Hydrological budget, carbon sources and biogeochemical processes in Lac Pavin (France): Constraints...

Hydrological budget, carbon sources and biogeochemical processesin Lac Pavin (France): Constraints from d18O of water and d13Cof dissolved inorganic carbon

N. Assayag a,b,*, D. Jézéquel c, M. Ader a,b, E. Viollier c, G. Michard c, F. Prévot c, P. Agrinier a,ba Laboratoire de Géochimie des Isotopes Stables, Institut de Physique du Globe de Paris, Université Paris 7, UMR CNRS 7154, 2, Place Jussieu,75251 Paris Cedex 05, FrancebCentre de Recherches sur le Stockage Géologique du CO2, Institut de Physique du Globe de Paris, 4, Place Jussieu, 75252 Paris Cedex 05, Francec Laboratoire de Géochimie des Eaux, Institut de Physique du Globe de Paris, Université Paris 7, UMR CNRS 7154, 2, Place Jussieu, 75251 Paris Cedex 05, France

a r t i c l e i n f o

Article history:Available online 22 April 2008

a b s t r a c t

Lac Pavin (French Massif Central) is a permanently stratified lake: the upper water layers(mixolimnion, from 0 to 60 m depth) are affected by seasonal overturns, whereas the bot-tom water layers (monimolimnion, from 60 to 90 m depth) remain isolated and are nevermixed. Hence, they are capable of storing important quantities of dissolved gases, mainlyCO2. With the aim of better constraining the water balance and of gaining new insights intothe carbon cycle of Lac Pavin, an isotopic approach is used. The d18OH2O profiles lead theauthors to give a new evaluation of the evaporation flow rate (8 L s!1), and to proposeand characterize two sub-surface springs. The sub-surface spring located at the bottomof the lake can be deduced from the 1% isotopic difference between the upper water layers(mean d18OH2O value: !7.3‰) and the bottom water layers (d18OH2O " !8:4‰). It is arguedthat this sub-surface spring has isotopic and chemical characteristics similar to those of themagmatic CO2-rich spring (i.e. Fontaine Goyon, d18OH2O " !9:4‰), and we calculate itsflow rate of 1.6 L s!1. The second sub-surface spring is located around 45 m depth, witha composition close to those of the water surface streams (d18OH2O < !7:6‰).Methane (4 mM) and dissolved inorganic carbon concentrations (#14 mM) allow the re-estimation of the relative DIC contributions in the bottom of the lake (90 m depth): 1/3deriving from methanogenesis (d13CDIC # +7‰) and 2/3 from the magmatic CO2-rich spring(d13CDIC # !5‰). Above 80 m depth, the variations in DIC concentrations (ranging from 0.5to 10 mM) and d13CDIC values (ranging from !6.5‰ to 4.4‰) are partly explained by theusual methanotrophy, organic matter oxidation, photosynthesis and CO2 equilibrium withatmosphere. The unusually high d13CDIC values in the upper water layers (ranging from!6‰ to 0‰) compared to the expected d13CDIC values assuming only organic matter oxida-tion, demonstrate the leakage of 13C-enriched DIC from the bottom water layers of LacPavin (d13CDIC values ranging from !5‰ to 3‰).

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1. Introduction

Lac Pavin, a permanently stratified crater lake, is theyoungest volcanic structure exposed in the French MassifCentral (De Goër de Hervé, 1974). Due to its limnologicand geologic setting, its bottom water layer (i.e. monimo-limnion) stores several dissolved gases: CH4, He, H2S andmore particularly about 2000 tons of CO2 (Restituito,

0883-2927/$ - see front matter ! 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.apgeochem.2008.04.015

* Corresponding author. Present address: Department of Earth Sci-ences, University of Cambridge, Downing Street, CB3 0FT, UK. Fax: +441223 333450.

E-mail address: [email protected] (N. Assayag).

Applied Geochemistry 23 (2008) 2800–2816

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1984; Camus et al., 1993; Aeschbach-Hertig et al., 1999,2002; Olive and Boulègue, 2004; Lehours et al., 2005).

Studies of Lac Pavin (particularly the monitoring of thephysico-chemical parameters) were undertaken followingthe lethal gas disasters caused by the sudden release of alarge amount of CO2 stored in the bottom water layers ofLake Monoun and Lake Nyos, in the 1980s (Kling et al.,1989; Sigurdsson et al., 1987; Kusakabe et al., 1989; Sig-valdason, 1989; Giggenbach, 1990; Giggenbach et al.,1991; Sano et al., 1990; Tanyileke et al., 1996). Despitethe fact that Lac Pavin has volcanic structures similar tothose of Cameron lakes, its CO2 outburst risk was assessedto be very low since the total equivalent pressure of dis-solved gases is about 1/3 of the hydrostatic pressure anduntil now, no significant accumulation of CO2 or othergases has been detected in its deep-water layers (Camuset al., 1993; Aeschbach-Hertig et al., 1999, 2002).

Previous works on Lac Pavin have also revealed its po-tential as a case-study for the stratified anoxic systemscontrolled by biogeochemical and hydrological interac-tions (Meybeck et al., 1975; Restituito, 1984; Martin,1985; Camus et al., 1993; Michard et al., 1994; Schmid,1997; Viollier, 1995; Viollier et al., 1997; Aeschbach-Hertiget al., 1999, 2002; Albéric et al., 2000; Olive and Boulègue,2004; Lehours et al., 2005, 2007). Nevertheless, severalpoints regarding the hydrolological budget, the carbonsources and the biogeochemical processes of Lac Pavin re-main to be better constrained.

Although the water level has been stable for severaldecades (M. Poulain, personal communication), the differ-ence between the water inputs and outputs leads to anapparent deficit of about 20 L s!1 (Aeschbach-Hertiget al., 1999, 2002). In order to close the water balance,hydrological models postulate the existence of sub-surfaceinput(s) either in the monimolimnion and/or in the mixo-limnion (Glangeaud, 1916; Meybeck et al., 1975; Martin,1985; Camus et al., 1993; Schmid, 1997; Viollier et al.,1997; Aeschbach-Hertig et al., 1999, 2002; Olive andBoulègue, 2004). It appears important, then, to identifythe location, to constrain the flow rate and the water com-position (in particular for their dissolved gas contents) ofthese sub-surface input(s) since these characteristics con-tribute both to the hydrological and to the Dissolved Inor-ganic Carbon (DIC) budgets of the lake.

So far, the DIC budget has been studied with the objec-tive of quantifying the relative contributions of DIC sourcesin the monimolimnion. Opposite conclusions have beenreached: according to Camus et al. (1993), biogenic CO2

is the dominant carbon source, whereas according toAeschbach-Hertig et al. (1999, 2002) and Olive and Boulè-gue (2004), most of the CO2 is of magmatic origin. This is-sue concerns only one of the aspects of the DIC budget andcannot be assessed without a better description of the bio-geochemical processes within the whole water column.

Thus, the main objectives of this study are to improveunderstanding of the water balance and to bring new in-sights into the carbon biogeochemical processes withinLac Pavin.

