Alteration of As-bearing phases in a small watershed located on a high grade arsenic-geochemical anomaly (French Massif Central)

International audienceAt a watershed scale, sediments and soil weathering exerts a control on solid and dissolved transport of trace elements in surface waters and it can be considered as a source of pollution. The studied subwatershed (1.5 km2) was located on an As geochemical anomaly. The studied soil profile showed a significant decrease of As content from 1500 mg.kg-1 in the 135-165 cm deepest soil layer to 385 mg.kg-1 in the upper 0-5 cm soil layer. Directly in the stream, suspended matter and the <63 μm fraction of bed sediments had As concentrations greater than 400 mg.kg-1. In all these solid fractions, the main representative As-bearing phases were determined at two different observation scales: bulk analyses using X-ray absorption structure spectroscopy (XAS) and microanalyses using scanning electron microscope (SEM) and associated electron probe microanalyses (EPMA), as well as micro-Raman spectroscopy and synchrotron-based micro-scanning X-ray diffraction (μSXRD) characterization. Three main As-bearing phases were identified: (i) arsenates (mostly pharmacosiderite), the most concentrated phases As in both the coherent weathered bedrock and the 135-165 cm soil layer but not observed in the river solid fraction, (ii) Fe-oxyhydroxides with in situ As content up to 15.4 wt.% in the deepest soil layer, and (iii) aluminosilicates, the least concentrated As carriers. The mineralogical evolution of As-bearing phases in the soil profile, coupled with the decrease of bulk As content, may be related to pedogenesis processes, suggesting an evolution of arsenates into As-rich Fe-oxyhydroxides. Therefore, weathering and mineralogical evolution of these As-rich phases may release As to surface waters

Metals retention after weathering of a sulphide ore body: unexploited gossan versus mine tailings.

Numerous studies have highlighted the fact that the bioavailability of potentially toxic trace elements (PTTE), such as As, Cd, Cr, Cu, Ni, and Pb, is strongly related to the redox state and chemical form. Since dissolved forms of trace elements are the most bioavailable, the issue of greatest concern is to define the conditions under which these elements may be remobilized in solution up to levels higher than those recommended by National agencies as e.g. the Environmental Protection Agency (EPA). In order to understand and then ideally to prevent trace element mobilization, the characterisation of the contaminant sources, i.e. the solid fraction, is crucial

First paleomagnetic and 40Ar/39Ar study of Paleoproterozoic rocks from the French Guyana (Camopi and Oyapok rivers), northeastern Guyana Shield

In order to understand the Paleoproterozoic geographic evolution of the Guyana Shield, paleomagnetic and 40Ar/39Ar investigations were carried out on granitoids and volcano-sedimentary rocks from the Oyapok and Camopi rivers (French Guyana–Brazil frontier). Scanning electronic microscope, thermomagnetic and isothermomagnetic experiments show that magnetite is the main magnetic remanent carrier in most of the samples. The metavolcano-sedimentary rocks (Paramaca) show a weak magnetization and scattered magnetic directions. Therefore, no reliable magnetic component could be isolated from these samples. Samples taken from tonalite and meta-ultrabasite rocks yield a characteristic magnetic direction, carried by subautomorphous magnetite, that is well defined and distinct from that of the present Earth field and that of nearby Jurassic dikes. A virtual geomagnetic pole (VGP) deduced from this probable primary remanence was calculated, namely pole OYA, λ=28.0°S, φ=346.0°E, N=5, k=31.9 and A95=13.8°. Four 40Ar/39Ar ages, ranging from 2052 to 1973 Ma, were obtained from amphiboles and biotites of tonalite rocks, showing a relatively slow cooling rate of ca 4.8+2.6/−2.1°C Ma−1. The linear extrapolation of this cooling rate to the magnetite unblocking temperature (540 to 580°C) yields a magnetization age of 2036±14 Ma for pole OYA. Pole OYA differs significantly from available paleomagnetic results from Venezuela of the West Guyana Shield dated at 2000±10 Ma. This difference may indicate an important latitudinal movement of the Guyana Shield between 2036 and 2000 Ma with a velocity of 9±7 cm/year

North western Alps Holocene paleohydrology recorded by flooding activity in Lake Le Bourget, France and possible relations with Mont-Blanc glaciers fluctuations

International audienceA 14-m long piston core was retrieved from Lake Le Bourget, NWAlps (France), in order to provide a continuous record of flooding events of the Rhone River during the Holocene. The selection of the coring site was based on high resolution seismic profiling, in an area with limited mass wasting deposits and accumulated proximal Rhone River inter-and underflow deposits. The age-depth model of this core is based on (i) 14 AMS radiocarbon dates, (ii)radionuclide dating(137Cs) and (iii) the identification of historical data (flood events, eutrophication of the lake).The sedimentary record dates back to 9400 cal BP, and includes a thin mass wasting event deposited around 4500 cal BP. A multi-proxy approach was used to track the evolution and origin of clastic sedimentation during the Holocene, in order to identify periods of higher hydrologic al activity in the catchment area. Spectrophotometry was used to detect fluctuations in clastic supply and the study of clay minerals (especially the Illite crystallinity index) allowed locating the main source area of fine grained clastic particles settling at the lake after flood events. This dataset highlights up to 12 periods of more intense flooding events over the last 9400 years in Lake Le Bourget and shows that the main source area of clastic particles during this period is the upper part of the Arve River drainage basin. This part of the catchment area drains several large glaciers from the Mont-Blanc Massif, and fluctuations in Rhone River flood supply in Lake Le Bourget is interpreted as resulting essentially from Mont-Blanc Glacier activity during the Holocene.The comparison of clastic sedimentationin Lake Le Bourget with periods of increasing land use and periods of Alpine glacier and mid-European lake level fluctuations, suggest that the core LDB04 clastic record in Lake Le Bourget is a continuous proxy of the Holocene hydrologic al history of the NW Alps

