Vitesses de réaction de dissolution et précipitation au voisinage de l'interface oxydo-réducteur dans un lac méromictique : le lac Pavin (Puy de Dôme, France)

Abstract

Une étude à l'échelle centimétrique de l'interface redox situé à la limite entre mixolimnion et monimolimnion d'un lac méromictique (le lac Pavin) a permis d'observer très finement l'évolution de la concentration d'un certain nombre d'éléments chimiques. Nous avons choisi de présenter ici des résultats concernant 5 éléments qui présentent des comportements très contrastés : le rubidium, le fer, le baryum, le vanadium et le manganèse. La comparaison avec un élément conservatif, le sodium, montre que Rb est conservatif, que Fe, Ba et V sont précipités et que Mn est dissous dans cette zone.Une modélisation de ces concentrations en vue de préciser à quelle profondeur et avec quelle vitesse se produisent les réactions concernant ces éléments nécessite la détermination des paramètres de transport au voisinage de cet interface.Une représentation analytique des concentrations de sodium permet de calculer le coefficient de diffusion turbulente Kz en fonction de la profondeur. Au voisinage de l'interface redox, ce coefficient est très petit (0,0017m2/jour) et inférieur au coefficient de diffusion thermique moléculaire.Les concentrations des éléments étudiés ont pu être représentés avec précisions par des polynômes en fonction de la concentration en sodium.Cela permet d'estimer les vitesses des réactions de précipitation dissolution en fonction de la profondeur. Le rubidium n'est affecté par aucune réaction. Le fer précipite entre 63 et 65 m, le baryum entre 68 et 72 m tandis que le vanadium précipite à la fois dans ces 2 zones. Le manganèse réagit dans une zone très étroite : il est précipité entre 61,5 et 62 m et dissous entre 62,8 et 63,1 m.Une étude similaire de tous les éléments majeurs (y compris pH et COD) pourrait permettre d'élucider les processus qui conduisent à ces comportements complexes.Lake Pavin, French Massif Central, is the main meromictic lake in France and has been extensively studied from more than 50 years. The upper part (mixolimnion) at a depth of less than about 60 m behaves as an oligotrophic lake and is oxic during the major part of the year. The lower layer (monimolimnion) has a higher salinity and is permanently anoxic. Unlike the top of the mixolimnion, element concentrations in the monimolimnion can be considered at steady state. The boundary between mixolimnion and monimolimnion is a redox interface. At this interface, an important number of both chemical and biochemical reactions occur. This boundary, where element concentrations vary greatly, was studied at the centimeter scale between 58 and 64 m depth. The present paper is focused on five elements showing very different behaviour: rubidium, iron, manganese, vanadium and barium. Sodium was used as a reference element. Sodium and rubidium concentrations had similar patterns: a progressive increase began at 61 m depth and the maximal gradient was located at 63 m. They continue to increase towards the bottom of the lake. Iron concentrations were low (< 1 µmol/L) at a depth less than 62.8 m and increased very sharply below this depth. Manganes...

Biogeochemical modelling of anaerobic vs. aerobic methane oxidation in a meromictic crater lake (Lake Pavin, France)

“…Stimulation of potential anaerobic CH 4 oxidation by Mn oxides in anoxic sediments from Lake Mendota has been reported previously (Zehnder and Brock, 1980). Miura et al (1992) suggested that, in subsoils, CH 4 contributed to a significant proportion of Fe(III) reduction. Kumaraswamy et al (2001) have shown a stimulation of CH 4 oxidation under anaerobic conditions when Fe(III) was added to anoxic rice soil samples.…”

“…Methane concentrations increase sharply at approximately 55 m depth to reach 4 × 10 −3 mol L −1 at 92 m depth. Differences in CH 4 concentrations for the deepest layers are observed in June 2007 but globally from 30 years of observations, the monimolimnion is considered to have reached interannual steady state ( [Viollier et al, 1997] and [Michard et al, 2003] ). However, significant increase of CH 4 is also observed in the epilimnion (Fig.…”

Identification of microbial communities involved in the methane cycle of a freshwater meromictic lake

“…All parameters taken together, the profiles showed typical trends usually observed in Lake Pavin, i.e. O 2 depletion (detection limit <1 μM) and a sharp increase in C25 and CH 4 at around 60 m depth (Viollier et al , 1997; Michard et al , 2003). In July 2007, the maximum gradients for DO and C25 were located at 57 m depth (oxycline) and 62.1 m depth (mesolimnion), respectively.…”

“…It is divided into two permanent stratified layers (Fig. 1) (see Michard et al , 2003). The surface stratum, or mixolimnion, includes three different layers according to variations in depth, temperature, pH and dissolved O 2 (the epilimnion, up to 5–15 m depth, the metalimnion, from the bottom of epilimnion up to 20 m depth and the hypolimnion from approximately 20 to 61 m depth).…”

“…Hence, this meromictic lake has three water layers, an anoxic deep water layer (monimolimnion) separated from the oxygenated upper layer (mixolimnion) by an intermediate layer (mesolimnion) with strong chemical gradients. The sharp increase in the concentration of dissolved compounds within the mesolimnion leads to an increase in the density of the bottom water layers and consequently strengthens the stability of the physical stratification, despite a temperature increase of about 1 °C (Michard et al , 2003). The isolation of the monimolimnion (by the mesolimnion) results in a significant CH 4 accumulation (over 4 mM at 90 m depth).…”