First, d18OH2O data will be used in order to provide qual-itative and quantitative views on the hydrodynamics of LacPavin (mixing dynamics, stratification, sub-surface water

input(s), and evaporation) since it has already been shownto be a powerful tool in such contexts (Dincer, 1968; Fon-tes et al., 1979; Krabbenhoft et al., 1990; Ojiambo et al.,2001; Herczeg et al., 2003). Subsequently, the DIC andCH4 concentrations and the d13CDIC will be used to re-eval-uate the relative contributions of DIC sources (magmaticversus biogenic) in the monimolimnion and to follow thebiogeochemical processes (photosynthesis, respiration,methanogenesis, methanotrophy) in the water column ofLac Pavin. It is well known that these tools provide strongconstraints on biogeochemical processes involved in thecarbon cycle of lacustrine systems (Oana and Deevey,1960; Deevey and Stuiver, 1964; Takahashi et al., 1968;Pagé et al., 1984; Quay et al., 1986; Miyajima et al.,1995; Wachniew and Rozanski, 1997; Striegl et al., 2001;Herczeg et al., 2003; Bade et al., 2004; Lehmann et al.,2004; Myrbo and Shapley, 2006).

2. Lake setting, material and methods

2.1. Setting of Lac Pavin

Lac Pavin shows several characteristics that have at-tracted the attention of numerous scientists (in the lastdecades: Olivier, 1952; Alvinerie et al., 1966; Omaly,1968; Pelletier, 1968; Meybeck et al., 1975; Devaux et al.,1983; Amblard and Restituito, 1983; Restituito, 1984,1987; Martin, 1985; Martin et al., 1992; Camus et al.,1993; Cossa et al., 1994; Michard et al., 1994, 2003; Sch-mid, 1997; Viollier, 1995; Viollier et al., 1995a,b, 1997;Aeschbach-Hertig et al., 1999, 2002; Albéric et al., 2000;Olive and Boulègue, 2004; Lehours et al., 2005, 2007;Schettler et al., 2007).

The lake is located in the youngest volcanic area of theFrench Massif Central (De Goër de Hervé, 1974), 35 km SWof the city of Clermont-Ferrand, at the geographic locationN 45"29,7400 E 2"53,2800 (lake center), and at an altitude of1197 m a.s.l. (Fig. 1). It is set in a maar crater, mainly com-posed of basaltic, trachyandesitic, granitic and gneissicrocks, formed about 3500 to 6600 years ago according tovarious authors (Brousse, 1969; Camus et al., 1973, 1993;Meybeck et al., 1975; Guenet, 1986; Juvigné and Gilot,1986; Juvigné and Gewelt, 1987; Juvigné et al., 1988; Gue-net and Reille, 1991; Lavina and Del Rosso-d’Hers, 2006).

One of its major features, initially described by Pelletier(1968), is the presence of a stagnant anoxic deep-waterlayer called the ‘‘monimolimnion”. The stability of thislayer is favored by the hollow shape of the basin: with alake area of 0.445 km2 and a maximum depth of 92 m,the hollow coefficient (Dmax./area0.5) is 0.138 (Delebecque,1898). This value is above the limit value of 0.1 that maylead to meromicticity (Dussart, 1966). Hereafter, the termi-nology of the bottom water layers is revisited, due to re-cent insights into this part of the lake.

The profiles of the physico-chemical parameters (dis-solved O2, specific conductivity and dissolved compounds)evidence a zone with a strong chemical gradient fromabout 60 to 70 m depth (Viollier, 1995; Viollier et al.,1997; Michard et al., 1994, 2003). Thereafter, this layeris named ‘‘mesolimnion”, the depth of the maximum

N. Assayag et al. / Applied Geochemistry 23 (2008) 2800–2816 2801

conductivity gradient being the chemocline. The sharp in-crease in concentration of dissolved compounds withinthe mesolimnion leads to an increase in density of the bot-tom water layers and consequently strengthens the stabil-ity of the physical stratification, despite a temperatureincrease of about 1 "C (Aeschbach-Hertig et al., 2002).

The ‘‘true” monimolimnion located below 70 m depth isstrictly anoxic, enriched in reduced compounds and not af-fected by seasonal vertical mixing. It can be considered atsteady state (Viollier et al., 1997; Aeschbach-Hertig et al.,1999, 2002; Michard et al., 2003).

The overlying waters that represent the mixolimnion(from 0 to 60 m depth) are mainly oxic and affected by sea-sonal vertical mixing. Due to seasonal variation in temper-ature, the mixolimnion waters are expected to overturntwice a year, in the periods from November to Decemberand fromMarch to April. Ice usually covers the lake surfacefrom late December to mid March.

Regular visual observations indicate that the water le-vel of the lake has been stable, within an uncertainty of50 cm, for at least several decades (M. Poulain, personalcommunication). Only water inputs by direct precipita-tion onto the lake surface Qp (18 L s!1) are reasonablywell constrained (Meybeck et al., 1975; Martin, 1985;Aeschbach-Hertig et al., 2002). Water inputs from themain surface streams around the lake (Qr), determinedusing the bucket method, are estimated to be about20 L s!1 (Aeschbach-Hertig et al., 2002). Water outputsby evaporation Qev (7 L s!1) are determined from eddy-correlation (Aeschbach-Hertig et al., 2002). Water out-puts via the surface outlet (flowing into the Couze Pavinriver), determined using an OTT pygmy current meter,vary greatly from about 10 L s!1 to more than 240 L s!1.Tritium data (Alvinerie et al., 1966; Meybeck et al.,1975; Martin, 1985; Camus et al., 1993; Schmid, 1997)enables a mean value (Qout) of about 50 L s!1 to be

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Fig. 1. Bathymetric map (from Delebecque, 1898) and surroundings of Lac Pavin. L and P correspond to the surface streams ‘‘Fontaine du Loup” and ‘‘Sourcedes Prêtres”, respectively (the latter is divided into five distinct arrivals). The outlet of Lac Pavin ‘‘Couze Pavin” is located on the north shore. The FontaineGoyon spring is located 2 km in the NE of Lac Pavin.

2802 N. Assayag et al. / Applied Geochemistry 23 (2008) 2800–2816

inferred, close to the value of 55 L s!1 proposed by Sch-mid (1997).

The difference between water inputs and outputs leadsto a deficit of about 20 L s!1 (Aeschbach-Hertig et al., 1999,2002), which is assumed to be balanced by sub-surfacesprings (Glangeaud, 1916; Meybeck et al., 1975; Martin,1985; Camus et al., 1993; Schmid, 1997; Viollier et al.,1997; Aeschbach-Hertig et al., 1999, 2002; Olive andBoulègue, 2004).

2.2. Sampling periods

Three sampling campaigns were carried out. The firstone, in November 2002, took place just before the fall over-turn; the mixolimnion water column was well stratified, asshown by temperature profiles. The second one, in May2003, occurred a few weeks after the spring overturn; themixolimnionwater columnwas slightly stratified. The thirdone, in July2004, corresponded to an intermediate situation.

