Geochemical study of a crater lake: Pavin Lake, France — Identification, location and quantification of the chemical reactions in the lake

CHEMICAL GEOLOGY I,SOT¢IPI~," ( i E ( L S ' ( +//-,'.\ ( "I+"

f!LSEVIER

Chemical Geology 115 (1994) 103-115

Geochemical study of a crater lake: Pavin Lake,, France Identification, location and quantification of the chemical reactions in the lake Gil M i c h a r d , Eric Viollier, D i d i e r Jezequel, G 6 r a r d Sarazin Laboratoire de GOochimie des Eaux (Groupe du LA 196). UniversitO Paris 7 et IPG. F-75251 Paris Cedex 05, France

(Received June 7. 1993: revision accepted December 1. 1993 )

Abstract A study of the main geochemical characteristics of the Pavin crater lake was performed in September 1992. ~,ssociated with previous results on the hydrochemical budget of the lake, the present study points out that: ( 1 ) in the mixolimnion, the major reactions are photosynthesis, removal of silica b,y diatoms, iron oxidation and alkali and alkaline-earth incorporation in a solid phase;

(2) the major reduction reactions occur within the sediment and the products of the reaction are brought to the lake by diffusion; (3) some reactions: silica dissolution, manganese oxide reduction, sulfate-sulfide transformations, iron and phosphate precipitation occur within the monimolimnion. complete budget of transfers and chemical reactions is presented for some components such as silica.

I. Introduction

Contrary to marine and stream environments which are really complex, small and therefore well-defined ecosystems such as lakes can be useful to understand transfer processes and chemical reactions governing the chemistry of such systems. It can be also useful to estimate the rates of these processes. Some meromictic lakes have a hypolimnion which can be considered as being in a steady state. The interpretation of the results in these lakes are simpler than in dimictic lakes, as long as the water budget within the lake is known. Therefore, a meromictic crater lake such as Pavin [CAI

Lake is a simple ecosystem suitable for a better understanding of the chemical processes occurring in such systems. The Pavin crater lake (elevation 1197 m above sea level) is located at 45°55'N and 2°54'E in a remote area far from any important industrial activity (Martin, 1985 ). Its shape is a circle which diameter is 750 m and which area is 0.44 km 2. Its depth reaches 92 m. The area of the drainage basin is 0.8 km 2. Pavin Lake is characterized by the presence of two stratified layers: the mixolimnion which is affected by winter mixing and the deepest layer (monimolimnion) which is permanent and deoxygenated. Its chemistry has been partially studied by Meybeck et al. ( 1975 ), Martin ( 1985 ) and Camus et al. (1993). Planktonic communities have been studied by Devaux (1980) and

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104

G. Michard et al. / Chemwal Geology 115 (I 994) 103-115

Amblard and Devaux (1981), and their biochemistry by Amblard and Restituito (1983). One of the goals of this paper is to use the overall hydrogeological model previously established (Martin, 1985; Camus et al., 1993) and the results of a complete chemical analysis in order to improve our knowledge of the reactions occurring within the monimolimnion. Another goal is to estimate the rates of those reactions.

2. Sampling and analytical techniques Waters were sampled along a profile near the center of the lake in September and October 1992. pH and dissolved oxygen were measured using WTW ® probes equipped with 100-m cables, pH meter was standardized against pH 4 and 7 NBS technical buffers, oxymeter against oxygen equilibrated water (Oxycal ® system). Water was sampled at 5-m depth intervals with a vertical Van Dorn bottle. Analytical techniques used are those described by BoulSgue et al. (1977) for sulfurated waters. We give here only a brief summary of these techniques. Sulfide, ferrous iron, nitrite and alkalinity were measured immediately (within 20-30 s for the 4 parameters) by colorimetric techniques using a battery-operated spectrophotometer (Lasa Plus ®, Dr. Lange Corp. ). For the three first compounds, classical methods (methylene blue, ferrospectral without reducing agent, and Griess reaction) are used; alkalinity was measured using a mixture of bromophenol blue and formic acid (F. Podda and G. Michard, in prep. ). Different aliquots were filtered on 0.45-/tm Sartorius ® filters and stored after addition of: ( 1 ) hydrochloric acid for major cations; (2) nitric acid for trace elements; (3) sulfuric acid for phosphate and ammonium; and (4) cadmium acetate for sulfate. An aliquot is stored without addition for chloride and nitrate measurements. Phosphate and ammonium were analyzed within 2-3 hr by classical colorimetric techniques. All other measurements were performed in Paris: major anions by ionic chromatography

(Dionex ® 2000i ), major cations by flame atomic absorption spectrophotometry (GBC ® 902) and trace elements (Mn, Sr) by graphite furnace AAS (Hitachi ® 180-70). Total dissolved inorganic carbon was calculated from pH and carbonate alkalinity. Carbonate alkalinity was calculated from total alkalinity, total sulfide, total phosphate and pH. Reverse titration confirms that phosphate contribution as well as eventual organic acid contributions were low (<2%, i.e. within the analytical uncertainty). Waters sampled at > 70-m depths degas in arriving at atmospheric pressure. Dissolved gases (H2S, CO2, CH4, N2) are lost. CO2 content can be recalculated from pH and alkalinity, CH 4 and Nz were measured by Desgranges' method (reported in Camus et al., 1993) using pressurized sampling devices. However, values for total sulfide cannot be corrected from the gas loss and are minimal values.

