196,365 research outputs found
Lake Pavin Sedimentary Environments
No full textInternational audienceIn the recent years, Lake Pavin sedimentary basin has been intensively studied by several acoustic surveys (high resolution seismic refl ection profi ling, multibeam bathymetry) and gravity coring campaigns. This new data set combining acoustic images and multidisciplinary study of sediment cores allows characterizing contrasted subaquatic sedimentary environments along the littoral slopes, a subaquatic plateau (close to the lake outlet), steep slopes and its deep central basin. Two main types of lacustrine sediments are identifi ed (i) between the lake shore and 26 m water depth (massive organic rich sandy silts), and (ii) below 26 m water depth when the lake fl oor slopes are less than 15° steep (organic rich laminated diatomites). A large and recent slide scar is in particular clearly identifi ed at the edge of the plateau just above the deep central basin. Evidences of former gravity reworking phenomena within the crater ring draining into Lake Pavin also include a large subaquatic slump deposit accumulated on the subaquatic plateau and several small scale rock fall deposits originating from outcropping lavas within the crater ring. The identifi cation of two recent outstanding erosive sandy layers in Pavin littoral environment also suggests that some of this gravity reworking phenomena have been associated with unusually violent waves and/or abrupt lake level drop. Lake Pavin geomorphology and sedimentary environments are in addition compared to the ones of the nearby Lake Chauvet based on a similar acoustic and sedimentary data base in order to highlight the infl uence of maar age and geomorphology on the development of sedimentary environments and Natural Hazards in this volcanic region of the French Massif Central. Lake Chauvet is comparatively to Lake Pavin characterized by a shallower central basin, less steep slopes and no subaquatic plateaus. A recent and relatively large mass wasting deposit is, however, clearly identifi ed along the slopes of a small delta facing the only tributary of this maar lake. This work 366 suggest that maar lakes from the study area are concerned by subaquatic slope stabilities, especially in Lake Pavin were slope failures may in addition impact the development or the stability of its meromicticity
Étude biogéochimique d'un lac méromictique : le lac Pavin, France / Biogeochemical study of a meromictic lake: Pavin lake, France
No full textAbstract Following a volcanic eruption which killed almost 2000 people in Cameroon in 1986, the Délégation aux Risques Majeurs, a French Government Agency, requested a report evaluating the risks of a similar eruption of CO2 in Auvergne: this volcanic province of the French "Massif Central" has structures comparable with those which were at the location of the Cameroon eruption. The lake Pavin, 90 m deep, located in a volcanic crater, is of meromictic type: the annual circulation of water is incomplete with the deep layer, (60-90 m), remains stagnant year to year. This property was confirmed by ten profiles of the content of 3H of the water column. A weak inflow of water (about 0.1m3.m2.yr') at the bottom of the lake plays an important role in the geochemistry of the system. Application of the ofFick's laws estimates the influence of advection and diffusion in particular for conservative components such as I8O, Na*. Carbon isotopes measurements on total dissolved inorganic carbon and on dissolved gases (CO2 and CH4) showed that the deep carbon contribution comprising about 20 % due to fermentation of organic matter, 10% dissolved inorganic carbon in the advective water inflow and 70% by CO2 ofmagmatic origin. This contribution of mantle fluids was confirmed by 3He/4He measurement. If quantity of CO 2 dissolved at -90 m does not exceed 1 LCO2.LH2ó' whereas its solubility is, with 5 °C, of 1.5 LCO2.LH2O'.bar', it would be interesting to follow thepH deep water, not only of Pavin but also of the other deep