Environmental responses to Lateglacial climatic fluctuations recorded in the sediments of pre-Alpine Lake Mondsee (northeastern Alps)

Declakes Participants : Soumaya Belmecheri, Marc Desmet, Helmut Erlenkeuser, Bernard Fanget, Jérôme Nomade.International audienceInvestigation of the sedimentary record of pre-Alpine Lake Mondsee (Upper Austria) focused on the environmental reaction to rapid Lateglacial climatic changes. Results of this study reveal complex proxy responses that are variable in time and influenced by the long-term evolution of the lake and its catchment. A new field sampling approach facilitated continuous and precisely controlled parallel sampling at decadal to sub-annual resolution for µ-XRF element scanning, carbon geochemistry, stable isotope measurements on ostracods, pollen analyses and large-scale thin sections for microfacies analysis. The Holocene chronology is established through microscopic varve counting and supported by accelerator mass spectrometry 14C dating of terrestrial plant macrofossils, whereas the Lateglacial age model is based on δ18O wiggle matching with the Greenland NGRIP record, using the GICC05 chronology. Microfacies analysis enables the detection of subtle sedimentological changes, proving that depositional processes even in rather large lake systems are highly sensitive to climate forcing. Comparing periods of major warming at the onset of the Lateglacial and Holocene and of major cooling at the onset of the Younger Dryas reveals differences in proxy responses, reflecting threshold effects and ecosystem inertia. Temperature increase, vegetation recovery, decrease of detrital flux and intensification of biochemical calcite precipitation at the onset of the Holocene took place with only decadal leads and lags over a ca. 100 a period, whereas the spread of woodlands and the reduction of detrital flux lagged the warming at the onset of the Lateglacial Interstadial by ca. 500-750 a. Cooling at the onset of the Younger Dryas is reflected by the simultaneous reaction of δ18O and vegetation, but sedimentological changes (reduction of endogenic calcite content, increase in detrital flux) were delayed by about 150-300 a. Three short-term Lateglacial cold intervals, corresponding to Greenland isotope substages GI-1d, GI-1c2 and GI-1b, also show complex proxy responses that vary in time. Copyright © 2011 John Wiley & Sons, Ltd

Rochechouart 2017-Drilling Campaign: First Results

No abstract available

The environmental and evolutionary history of Lake Ohrid (FYROM/Albania). Interim results from the SCOPSCO deep drilling project

This study reviews and synthesises existing information generated within the SCOPSCO (Scientific Collaboration on Past Speciation Conditions in Lake Ohrid) deep drilling project. The four main aims of the project are to infer (i) the age and origin of Lake Ohrid (Former Yugoslav Republic of Macedonia/Republic of Albania), (ii) its regional seismotectonic history, (iii) volcanic activity and climate change in the central northern Mediterranean region, and (iv) the influence of major geological events on the evolution of its endemic species. The Ohrid basin formed by transtension during the Miocene, opened during the Pliocene and Pleistocene, and the lake established de novo in the still relatively narrow valley between 1.9 and 1.3 Ma. The lake history is recorded in a 584 m long sediment sequence, which was recovered within the framework of the International Continental Scientific Drilling Program (ICDP) from the central part (DEEP site) of the lake in spring 2013. To date, 54 tephra and cryptotephra horizons have been found in the upper 460 m of this sequence. Tephrochronology and tuning biogeochemical proxy data to orbital parameters revealed that the upper 247.8 m represent the last 637 kyr. The multi-proxy data set covering these 637 kyr indicates long- term variability. Some proxies show a change from generally cooler and wetter to drier and warmer glacial and interglacial periods around 300 ka. Short-term environmental change caused, for example, by tephra deposition or the climatic impact of millennial-scale Dansgaard–Oeschger and Heinrich events are superimposed on the long-term trends. Evolutionary studies on the extant fauna indicate that Lake Ohrid was not a refugial area for regional freshwater animals. This differs from the surrounding catchment, where the mountainous setting with relatively high water availability provided a refuge for temperate and montane trees during the relatively cold and dry glacial periods. Although Lake Ohrid experienced significant environmental change over the last 637 kyr, preliminary molecular data from extant microgastropod species do not indicate significant changes in diversification rate during this period. The reasons for this constant rate remain largely unknown, but a possible lack of environ- mentally induced extinction events in Lake Ohrid and/or the high resilience of the ecosystems may have played a role