3. Results

The results presented in Table l are generally in agreement with those obtained previously. Iron and manganese are in close agreement with data by Amblard and Restituito (1983), Martin (1985), and G. Michard and C. Rabouille (unpublished results, 1987 ). Manganese results by Cossa et al. (1994) on samples taken in August 1990 are also in agreement, but the corresponding values for iron are ~ 3 times lower. Alkalinity values are in agreement with 1987 data and data which were obtained in 1992 by classical titration method. Major elements are also in agreement with data obtained by the Centre de Recherches G6odynamiques, Thonon (Meybeck et al., 1975 ). Some analyses of isotopes which were carried out by Ph. Olive and coworkers, and of dissolved gases by Desgranges' method, published in Camus et al. ( 1993 ), are also used in the discussion.

105

G. Michard et al. / Chemical Geology 115 (I 994) I03-115 Table 1 Element concentrations (#mol 1-~ ) vs. depth in Pavin Lake (September 1992 ) Depth Temper- pH Im ) ature ( °C I

I 5 IO 12 15 20 25 30 35 40 45 50 55 57.5 60 62.5 65 67.5 711 75 80 85 88

15.3 15.3 13.0 9.1 7.2 5.5 4.7 4.3 4.1 4. I 4.1 4.1 3.9 3.9 4.1 3.9 4.1 4.4 4.5 4.6 4.6 4.7 4.7

8.06 8.07 8.30 8.48 7.51 7.06 6.94 6.79 6.70 6.61 6.54 6.47 6.35 6.24 6.18 6.15 6.07 6.03 6.03 6.02 6.03 6.03 6.03

Alkalinity

446 459 459 460 476 459 482 473 481 465 482 471 496 512 590 1,065 2.300 3,320 3,510 3,955 4,235 4.445 4.440

Depth (m)

CI

SO4

NO3

I 5 10 12 15 20 25 30 35 40 45 50 55 57.5 60 62.5 65 67.5 70 75 80 85 88

49.2 47.5 47.3 47.5 48.6 46.0 46.7 47.3 46.6 46.7 46.8 47.1 46.7 46.5 46.8 49.5 53.5 56.4 58.8 59.9 60.5 60.9 61.1

15.6 15.4 15.5 15.7 15.6 16.4 16.2 16.5 16.8 16.8 16.6 16.8 16.8 16.9 16.9 5.8 2.8 2.4 2.1 2.7 3.3 2.5 4.2

O. 1 0.7 0.4 0.6 0.1 O. 1 0.6 0.1 0.3 2.3 2.1 2.6 2.2 1.3 0.3 0.5 0.5 0.4 0.4

NOz

0,3

Carbonate alkalinity

~Y'CO2

446 459 459 460 476 459 482 473 481 465 482 471 496 512 590 1,061 2,287 3,303 3,486 3,926 4,203 4.408 4.405

453 466 461 460 5t7 575 648 702 774 813 907 951 1,171 1,409 1,771 3,349 8,186 12,570 13,245 15,140 15,937 16,683 16,670

H2CO3

Pc02

02

Na

K

Ca

Mg

219 220 225 231 226 225 225 226 226 229 226 245 235 238 245 272 367 423 435 454 461 464 457

88 89 88 91 89 91 9O 95 91 93 92 98 94 02 00 13 57 83 96 212 220 222 227

61 63 60 64 63 63 62 60 62 63 61 67 69 67 78 92 164 183 197 218 226 235 242

55 65 62 62 62 63 61 60 62 62 66 65 75 84 82 118 194 281 283 302 316 331 321

(lmtm)

(gmol) (%sat.)

NH4

PO4

0.3 1.2 0.3 0.3 0.3 0.2 0.2 0.2 0.2 0.2 0.4 0.6 0.6 2.3 26.8 92.0 350.0 570.0 620.0 770.0 910.0 940.0 1,020.0

0.25 0 0 0 0 0 0 0 0.17 0.4 0.55 0.62 1 1 1.5 28 118 157 221 277 303 353 336

9 9 6 4 41 116 166 229 293 348 425 480 675 897 1,181 2,288 5,899 9,268 9,759 11,214 11,734 12,275 12,265

198 198 123 72 691 1,842 2,561 3,483 4,424 5,255 6,418 7,248 10,119 13,447 17,833 34,300 89,076 141,472 149,504 172,413 180,407 189,404 189,249

259 258 391 413 375 324 292 256

226 213 187 175 103 9 6 0 0 0 0 0 0 0 0

95 94 137 131 113 94 83 72 63 60 52 51 29 3 2 0 0 0 0 0 0 0 0

HS

Stotal

Fe(II)

Mn

SiO2

Sr

Li

A1

0.6 13.6 14.2 17.5 14.5 15.2 14.2 12.7 14.5

15.6 15.4 15.5 15.7 15.6 16.4 16.2 16.5 16.8 16.8 16.6 16.8 16.8 16.9 17.5 19.4 17.0 19.9 16.6 17.9 17.5 15.2 18.7