2.3. Sampling procedures

The water was sampled near the center of the lakewhere the depth is close to its maximum (92 m), usingan automatic syringe sampler, equipped with a depthgauge. This prototype device was previously described inViollier (1995). Once the water was sampled, a Sartorius#

syringe-filter (0.2 lm pore size with cellulose acetatemembrane) was fitted on the syringe (Norm-Ject 50 mLsyringe); the filtered water was collected in 12 mL glasstubes, filled to the brim and closed with rubber septa (Lab-co Exetainer# tubes), in order to ensure good airtightness.Water samples were stored in the dark and at 4 "C, untilchemical and isotopic analyses were carried out.

A mineral spring called Fontaine Goyon, spouting outabout 2 km NE of the lake, at an elevation of 1085 m a.s.l.(i.e. 20 m below the lake bottom), was chosen as an ana-logue of the sub-surface spring, which is assumed to feedthe bottom of the lake (Fig. 1). This mineral spring wassampled in November 2002, May 2003 and March 2004(Table 1). In addition to water sampling, CO2 degassingfrom this spring was collected in a gas-tight plastic bagplaced over the spring outlet, enabling a glass flask to befilled for further gas analysis. Carbon dioxide was foundto make up almost 100% of the gas phase. The CO2 flux isabout 1.3 L min!1, whereas the water flow is about 5.6 Lmin!1 (determination of March 2004).

Surface streams flowing into the lake (Fontaine du Loup,L, and Source des Prêtres, P, were sampled in July 2004(Fig. 1 and Table 2).

2.4. Field measurements

In situ measurements of parameters relative to biologi-cal activity (dissolved O2, pH) and physical stratification(temperature, specific conductivity) of this aquatic systemwere performed with three different multi-parameterprobes. For the three sampling campaigns, the WTW 196pH–T probe was used for pH and temperature measure-ments. The dissolved O2 and the specific conductivity weremeasured using a YSI 6600 CTD-O2 for the sampling cam-paigns of November 2002 and May 2003, and a SeabirdSeacat Profiler SBE19 for the sampling campaign of July2004. For the YSI and WTW probes, measurements lasted2 min at each selected depth, in order to ensure stablereadings. The Seabird probe was lowered at the recom-mended speed (20 cm s!1). The O2 and pH sensors are sub-ject to long-term drift that requires regular calibrations:the O2 sensor was calibrated against the results of Winklermeasurements on water samples, and the pH sensor wascalibrated with standard buffer solutions. Alkalinity mea-surements were performed by the bromophenol blue col-orimetric method (Podda and Michard, 1994), using aMerck SQ300 spectrophotometer.

2.5. Isotopic and chemical measurements

2.5.1. d13C of the dissolved inorganic carbon (d13CDIC)The analytical protocol has been detailed in a previous

study (Assayag et al., 2006) and is only summarized here.An aliquot of the water sample is injected into a H3PO4-pre-loaded and He-flushed Labco Exetainer tube. Afterthe dissolved CO2 has equilibrated with the headspacegas (for one night), the headspace gases (i.e. He and CO2)are sampled and introduced into a gas chromatography–isotope ratio mass spectrometer (AP2003). The measure-ments of the carbon isotope ratio of the released CO2(g)

are calibrated using CaCO3 standards to yield the d13CDIC

versus the PDB scale. The d13CDIC values are determinedwith a precision (1r) better than ±0.1‰ and ±0.25‰, forDIC concentrations between 1 and 25 mM and lower than1 mM, respectively.

2.5.2. d18O of waterThe d18O of water is measured by equilibrating 1 mL of

the water sample with a gas mixture (i.e. He and CO2).After the water sample has equilibrated with the gas mix-ture (for one night), the headspace gases (i.e. He and CO2)are sampled and introduced into a gas chromatography–isotope ratio mass spectrometer (AP2003). The measure-ments of the oxygen isotope ratio are calibrated using a

Table 1Chemical element measurements (lM): Na, Cl and K for Fontaine Goyon spring (March 2004) and Lac Pavin at 90 m depth (November 2002)

Name C25 (lS/cm) T ("C) pH DIC (mM) d18O (‰) d13C (‰) Na (lM) K (lM) Cl (lM) Na/K Na/Cl

Fontaine Goyon 1038 4.9 4.5 80 !9.4 !4.3 744 286 100a 2.6 7.4Lac Pavin (90 m depth) 435 197 64b 2.2 6.8

DIC concentration and d13CDIC value of the Fontaine Goyon were calculated from the sum of CO2 and d13C collected in the gas phase and in the residualdissolved phase.

a Measurements from Pauwels et al. (1997).b Unpublished results, September 1993.


set of water standards to yield the d18OH2O versus theSMOW scale. The d18OH2O values are determined with aprecision (1r) better than ±0.1‰.

2.5.3. DIC concentrationsDIC concentrations $DIC " %H2CO3& ' %HCO!

3 & ' %CO2!3 &(

of all the water samples are determined using the analyti-cal technique developed by Assayag et al. (2006), and fromalkalinity and in situ pH measurements. In the mixolim-nion, DIC concentrations, determined from the two differ-ent protocols, are in agreement within 15% ofuncertainty, confirming the integrity of DIC concentrationduring the sampling.

For water samples collected in the monimolimnion, gasbubbles are seen to form because the total gas pressure iswell above atmospheric pressure (about 880 mbar for thelake elevation). In this case, only DIC concentrations deter-mined from the alkalinity (which is independent of gaslost) and in situ pH measurements are taken into account.As the absolute accuracy of the pH data is about 0.1 unitsand the presence of H2S in the monimolimnion affectsthe pH precision of the WTW 196 pH–T probe, the uncer-tainty in DIC concentrations is no better than 20%. For rea-sons of consistency, only the DIC concentrations calculatedfrom the alkalinity and in situ pH measurements are dealtwith. It is assumed that the CO2 degassing did not signifi-cantly change the d13CDIC values.

2.5.4. Methane measurementsDissolved gases are extracted from water by expansion

in He-filled Labco Exetainer tube. The gas phase is thensampled with a glass syringe and injected into a FisonsInstruments 800GC gas chromatograph for the quantitativeanalysis of CH4. The measurements of CH4 are calibratedusing gas standard Supelco (4% in volumetric proportion,for CH4). The precision of the measurements is about 5%of the measured value (Table 3).

2.5.5. Chemical element measurementsConcentrations of major and minor cations are mea-

sured by ICP-AES (Optima 3000), and anions by ionic chro-matography (Dionex AS4A), in the water samples fromFontaine Goyon (March 2004) and Lac Pavin (at 90 mdepth; November 2002) (Table 1). Sodium, a conservativeelement used as a mixing tracer, is measured with anAtomic Absorption Spectrometer (Solaar M6 Unicam), inthe water samples from Lac Pavin collected in May 2003.The precision of the measurements is about 5% of the mea-sured value.

3. Results

3.1. Physico-chemical stratification of the water column

Along the depth profiles, the variations in dissolved O2

and pH are expected to be characteristic of biological activ-ity (Fig. 2b and 2c), and the variations in temperature andspecific conductivity to display the physical stratificationof the water column (Fig. 2a and 2d).

