Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
A new portable sampling system was developed to extract Radium isotopes from large volumes (up to 300 L) of fresh surface- and ground-waters of low Ra-activities (<5 mBq/L). Ra is quantitatively adsorbed on a small amount (6.5 g) of MnO2-coated acrylic fibers, which are then dried and burned at 600 °C in the laboratory. The resulting Mn-oxide powder (about 2 cm3 when compacted) is then analyzed through gamma-ray spectrometry which allows measurement of the whole Ra quartet (226Ra, 228Ra, 224Ra and 223Ra) in a single counting of a few days. The usual relative standard combined uncertainties (1σ) are 2–3% for 226Ra, 228Ra and 224Ra; and less than 10% for 223Ra. This method was applied to the analysis of Ra in karstic waters of the Lez aquifer, and surface- and ground-waters of the upper and middle Vidourle watershed (South of France). The analyzed waters have relatively low 226Ra activities (1–4 mBq/L) in both cases, regardless of the contrasted geology (Mesozoic limestone vs crystalline Variscan basement), but clearly distinct (228Ra/226Ra) ratios in agreement with the differences in Th/U ratios of the two drained areas. Short-lived Ra isotopes (224Ra and 223Ra) appear to be mainly influenced by near-surface desorption/recoil processes for most of the sampling sites.
ISSN: 0969-8043

hal-01491281
https://hal.archives-ouvertes.fr/hal-01491281
DOI : 10.1016/j.apradiso.2016.12.010

Éditeur(s) : Elsevier Science Bv
Résumé : The breakup of Pangea and onset of growth of the Pacific plate led to several paleoenvironmental feedbacks, which radically affected paleoclimate and ocean chemistry during the Jurassic. Overall, this period was characterized by intense volcanic degassing from large igneous provinces and circum-Panthalassan arcs, new oceanic circulation patterns, and changes in heat and humidity transports affecting continental weathering. Few studies, however, have attempted to unravel the global interactions linking these processes over the long-term. In this paper, we address this question by documenting the global changes in continental drainage and surface oceanic circulation for the whole Jurassic period. For this purpose, we present 53 new neodymium isotope values (εNd(t)) measured on well-dated fossil fish teeth, ichthyosaur bones, phosphatized nodules, phosphatized ooids, and clastic sediments from Europe, western Russia, and North America. Combined with an extensive compilation of published εNd(t) data, our results show that the continental sources of Nd were very heterogeneous across the world. Volcanic inputs from a Jurassic equivalent of the modern Pacific Ring of Fire contributed to radiogenic εNd(t) values (− 4 ε-units) in the Panthalassa Ocean. For the Tethyan Ocean, the average surface seawater signal was less radiogenic in the equatorial region (− 6.3), and gradually lower toward the epicontinental peri-Tethyan (− 7.4), western Russian (− 7.4) and Euro-Boreal seas (− 8.6). Different Nd sources contributed to this disparity, with radiogenic Nd influxes from westward Panthalassan currents or juvenile volcanic arcs in open oceanic domains, and substantial unradiogenic inputs from old Laurasian and Gondwanan shields for the NW Tethyan platforms. Overall, the εNd(t) values of Euro-Boreal, peri-Tethyan, and western Russian waters varied quite similarly through time, in response to regional changes in oceanic circulation, paleoclimate, continental drainage, and volcanism. Three positive shifts in εNd(t) values occurred successively in these epicontinental seas during the Pliensbachian, in the Aalenian-Bathonian interval, and in the mid-Oxfordian. The first and third events are interpreted as regional incursions of warm surface radiogenic currents from low latitudes. The Aalenian-Bathonian shift seems linked to volcanic outbursts in the NW Tethys and/or circulation of deep currents resulting from extensional events in the Hispanic Corridor and reduced influences of boreal currents crossing the Viking Corridor. In contrast, the εNd(t) signals decreased and remained very low (< − 8) during the global warming events of the Toarcian and Late Oxfordian - Early Tithonian intervals. In these greenhouse contexts, a latitudinal expansion of humid belts could have extended the drainage pathways toward boreal Nd sources of Precambrian age and increased the supply of very unradiogenic crustal-derived inputs to seawater. Finally, a brief negative εNd(t) excursion recorded in parallel with regional drops in seawater temperature suggests that southward circulation of cold unradiogenic Arctic waters occurred in the NW Tethys in the Callovian - Early Oxfordian. All these results show that changes in surface oceanic circulation resulting from the Pangean breakup could have regionally impacted the evolution of seawater temperatures in the NW Tethys.
Gondwana Research (1342-937X) (Elsevier Science Bv), 2015-06 , Vol. 27 , N. 4 , P. 1599-1615