0.4 0.2 0.3 0.4 0.3 0.4 0.3 0.3 0.6 0.3 0.2 0.5 0.4 0.6 2.1 123.0 433.0 726.0 847.0 989.0 1,084.0 1,164.0 1,184.0

0.03 0.03 0.05 0.03 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.03 0.25 1.48 9.40 17.40 24.00 24.30 24.50 24.70 25.90 25.70 24.30

241 241 265 230 230 232 235 241 253 265 283 295 313 325 36/) 566 830 994 1,030 1,090 1,110 1,080 1,130

0.20 0.20 0.21 0.21 0.21 0.20 0.22 0.20 0.21 (I.22 0.22 0.23 0.23 (/.24 0.23 0.28 0.46 0.64 0.70 0.82 0,86 0.88 0.92

0.45 0.45 0.45 0.47 0.48 0.49 0.46 0.49 0.44 0.50 0.48 0.50 0.54 0.57 0.62 0.80 1.37 1.71 1.84 1.96 2.02 2.06 1.95

0.23 0.16 0.13 0.10 0.05 0.11 0.08 0.04 0.05 0.05 0.03 0.08 0.08 0.07 0.06 0.06 0.08 0.08 0.05 0.09 0.12 0.13 0.76

G. Michard et al. / Chemical Geology 115 (1994) 103-115

106

4. Discussion

mixolimnion are large e,nough for using crude approximations of mixolimnion concentrations without introduction of large uncertainties for the important fluxes.

4.1. The water balance The water budget derived from estimation of input by rainfall and rivers and of output by evaporation and outflow (Glangeaud, 1916; Meybeck et al., 1975; Martin, 1985) suggested an additional input to the lake by underlacustrine springs. The 5t80-values, different in the monimo- and mixolimnion, indicate a different origin for waters of the two layers and support the hypothesis of existence of underlacustrine springs. A modelling using the distribution of tritium in the lake (P. Hubert et al. in Martin, 1985 ) allows to present a schematic model of the lake. Using a box model and new results on tritium (measurements of 1981 and 1987), Camus et al. ( 1993 ) obtain slightly different values for the different fluxes in the lake (Fig. 1). This model will be used to calculate the fluxes of chemical elements. In September, the upper reservoir in the lake ("mixolimnion") is stratified and oxygen concentration decreases smoothly from the highest productivity depth ( ~ 12 m) to the limit of the mixolimnion (60 m). Mean values of concentrations are considered. The differences between mixo- and monimolimnion concentrations for elements which exhibit variations within the PAVIN LAKE 2074

5184 m+/day MIXOI .l~.~qlON %,~lbcles

>

3110 , MONI~4OI.IMNION

Tvans{ ors :S F~ i !~

112000 ] 4280 / 3165 3 !,0 . __~

(mol. dny -1 )

t4q 930 Na 710 Ca 560 Mn ] O0 . . . .

CI S Li $r 02

(80) (20) 57 25 1370

Fig. l. Water fluxes in m 3 day-~ and particulate element transfer in Pavin Lake.

4.2. Box models and first estimation of Jluxes According to Martin (1985) and Camus et al. (1993 ), the lake is divided into two boxes corresponding to the mixolirnnion and monimolimnion; the water fluxes are given in Fig. 1. The input (24 1 s t) includes rainfall (19 1 s- 1) and fluxes of small rivers (5 1 s- ~). The output ( 60 1s- ~) includes outflow ( 551 s- J ) and evaporation (5 1 s-~ ). Input of chemical elen;tents to the lake occurs by: (1)rainwater; ( 2 ) c h e m i c a l load of rivers; (3) deposition of particulate matter; and (4) chemical load of underlacustrine springs. Fluxes of dissolved elements between the reservoirs can be obtained from the mean concentration of each element in the different boxes. The same is valid for the outflow of the lake: its composition is the same as the composition of the surface layer of the lake. For almost all the elements, the main fluxes are: U, the flux between monimo- and mixolimnion; and S, the outflow. These two fluxes and D, the flux between mixoand monimolimnion, the input by small rivers, R, can be accurately quantitated. Fluxes F put in by underlacustrine springs is unknown and can be derived from the box model, as well as q), flux of particles formed in the mixolimnion and dissolved in the monimolimnion. For box model calculations, P (input by rainwater) and H (amount resulting of dissolution of particulate matter) are necessary. However, these two contributions are minor and a rough estimate is sufficient even for a precise calculation of the unknown quantities in the model. An exception is for silica. The sediment is principally formed of diatom skeletons and the flux of particulate silica is not neghgible in the budget. An analytical survey of rainwater was conducted in an area of similar geology and morphology ~ 50 km south of Pavin Lake. As this contribution is rather small, it has been consid-

G. Michard et al. / Chemical Geology 115 (1994) 103-115

ered as an attempt to represent the chemical input by rain. Martin (1985) gave values of the annual deposition of some trace elements after and before 1850 on the lake floor. Ancient values considered as natural ones are used to estimate deposition of other elements, considering that solid phase is mainly basalt associated with diatom shells ( ~ 80% of the sediment; Martin, 1985). As we shall see, the total amount of particulate matter entering the lake is small and only a small part of it will dissolve and participate to the cycle. Therefore, it will be neglected in the budget, except for silica (see Table 2). From the box model of Fig. 1, residence times in the mixolimnion and monimolimnion are estimated to 8.3 and 3.7 yr, respectively. Chemical analyses from 1975 to 1992 indicate only small variations, with the exception of Cossa et al.'s (1994) results on iron. As all other data are in good agreement, Cossa et al.'s results are not considered. It is therefore reasonable to consider that the lake composition has reached a steady state.