The variations with depth in dissolved O2, pH, temper-ature and conductivity make it possible to differentiatethree different layers within the mixolimnion:

(1) The epilimnion corresponds to the surface mixedlayer, with seasonal variations in temperature andthickness. In November 2002 and July 2004, the epi-limnion extended from 0 to 12 m depth and from 0to 3 m depth, with a temperature of about 9.7 and16.5 "C, respectively. In May 2003, the epilimnionwas not yet in place to any significant extent. Thedepth of light penetration ranges between 3.5 and8 m (Meybeck et al., 1975).

(2) The metalimnion is characterized by a steep temper-ature gradient and as a consequence by a verticalturbulent diffusion coefficient Kz(metalimnion) muchlower than that of the epilimnion Kz(epilimnion) (Table4; Aeschbach-Hertig et al., 2002). In November 2002and July 2004, the metalimnion extended from 12 to20 m depth and 3 to 10 m depth, respectively. Thethermocline (i.e. the depth where the temperaturegradient is maximum) was located at a depth ofabout 15 and 8 m, respectively. In May 2003, theupper layer (0–20 m depth) corresponded to thethermal gradient zone (i.e. it could be assimilatedto the metalimnion), the thermocline depth beinglocated at 6 m depth.In the epilimnion and the metalimnion, the increasein dissolved O2 and pH is essentially due to photo-synthetic processes.The peaks observed in the metalimnion, in the dis-solved O2 and pH profiles, for the November 2002and July 2004 periods, are linked to a dynamic con-text. In this zone of the lake, Kz(metalimnion) values are

Table 2Physico-chemical parameters, DIC, d18O and d13CDIC values of the surfacestreams

Name C25

(lS/cm)T("C)

pH DIC(mM)

d18O(‰)

d13C(‰)

Source des Prêtres ‘‘P1” 94.0 12.1 7.7 0.56 !9.1 !13.9Source des Prêtres ‘‘P2” 70.0 6.7 7.6 0.58 !9.2 !17.2Source des Prêtres ‘‘P3” 69.7 6.6 7.6 0.55 !9.2 !17.4Source des Prêtres ‘‘P4” 68.6 6.5 7.8 0.53 !9.2 !18.5Source des Prêtres ‘‘P5” 68.0 7.0 7.8 0.42 !9.1 !16.0Fontaine du Loup ‘‘L” 63.7 5.6 7.6 0.47 !8.9 !16.1

Table 3Methane concentrations (mM) from Camus et al. (1993) (a), Aeschbach-Hertig et al. (1999) (b), Lehours et al. (2005) (c) and this study (d)

Depth(m)

Camus et al.(1993)(a)

September1994(b)

September1996(b)

Lehours et al.(2005)(c)

May2003(d)

58 0.2160 3.4 0.05 0.2262 0.7464 0.8965 1.08 0.97 1.7666 1.2168 2.37 1.9570 2.14 2.57 3.3072 2.5575 2.75 3.50 2.8480 18 3.05 3.35 4.3085 3.41 3.46 4.25 4.1290 13 3.94 4.46 4.02


very low, i.e. transport processes by turbulent dis-persion are relatively reduced. This feature gener-ates a confined environment (i.e. accumulationzone of dissolved O2) where the effects of photosyn-thesis are greatest.

(3) The hypolimnion is a layer where the temperature ispractically stable, close to 4 "C (i.e. the temperaturecorresponding to the maximum water density). Forthe three periods, the hypolimnion extendedapproximately from 20 to 60 m depth. In this zone,

photosynthetic activity decreases due to low lightpenetration, while respiration (i.e. organic matteroxidation) becomes dominant, inducing a decreasein dissolved O2 and pH.A transition zone, the ‘‘mesolimnion”, separates themixolimnion from the monimolimnion.

(4) The mesolimnion extends between about 60 and70 m depth; it is characterized by a strong increasein specific conductivity and by an increase in tem-perature of about 1 "C. Dissolved O2 decreases to

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Fig. 2. Temperature (a), dissolved O2 (b), pH (c) and conductivity (d) profiles. Dissolved O2 and pH are mainly characteristic of biological processes;temperature and conductivity emphasize the physical stratification of the water column.


0.1 mg L!1 at 58.8 m depth (November 2002),61.4 m depth (May 2003) and 62.6 m depth (July2004), emphasizing the transition from oxic toanoxic conditions.

(5) The monimolimnion, below about 70 m depth, is thedeepest part of the lake, where none of the mea-sured physico-chemical parameters vary whateverthe season or the year.

3.2. d18O in the water column

Fig. 3 provides the d18OH2O profiles for the three periods.In the mixolimnion, the d18OH2O values range from

!7.8‰ to !6.8‰; the three d18OH2O profiles are not super-imposed, confirming that this zone is affected by seasonalvariations. For the November 2002 profile, the physicalstratification of the mixolimnion is evidenced by a sharpd18OH2O decrease from !7‰ to !7.7‰, separating the epi-limnion from the hypolimnion. For the other two profiles(May 2003 and July 2004), the contrast in d18OH2O is justbeginning.

In the monimolimnion and the mesolimnion, thed18OH2O profiles are superimposed within uncertainties,with d18OH2O values decreasing sharply downward from!7.7‰ to !8.6‰ in the mesolimnion and then remainingstable around !8.6‰ in the monimolimnion.

As previously reported by Martin (1985), Martin et al.(1992), Camus et al. (1993) and unpublished data (CRGThonon 1970–1986), a d18OH2O shift larger than 1‰ sepa-rates the mixolimnion and the monimolimnion, confirmingthe meromictic character of the lake.

3.3. d13C and concentration of dissolved inorganic carbon inthe water column

Figs. 4a and 4b show the DIC concentrations and d13CDIC

profiles for the three periods.In the mixolimnion, the DIC concentrations range be-

tween 0.4 and 1.5 mM. For each profile, the DIC concentra-tions decrease slightly within the epilimnion and increasesteadily with depth in the hypolimnion. The chemicalstratification of the mixolimnion is clearly revealed bythe d13CDIC profiles, the mixolimnion being divided into

three layers. The epilimnion, from approximately 0 to20 m depth, is characterized by high d13CDIC values rangingfrom !0.3‰ to 3.2‰. The metalimnion has a d13CDIC valuethat rises to 4.4‰ in the summer period. The hypolimnion,below 20 m depth, shows low d13CDIC values ranging from!2.6‰ to !6.5‰.

In the mesolimnion and the monimolimnion, the DICconcentrations and d13CDIC profiles are superimposed with-in uncertainties, underlining once again the fact that thiszone of the lake is not affected by seasonal variations.The d13CDIC and DIC values increase very sharply withdepth in the mesolimnion from !6.5‰ to 2.3‰ and from1.5 to 11 mM, respectively, and more smoothly in the mon-imolimnion from 2.3‰ to 3.2‰ and from 11 to 16 mM,respectively.

