36Cl contribution of the radiological background in Meuse/Haute-Marne : distribution of current pools and transferts into the several compartments of the terrestrial ecosystem

Le 36Cl est utilisé dans les études hydrologiques en raison de son caractère conservatif. Il apparaît cependant que le Cl- participe à un cycle biogéochimique complexe associé au turnover de la matière organique. Le flux de 36Cl atmosphérique a été déterminé (suivi mensuel des eaux de pluie sur deux ans) et varie saisonnièrement avec des valeurs accrues au printemps - été. L'importance de la période d'échantillonnage a été mise en évidence indiquant qu'une période de 6 mois est recommandée afin de lisser les variations saisonnières et les pics sporadiques du flux de 36Cl. Le flux atmosphérique moyen du 36Cl à notre site, constituant le flux d'entrée majoritaire dans son cycle biogéochimique, est de (75±6) atoms.m-2.s-1.La distribution du Cl et du 36Cl a été étudiée dans l’écosystème forestier grâce au développement des protocoles adéquats. 72% du Cl et 80% du 36Cl sont présents sous forme organique dans le sol alors qu'ils sont principalement sous forme inorganique dans la végétation. Les rapports mesurés dans la végétation (~ 200 x 10-15 at.at-1) sont similaires à ceux provenant des apports atmosphériques (~ 180 x 10-15 at.at-1) et ceux mesurés dans le sol sont un ordre de grandeur supérieur. Ces derniers augmentent significativement à 5-15 cm dans la fraction organique du profil de sol alors que dans la fraction inorganique une faible augmentation apparaît à 15-30 cm. Nous attribuons ces observations au recyclage du 36Cl du pic nucléaire des années 1950-1960. Ces résultats indiquent que la distribution du 36Cl diffère de celle du Cl suggérant des processus dynamiques d’immobilisation et de remobilisation du 36Cl liés au renouvellement de la matière organique.36Cl is widely used in hydrological studies since it is considered as a conservative tracer. However it appears that Cl- participates in a complex biogeochemical cycle linked to the organic matter turnover. The 36Cl atmospheric fallout rate has been determined (monthly record of rainwater samples during two years) and varies seasonally with higher values during spring – summer. The sampling period should at least span 6 months to avoid any biais in the fallout rate determination due to the monthly variations or any sporadic bursts of 36Cl. The mean 36Cl fallout rate at our site, corresponding to the main input of its biogeochemical cycle, equals (75±6) atoms.m-2.s-1. The distribution of Cl and 36Cl has been studied into the ecosystem thanks to the development of appropriate protocols.72% of the Cl and 80% of the 36Cl are contained in the organic fraction in soil while they are mainly in inorganic form in the vegetation. 36Cl/Cl measured in vegetation (~200 x 10-15 at.at-1) are within the range of the ones measured in the rainwater samples (~180 x 10-15 at.at-1), while the 36Cl/Cl ratios are 10 times higher in soil. 36Cl/Cl in soil increase significantly at a depth of 5-15 cm in the organic fraction while a slight increase is observed in the inorganic fraction at 15-30 cm deep. This suggests that 36Cl originating from the massive input of 36Cl introduced in the atmosphere more than 50 years as a consequence of nuclear tests, might still be recycling.These results emphasize that the distribution of 36Cl and Cl are not similar suggesting a possible occurrence of dynamic processes of 36Cl accumulation and release associated with the turnover of the organic matter

Mobility of 137Cs, 10Be and 210Pb in soil as a function of the soil texture, organic matter content and pH

Vertical matter transfer processes (bioturbation and <2μm translocation) in soils and their dynamics are up to now poorly constrained although they are responsible for the temporal evolution of the ecosystem services. In order to better constrain and quantify these processes, a kinetic quantification of the involved transfers based on vertical profiles of 210Pb (xs), 10Be and 137Cs has been developed for Luvisols in which soluble transfers could be neglected. The range of applicability of this type of approach was nevertheless not tested. In this work, we analyzed 210Pb (xs), 10Be and 137Cs in soils exhibiting gradients of organic matter, pH and < 2 μm fraction (Podzol, Andosol Ferralsol, Leptosol). Acidic soils and large organic matter contents were considered. The depth distribution of the studied isotopes was then interpreted as a function of the organic matter content, the soil pH and its < 2 μm fraction. This demonstrates that 10Be and 137Cs losses occur under soluble form in the Podzol, especially under forest, more acid, for the latter. Soluble 10Be transfers were also identified in the Andosol when they were considered as negligible in the Leptosol. For 210Pb (xs), soluble transfers are also probable for soils with pH less than 5.5 (Andosol and Ferralsol) and strong affinity for organic matter has been demonstrated. Thus, under acidic conditions and low < 2 μm fraction, soluble transfer cannot neglected anymore

Monthly record of the natural 36Cl fallout rate in a deciduous forest ecosystem in NE France between 2012 and 2014

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Monthly record of the Cl and Cl-36 fallout rates in a deciduous forest ecosystem in NE France in 2012 and 2013

International audienceThis study aims at determining the chlorine and chlorine-36 fallout rates in an experimental beech forest site located in NE France (48 degrees 31'55 `' N, 5 degrees 16'8 `' E). A monthly record of Cl and Cl-36 concentrations in rainfall samples collected above the canopy was performed during two years, from March 2012 to February 2014. The results show that the Cl concentrations mainly originate from sea-spray while the Cl-36 concentrations originate from the stratosphere and therefore present a seasonal dependency. Abrupt and important inputs of Cl-36 from the stratosphere indeed yield sharp increases of the recorded concentrations during the spring-summer. We also show that a too short sampling period might bias the determined Cl-36 fallout rate. To smooth the seasonal and sporadic bursts of Cl-36, a minimum of 6 months sampling period is required. A mean Cl-36 fallout rate of (77 +/- 21) atoms m(-2) S-1 can be deduced from our study, which is 45% higher than the modelled value. This discrepancy suggests more studies aiming at measuring the Cl-36 fallout rate worldwide are necessary. (C) 2016 Elsevier B.V. All rights reserved

36Cl, a new tool to assess soil carbon dynamics

Abstract Soil organic carbon is one of the largest surface pools of carbon that humans can manage in order to partially mitigate annual anthropogenic CO2 emissions. A significant element to assess soil sequestration potential is the carbon age, which is evaluated by modelling or experimentally using carbon isotopes. Results, however, are not consistent. The 14C derived approach seems to overestimate by a factor of 6–10 the average carbon age in soils estimated by modeling and 13C approaches and thus the sequestration potential. A fully independent method is needed. The cosmogenic chlorine nuclide, 36Cl, is a potential alternative. 36Cl is a naturally occurring cosmogenic radionuclide with a production that increased by three orders of magnitude during nuclear bomb tests. Part of this production is retained by soil organic matter in organochloride form and hence acts as a tracer of the fate of soil organic carbon. We here quantify the fraction and the duration of 36Cl retained in the soil and we show that retention time increases with depth from 20 to 322 years, in agreement with both modelling and 13C-derived estimates. This work demonstrates that 36Cl retention duration can be a proxy for the age of soil organic carbon