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) Dissolution and precipitation reactions at the redox interface in a meromictic lake (Pavin lake, France)

Résumé

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 K_z 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.

Abstract

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. Manganese concentrations were very low in the mixolimnion(<0.01 µmol/L), exhibited a peak between 62.4 and 63.5 m depth (up to 60 µmol/L at 63 m) and reached a value of about 30 µmol/L at 85 m. Barium concentrations began to increase only at depths greater than 65-67 m. Vanadium concentrations in the mixolimnion were about 14 nmol/L, decreased to a minimum below the detection limit at 62.2 m and then increase drastically (150 nmol/L at 85 m).

In order to derive the accurate location of the chemical reactions and an estimation of their rates from the concentration profiles, knowledge of the transport parameters was needed. As advection can be considered to be negligible, the major parameter of interest is the vertical eddy diffusion coefficient K_z. Na is assumed to be unreactive in the studied layer. Its concentrations can be represented by an analytical function

      Cmax - Cmin                 Cmax + Cmin

C = ___________ * th [P(z)] + ___________

             2                                    2

with P(z)=0.0016 * (z-z_o)³ - 0.0493 * (z-z_o)² + 0.5735 * (z-z_o) - 0.4811

This allows the determination of the coefficient K_z.

K_z = λ ch² [P(z)]/ [P'(z)]

λ is determined from the value of K_z at 85 m depth previously obtained from an hydrodynamic study of the lake (Aeshbach-Hertig et al., 1999). This coefficient is about 0.1 m²/day at the bottom of the monimolimnion. It is very low at the redox interface (0.0017 m²/day), far below the molecular thermal diffusion coefficient. It increases very sharply at the bottom of the mixolimnion. The K_z profile is in fair agreement with the results obtained from the earlier hydrodynamic study.

A quantitative study of the dissolution-precipitation reactions at the center of the lake at depths between 55 and 85 m can then be undertaken. The 55 m limit corresponds to a depth where inputs of fresh water can occur. The 85 m limit is about 7 m above the bottom of the lake. Below this depth important inputs from the pore waters occur which are not taken into account by the present modeling. Concentrations of Rb, Fe, Ba and V can be accurately represented by polynomial functions of the Na concentration. The parameter u=th[P(z)] represents the concentrations of these 4 elements by polynomials :

          N

X(u) = Σ a_n * uⁿ

          0

The rate of dissolution-precipitation for each element as a function of depth can be derived.

                                       N

R = - λ [P'(z)] * ch^-2[P(z)] Σ a_nn(n-1) * th^n-2[P(z)]

                                        0

Rb concentrations are a linear function of the Na ones and therefore rubidium is not reactive. Fe concentrations can be related to sodium concentrations by a parabolic relationship. From this relationship, it can be derived that strong iron precipitation occurs in the 63 - 65 m depth layer. V concentrations are related to sodium ones by a 4^th degree polynomial. It can be derived that V deposition occurs at depths of 63-65 m and at 70 m.

Ba precipitates around 70 m depth. Mn concentrations are represented by

[Mn]=a₀ +a₁ u + b₁ exp[-(z-z°)²/z^*2]

and the derivation shows that Mn is strongly dissolved between 62.8 and 63 m and precipitated just above. These results are in good agreement with a previous study of particles fluxes derived from sediment trap analysis (Viollier et al, 1997).

This study shows the complexity of this interface and more comprehensive studies including all major elements, dissolved organic carbon (DOC) and pH are needed.