4.2.1. Box model for iron If the lake composition is in steady state, inputs in each box are equal to outputs. The different fluxes of Fe can be estimated from the composition of lake and rivers (Table 2 ): it appears clearly that all fluxes are negligible with respect to U ( 1500 kmol y r - 1, all the others being < 1 kmol yr-~ ). The flux of particulate iron can be estimated at ~ 1 mmol s- t (31 kmol yr- 1), but iron is almost completely insoluble in an oxic environment, and cannot be dissolved in the mixolimnion. Thus, the input to the mixolimnion of ~ 1500 kmol yr- 1can be balanced. It can be done by the precipitation of an equal amount of iron - - the ferrous iron coming from the lower compartment being oxidized by oxygen in the mixolimnion. The iron budget in the lake is achieved by a particulate transfer • from the mixolimnion to the monimolimnion. This is the well-known "iron wheel" (Campbell and Jorgensen, 1980).

107

From the balance in the monimolimnion, the input by underlacustrine springs:

F=U-O-D can be estimated to be 14 pmol s- ~ and the concentration in the spring, 0.4 pmol 1-1. However, these values are highly uncertain because the calculated fluxes are two orders of magnitude lower than the flux of particles and we were unable to estimate the proportion which can be dissolved.

4.2.2. Fluxes for other elements The same method is used for all the elements analyzed in this study to calculate exchanges of dissolved matter betwee,n the different boxes. Results are presented in Table 2. As for iron, budgets for carbon, nitrogen and phosphorus are not balanced in the mixolimnion and steady state implies, that precipitation occurs in this layer. The solid phase formed is essentially living matter; after the death of organisms, a part of this matter is oxidized within the mixolimnion and the other part goes down in the monimolimnion by gravity. A precipitation is also necessary for balancing silica (diatom shell formation), manganese (oxidization in MnO2) and also alkali (Na, K, Li, Rb) and alkaline-earth ions (Mg, Ca, Sr). For the two last groups, the nature of the precipitated phase is unknown. A coupling between redox conditions and the aquatic chemistry, of alkali metals (Na, K, Cs) and alkaline-earth elements (Mg, Ca, Ba) was already observed in lakes (Sholkovitz and Copland, 1982a, b; Evans et al., 1983; Sholkovitz, 1985). Several hypotheses are proposed to explain this coupling: (a) Concomitant release from pore waters. (b) Association of alkalis and alkaline earths with organic matter formed in the epilimnion. (c) Ion exchange between particulate (clay minerals) and dissolved species: K, Cs, Ba and Ca present in the solid phase are exchanged with Fe 2+ and NH + which are very abundant in reduced waters. (d) Adsorption-desorption of alkalis and alkaline earths on ferric and manganese hydroxides.

G. M ichard et al. / Chemical Geology 115 (I 994) 103- 115

108

rable 2 Estimated fluxes between mixo and monimolimnion (see Fig. 13 for symbols) Element

H

P

R

U

S

D

q~

F

m

91 0.006 39 17 7.2 0.8 58 8.5 3.5 0.5

1,565 1,565 666 319 456 326 42,460 87 26 478

417 0.5 376 159 103 103 1,252 78 27 0 434

87 0.1 78 33 21 21 260 16 5.5 0 87

1,153 1,5,64 261 131 339 2,34 41,0'30 31 6 479

325 n.d. 328 154 96 100 1,200 39 15 n.d.

290 n.d. 300 140 87 91 1,100 35 13 n.d.

2 32 79

36,200 1,232 2,898

50 350 750

10 73 158

36,250 923 2,070

n.d. 233 671

n.d.

(kmol yr - j ): Si Fc Na K Mg

('a (' ('1 S P ()~

480 35 5.6 0.6 15 13

0.5

0.1 0.1 8.5 3.4 0.6 2.3 1.1 15 8.5 0.1

(mol yr --~ ): Mn Sr ki

172 31 10

113 85 0

0.21 0.6

m =calculated molality (#mol 1-~ ) of element in underlacustrine springs, n.d. indicates that the values are not significantly different from zero.

