doctoral ministériel)
 candidature auprès d’un organisme (préciser nom de l’organisme)
Spécialités de l’école doctorale : (cocher une seule spécialité sans la modifier)
 Astrophysique, Sciences de l’Espace, Planétologie
 Climat, Océan, Atmosphère, Surfaces Continentales
 Ecologie Fonctionnelle
X Hydrologie, Hydrochimie, Sol, Environnement
 Sciences de la Terre et des Planètes solides
Nom et statut (PR, DR, MCf, CR, …) du (des) responsable(s) de thèse (préciser si HDR) :
Yves Goddéris DR CNRS (HDR)
David Labat Prof UPS (HDR)
Coordonnées (téléphone et e-mail) du (des) responsable(s) de thèse :
yves.godderis@get.obs-mip.fr (poste 2615)
David.labat@get.obs-mip.fr
Résumé du sujet de la thèse (le descriptif ne doit pas dépasser une page recto/verso)
Contexte scientifique général, Compétences souhaitables,…
On Earth, the thin layer between the rocks and the atmosphere is a chemical interface that
regulates the Earth’s climate at all scales (through consumption of CO2 by ecosystems and rock
dissolution) and is critical for the future of humanity. This zone is now called the Critical Zone by the
Earth Scientists. This is where humanity lives, the location of most of our water and soil resources and
a zone submitted to a unique forcing since the beginning of the anthropocene. A recent study has
emphasized the sensitivity of the land-to-ocean carbon fluxes to the anthropogenic forcing, including
climate change and land use change (Regnier et al., 2013). Among those fluxes, the dissolution of
continental rocks (continental weathering), a carbon-consuming and alkalinity-generating process,
appears to respond rapidly to the global environmental changes. Continental weathering is a function
of the lithology of the exposed surfaces (Amiotte-Suchet et al., 2003), among many other parameters.
This multi-parameter dependence makes it difficult to assess the response of continental weathering to
climate change at the decadal to centennial scale, a timescale at which critical zone processes operate
in an out-of-steady-state mode.
We are currently developing at the GET a mechanistic model describing the weathering
processes in the natural environment based on laboratory kinetics law: the WITCH model (Goddéris et
al., 2006, for the original version). The WITCH model is designed to simulate weathering processes
from the spatial scale of a weathering profile to the continental watersheds. This mechanistic method
allows to capture the potential dynamics of the weathering processes inside the critical zone. One of
ED 173 - SDU2E
the main innovation of the WITCH model compared to previously published models (such as SAFE,
CRUNCHFLOW) is its incorporation into a cascade of models to account for the role of the climate,
of the biosphere and of hydrology on the biogeochemistry of the critical zone, and hence on
weathering processes. The WITCH model has been validated on numerous catchments under various
climates and lithological conditions, demonstrating its ability to simulate the export of main cations as
well as the CO2 consumption by weathering.
We recently used the WITCH numerical model together with a climate and a continental
biosphere models to explore the response of the weathering processes to the climate change at the
century timescale. Our studies point at the key role played by carbonate mineral dissolution. We
demonstrate that about 80 % of the weathering response to an increase in atmospheric CO2 from preindustrial levels up to 560 (Beaulieu et al., 2012) over the Mackenzie watershed is driven by carbonate
dissolution. This result seems to be valid also for lithologies displaying only a few percent carbonate
in volumetric abundances (shales, loess) (Goddéris et al., 2013). While carbonate weathering is
inefficient in terms of CO2 consumption at the geological timescale, it consumes efficiently
atmospheric CO2 at timescales below the mixing time of the ocean (Calmels et al., 2007).
Carbonate rock outcrops cover 10.4% of the land surface of the Earth (Dürr et al., 2005). Carbonate
minerals are also present in a wide variety of lithologies, including loess and more generally
sedimentary rocks (36% of the continental surfaces). Within the framework of this PhD, we aim at
modeling the response of carbonate weathering to the ongoing climate change. Relying on a close
collaboration with Meteofrance, we we will use global output of the ARPEGE General Circulation
Model. Those output will be use as forcing of the biospheric/land surface model ISBA-Ags for the
next century under a variety of climate change scenarios, and ultimately the chemical weathering
model WITCH to evaluate the response of the carbonate weathering at the global scale. The main
outcome of this work will be the answer to the following question : will carbonate weathering be a
mitigator or an amplifier of the anthropogenic climate change ?