lakes from Auvergne (Godivelle, Issarlès, Tazenat, Chau- vet) likely to transform into carbon dioxide tank.Résumé À la suite d'une éruption volcanique qui provoqua la mort de près de 2000 personnes au Cameroun en 1986, la Délégation aux Risques Majeurs demanda un rapport sur l'évaluation des risques d'une éruption de CO2 en Auvergne dans la mesure où cette province volcanique du Massif Central français présente des appareils et des lacs comparables à ceux qui furent à l'origine de la catastrophe de Nyos. Le lac Pavin, profond de 90 m, situé dans un cratère volcanique est de type méromictique car la circulation annuelle des eaux est incomplète et la couche profonde, de -60 à -90 m, reste stagnante d'une année sur l'autre. Cette méromicticité a été confirmée par une dizaine de profils de la teneur en 3H de la colonne d'eau. L'étude de ces profils de 3H a permis de mettre en évidence un flux d'eau profonde à la base du monimolimnion qui, s'il est faible (0,1 m3.m2.an"'), joue un rôle important dans la géochimie du système par l'apport d'éléments en solution. L'application des lois de Fick permet d'estimer les effets d'advection et de diffusion notamment pour les éléments ayant un comportement chimique conservatif (18O, Na% etc.). La mesure des isotopes du carbone sur le carbone minéral total dissous et sur les gaz (CO2 et CH4) a permis de montrer que l'apport profond de carbone comprend trois sources : 20 % dus à la fermentation méthanique de la matière organique, 10 % de carbone minéral total dissous dans le flux d'eau advectif du fond et 70 % de CO2 d'origine magmatique. Cet apport de fluides mantelliques est confirmé par la mesure du rapport isotopique 3He/4He. Si la quantité de CO2 dissous à -90 m ne dépasse pas 1 Lcqj.Ljko"1 alors que sa solubilité est, à 5 °C, de 1,5 LCO2.LH2O1.bar1, il serait intéressant de suivre le pH des eaux profondes, non seulement du lac Pavin mais aussi des autres lacs profonds d'Auvergne (Godivelle, Issarlès, Tazenat, Chau- vet) car ils sont susceptibles de se transformer en réservoir de gaz carbonique.Olive Philippe, Boulègue Jacques. Étude biogéochimique d'un lac méromictique : le lac Pavin, France / Biogeochemical study of a meromictic lake: Pavin lake, France. In: Géomorphologie : relief, processus, environnement, Octobre-décembre, vol. 10, n°4. pp. 305-316
Reduced sulfur and iron species in anoxic water column of meromictic crater Lake Pavin (Massif Central, France)
No full textInternational audienceThe vertical distribution of reduced sulfur species (RSS including H2S/HS-, S-0, electroactive FeS) and dissolved Fe(II) was studied in the anoxic water column of meromictic Lake Pavin. Sulfide concentrations were determined by two different analytical techniques, i.e. spectophotometry (methylene blue technique) and voltammetry (HMDE electrode). Total sulfide concentrations determined with methylene blue method (Sigma H2SMBRS) were in the range from 0.6 mu M to 16.7 mu M and were substantially higher than total reduced sulfur species (RSSV) concentrations determined by voltammetry, which ranged from 0.1 to 5.6 mu M. The observed difference in the sulfide concentrations between the two methods can be assigned to the presence of FeS colloidal species.Dissolved Fe was high (>1000 mu M), whereas dissolved Mn was only 25 mu M, in the anoxic water column. This indicates that Fe is the dominant metal involved in sulfur redox cycling and precipitation. Consequently, in the anoxic deep layer of Lake Pavin, "free" sulfide, H2S/HS-, was low; and about 80% of total sulfide detected was in the electroactive FeS colloidal form. IAP calculations showed that the Lake Pavin water column is saturated with respect to FeSam phase. The upper part of monimolimnion layer is characterized by higher concentrations of S (0) (up to 3.4 mu M) in comparison to the bottom of the lake. This behavior is probably influenced by sulfide oxidation with Fe(III) oxyhydroxide specie
Phylogenetic diversity of archaea and bacteria in the anoxic zone of a meromictic lake (Lake Pavin, France).