In order to investigate the relationship between DIC andd13CDIC, the d13CDIC values are plotted versus the inverse ofDIC concentration in Fig. 5. Two linear trends organized ina V-shape can be identified: the mixolimnion trend and themonimolimnion trend, which are denoted by dotted con-tours. As shown by the arrow in Fig. 5, the data points onthis V-shape are ordered according to their samplingdepth, which increases while 1/DIC decreases.

0

10

20

30

40

50

60

70

80

90

Dept

h (m

)

-8.9 -8.7 -8.5 -8.3 -8.1 -7.9 -7.7 -7.5 -7.3 -7.1 -6.9 -6.7 -6.5

18O (‰)"


Fig. 3. d18O profiles. These profiles highlight the meromictic character ofLac Pavin and the seasonal variations in its mixolimnion.


0

10

20

30

40

50

60

70

80

90

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0DIC (mM)

Dept

h (m

)

(a)

(b)

Fig. 4a. DIC concentrations profiles. The uncertainties in DIC measure-ments are about 15%, for water samples collected in the mixolimnion (a),and about 20%, for water samples collected in the monimolimnion (b)(the error bars are due to the precision of pH data).

0

10

20

30

40

50

60

70

80

90

-8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5

13CDIC (‰)

Dept

h (m

)

"


Fig. 4b. d13CDIC profiles (for analytical reasons (i.e. detection limit), inMay 2003, d13CDIC measurements for the first 10 m are not available).


The mixolimnion trend extends from a low DIC concen-tration (1/DIC # 2.2) and high d13CDIC (#+4‰) endmemberto an intermediate DIC concentration (1/DIC # 1.7) and lowd13CDIC (#!6‰) endmember; it has an intercept atd13C # !12‰. On this trend, the July 2004 and May 2003data are shifted towards the low DIC concentration ! highd13CDIC trend side, whereas the November 2002 data areshifted towards the high DIC concentration ! low d13CDIC

trend side.The monimolimnion trend extends from a high DIC con-

centration (1/DIC # 0.5) and low d13CDIC (#!6‰) endmem-ber to a very high DIC concentration (1/DIC # 0.03) andhigh d13CDIC (#+3‰) endmember, at the bottom of the lake.Note that this trend shows a small but significant curvature.

The mixoliminion–monimolimnion transition zonedefines a variable secondary linear trend (dotted lines)for the November 2002 (between 40 and 58 m depth)and May 2003 profiles (between 56 and 62 m depth),which connects the arms of the two major linear trends.No such secondary trend is seen for the July 2004 profile.

3.4. Methane in the water column

Methane measurements from this study and those fromthe literature are reported in Table 3. Above 58 m depth,CH4 concentration is not detected. Between 58 and 80 mdepth, CH4 concentrations increase with depth, from 0.21to 4.46 mmol L!1, and remain almost constant below80 m depth (around 4 mmol L!1). The results are in goodagreement with those from Aeschbach-Hertig et al.(1999) and Lehours et al. (2005). The higher CH4 concen-trations reported by Camus et al. (1993) are likely to bedue to differences in the extraction protocols and samplingimperfections.

3.5. Fontaine Goyon and surface streams

The Fontaine Goyon has a mean d18OH2O value of !9.4‰,very close to the previous values reported by Pauwels et al.(1997) (!9.3‰) and by Olive and Boulègue (2004)

(!9.5‰). This d18OH2O value falls within the narrowd18OH2O range (!9.5 ± 0.5‰) of the CO2-rich spring locatedin the vicinity of Lac Pavin (Olive and Boulègue, 2004) andelsewhere in the French Massif Central (Matthews et al.,1987; Pauwels et al., 1997). The DIC value of 80 mM is con-sistent with the previous measurement by Olive andBoulègue (2004). The d13CDIC value of !4.9‰ is character-istic of magmatic CO2 in the French Massif Central area(Matthews et al., 1987) and consistent with that of theother local CO2-rich springs (Table 1).

Around the lake, up to 16 surface streams were listed,some of which are non-permanent. The main water inputscorrespond to the five surface streams named ‘‘Sources desPrêtres” located on the SE shore. The Sources des Prêtres,with an inflow value ranging between 15 and 20 L s!1,have d18OH2O values around !9.1‰ which fall within thed18OH2O range of local precipitation (!8.5 ± 2.4‰; Buéno-Roméro, 1969). The d13CDIC values range from !18.5‰ to!13.9‰ and DIC values are around 0.5 mM. The ‘‘Fontainedu Loup” surface stream, with an inflow value ranging be-tween 2 and 4 L s!1, has a d18OH2O value of !8.8‰, and ad13CDIC value of !16.1‰ (Table 2).

4. Discussion

4.1. Hydrodynamics and water balance of the lake

The d18O is used here to provide qualitative and quanti-tative views of the hydrodynamics and the water balanceof Lac Pavin (mixing dynamics, stratification, water inputsand evaporation). In particular, the nature and the flowrate of the sub-surface springs (located within the mixo-limnion and the monimolimnion), which are assumed tocompensate for the apparent deficit of the water balance,will be constrained in the following sections.

4.1.1. Mixing in the mixolimnionFor the 3 sampling periods (November 2002, May 2003

and July 2004), the weighted average d18OH2O value of themixolimnion yields identical values within uncertainties:!7.32 ± 0.05‰, !7.38 ± 0.08‰ and !7.27 ± 0.07‰, respec-tively (the bathymetric data between 0 and 60 m depth arefrom Delebecque, 1898). The similarity of the mean d18OH2O

values for November 2002 and May 2003, and the relativeconstancy of the d18O profile of May 2003 compared to thestratified d18OH2O profile of November 2002 (Fig. 3), showthat the water column of the mixolimnion (the top 60 m)was fully mixed during the winter–spring 2002–2003 per-iod. For the period from 1994 to 1996, Aeschbach-Hertiget al. (2002) suggested that the seasonal overturn mighthomogenize only the top 30 m. Hence, the complete mix-ing of the mixolimnion would not be achieved every year.The amplitude of the mixing could be limited in depth,probably depending on climatic parameters such as wind,air temperature, ice cover, etc., that may change fromone year to another.

4.1.2. Sub-surface spring in the mixolimnionFrom temperature, 3H and CFC-12 profiles, Aeschbach-

Hertig et al. (2002) underlined the existence of a sub-sur-

-12

-10-8

-6

-4-2

0

24

6

0 0.2 0.4 0.6 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6

July 04May 03November 02Orga

nicmatt

er

oxida

tion

Photo

synth

esis

13C

DIC

(‰)

"

1/DIC (mM )-1

s

Mixolimnion trend

Monimolimnion trend

Increasing depth

0.8 1

Fig. 5. d13CDIC versus 1/DIC diagram. Dashed contours underline thelinear trends of the mixolimnion and the monimolimnion. (Line equa-tions: y = 6x ! 11‰ for the mixolimnion, y = !16x + 3.4‰ for the mon-imolimnion). The organic matter oxidation–photosynthesis ‘‘OMO–P line”represents the theoretical locus of data points, if organic matter oxidationand photosynthesis process were the only active processes.


face spring in the mixolimnion, between 40 and 50 mdepth (Q45). Aeschbach-Hertig et al. (2002) and Viollieret al. (1997) suggested that this sub-surface spring wouldbe of surface water type, at least from the major ions andFe concentrations.