Droits : 2014 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved
http://archimer.ifremer.fr/doc/00179/29015/27443.pdf
DOI:10.1016/j.gr.2014.02.006
http://archimer.ifremer.fr/doc/00179/29015/

Éditeur(s) : HAL CCSD Elsevier
Résumé : Our method for the simultaneous determination of the four natural Ra isotopes (Ra-226, Ra-228, Ra-224 and Ra-223) in thermal waters involves a separation of Ra on a selective filter (3 M EMPORE (TM) Radium Rad disk), and a single counting using a broad-energy HPGe detector (BE Ge manufactured by CANBERRA (TM)). The calculation of Ra-223 and (228)a activities requires interference and cascade summing corrections. The Ra-226 activities in CO2-rich thermal waters of the Lodeve Basin (South of France) range from 530 to 2240 mBq/l. The low (Ra-228/Ra-226) activity ratios (0.19-0.29) suggest that Ra is mostly derived from the aquifer carbonates. The short-lived Ra-224 and Ra-223 are probably added to the water through recoil or desorption processes from Th-enriched coatings on the fracture walls.
ISSN: 0969-8043

hal-00475587
https://hal.archives-ouvertes.fr/hal-00475587
DOI : 10.1016/j.apradiso.2009.10.056

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The behaviour of the four Ra isotopes (measured by gamma spectrometry) and 222Rn (measured with an AlphaGUARD ionisation chamber) has been investigated in NaCl-rich thermal waters of Balaruc-les-Bains on the Mediterranean coast (South of France). This study allows identification of the deep thermal water signature and reveals the influence of seawater or karst water inflows on Ra isotopes and 222Rn. The deep thermal water has 226Ra and 222Rn activities of 840 and 1900 mBq/l, respectively, a (228Ra/226Ra) activity ratio of 0.59, and low (224Ra/228Ra) and (223Ra/226Ra) ratios of 0.67 and 0.025, respectively. Several arguments suggest a dominant role of radioactive decay for short-lived Ra isotopes and Rn during the relatively rapid ascent of thermal water through wide open fractures and drains. The low (223Ra/226Ra) ratio constrains the maximum ascent velocity of thermal water to 8-10 m/h. Seawater inflow into the hydrothermal system results in an enrichment in short-lived Ra isotopes and 222Rn. The high (223Ra/226Ra) ratio (0.23) suggests that 223Ra, 224Ra and 222Rn produced by alpha-recoil in the upper-Jurassic limestone are entrained by seawater percolation through the coastal basement, due to active pumping in one of the near shore production well. This process would be enhanced by a low water/rock ratio (i.e. in a low-porosity, micro-fractured limestone). Mixing of this enriched seawater with thermal water induces barite precipitation with co-precipitation of a large fraction of Ra. Short-lived Ra isotopes may thus be useful tracers of seawater flux towards the continent. Conversely, the inflow and mixing of karst groundwater result in a general dilution of all dissolved elements, only a small enrichment in 224Ra and 223Ra, but a large enrichment in 222Rn (up to 26 Bq/l). The combination of Ra isotopes and 222Rn data provides a good way to trace the dynamics of different water masses along coastal areas.
ISSN: 0016-7037

hal-00795512
https://hal.archives-ouvertes.fr/hal-00795512
DOI : 10.1016/j.gca.2012.09.010