No full textInternational audienceThe compositions of archaeal and bacterial populations at different depths (60 m [mixolimnion-chemocline interface], 70 m [chemocline-subchemocline interface], 90 m, and 92 m [the water-sediment interface]) in the anoxic zone of the water column in Lake Pavin, a freshwater permanently stratified mountain lake in France, were determined. Phylogenetic trees were constructed from sequences to assess archaeal and bacterial diversity at the four sites
Lake Pavin Mixing: New Insights from High Resolution Continuous Measurements
No full textInternational audienceAs a meromictic lake, Lake Pavin mixing is very specific. The chemocline located at about 60 m depth separates the mixolimnion (fully or partially mixed according to the year) and the monimolimnion. Deep layers are geothermally heated and stability is ensured at the bottom of the lake by the increasing dissolved substance concentration. The monimolimnion forms a compartment which has its own specific dynamics but that may interact with the mixolimnion at large time scales. Understanding of physical mixing processes is crucial to study further geochemical processes.Temperature and turbulence were investigated in 2006 and 2007 using continuous measurements, a CTD and a high resolution temperature microstructure profiler (SCAMP). Continuous measurements give the evidence of a sublacustrine spring discharging intermittently into the mixolimnion around 55 m depth. This cold water input was observed using thermistor chains at different depths in 2007. Because of its low saline content, the spring water input rises in the water column by saline convection. The use of a simple conceptual model, representing turbulent diapycnal diffusivity and convection shows its role in maintaining the meromixis characteristic of the lake on the intra-annual time scale. The spring also influences seasonal overturns and thus contributes to establish the depth of the mixolimnion–monimolimnion interface on the interannual time scale.Using SCAMP, vertical dispersion coefficients are estimated by different methods. Vertical dispersion coefficients show a high space and time variability. The use of these data in the geochemical model AQUASIM applied to Lake Pavin shows a variability of model outputs directly depending on mixing inputs and their variability
Genomic fragments obtained from Lake Pavin using the specific-gene capture approach targeting DEH 16S rRNA genes.
No full text<p>The genomic fragments were recovered at 68 m depth by specific-gene capture approach using probes targeting the 16S rRNA genes of DEH phylotypes (first capture experiment) and a hypothetical protein (second capture experiment) identified in Lake Pavin water column. 16S-pos8, 16S-pos50, 16S-pos1250, H9-1 and H9-2: capture probes; E1-1R: PCR primer.</p
Modelling CO2 dispersion in the air during potential limnic eruption at the lake Pavin (France)
No full textRisk mitigation in long-dormant volcanic provinces is a challenge due to the absence of collective memory of past disasters as well as the scarcity, and subtlety, of unrest signals that can be monitored. In this study, the impact of a potential limnic eruption is assessed at the 92-m-deep lake Pavin (French Massif Central). The lake is hosted in a maar crater formed during the last eruptive event in metropolitan France (ca. 7 ka) and contains dissolved CO2 in the deepest water layer, below 60 m. Carbon dioxide (CO2) emissions measured at the lake surface (0.44 km2) reach up to 10.1 tons/day during the winter. Beyond this (limited) continuous degassing of the lake, the current CO2 budget in the monimolimnion layer (at a depth of 60 m to 92 m) was estimated at 1750 tons, of which about 450 tons are available for release in case of overturn of the lake. Scenarios for CO2 dispersion in the lower atmosphere were simulated with the DISGAS and TWODEE-2 models by varying (i) meteorological conditions, (ii) the amount of CO2 released, (iii) and the mechanisms of degassing during a potential limnic eruption. The simulations allowed identification and delimitation of areas potentially impacted by hazardous CO2 levels in the air down-valley from the lake and directly around the lake. The spatio-temporal evolution of the potential CO2 cloud raises issues regarding the impacts of such a hypothetical event in the close vicinity of the lake and, given the area is populated and highly visited, needs to be considered in future risk mitigation strategie
A 7000-year environmental history and soil erosion record inferred from the deep sediments of Lake Pavin (Massif Central, France)
