Our laboratory specializes in isotopic geochemistry of terrestrial surface and subsurface environments in the past and present. The main research carried out relates to the reconstruction of past climates, major biogeochemical cycles and the effect of anthropogenic activities on the environment.
Our research calls for high precision measurements of relative or absolute abundances of isotopes present in the environment, in trace (μg/g) or ultra-trace concentrations (ng/g or less). The applications fall into two main categories: isotopic tracing of sources (isotopic signatures in strontium, lead, neodymium, iron, silicon, cadmium, uranium and radium) and absolute dating by radiochronology (radioactive imbalances uranium-thorium and uranium-lead).
Technical resources include a clean room for sample preparation, a plasma and multicollection source mass spectrometer (MC-ICP-MS), a thermal ionization mass spectrometer (TIMS) and a high resolution plasma source mass spectrometer (HR-ICP-MS) equipped with a laser ablation ion source (LA). We are also equipped with a microdrill for sampling of solid samples at high spatial resolution and an ion chromatography apparatus (IC) for elementary analysis of major ions in solution.
This clean room suite is intended for the preparation of samples (purification and pre-concentration) and has an ISO level 5 ultra-clean atmosphere. There are five separate rooms occupying a total surface of 80 m2, with seven work stations under laminar flow hoods with all associated equipment: fume hoods, ultra-pure water production systems, high-precision balance, evaporation boxes under filtered atmosphere, reagent purifiers, etc.
Our “Neptune Plus” ThermoFisher Scientific instrument is equipped with the “Jet Interface” option, nine Faraday collectors (with a range of resistance amplifiers from 1010, 1011 and 1012 ohms), a secondary electron multiplier (SEM) and four compact discrete dynode detectors (CDD). This instrument is dedicated to high precision isotopic analysis for elements such as strontium, neodymium, uranium, thorium, lead, iron, copper, zinc and cadmium, as well as, thanks to its high resolution, silicon.
This VG Sector 54-30 model device is equipped with seven Faraday collectors and an extension consisting of an electrostatic energy filter followed by a Daly detector configured in ion counting mode. It has excellent separation efficiency, sensitivity and linearity, and it is used for the analysis of isotopes of strontium, lead, thorium and uranium.
This Element-XR instrument performs elementary and isotopic analysis on solutions and solids. In the latter case, it is coupled to a laser ablation sampling system (193 nm) which allows in situ analysis of surfaces prepared in polished or thin sections. The laser improves the spatial resolution by a scale reduced to 10 μm (commonly 150 μm).
A Geomill-type column micro drill can be used to sample rock veins with one millimeter thickness. The sample taken from a smooth surface is recovered as a powder, before being purified in a clean room and analyzed by a isotopic mass spectrometer.
This instrument used for elementary analysis measures the concentration of major ions dissolved in a solution. Our model is an Aquion made by Dionex (Thermo Scientific) and equipped with sample changer and self-healing suppressor. It currently analysis anions and cations in a concentration range from 5 ppb to 50 ppm.
The research we do with our instrumental platform can be grouped into several themes:
Heavy isotope dating techniques, particularly uranium-lead method, are well suited for carbonate minerals studies. They make it possible to study the evolution of petrophysical properties of petroleum reservoirs over relatively long time scales (from a few million to several hundred million years). Different phases of fluid circulation, as well as the precipitation or dissolution of carbonate mineral phases which result from it, can greatly modify the permeability and porosity of these reservoirs over the time (Godeau et al., 2018). The possibility of bringing absolute time constraints to these diagenetic episodes, sometimes directly linked to the migration of hydrocarbons, is therefore of crucial importance for understanding the chronology of petroleum systems.
Over much shorter time periods (a few thousand to a few hundred thousand years), the uranium-thorium method is of major use in paleoclimatology. It is used for reconstructing sea-level fluctuations over glacial-interglacial cycles by dating coral reefs (Bard et al., 1996 a, b; 2010). For example, we were able to highlight a brief acceleration of sea level rise during the last deglaciation (Deschamps et al., 2012). In addition, comparing uranium-thorium and carbon-14 isotopic analysis on fossil corals improved the accuracy of these two dating methods over the last 16,000 years (Durand et al., 2013). Studying corals also has paleoceanographic applications, such as the multi-annual evolution of ocean temperatures (Felis et al., 2012) and relationships between climate and tectonics gleaned from microatolls (Weil-Accardo et al., 2016).
Uranium-thorium and uranium-lead methods can also be applied to material transfer rate measurements in geological systems. Dating speleothems is efficiently done by the uranium-thorium method (Pons-Branchu et al., 2010). Other applications exist such as studying the mechanical aspects of travertine crystallization (Gratier et al., 2012), or permeability evolution (Frery, 2015), or geodic calcite formation in a sedimentary basin (Pisapia et al., 2017).
Hydrogeologists also use isotopic analysis for geographic tracing of material flows. Studying continental, lacustrine, marine or oceanic water circulation commonly uses strontium, neodymium, lead, uranium and radium isotopes. These isotope systems make it possible to study the water supply in lagoons (Gattacceca et al., 2011), hydrological regime evolution in deltas (Flaux et al., 2013) or lake functioning. The case of Lake Chad was an opportunity to assess the complete water budget in this system (Amaral et al., 2013; Bouchez et al., 2016; Poulin et al., 2019; Mahamat-Nour et al., 2019). The resilience of large underground aquifers can also be estimated using isotopic analysis (Petersen et al., 2013). Measurements of strontium and neodymium isotopes can help to determine the origin of calcium in carbonate nodules (Dietrich et al., 2017).
Dating anthropized sites is another possible application for isotopic measurements of strontium, neodymium and lead. Occupation of old coastal sites has thus been traced in the case of the ancient port of Sidon (Véron et al., 2009).
Finally. measurements of silicon, zinc, copper and iron isotopes in soils make it possible to better understand active geochemical processes in sites polluted by metallurgical industries (Gelly et al., 2019).
Responsables scientifiques : Pierre Deschamps (CR), Bruno Hamelin (PR), Edouard Bard (PR)
Responsables techniques des salles blanches : Hélène Mariot (AI), Marion Defrance (IE)
Responsable technique du MC-ICP-MS : Abel Guihou (IGR)
Responsable technique du TIMS : Wulfran Barthélemy (IE)