This sub-surface spring cannot be identified directlyfrom the d18OH2O profiles established in the center of thelake since none of them displays any d18OH2O anomaly be-tween 40 and 50 m depth. However, seasonal variations inthe d18OH2O profiles allow a d18O constraint to be deducedon this water input. Due to evaporation processes, the epi-limnetic d18OH2O values should increase every year andconsequently, the mean d18OH2O value of the mixolimnionshould also increase every year. As the mean d18OH2O val-ues of the mixolimnion remain unchanged betweenNovember 2002 and July 2004, the 18O enrichment of theepilimnion water has to be compensated by 18O-depletedinputs. These low 18O inputs are also evidenced by the de-crease of the hypolimnetic d18OH2O values between May2003 and July 2004 (Fig. 3).

These features lead to the conclusion that a sub-surfacespring located in the hypolimnion should supply waterwith a d18OH2O signature lower than about !7.6‰, the low-est d18OH2O value in the hypolimnion. The nature of thissub-surface spring remains compatible with that of surfacestreams, for which d18OH2O values range between !8.9‰and !9.2‰ (Table 2).

The turbulent water exchange between the monimo-limnion and the mixolimnion may also contribute to de-crease hypolimnetic d18OH2O values but not significantly,since as demonstrated below, its contribution (about1 L s!1) is too small compared to that of the sub-surfacespring (Q45 # 20 L s!1).

4.1.3. Sub-surface spring in the monimolimnionAs determined in the previous section, the mixolimnion

has a mean d18OH2O value of !7.3‰, whereas the monimo-limnion has a mean d18OH2O value of !8.6‰. This d18OH2O

difference of over 1‰, already noted by Martin (1985),Martin et al. (1992), Camus et al. (1993) and unpublisheddata (CRG Thonon 1970–1986), has been maintained fora period of more than 30 years. The most likely way ofmaintaining such an isotopic contrast is to envisage awater input, in the bottom of the lake, with a d18O valuelower than !8.6‰.

Geographically, a likely candidate would be the Fon-taine Goyon, situated 20 m in altitude below the bottomof the lake. Such Fontaine Goyon-type water, with ad18OH2O value close to !9.4‰ (Table 1), could mix withthe monimolimnion water. This Fontaine Goyon-typewater inflow should be small; otherwise the d18OH2O valueof the monimolimnion would have reached a value close to!9.4‰.

This inference is also supported by comparing the ratioof conservative tracers (87Sr/86Sr, Na/K, Na/Cl) in the bot-tom water layers of Lac Pavin with those of the FontaineGoyon water. Like the d18O values, the difference in87Sr/86Sr ratio between the monimolimnion (0.70685 ±0.00001) and the mixolimnion (0.70607 ± 0.00001) is alsoconsistent with a Fontaine Goyon-type water input in themonimolimnion since its 87Sr/86Sr ratio is 0.71230 (data

from A. Michard, and reported in Viollier, 1995). Moreover,the Na/K, Na/Cl ratios are very similar in the monimolim-nion waters (at 90 m depth) and in the Fontaine Goyonwater (Table 1).

The oxygen isotope balance can provide an evaluationof this Fontaine Goyon-type water flow rate: at steadystate, in order to maintain the d18OH2O contrast betweenthe mixolimnion and the monimolimnion, the followingrelationship must hold:

Q90$d18OFontaine Goyon ! d18Omonimo(

" AKz

e$d18Omonimo ! d18Omixo( $1(

where AKze is the turbulent water exchange term between

the monimolimnion and the mixolimnion. Assuming thevertical turbulent diffusion coefficient, Kz(mesolimnion) =(4.5 ± 2) ) 10!8 m2 s!1 (Aeschbach-Hertig et al., 2002),the area of the mixolimnion-monimolimnion interface,A = (2.0 ± 0.1) ) 105 m2, the thickness of the mixolim-nion–monimolimnion interface (i.e. mesolimnionthickness), e = 9 ± 1 m; d18OFontaineGoyon = !9.4 ± 0.1‰,d18Omonimo = !8.6 ± 0.1‰ and d18Omixo = !7.3 ± 0.1‰; thecalculation yields a Fontaine Goyon-type water flow rate,Q90, of about 1.6 ± 0.6 L s!1, the uncertainty being deter-mined by the Monte-Carlo method.

4.1.4. EvaporationIn the epilimnion, the d18OH2O variations may be due to

water inputs by surface streams, precipitation and wateroutputs by evaporation. Water inputs by surface streams(d18OH2O # !9 ± 0.1‰) and precipitation (d18OH2O #!8.5 ± 2.4‰; Buéno-Roméro, 1969) cannot be responsiblefor the d18O increase in the epilimnion since they haved18OH2O values lower than the epilimnetic d18OH2O values(d18OH2O # !7‰). Evaporation can only be responsible forthe d18OH2O increase in the epilimnion, since during sucha process 16O goes preferentially into the vapor phase,enriching the residual liquid phase in 18O.

In November 2002, the epilimnion waters were signifi-cantly enriched in 18O compared to those of the hypolim-nion, recording the cumulative effects of evaporationover the spring to fall 2002 period. In May 2003, the con-trast in d18OH2O between the epilimnion and the hypolim-nion was just beginning. The evaporation was still weakat this time of the year, and only occurred for a short per-iod after the spring overturn. In July 2004, the d18OH2O pro-file was halfway between those of November 2002 andMay 2003, highlighting the progress in the physical strati-fication of the lake (Fig. 3).

An evaluation of the minimum evaporation flow ratemay be deduced from the oxygen isotope balance:

Qpd18Op ' Q rd

18Or ' Q45d18O45

' $AKz=e($d18Omonimo ! d18Omixo( ' Q90d18Omonimo

" Qoutd18Omixo ' Qev$d18Omixo ! D$L—V(( $2(

Using the following values: d18Op = !8.5 ± 0.5‰ (Buéno-Roméro, 1969), d18Or = !8.5 ± 0.5‰, d18O45 = !8.5 ± 0.5‰,d18Omixo = !7.3 ± 0.1‰ (mean d18O value), d18Omonimo =!8.6 ± 0.1‰, Qp = 18 ± 2 L s!1 (Aeschbach-Hertig et al.


(2002)), Qr = 20 ± 2 L s!1 (Aeschbach-Hertig et al. (2002)),Q45 = 18.4 ± 0.6 L s!1, Q90 = 1.6 ± 0.4 L s!1, Qout = 50 ±4 L s!1, AKz/e = 1 ± 0.5 L s!1, D(L–V) # 10 ± 1‰ (Majoube,1971), the oxygen isotope balance yields an evaluation of8 ± 3.5 L s!1.

This value is in good agreement with previous estima-tions from Martin (1985) and Aeschbach-Hertig et al.(2002), who proposed an evaporation flow rate ranging be-tween 6 and 7 L s!1, based on hydrological methods (eddycorrelation).