(e) Mineral dissolution enhanced by the large amounts of carbon dioxide. In Pavin Lake, the last hypothesis can be ruled out since carbonates are absent and the detrital phase is negligible. Hypothesis (b) is not the major process since C1 is present in biological tissues at about the same molar content as Na and because C1 is not implied in the recycling process observed in Pavin. Hypothesis (a) is not relevant in the present situation. Cation selectivity during ion exchange or adsorption onto various substrates has been discussed by Kinniburgh and Jackson ( 1981 ). Alkali ions exhibit little specific adsorption and can show either a "normal" sequence (Cs> R b > . . . > L i ) or a reverse sequence; alkaline earths may also present both sequences. The selectivity in Pavin Lake can be estimated from the ratio of an element incorporated in the solid phase formed in the mixolimnion (proportional to q~) to its concentration as dissolved species in the mixolimnion. The following sequence is observed, where the values are normalized to sodium:

Mg ( 4 . 5 ) > S r ( 3 . 8 ) ~ L i ( 3 . 7 ) > C a (2.7) >K (1.3)>Na

(1)

The rather low variations are in agreement with the lack of specificity of adsorption of all these ions on oxides, but this argument is not sufficient for ruling out the possibility of exchange with clays. For many elements (except phosphorus, iron and manganese), the calculated flux F brought by underlacustrine springs is higher by an order of magnitude than the annual amount brought by particulate matter. The contribution of dissolution of this particulate phase can be neglected and a value of the concentration of the elements in the underlacustrine springs can be calculated (column m in Table 2 ). The concentrations are similar to those in the rivers and in the mixolimnion. If any thermomineral water, which is common in this area, is present, its contribution to the total of underlacustrine springs will be negligible.

G. M ichard et al. / Chemical Geology 115 (1994) 103- I 15

4.3. Tube models and location o f the reactions

60

Within the monimolimnion, the curves of concentration vs. depth are highly nonlinear. As we assume steady-state conditions, and for a dispersion coefficient independent of depth, the curve must be linear for an element neither produced nor consumed within the layer, unless an advection term is present (Craig, 1969). Especially, temperature, 5180 and C1 (Figs. 2-4), which are conservative tracers, exhibit nonlinear relationships with depth. Therefore, we will consider the advection-diffusion reaction model or "tube model" used by Craig ( 1969 ) for deep seawater. At steady state, the equation of concentration change will be written: .

.

.

.

.

.

.

~7(, 75l

Pavin lake 18 oxygene

]

1~5 I

70 [

~

/

,

75

i

80 ] I

m

i

ssl

{~5

t)

85

%

~

7 5

-7

Fig. 4. 5~80 vs. depth in the monimolimnion (squares indicate the observedvalues, the curve the modelled values).

a2C waC+R=OC=o Oz

Ot

(1)

In this equation, D is the eddy diffusion coefficient (Craig, 1969) which includes molecular diffusion and dispersion (the coefficient is the same for all elements; it can be applied also to the temperature); w is the advection rate, positive if turned toward the top; and R is the chemical reaction term.

Pavin lake chloride

65~

~

E i

D~60

109

1

'45

4.3.1. Conservative elements 45

~II

50

55

cont cntration

65

60

(~M)

Fig. 2. Chloride concentration vs. depth in the monimolimnion (squares indicate the observed values, the curve the modelled values ).

,0

~-..,~

Pavin lake

gll i

[

i ? 5

3 7

39

4.1

4.3

4.5

47

{°c) Fig. 3. Temperature vs. depth in the monimolimnion (squares indicate the observed values, the curve the modelled values ).

For conservative elements R = 0, the solution is: C-Co=

(C m -Co)

exp(z/z*) - 1 exp ( 1/z* ) - 1

(2)

where z*= D/w; I is the thickness of the layer; Co and Cm are the concentrations of the tracer at the bottom ( z = 0 ) and at the top ( z = l ) , respectively, of the monimolimnion. Co and Cm are the boundary conditions equal to the observed values, z* is determined by the fit of the observed curve. The conservative tracers can be temperature, 180 and very soluble elements such as CI. The fit was achieved on the C1 curve and give z*= 5 m. A good fit is obtained with the same z*-value for temperature and 6180 vs. depth curves. The positive value of z* indicates that the advection velocity is from the bottom to the top of the layer, which is consistent with the upflow

I I0

G. Michard et al. / Chemical Geology 1 I5 (1994) 103-115

from monimolimnion to the mixolimnion derived from the tritium results. The fit is also fair for total carbonate, potassium and magnesium curves (Figs. 5-7 ), and sodium and lithium, suggesting that production or consumption of these elements are negligible within the monimolimnion. It is not possible to derive from the model the values of w and D. However, we can estimate these parameters from the flux calculated either from the "tube model" or from the previous box model. As the box model calculations use a mean value close to the value at a depth of 80-85 m and as in this case the diffusive flux is small, the velocity w corresponds to the water flux ( ~ 45 l s -l ) divided by the mean area of the lake be6(PF [

~

Pavin lake

v:'- ~51 .

stum

-

g

'

l

i

~ 71) 7S 80 ~5

5(I

100

150

200

2511

concentration (/*M)

Fig. 5. Total C O 2 concentration vs. depth in the monimolimnion (squares indicate the observed values, the curve the modelled values).

60 - ~s[

~

Pavin lake !

magnesium i =3 711i v5

! l

9/) 0

511

100

150 200 concentration (p.M)

250

300

3511

Fig. 6. Potassium concentration vs. depth in the monimolimnion (squares indicate the observed values, the curve the modelled values).