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We present in this study the first data on Ra isotopes, 222Rn, and 210Pb in waters from the Lez karst system, one of the main ground-water resources in the South of France. The low Ra isotope activities of the Lez system waters (from 0.35 to 3.45 mBq/L of 226Ra) were measured by gamma spectrometry using the method developed in our previous work. Our data reveal the coherent behavior of 226Ra in Lez spring waters, well correlated to other major and trace elements, like Cl, Na, K, Mg, Li, Rb and Ba. The (228Ra/226Ra) ratios are distinct between the Lez spring (0.60 to 0.72) and two neighboring springs (0.52 to 0.65), showing the spatial variability in karst systems. In spite of their small variations in the Lez spring, it was possible to trace different water components and mixing processes from Lower-Cretaceous, Upper-Jurassic and Middle Jurassic end-members, in agreement with previous studies based on major and trace elements. Data obtained on waters derived from the Upper-Jurassic karst reveal that their (228Ra/226Ra) ratios are not related to the Th/U ratio of the Jurassic limestone, but are rather influenced by the soil and alterite cover of the recharge area.222Rn activities in the Lez spring are relatively low (0.5 to 3.7 Bq/L) compared to other karst regions. The highest 222Rn activities are found in diluted waters following heavy rainfall episodes, suggesting a subsurface origin of Rn. Soil and alterite samples that have much higher 226Ra activities (from 51 to 130 Bq/kg), compared to those from the main aquifer rocks (from 2.9 to 16 Bq/kg), are probably the main source of Rn, rather than the rocks of the karst itself. The study of several flood episodes reveals the correlation between Rn peak-values or total activity discharged at the spring with cumulative rainfall. Finally, 222Rn dissolved in water decays to 210Pb, and our data show that constraints on transfer times of underground waters can potentially be obtained from their coupled measurement.
ISSN: 0009-2541

hal-01622749
https://hal.archives-ouvertes.fr/hal-01622749
DOI : 10.1016/j.chemgeo.2017.06.022