4.1.5. Model of the water balanceTo describe the water balance of Lac Pavin, two main

models have been proposed, based on different parametersets (Table 4).

The first model from Martin (1985) and Camus et al.(1993) (Table 4) postulates the existence of a mineral-ized spring discharging into the monimolimnion, repre-senting the total sub-surface water inputs. However,the estimated water flow rate for this sub-surface spring

and the value of the vertical turbulent diffusion coeffi-cient Kz, in the hypolimnion are too high, and cannot ex-plain the He profile in the monimolimnion (Aeschbach-Hertig et al., 1999, 2002). Moreover, such a high deep in-flow should imply a high Fe(II) flux from the monimo-limnion towards the mixolimnion and, as the dissolvedFe(II) concentration is very low in this aerobic part ofthe lake, it should imply a high Fe(III) flux of settlingparticles. Viollier et al. (1997) showed from sedimenttrap recordings that the Fe(III) settling flux data allowthe hypothesis of a main spring in the monimolimnionto be discarded.

Conversely, the second model proposes a main waterinput in the mixolimnion, a null (Aeschbach-Hertig et al.,2002) (Table 4) or small (Viollier et al., 1997) water inputin the monimolimnion and a more realistic value for thevertical turbulent diffusion coefficient Kz in the hypolim-nion. This model yields better fits with the He profiles,and a lower Fe(III) settling flux $* 0:365 mol m!2 a!1(,although still superior by a factor of 2 compared to themeasured flux by Viollier et al. (1997).

In this study, the d18O data supports the presence of awater input in the monimolimnion. In order to check ifthe sub-surface spring input is compatible with the hydro-logical model of Aeschbach-Hertig et al. (2002), their mod-el was updated by including a Fontaine Goyon-type waterinput in the monimolimnion, with a water flow rate Q90

ranging from 0 to 10 L s!1. The fitting of this updatedmodel to the He profile data from Aeschbach-Hertig et al.(1999, 2002), performed with the Aquasim programfrom Reichert (1994), shows that for a water flow rateranging from 0 to 2 L s!1, the adjusted Kz(hypolimnion) andKz(mesolimnion) values still remain realistic and comparableto those of Aeschbach-Hertig et al. (2002). The range of thiswater flow rate (0–2 L s!1) remains consistent with theFontaine Goyon type-water flow rate evaluated in thisstudy (1.6 L s!1).

Fig. 6 summarizes the hydrological budget of Lac Pavin,with the water inputs: Qr, Qp, Q45, Q90, and water outputs:Qev, Qout, and their respective water flow rates and d18Ovalues. Accordingly, the water residence times are around

MESOLIMNION+

MONIMOLIMNION

MIXOLIMNION

AK z/e:

" O: -8.6

" O: -9.4

Qr : 20 L.s-1

Qp : 18 L.s -1

Qe: 8 + 3 L.s-1

Q90 : 1.6 + 0.4 L.s-1

Q45 : 18.4 L.s-1

18

18

" O: -9.218Qout: 50 L.s-1

" O< -7.718

‰

‰

‰

‰

(a)(b)

(b)

(b)(b)

1L.s-1

Fig. 6. Summary figure recapitulating the hydrological budget of Lac Pavin, with the water flow rates and d18O values, corresponding to input and outputwaters. (a) Data from Buéno-Roméro (1969). (b). Data from Aeschbach-Hertig et al. (2002).

Table 4Parameter set for the hydrological models of Lac Pavin

Martin(1985)

Camus et al.(1993)

Aeschbach-Hertiget al. (2002)

Qp (L s!1) 19 18 18Qr (L s!1) 34 5 20Qev (L s!1) 0 0 20Q45 (L s!1) 0 0 20Q90 (L s!1) 40 35 0Kepi (m2/day) – – 864Kmeta (m2/day) 7.8 86 0.43Khypo (m2/day) 173 17.3 0.78Kmeso (m2/day) 0.0052 0.0069 0.0045Kmoni (m2/day) 2.6 8.6 0.052

According to Martin (1985), Camus et al. (1993) and Aeschbach-Hertiget al. (2002): water flow rates from precipitation (Qp), surface streams(Qr), evaporation (Qev), sub-surface springs located in the mixolimnion(Q45) and in the monimolimnion (Q90). K: vertical turbulent diffusivitycoefficient in the following water layers: epilimnion (epi: 0–10 m),metalimnion (meta: 10–20 m), hypolimnion (hypo: 20–60 m), mesolim-nion (meso: 60–70 m) and monimolimnion (moni: 70–90 m).


10 years in the mixolimnion and 70 years in themonimolimnion.

4.2. Processes and sources controlling the DIC concentrationsand d13CDIC

The d13CDIC and DIC concentrations are used here toprovide new insights into the main processes which affectthe DIC and its d13C in the different compartments of LacPavin: monimolimnion, mesolimnion and mixolimnion.In order to illustrate how a process might modify the DICand its d13C, in a diagram of d13CDIC versus 1/DIC (Fig. 7),the trajectory of DIC removals (2) photosynthesis and DICadditions (3) organic matter oxidation, (4) CH4 oxidation,(5) magmatic CO2 influx and (6) methanogenic CO2 influx,for a water mass in isotopic equilibrium with the atmo-sphere (1) have been represented.

4.2.1. Characterization of the DIC sources at the bottom of themonimolimnion

The 14C measurements of the DIC of the monimolim-nion, from Saliège (personal communication, 1998) and Ol-ive and Boulègue (2004), indicate that the percentages ofmodern carbon (pmc) are 36.0 ± 0.8% (80 m depth) and,31.7 ± 0.6% (75 m depth), 32.9 ± 0.4% (90 m depth), respec-tively. These values are far below the modern 14C activityand reveal the input of ‘‘dead” carbon of magmatic origin.

As argued in the previous section, a Fontaine Goyon-type water input in the monimolimnion is required tomaintain the monimolimnion–mixolimnion d18O contrast.This water input should carry some DIC (80 mM, if a simi-lar DIC concentration as the Fontaine Goyon is assumed)with a d13CDIC value of !4.9‰ (Table 1). Such a contribu-tion of magmatic CO2 is consistent with the mantle He in-put identified by Aeschbach-Hertig et al. (1999, 2002).From the measured DIC concentration of the Fontaine Go-yon (80 mM), the magmatic CO2 flux is about180 ± 66 ton yr!1, i.e. higher than the 70 ± 28 ton yr!1 esti-mated by Aeschbach-Hertig et al. (1999, 2002).

However, it might be questioned whether the FontaineGoyon-type water input provides a DIC content of 80 mMor less. Unlike conservative tracers (d18O, Na, etc.), DIC ishighly reactive and its content may vary from one springto another, depending on their water-rock interactions be-fore they reach the surface. Modeling the DIC transport be-tween the monimolimnion and the mixolimnion wouldgreatly help to estimate the magmatic carbon input tothe monimolimnion.