60

. . . . . Pavin lake

~ a ~ - - - . . .

total dissolved carbonate

75t

()

5

It) c~nccrltrati(~n (him }

15

21)

Fig. 7. Magnesium concentration vs. depth in the monimolimnion (squares indicate the observed values, the curve the modelled values ).

tween 60- and 88-m depth, i.e. 2.2.105 m 2, and equals 1.9.10 - 7 m s-I. From z = D / w , a D-value of 9.5- 10- 3 cm 2 s- l is derived. This value is close to the value derived in a completely independent way (Michard, 1993 ) in the hypolimnion of Aydat Lake situated in the same area as Pavin Lake (Massif Central, France), which is also reduced and has the same chemical composition and the same temperature. Thus the D-value is not unreasonable. As shown previously, the monimolimnion is supplied by underlacustrine springs, by downflow from the mixolimnion, and by dissolution of solids formed within the mixolimnion. The discussion in this section suggests that, in the central part of the lake, only upwards advection occurs and that no water input can be detected at any intermediate depth. We consider that downflow and input of springs occur on the lake periphery. The composition of the bottom water includes therefore elements diffusing from the pore waters, elements present in the upper layer of the lake and elements present in the underlacustrine springs. For all conservative elements, fluxes are the same at the bottom and at the top of the monimolimnion. Particulate matter sinking down from the mixolimnion (organic matter, ferric hydroxide, etc. ) is not dissolved within the monimolimnion, but sedimented and dissolved within the sediment.

G. Michard et al. / Chemical Geology 1 I5 (1994) 103-115

4.3.2. Non-conservative elements In contrast, theoretical curves (with z*= 5 m) do not fit the data for elements such as Fe, Si, Mn, Sr, P or S species. Observed Si, Mn and S2are above the theoretical curve, suggesting that these elements are brought into solution by reactions: dissolution of silica skeletons (diatoms) and of particulate Mn (III )- or Mn ( IV ) -oxides, and by sulfate reduction. Sulfate is removed from the solution by reduction. It is less obvious to indentify the reaction that removes iron from the solution: in the global reaction iron is added to the solution by the reduction of ferric hydroxide as soluble ferrous iron, organic matter being the reduction agent; waters from the monimolimnion are supersaturated with respect to ferrous sulfide, ferrous carbonate and ferrous phosphate (Table 3). Therefore, precipitation of one of these species is possible. The amount of total S in the system is too low to precipitate more than 20 #tool 1- ~Fe, solutions are only slightly oversaturated with siderite. As the concentrations of phosphorus are large, and as P is also lower than predicted by the advection-diffusion model, the formation of vivianite is the most probable.

111

~0, Pavin

•

lake

iron

70

0

75

e'x 5

g[I

II

t)f) . 0

.

. 200

.

. 400

.

. 600

8(X)

I [IIX)

I 200

1 400

c o n c e n t r a t k ) n (txM)

Fig. 8. Iron concentration vs. depth in the monimolimnion (squares indicate the observed values, the curves the modelled values). The index on the curve is the value of R / w (mot m - 4 ) . 60~ Pavin

lake

phosphorus ~ 70t

\4

i 75~

7

~

80i

85 i i

I

4.3.2.1. Constant production A first model could be considered assuming that R is constant. In that case, the solution of Eq. 1 is: z/z*: C=[C m __ C o _ ~ ]

Rz +Co + . - w

exp(z/z*)--I e~p(p( 1 ~ ) - - 1 (3)

which depends on the adjustable quantity R/w. Figs. 8 and 9 show that this model is successful for iron and phosphorus, respectively. It is also Table 3 Saturation indices (log Qs/Ks) for some ferrous compounds ( organic complexes are assumed to be negligible ) FeCO3 FeS Fe3(PO4)2"8HzO

0,5 1.1 5,5

90 I 0

1O0

200

300

400

concentration (gM)

Fig. 9. Phosphorus concentration vs. depth in the monimolimnion (squares indicate the observed values, the curves the modelled values). The index on the curve is the value of R~ w (mol m - 4 ) .

successful for Sr, but it does not explain the rapid change of Si, HS- and Mn close to the upper part of the monimolimnion. The best value of R / w is about - 6 mol m - 4 for iron and - 4 mol m -4 for phosphorus, a ratio which corresponds exactly to the vivianite formula Fe3 ( P O 4 ) 2 . The total amount of vivianite in the lake formed annually is: Q= - (R/w)wlA =2.3 mmol s-l =73 kmol y r - l where A is the mean area of the monimolimnion estimated to 0.22 km 2.

112

G. Michard et al. / Chemical Geology 115 (1994) 103-115

Since there is a consumption within the layer, the fluxes are different at the bottom and at the top of the layer and can be calculated by:

6{I

Pavin

lake

manganese

@ 7o B

'\

dzJ

'nn

75 A

i

"'f{C ='~\

exp(1/z*)

1

°exp(1/z*)-l-Cmexp(1/z*)

)

8o

-1 85

90

.

.

.

0

. I0

.

. 21)

3[)

411

c o n c c w ra tio n {#M)

+(R/w) z-Z*+exp(1/z,)_l

(4)

For instance, the fluxes for Fe and P respectively are 0.225 and 0.067/~mol m -z s-l at the bottom, and 0.2 and 0.049/zmol m -2 s-~ at the top of the layer. Corresponding total amounts are 1560, 464, 1340 and 340 kmol y r - 1. The flux at the top, arriving in the mixolimnion, is obviously less than at the bottom; the difference is the amount precipitated within the monimolimnion.

