This thesis aims to establish the control factors and mechanisms at the origin of the recording of isotopic signatures during the precipitation of calcareous concretions at the level of the sources. From targeted laboratory experiments, a numerical modeling of the coupled behavior of the chemistry of major elements (Ca, Mg) and isotopologues (13C-12C, 18O-16O) will be proposed (a modeling of the behavior of trace elements (Sr, Ba, Ra) and stable isotopes of strontium may also be considered if the opportunity arises). It aims to transcribe the variations of chemical and isotopic compositions of calcareous concretions into variations of hydrological factors (flow of sources and recharge rate). This modeling approach will be extended to data from travertine deposits (Turkey, Spain, France, Poland) to reconstruct the response of sources to recent climate changes and groundwater abstraction.
Keywords: Hydrochemistry, Isotopy, Isotopic fractionation, Carbonate precipitation
PhD supervisor: Christophe Emblanch (UMR EMMAH, Avignon université)
Co-PhD supervisor : Florent Barbecot
Co-supervisors : Marina Gillon (UMR EMMAH, Avignon Université), Elisabeth Gibert-Brunet (UMR GEOPS, Université Paris Saclay)
Candidate profile: The candidate must have a master’s degree (or equivalent) in Earth Sciences or a related discipline with essential hydrochemistry-isotopy knowledge. He/she must demonstrate autonomy and a real ability to work in a team as part of a multidisciplinary project. He/She will have to develop a mathematical approach. He/She will also have to go to the study sites to acquire new data and he/she will have to develop laboratory experiments. Required skills: hydrochemistry-isotopy, numeric development; appreciated skills: Field work and laboratory work.
The candidate must send their CV, cover letter and references to Marina Gillon (marina.gillon [at] univ-avignon.fr">marina.gillon [at] univ-avignon.fr - Phone: 0490144467).
Context and challenges:
The Anthropocene is accompanied by major environmental changes that affected the underground part of the water cycle of continental hydrosystems (de Graaf et al., 2019). However, in the absence of physical records (flows, temperatures), it is impossible todetermine whether the reductions in flow recognized today are part of the natural variability of these systems or if we are faced with a major reduction of their functioning. This issue is important because groundwater supports the functioning of most wetlands and the base flow of rivers (Blöschl et al., 2019). Many authors have attempted to use carbonate deposits from sources for paleoclimatic reconstructions, namely past emergence regimes and temperatures (e.g. Gonfiantini et al, 1968; Matsuoka et al, 2001; Andrews, 2006 ; Brasier et al, 2010). However, the isotopic fractionations associated with calcite precipitation are not necessarily at equilibrium, but are rather kinetic, which complicates their interpretation, especially since they
are still poorly understood (Affek et al., 2014; Andrews et al., 2006, Fleurent, 2015; Gonfiantini et al., 1968). This information opens the way to new joint interpretations of chemical and isotopic tracers of these recordings. For this, we intend to develop controlled precipitation pilots in the laboratory and thus model the behavior of isotopologues during calcite precipitation, in order to use their isotopic composition as a marker of changes in the functioning of aquifer systems, which they are climatic or anthropogenic, and thus assess the sustainability of these systems. This thesis work falls within the framework of the issue of a major axis of UMR EMMAH: the vulnerability of water resources to environmental changes.
Much data were acquired during the thesis of L. Fleurent (2015) both in the laboratory and in the natural environment (spring of Ours, Massif-Central, France). These data will be the basis of a mathematical development which aims to identify the processes explaining the non-equilibrium isotope fractionations and to put them in the form of equations. Preliminary work made it possible to initiate these equations but questions remain unanswered: How to relate the calibration parameters to measurable in-situ variables (temperature, chemistry, thickness of the water film, etc.)? What factors influence the isotope fractionation values required to match the simulated data to the measured data? Further investigations in the field and in the laboratory are therefore necessary to answer these questions by completing the existing dataset. Experiments at the hydrogeochemistry laboratory of the Avignon University will allow us to study the role of the water-gas exchange surface and the thickness of the water layer on the
process of CO2 diffusion, which is the origin of calcite precipitation, on the rate of calcite precipitation and on the associated isotopic fractionations (13C18O). Under a controlled atmosphere, different water-gas exchange surfaces will be tested as well as different thicknesses of the water layer. Each case will be repeated to ensure the validity of the results.
For different time steps (which will have to be defined), the water, gas and calcite will be sampled for analyzes of the chemistry of the water, the partial pressure of ambient CO2, the amount of calcite which has precipitated, and the isotopic composition (18O and 13C) of water, carbon dissolved in water, calcite and CO2. Samples for the analysis of trace elements (Sr, Ba, Ra) and stable isotopes of strontium could also be considered to complete the tracers and refine the approach. The student will then make trips to the GEOTOP laboratory for the measurements of these elements. At the same time, in situ monitoring of sources in the gallery of the Underground Laboratory at Low Noise in Rustrel will be undertaken. At these sources, the calcite precipitation is very localized (eliminating the problem of water flow migration), the
constant temperature, only the flow regime and the chemistry of the water constrain the isotopic recording. This monitoring will therefore be complementary to laboratory
experiments by targeting the role of flow, an essential point since a flow variation indicates a variation in the recharge conditions of the aquifers feeding the source. The impact of biofilms on isotopic fractionations can also be investigated depending on the progress of work on environmental factors and the results obtained.
All the results will enrich the data necessary for the development of the numeric approach. A model of the coupled behavior of the chemistry of major elements (Ca, Mg) and isotopes (13C-12C, 18O-16O) will be proposed as well as that of the behavior of trace elements (Sr, Ba, Ra) and stable isotopes of strontium depending on the data that may have been acquired for these elements. The purpose of this approach is (1) to model geochemical processes and (2) to be able to have equations allowing, from current observations of hydrosystems and isotopic recordings (18O, 13C) in carbonate deposits to reconstruct the variations water conditions for the recent period when climatic and anthropogenic pressures on hydrosystems have been particularly strong. An application of these equations to data from travertine deposits (Turkey, Spain, France, Poland) acquired during previous work and collaborations will validate the approach and thus propose hydro-environmental reconstructions on a more global scale.
- Affek H. P.,Matthews A., Ayalon A., Bar-Matthews M., BurstynY., Zaarur S., Zilberman T. (2014) Accounting for kinetic isotope effects in Soreq Cave (Israel) speleothems, Geochimica et Cosmochimica Acta 143, 303–318.
- Andrews J.E., 2006. Palaeoclimatic records from stable isotopes in riverine tufas: Synthesis and review. Earth-Science Reviews, 75, 85-104.
- Blöschl, G., Bierkens, M. F. P., Chambel, A., Cudennec, C., Destouni, G., Fiori, A., Zhang, Y. (2019). Twenty-three unsolved problems in hydrology (UPH) – a community perspective. Hydrological Sciences Journal, 64(10), 1141-1158. doi:10.1080/02626667.2019.1620507
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