General information
Offer title : PhD in geosciences : “The alkaline waters of the Okavango wetlands: origin(s), composition(s), ecosystem(s)” (Alkaline Wetlands Project) M/F (H/F)
Reference : UMR7154-SABGAL-037
Number of position : 1
Workplace : PARIS 05
Date of publication : 11 July 2024
Type of Contract : PhD Student contract / Thesis offer
Contract Period : 36 months
Start date of the thesis : 1 October 2024
Proportion of work : Full time
Remuneration : 2 135,00 € gross monthly
Section(s) CN : Interactions, particles, nuclei, from laboratory to cosmos
Description of the thesis topic
Missions :
Semi-arid regions are strongly impacted by global change and increasing anthropogenic pressure. In the heart of the Kalahari Desert, Southern Africa, the unique oasis of biodiversity formed by the Okavango Delta wetlands – a UNESCO World Heritage site – is threatened by global change and growing anthropogenic pressure. While global warming decreases the amount of water sustaining wetlands in semi-arid regions, threatening their existence, changes in the quality of that water (increased salinity, potential microbiological contamination, anthropic pollution, etc) also play a major role in the degradation of the ecosystems. On top of climate-driven vegetation shifts, increased evaporation rates or water temperature could also impact the soil and water microbiota which will probably have strong consequences on the water chemistry. Understanding the complex natural processes that govern the fragile equilibrium of such a hot-spot of biodiversity is a major challenge requiring us to increase our scientific capacities to detect, forecast and prevent ecological threats linked to global change.
The Okavango wetlands are sustained by annual floods bringing ~11 km3 of freshwater into the Delta from the highlands of Angola to the North. Because of the very low regional topographic gradient and the distance of about 700 km between the headwaters and the Delta, the inundation occurs during the dry season (May – September). This water input is complemented by about 450 mm/year rainfalls over the Delta during the period November – March. The Delta is dotted with about 150,000 islands of all shapes and sizes. The most intriguing characteristic of these islands is the occurrence, at least in some of them, of a highly alkaline (pH>8) aquifer resting some 2 to 3 m below the surface. These waters are extremely enriched in metals and metalloids such as Cr, U, V, or As (the latest reaching concentrations of 35 mg.l-1). However, this contamination is entirely natural. A model based on refilling of the groundwater aquifer by infiltration from the floodplain during flooding, followed by progressive elements concentration through evapotranspiration in the central part of the island disconnected from the floodplains during the low-flood season, has been extensively quoted to explain the chemical composition of this aquifer. The concentration process would lead to the formation of a dense brine-drop sinking below the centre of the islands.
The model has then been applied to all the islands of the Delta showing a brine- type aquifer at depth although the presence of a salt crust on the surface was not attested. The research team participating in Alkaline Wetlands recently questioned the brine-drop model, demonstrating that the peculiar Rare Earth Elements (REE) patterns (strongly enriched in Heavy REE) of the brine-type groundwaters could not result from the concentration of the river water through evaporation or evapotranspiration. We suggested that the salted aquifer is completely different and disconnected from the surface aquifer, possibly inherited from a previous lacustrine phase. Our new interpretation implies that the salted aquifer in the Delta may not be restricted to the island centers but, might cover larger surfaces. If so it would represent a unique reservoir of highly concentrated chemical elements and a potential environmental threat, should it be released to the surface either by tectonic movements or climate change-related modifications of the alluvial fan dynamics.
In the eastern, permanently flooded part of the Delta, many of the islands surrounding the lagoons are covered with efflorescent salt. The pH of the water within the lagoon reaches pH>8, a value similar to the one obtained from the brine-type groundwaters in the previously explored islands. This system has never been described in the literature. Although one could consider massive evaporation and evapotranspiration of river water, the mechanism controlling the concentration of salt in the lagoons remains unknown as well as the kinematic of water exchange between the lagoon and the river / floodplain. Similarly the chemical composition of the water in the lagoon, the exact composition of the salt and the potential similarities between this alkaline water, the brine-type groundwater in the western part of the Delta and the brine waters in the Makgadikgadi pan salt lakes that form the central part of the Kalahari Basin have to be assessed.
