Datation radiométrique

Résumé

Une datation radiométrique (dite « par radiochronologie ») est une méthode de datation absolue utilisant la variation régulière au cours du temps de la proportion de radioisotopes dans certains corps. La plus connue est sans doute la datation par le carbone 14, mais il en existe bien d'autres. Toutes ne font pas appel aux mêmes raisonnements physiques et géologiques, et leurs précisions varient. Les méthodes de datations radiométriques constituent le cœur de la géologie isotopique. Historique Avant la découverte de la radioactivité, Lord Kelvin avait estimé l'âge de la Terre à quelque vingt millions d'années, en supposant que la seule source d'énergie capable de s'opposer au refroidissement était la chaleur résiduelle, initialement produite par la formation de la Terre. Un âge de seulement quelques dizaines de millions d'années fut considéré beaucoup trop court par les géologues, et un débat assez virulent s'ensuivit entre la communauté des géologues et celle des physiciens

Source officielle

https://fr.wikipedia.org/wiki/Datation_radiom%C3%A9trique

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The predicted climate changes on Earth will significantly impact the environment and human society. The climate patterns observed during the past centuries led to a better understanding of the driving forces for such changes. However, more studies using new, innovative techniques are necessary to broaden the knowledge on the involved processes and accomplish a reliable scientific basis for decision-makers.On this subject, radiometric dating is a well-established contemporary technique whereby various time periods (from a few years to millions of years) can be covered with this method. Notably, a dating gap exists so that the period from 100 to 1000 years (before today) is currently not covered. However, this period is of great interest, as it allows for studying and understanding environmental processes such as glacier dynamics, ocean, and atmospheric circulation. Interestingly, a potential nuclide that could fill this dating gap is available: Silicon-32 (Si-32). The problem, however, is the currently very inaccurately determined T1/2 of around 150 years so that a use for radiometric dating is hindered.Hence, the aim of the SNSF-funded SINCHRON (Si-32: a new chronometer) project, and therefore the primary goal of this thesis, concerns the re-determination of the Si-32 half-life. Previous half-life determinations were limited to using samples with low activities. To overcome the issue with the small amounts, Si-32 was artificially produced at the Paul Scherrer Institut (PSI). Subsequently, an innovative wet chemical separation system was developed to allow for the selective removal of Si-32 from an irradiated vanadium matrix. As a result, 20 mL of an ultra-pure Si-32 solution could be produced, fulfilling the desired parameters related to the half-life re-determination.Within the framework of this work, the determination of the half-life via the direct method was applied; i.e., both the determination of the number of atoms (N), in combination with the activity (A), are required. Within the SINCHRON-collaboration, several independent measurements were performed between various multinational metrological institutes. Based on the different requirements, the Si-32 solution was manufactured accordingly: (I) the solution's activity concentration was confirmed to be greater than 100 kBq/g, (II) a chemically very stable Si species was chosen, and (III) ultra-traces of S-32 were removed. Besides, (IV) solid samples for accelerator mass spectrometry (AMS) could be prepared from the stock solution, too. As a result, we are getting close to providing a new, recommended value with low uncertainty (less than 5%). Within the scope of this work, a preliminary T1/2 for Si-32 of 125 +/- 5 years has been determined. Additionally, we studied vanadium as a target material and determined the production cross-sections of Ti-44, Ca-41, and Al-26. These radionuclides are produced as by-products during the irradiation process. For the latter two nuclides, the presented data describe the very first experimental determination of the cross-section for vanadium as a target. In this context, two independent gamma spectrometric measurement systems were used for the activity determination of Ti-44, and no prior chemical separation of Ti-44 from the matrix was required. Contrarily, Ca-41 and Al-26 were successfully separated using a selective and robust chromatographic wet chemical separation scheme. The activity of these nuclides could then be determined using AMS.