4.3.2.2. Manganese For Mn, the increase in solution results from the dissolution of manganese dioxide particles sinking down from the mixolimnion. If the dissolution is fast enough, the amount of particles will decrease exponentially and the reaction term can be written:

R=Roexp[ol(z- 1 ) ]

(5)

The solution of Eq. 1 is:

C=Co +[C, - C o -

Ro wol - Da 2

solved at the top of the monimolimnion is 81 kmol yr -I. The Mn flux at the bottom and top is ~ 33 and ~ 114 kmol yr- ~, respectively.

4.3.2.3. Silica Although the same model could be applied to silica, it is not reasonable to do it. An important amount of silica is deposited at the bottom of the lake and unless imagining several kinds of diatoms having different dissolution rates, the hypothesis of the complete dissolution of the particles sinking from the upper layer is obviously wrong. The silica concentration at the bottom of the monimolimnion is close to the equilibrium value with amorphous silica ( 1150/tmol 1- ~ at 5 °C; Fritz, 1981 ). The dissolution reaction will be then considered as reversible and the production term R can be written:

R=k(C~q - C )

]] exp(z/z*) - 1

E l - e x p ( - - ) JeTp(iT)5

Fig. 10. Manganese concentration vs. depth in the monimolimnion (squares indicate the observed values, the curves the modelled values). The curves A, B and C refer to model with no reaction, constant rate reaction and reaction with a limited amount of manganese dioxide, respectively (see text).

(7)

The solution of Eq. 1 is:

1

C=Ceq -~

R

+wol_Da2[exp{ol(z-- 1 ) } - - e x p ( - a ) ]

(6) where a is an adjustable parameter. The fit of the observed curve yields values of 0.7 m - ~ for a and 43 for R / w (Fig. 10). From these values, the total amount of manganese dioxide dis-

Cm--C., exp (rl l) - exp (r2 l)

× [exp(r, z) - e x p ( r 2 z ) ] with

w+x/w2 +4kD rl 2D and

(8)

G. Michard et al. / Chemical Geology 115 (1994) 103- 115

4.3.2.4. Sulfur species

w-~/W~-1-4kD r2 --

113

2D

A similar model was developed by Vanderborght et al. (1977) for organic-rich sediments of the Belgian coast of the North Sea. However, the equilibrium value deduced from the model was ~ 400 #mol 1-~, far below the true equilibrium value. This discrepancy was discussed by Schink et al. (1975): they showed that the asymptotic concentration at great depth was related to bioturbation in the superficial zone. It is obviously not the case here, because: (1) the study concerns free water and not pore water; and (2) the equilibrium value used in our model is close to the true equilibrium value. The best fit is obtained for q = 0 . 2 7 (and r 2 = - 0 . 0 7 ) corresponding to a k-value of 1 . 8 " 1 0 - 2 S - J (Fig. 11). The flux of silica dissolution is 5.5.10-5 mmol m -2 s -l so the total amount for the lake is 360 kmol y r - ~. The flux of dissolved silica across the mixolimnion-monimolimnion boundary is 2.66- 10 -4 mmol m s- t or 1860 kmol y r - I. The flux at the bottom interface is 1500 kmol yr-~. As previously stated, it corresponds to silica coming from pore waters, from underlacustrine springs and from the mixolimnion. 6(I

. . . . . . . Pavin

Since the measurements of sulfide are largely uncertain, the total sulfur ,content can be considered as being approximately constant within the monimolimnion. The reaction is assumed to be a reduction of sulfate (Amblard and Restituito, 1983). It has been established (Froelich et al., 1979) that the oxidants of organic matter are utilized in the decreasing order of their redox potential (Sill6n, 1967): oxygen, nitrate, manganese dioxide, ferric hydroxide and sulfate; then disproportionation of organic C into carbonate and methane occurs. In Pavin Lake, sulfate seems to be reduced at the mixolimnion-monimolimnion interface (Fig. 12), before ferric hydroxide which is reduced in the sediment. The reaction can also be an oxidation of sulfide coming from the bottom of the lake. It occurs close to the redox change, so the oxidation agent can be oxygen. The sulfur can be supplied to the sediment as sulfate by underlacustrine springs and downflow from the mixolimnion, and reduced within the sediment. It will be interesting to test these alternative hypotheses by 34S measurements, or by identification of bacteria (Desulfiwibrio desulfuricans or

thiobacterium ). 4.3.2.5. Modified fluxes This section shows that the estimation of fluxes derived from the box model obtained from the tritium results must be improved by considering

lake

silica

K

Pavin

~o 70;

lake

~2~5i

75

71) i i f i

75;

/ •

i

8[)

t

91) 2(1¢)

40fl

6{}0

8(10

I 000

1 211(I

I 400

c o n c e n t r a t i o n (~zM)

Fig. 11. Silica concentration vs. depth in the m o n i m o l i m n i o n (squares indicate the observed values, the curves the modelled values). The curves A, B and C refer to model with no reaction, constant rate reaction and equilibrated reaction, respectively (see text ).

0

5

I0

15

2(I

25

¢ o n c e n t r a t : o n (b~M)

Fig. 12. Sulfate concentration vs. depth in the monimolimnion (squares indicate the observed values, the curve the modelled values ).