The PhD researches will aim at :
1) Establish a “geochemical passport” for the different alkaline waters of the Delta and of the salt lakes in the Kalahari Basin. This will include major and trace elements compositions (especially the REE patterns that seem specific) and stable isotopes indicators such as 13C (as a proxy to bacterial activity such as anoxic production of methane) or 37Cl (as a proxy to evaporation processes).
2) Decipher the mechanisms that lead to the formation of those waters: how are the elements concentrated? Which mechanism fractionates the REE? Is the biological/microbiological activity playing a role?
Activities :
The PhD student will work in close collaboration with the researchers and engineers at IPG Paris, Géosciences Rennes and Okavango Research Institute. She/he will be responsible for collecting the samples (water, soil, salt, etc) in the field, conducting the relevant chemical analysis and their scientific interpretation. An important aspect of the project will be to map the occurrence of salt deposits in the Delta, in order to understand their precise distribution and possible origin. This part of the project will involve field mapping and the use of remote sensing analysis.
Expected Skills :
Hability to conduct multidisciplinary-team fieldwork in remote areas sustaining wild megafauna and in areas requesting close collaboration with local communities is compulsory. The research will require the PhD to be fluent in English, both oral and written. The international, multidisciplinary framework will also request developing and maintaining contacts with various researchers of different disciplines and culture. In terms of technical skills, the candidate will have a good background in geochemistry (sample collection, preparation and measurements), especially the analysis of trace elements using ICP-MS. Finally, a background in remote sensing analysis (including the development of analytical codes in Python) and GIS softwares (Qgis) will be appreciated.
Work Context
The institut de physique du globe de Paris is a world-renowned geosciences organisation, the IPGP is associated with the CNRS and an integrated institute of the Université Paris Cité. Bringing together more than 500 people, the IPGP studies the Earth and the planets from the core to the most superficial fluid envelopes, through observation, experimentation and modelling.
The research aeras are structured through 4 main unifying themes: Interiors of the Earth and Planets, Natural Hazards, Earth System and Origins.
The IPGP is in charge of labelled observation services in volcanology, seismology, magnetism, gravimetry and erosion. And the IPGP’s permanent observatories monitor the four active French overseas volcanoes in Guadeloupe, Martinique, Réunion Island and Mayotte.
The IPGP hosts powerful computing resources and state-of-the-art experimental and analytical facilities and benefits from first-class technical support. The IPGP provides its students with geosciences training that combine observation, quantitative analysis and modelling, and that reflects the quality, richness and thematic diversity of the research conducted by the IPGP teams.
Team Department :
The PhD will be held in the Tectonics and Mechanics of the Lithosphere team which primary objective of the IPGP Tectonics team is to pursue fundamental research to push frontiers of our knowledge of the mechanics of the lithosphere, and on the interactions of deep-seated tectonic processes with surface processes. The team includes specialists in structural geology, sedimentology, geodesy, remote sensing and social sciences. IPG Paris is a highly interdisciplinary institute and the PhD student will also interact with researchers in geochemistry, geophysics and hydrology. Furthermore, the project is proposed in collaboration with the geochemistry team of Géosciences Rennes laboratory (Université Rennes, France) and the PhD student will have to spend some research time in Rennes. Finally, the co-PI of the project, N. Mazrui belong to the Okavango Research Institute (ORI) in Maun (Botswana). ORI is a multidisciplinary institute focusing mainly on biology, ecology and social analysis of the Okavango Delta. Aside from fieldwork in Botswana, the PhD may also be spending research time at ORI for specific analytical tasks.
Constraints and risks
The working hours will in general be those of the IPG Paris (legally 35 hrs per week). However, working hours during fieldwork will be strongly dependent of the local constrains and activities. Similarly, during stays at Géosciences Rennes or ORI, the working hours will be those of the hosting institute. Costs of professional trips will be covered by the project based on the CNRS official per-diem rates. Potential risks are associated with the use of chemical products in the lab and with fieldwork in remote areas. All those risks will be minimized : as a rule in the CNRS, the PhD will not be allowed to work alone in the clean rooms or in the field. Specific training will be organized for any new task to be performed that may involve risks. The laboratories all have specific procedures to minimize risks associated to gender conflicts. During fieldwork, the PIs of the project (Nashaat Mazrui from ORI and Marc Jolivet from IPG-Paris) will make sure that both genders are represented by at least two representatives.
Additional Information
The candidate will have a Master in geosciences (preferentially in the theme of geochemistry), delivered since less than 1 year.