No full textInternational audienceA 14-m long sedimentary sequence (core PAV12) was collected in the deepest part of Lake Pavin, a maar lake located in the French Massif Central. The PAV12 sedimentary sequence documents the lake's environmental evolution since its formation 7000 years ago. The relationships between the catchment's vegetation cover, erosion processes and changes in trophic status were shown using a multi-proxy characterization of mineral and organic fractions supported by palynological data. The record shows a succession of lithological units starting at the base, with volcanoclastic material corresponding to the early stage of Lake Pavin. The deposition of organic-rich and diatomaceous sedimentary units above volcanoclastic material indicates an evolution toward a pristine lacustrine state. The Late Holocene environmental history of this lake is marked by two tipping points reflecting major environmental disturbances at ca. 4000 cal BP and after the deposition of erosive mass-wasting deposits (MWDs) at 1350 cal BP (CE 600) and 650 cal BP (CE 1300). The upper unit of core PAV12, which corresponds to the past 700 years, indicates that one of these MWDs was likely the driving force behind a major limnological change marked by a shift in redox-sensitive elements (i.e., current meromictic lacustrine state). The palynological diagram indicates a forested catchment where woodland clearances and agro-pastoral activities have remained limited except over the last 700 years. These findings suggest restricted human impact within the watershed compared to other regional archives. The reconstruction of the Lake Pavin erosion record determined from titanium and red amorphous particle fluxes highlights phases of enhanced erosion at ca. 6.5–5.5, 4.1–3.8, 3.5, 2.8–2.6, 1.6–1.4 cal kyr BP and during the Little Ice Age (LIA). A comparison between this erosion record, palaeoenvironmental archives from Western Europe and palaeoclimatic data supports an Atlantic signal driving precipitation patterns over Lake Pavin at centennial to millennial timescales. The influence of local human activities, even on a small scale, cannot be completely discounted as their impact on erosional processes may be amplified in a steep catchment such as that found in Lake Pavin
Seasonal depth-related gradients in virioplankton: standing stock and relationships with microbial communities in Lake Pavin (France)
No full textInternational audienceThis study presents a depth-related survey of virioplankton abundance in Lake Pavin (Massif Central, France), in relation to the abundances of heterotrophic prokaryotes, picocyanobacteria (Pcy), autotrophic picoeukaryotes (Peu), and of autotrophic (ANF) and heterotrophic (HNF) nanoflagellates. The sampling strategy was designed to be representative of the physico-chemical gradients of the whole water column of the lake, and the seasonal variability as well. In mixolimnic surface waters, all communities were present and viral abundance peaked in summer and autumn. Viral abundance was significantly correlated (p < 0.001) with Pcy, Peu, and ANF, indicating that cyanophages and perhaps other phytoplankton viruses represent a significant pool of viral standing stocks in the mixolimnion of Lake Pavin. Microautotrophs were absent in the deep monimolimnic water masses, where viruses and heterotrophic prokaryotes exhibited highest seasonal abundances in summer and/or autumn and were significantly correlated (p < 0.001) to each other. This indicates that the anoxic monimolimnion of Lake Pavin is an exclusive habitat for viruses and heterotrophic prokaryotes. We conclude that in this habitat, host availability is prevalent over other factors (temperature, oxygen, nutrients, grazers) in favoring viral proliferation
Multidisciplinary characterisation of sedimentary processes in a recent maar lake (Lake Pavin, French Massif Central) and implication for natural hazards
No full textSedimentation processes occurring in the most recent maar lake of the French Massif Central (Lake Pavin) are documented for the first time based on high resolution seismic reflection and multibeam bathymetric surveys and by piston coring and radiocarbon dating on a sediment depocentre developed on a narrow sub aquatic plateau. This new data set confirms the mid Holocene age of maar lake Pavin formation at 6970&plusmn;60 yrs cal BP and highlights a wide range of gravity reworking phenomena affecting the basin. In particular, a slump deposit dated between AD 580–640 remoulded both mid-Holocene lacustrine sediments, terrestrial plant debris and some volcanic material from the northern crater inner walls. Between AD 1200 and AD 1300, a large slide scar mapped at 50 m depth also affected the southern edge of the sub aquatic plateau, suggesting that these gas-rich biogenic sediments (laminated diatomite) are poorly stable. Although several triggering mechanisms can be proposed for these prehistoric sub-aquatic mass wasting deposits in Lake Pavin, we argue that such large remobilisation of gas-rich sediments may affect the gas stability in deep waters of meromictic maar lakes. This study highlights the need to further document mass wasting processes in maar lakes and their impacts on the generation of waves, favouring the development of dangerous (and potentially deadly) limnic eruptions
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