Résumé : La forêt tropicale humide guyanaise est située dans le grand ensemble Amazonien, qui est un point chaud des enjeux sur la biodiversité et la conservation des forêts tropicales. Une connaissance de l’âge et des taux d’accroissement des arbres est primordiale pour comprendre la dynamique de ces écosystèmes Cependant, l’absence de conditions hivernales sous les tropiques rend difficile la lecture de l’histoire dans les bois de ces forêts. En se plaçant dans le champ de l’anatomie fonctionnelle, cette thèse à caractère exploratoire avait pour but, à travers différentes approches, i) de suivre l’activité cambiale de quatre espèces ligneuses durant deux ans par prélèvements de micro-carottes, ii) de faire une étude dendrochronologique sur une espèce guyanaise, iii) d’utiliser la variation de la signature isotopique comme marqueur de croissance (ou d’histoire) et iv) de tester la pollution aérienne consécutive aux lancements de la fusée Ariane comme marqueurs de la croissance. L’examen bi-mensuel de l’activité cambiale a montré que tous les arbres présentent des alternances de quasi repos et de forte activité du cambium durant les deux années. Ces périodes de ralentissement ont été plus marquées chez Parkia nitida (décidue), Parkia velutina (décidue) et Schefflera morototoni (sempervirente). On observe donc des périodes de ralentissement de l’activité du cambium aussi bien chez deux espèces décidues que chez une espèce sempervirente. Il n’y a pas de patron universel de liaisons deux à deux entre pluviométrie intra-annuelle, phénologie de la feuillaison et activité cambiale même si des analogies fortes existent au cas par cas entre deux espèces pour une relation donnée. L’étude dendrochronologique chez Parkia nitida a mis en évidence la présence de cernes de croissance bien distincts et annuels. La limite de cerne se caractérise par une fine bande de parenchyme marginal renfermant de nombreux cristaux d’oxalate de calcium. A l’échelle de l’arbre, la croissance secondaire est homogène et elle est relativement synchrone au sein d’individus de cette espèce dans le même lieu. Cependant, l’influence des paramètres climatiques testés, comme la température et les précipitations, sur la croissance radiale annuelle est faible sur la période considérée. Ces deux premières études montrent que, dans le contexte de la Guyane française, la saisonnalité annuelle très marquée de la pluviométrie ne se traduit pas par un signal suffisamment fort pour synchroniser l’ensemble du fonctionnement des arbres. Néanmoins, elle influence clairement, au cas par cas soit la feuillaison, soit la floraison, soit la croissance cambiale. Les autres méthodes de marquage de la croissance (i.e. isotopie et chimie) entreprises demandent à être approfondies. Les bois des espèces de forêt tropicale humide se caractérisant en moyenne par un pourcentage élevé du plan ligneux occupé par les parenchymes axiaux, il serait important d’entreprendre une étude approfondie de leur rôle en tant qu’accumulateurs de substances potentiellement utiles pour la défense du bois dans l’arbre contre les xylophages. L’évolution dans le temps (périodicité, vieillissement chimique) des solutions de défense mises en place dans le bois est une piste intéressante pour la dendrochronologie
The rainforest of French Guiana is located in the Amazonian region, which is a hotspot of biodiversity, thus leading to several issues about conservation in tropical forests. Knowledge of the age and growth rate of trees is essential to understand the dynamics of these ecosystems. However, due to the lack of winter conditions it is difficult to read the story in the wood of these tropical forests. In the topic of functional anatomy, this thesis intends through different approaches i) to monitor the cambial activity of four tree species during two years sampling micro-cores, ii) to make a dendrochronological study on a Guyanese species iii) to use the variation of the isotopic signature as a marker of growth (or history) and iv) to test air pollution following launches of the Ariane rocket as markers of growth. The bi-monthly review of cambial activity showed that all trees have almost alternating rest and high activity of cambium in both years. These downturns have been more pronounced in Parkia nitida (deciduous), Parkia velutina (deciduous) and Schefflera morototoni (evergreen). We observe periods of cambial activity downturn in two deciduous species as well as in an evergreen species. There is no universal pattern of connections in pairs between rainfall intra-annual phenology of leafing and cambial activity although strong similarities exist in each case between two species for a given association. The dendrochronological study in Parkia nitida highlights the presence of distinct annual growth rings. The limit is characterized by a thin band of marginal parenchyma containing numerous crystals of calcium oxalate. At tree scale, secondary growth is homogeneous and relatively synchronous within individuals of this species in the same location. However, the influence of tested climatic parameters, such as temperature and precipitation on the annual radial growth, is low over the considered period. The first two studies showed that, in the environment of French Guiana, marked annual seasonality of rainfall does not affect in a strong way the behavior of the trees. Nevertheless, those rainfalls have an influence either on leafing, flowering or cambial growth. Other methods of marking growth (i.e. isotopic and chemical) intended require further investigations. Axial parenchyma represents a high average percentage in the wood of rainforest species: it could be important to investigate more precisely its role as accumulators of potentially useful substances involved in wood defense against borers. The evolution throughout time (periodicity, chemical aging) of defense mechanisms in woods is an interesting way for dendrochronology.

http://www.theses.fr/2013AGUY0605/document

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The present study reports data on CO2-rich geothermal waters and their associated carbonate deposits from the SW part of the Permian Lodève Basin (South of France). Both waters and carbonates are enriched in 226Ra, with activities up to 2.2 Bq/L and 3.5 Bq/g respectively. A series of carbonate samples precipitated around the studied wells since 1990 were dated through the 226Ra-210Pb and 228Ra-228Th methods, and the results are discussed in a new Concordia diagram. While both methods give comparable dates for many samples, Ra-Pb ages are sometimes much older than Ra-Th ages. We propose that these discrepancies result from the adsorption of 222Rn daughter nuclides in such a Rn-rich environment. The (228Ra/226Ra)0 activity ratios calculated at the time of deposition are remarkably constant for about 12 years, suggesting the presence of a steady-state aquifer. The low (228Ra/226Ra)0 ratios (0.348 ± 0.008) are best explained if most of the Ra derives from the Cambrian carbonate series (mainly dolostone) underlying the Permian Basin, which have low Th/U and similar (228Ra/226Ra) ratios. This study suggests that Ra and other alkaline-earth elements do not necessarily originate from the rocks surrounding the deep geothermal reservoir, but rather may be introduced into the CO2-rich water during its transfer towards the surface and interaction with carbonate rocks.
ISSN: 0009-2541

hal-01053715
https://hal.archives-ouvertes.fr/hal-01053715
DOI : 10.1016/j.chemgeo.2014.02.008