EPFL2022

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Sedimentological processes in the Rhone Delta subaquatic canyons (France-Switzerland)

Nicolas Le Dantec

Deltas are very sensitive environments and highly vulnerable to variations in water discharge and the amount of suspended sediment load provided by the delta-forming currents. Human activities in the watershed, such as building of dams and irrigation ditches, or river bed deviations, may affect the discharge regime and sediment input, thus affecting delta growth. Underwater currents create deeply incised canyons cutting into the delta lobes. Understanding the sedimentary processes in these subaquatic canyons is crucial to reconstruct the fluvial evolution and human impact on deltaic environments, and to carry out a geological risk assessment related to mass movements, which may affect underwater structures and civil infractructure. Recently acquired high-resolution multibeam bathymetry on the Rhone Delta in Lake Geneva (Sastre et al. 2010) revealed the complexity of the underwater morphology formed by active and inactive canyons first described by Forel (1892). In order to unravel the sedimentary processes and sedimentary evolution in these canyons, 27 sediment cores were retrieved in the distal part of each canyon and in the canyon floor/levee complex of the active canyon. Geophysical, sedimentological, geochemical and radiometric dating techniques were applied to analyse these cores. Preliminary data show that only the canyon originating at the current river mouth is active nowadays, while the others remain inactive since engineering works in the watershed occurred. However, alternating hemipelagic and turbiditic deposits in the easternmost canyons, evidence underflow processes during the last decades. Two canyons, which are located close to the Rhone river mouth, correspond to particularly interesting deeply incised crevasse channels formed when the underwater current broke through the outer bend of a meander in the proximal northern levee. In these canyons, turbidites were observed in the sediment record indicating ongoing sediment dynamics during whether extreme flood events or mass-movements due to deltaic scarp failures. The active canyon shows a classic turbiditic system with frequent spillover processes in the canyon floor/levee complex. Geotechnical measurements, a decrease in the frequency of turbidites and a fining upward sequence along the levee suggest that erosion dominates sedimentation in the canyon floor, while sedimentation dominates in the rapid levee building-up process, with sedimentation rates that exceed 2.5 cm/yr in the proximal areas. Therefore, mechanisms controlling the sedimentary evolution on the active canyon represent a complex interplay between erosion and sedimentation.

2012

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Sediment dynamics in the subaquatic channel of the Rhone delta (Lake Geneva, France/Switzerland)

Nicolas Le Dantec

With its smaller size, well-known boundary conditions, and the availability of detailed bathymetric data, Lake Geneva's subaquatic canyon in the Rhone Delta is an excellent analogue to understand sedimentary processes in deep-water submarine channels. A multidisciplinary research effort was undertaken to unravel the sediment dynamics in the active canyon. This approach included innovative coring using the Russian MIR submersibles, in situ geotechnical tests, and geophysical, sedimentological, geochemical and radiometric analysis techniques. The canyon floor/levee complex is characterized by a classic turbiditic system with frequent spillover events. Sedimentary evolution in the active canyon is controlled by a complex interplay between erosion and sedimentation processes. In situ profiling of sediment strength in the upper layer was tested using a dynamic penetrometer and suggests that erosion is the governing mechanism in the proximal canyon floor while sedimentation dominates in the levee structure. Sedimentation rates progressively decrease down-channel along the levee structure, with accumulation exceeding 2.6 cm/year in the proximal levee. A decrease in the frequency of turbidites upwards along the canyon wall suggests a progressive confinement of the flow through time. The multi-proxy methodology has also enabled a qualitative slope-stability assessment in the levee structure. The rapid sediment loading, slope undercutting and over-steepening, and increased pore pressure due to high methane concentrations hint at a potential instability of the proximal levees. Furthermore, discrete sandy intervals show very high methane concentrations and low shear strength and thus could correspond to potentially weak layers prone to scarp failures.

Springer Verlag2014

Chargement

Sedimentological processes in the Rhone Delta subaquatic canyons (France-Switzerland)

Nicolas Le Dantec

2012

EPFL2022