114

G. Michard et al. / Chemical Geology 115 (1994) 103- 115

Table 4 Improved estimation of element fluxes (see Fig. 13 for symbols) P

R

U

S

D

A

qb

F

X

E

91 <0.l n.d. 0.5

1,860 1,325 1,141 320

417 0.5 n.d. n.d.

87 0.1 n.d. n.d.

360 -220 81 147

1,450 1,324 114 321

745 (19) n.d. n.d.

480 50 n.d. n.d.

1,500 1,565 33 465-147

( kmol yr- ~): Si Fe Mn P

0.1 O. 1 n.d. 0.1

n.d. = not determined.

IP R

)

MIXOLIMNI©N

PAVIN LAKE :

°1

SILICA TRANSFERS

D

/n LIMNION ~ A F

)l

~

i

SEDIMENT

~z silica transfers 03 249 5092 1142

D A @ F

238 986 3962 2045

(mol.day 1) Z E

1314 4106 161

Fig. 13. Improved scheme for transfer budget in Pavin Lake,

the fluxes of dissolution-precipitation within the monimolimnion. Fig. 13 results from this improvement and replaces Fig. 2 for all "reactive" elements. The corresponding values of the fluxes are presented in Table 4.

derlying sediment. The main reactions linked with the oxidation of organic matter (methanogenesis, iron (II) production, liberation of alkali and alkaline-earth ions, etc. ) occur within the sediment in the pore waters and the enrichment observed in the monimolimnion results from the diffusion of dissolved species from the sediment. Reactions related to oxidation by oxygen and biological production occur obviously in the mixolimnion. However, some reactions (silica and manganese oxide dissolution, oxido-reduction of the sulfur species, ferrous phosphate precipitation) occur within the monimolimnion and a quantification of their rates is possible. For a better understanding of the chemistry of the lake, numerous studies including chemistry of solid phase and pore waters of the sediment, particulate matter in the lake, and also behaviour of trace elements are still needed. Some of them are planned in the near future.

5. Conclusions Acknowledgements The chemistry of the lower layer of Pavin Lake presents all characteristics previously observed in many other dimictic or meromictic lakes: large enrichment in total carbon, iron, manganese, phosphorus and silica, and also alkali and alkaline-earth ions. The knowledge of the water budget associated with the steady-state assumption for the monimolimnion allows a quantification of the chemical reactions occurring in the lake and in the un-

Christian Amblard and Gilles Bourdier provided important help for sampling and field measurements. Their constructive criticisms on a first draft of the manuscript is also acknowledged. Monique P~pe, Marc l~vrard and Dominique Lavergne helped with the chemical analyses. The English was improved by Sophie Leboulleux.

G. Michard et al. / Chemical Geology I 15 (I 994) 103-115

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D., Hartman, B. and Maynard, V., 1979. Early oxidation of organic matter in pelagic sediments of eastern equatorial Atlantic. Geochim. Cosmochim. Acta, 43: 1075-1090. Glangeaud, P., 1916. Le cratbre lac du Pavin et le volcan de Montchalm. C.R. Acad. Sci. Paris, 162: 428-43t. Kinniburgh, D.G. and Jackson, M.L., 1981. Cation adsorption by hydrous metal oxides and clays. In: M.A. Anderson and A.J. Rubin (Editors), Adsorption of Inorganics at Solid-Liquid Interface. Ann Arbor Sci. Publ., Ann Arbor, Mich., pp. 91 - 160. Martin, J.M., 1985. The Pavin crater lake. In: W. Stumm (Editor), Chemical Processes in Lakes. Wiley, New York, N.Y.,pp. 169-188. Meybeck, M., Martin, J.M. and Olive, Ph., 1975. G6ochimie des eaux et des s6diments de quelques lacs volcaniques du Massif Central. Vehr. Int. Vet. kimnol., 19:1150-1165. Michard, G., 1993. Estimation des flux de mati6re dissoute l'interface eau-sediment. C.R. Acad. Sci. Paris, S6r. II, 317: 783-787. Schink, D.R., Guinasso, N.L. aad Fanning, K.A., 1975. Processes affecting the concentration of silica at the sediment-water interface. J. Geophys. Res., 80:3013-3031. Sholkovitz, E., 1985. Redox related geochemistry' in lakes: alkali metals, alkaline earth elements and Cs. In: W. Stumm (Editor), Chemical Processes in Lakes. Wiley, New York, N.Y., pp. 119-142. Sholkovitz, E. and Copland, D., 1982a. The chemistry of suspended matter in Esthwaite lake, a biologically productive lake with seasonally anoxic hypolimnion. Geochim. Cosmochim, Acta, 46: 393-410. Sholkovitz, E. and Copland, D., 1982b. The major element chemistry of suspended particles in the north basin of Windermere. Geochim. Cosmochim. Acta, 46: 19211930. Sill6n, L.G., 1967. Master variables and activity scales. In: Equilibrium Concepts in Natural Water Systems. Adv. Chem. Set., No. 67, pp. 45-56. Vanderborght, J.P., Wollast, R. and Bitlen, G., 1977. Kinetic models of diagenesis in disturbed sediments, I. Mass transfer and silica diagenesis. Limnol. Oceanogr., 22: 787793.