Besides the magmatic carbon input, DIC produced bybiogenic processes contributes to the DIC budget in themonimolimnion. The monimolimnion receives particulateorganic carbon (POC) settling from the mixolimnion withan annual mean flux of about 35 g m!2 yr!1 (Viollieret al., 1997). At the bottom of the monimolimnion and inthe sediments, this POC undergoes a subsequent anaerobicbacterial mineralization by methanogenesis, generatingmainly CO2 (i.e. DIC) and CH4 (Lehours et al., 2005). A min-or amount of DIC may also be generated by organic matteroxidation (mediated mainly by Fe oxyhydroxides and to aminor extent by Mn oxides, present in low concentrationsin the monimolimnion (Camus et al., 1993)).

The C isotopic compositions of CH4 (d13CCH4) and meth-anogenic CO2 (d13CmethanogenicCO2 ) have not been measuredhere. Nevertheless, from a carbon isotope balance, usingtheir measured d13C value of CH4 (!63.2 ± 2‰, at 90 mdepth, a value within the range of CH4 of bacterial origin;Whiticar, 1999) and that of the settling organic matter(d13Corganicmatter = !28.3 ± 2‰), Camus et al. (1993) evalu-ated the d13C of the DIC generated by acetate fermentation:d13CmethanogenicCO2 # +7 ± 3‰. This value is in agreementwith the range of known d13CmethanogenicCO2 values reportedelsewhere (Oana and Deevey, 1960; Myrbo and Shapley,2006).

4.2.2. Relative DIC contributions at the bottom of themonimolimnion

In the absence of other significant sources below 80 mdepth, the DIC results from the contributions of the meth-anogenic 13C-enriched CO2 and the magmatic CO2 input.The relative contributions of these DIC sources have al-ready been evaluated (Camus et al., 1993; Aeschbach-Her-tig et al., 1999, 2002; Olive and Boulègue, 2004) but theystill remain a matter for debate. In this section, new evalu-ations are proposed based on the present CH4, DIC data setand compare them to the previous ones.

4.2.2.1. The [CH4]/[DIC] molar ratio. Provided that the pro-duction of CH4 can be related to that of the methanogenicCO2, the [CH4]/[DIC] molar ratio can be used to deduce therelative proportions of the methanogenic and magmaticCO2. Methanogenesis can mostly take place via two meth-anogenic pathways: acetate fermentation (i) and CO2

reduction (ii) (Whiticar et al., 1986; Whiticar, 1999):

CH3COOH ! CH4 ' CO2 $i(

CO2 ' 4H2 ! CH4 ' 2H2O $ii(

From molecular characterization of microbial commu-nities, Lehours et al. (2005, 2007) showed that an Archaeacommunity affiliated with Methanosaeta species uses

-65-55-45-35-25-15-55

1525

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Photosynthesis (2)Organic matter oxidation (3)

Magmatic CO2 addition (5)Methanogenic CO2 addition (6)

13C

DIC

(‰)

"

1/DIC (mM )-1

(3)

(4) Methane oxidation (4)

(6)(5)

(2)

(1)

Water in equilibrium with atmospheric CO2 (1)

Fig. 7. This diagram displays the path of the main processes involvingDIC. The point (1), set as a fixed endmember, represents water in isotopicequilibrium with atmospheric CO2. The path specific to each processtends towards the d13C of the added DIC sources (symbolized by greysquares on the y-axis): (2) photosynthesis, (3) organic matter oxidation,(4) methane oxidation, (5) magmatic CO2 addition and (6) methanogenicCO2 influx.


acetate as a metabolite to generate CH4 and CO2 within LacPavin. The relationship between the abundance of Methan-osaeta and CH4 leads them to conclude that acetate fer-mentation is the dominant methanogenic pathway,without excluding the presence of the other methanogenicpathway.

Assuming that acetate fermentation is the main metha-nogenic pathway, the [CH4]/[CO2]methanogenic ratio would be1:1 (i.e. [CH4] = [CO2]). Since the [CH4]/[DIC] ratio is 4/14(at 90 m depth), the relative contributions of these CO2

sources are therefore XmethanogenicCO2 = 30 ± 8% andXmagmaticCO2 = 70 ± 8%. The large uncertainties in these pro-portions, estimated via Monte-Carlo simulation, are dueto the uncertainties in the DIC concentrations (which areat least 20%).

From their [CH4]/[DIC] ratio, Aeschbach-Hertig et al.(1999) determined a contribution of methanogenic CO2

of about 22 ± 5% and from their 3He/4He and C/3He ratios,they estimated a contribution of magmatic CO2 of about78 (±40%), in agreement within uncertainties, with thepresent determinations.

Note that the relative DIC contributions (75% of metha-nogenic CO2 versus 25% of magmatic CO2), estimated usingthe [CH4]/[DIC] ratio from Camus et al. (1993), are not con-sidered here because of the large discrepancy betweentheir CH4 concentrations and those reported by othersauthors including the present ones (Table 3).

4.2.2.2. The 13C/12C isotope balance. From the C isotope bal-ance (14C, DIC), Olive and Boulègue (2004) determinedthree different DIC sources: 23% from methanogenic CO2,10% from groundwater input with a low DIC content and67% from magmatic CO2. Since the groundwater has a car-bon isotopic signature of biogenic origin (d13CDIC # !17‰),the biogenic contribution can then be reevaluated to:23 + 10 = 33%. Although their estimations remain consis-tent with the present one, Olive and Boulègue (2004) as-sumed a percentage modern carbon of 100% for thebiogenic CO2 contribution; they did not take into accounta possible recycling of carbon in the whole water column.In fact, the phytoplankton would incorporate DIC with a14C level resulting from atmospheric CO2 (100 pmc) andCO2 diffusing upwards from the monimolimnion(<100 pmc). The organic matter settling into the monimo-limnion (i.e. sedimentation of phytoplankton) would carrycarbon with 14C lower than 100 pmc. Hence, the mean per-centage of modern carbon (33 pmc) only yields the mini-mum proportion of biogenic carbon (1/3) and themaximum proportion of magmatic CO2 (2/3). Additional14C measurements both in the mixolimnion and for phyto-plankton are required to estimate the true proportions ofbiogenic carbon versus that of magmatic carbon.

Note that the relative DIC contributions may also be esti-mated from the 13C/12C isotope balance between the mag-matic and the methanogenic CO2 sources, using the dataset from this study (d13CDIC = +3 ± 1‰, at 90 m depth;d13CDICFontaineGoyon # !5 ± 1‰) and a previous oned13CmethanogenicCO2 # 7 ± 3‰, computedbyCamus et al., 1993):

d13CDIC " XmagmaticCO2d13CmagmaticCO2

' XmethanogenicCO2d13CmethanogenicCO2

$3(

with

XmagmaticCO2 ' XmethanogenicCO2" 1 $4(

However, this 13C/12C isotope balance yields a poorly con-strained value for the XmagmaticCO2 (#33 ± 39%) and theXmethanogenicCO2 (#67 ± 39%). In the present state of knowl-edge, this approach is not reliable enough to be taken intoaccount and presents two main sources of difficulties. First,a sensitivity analysis of the carbon isotope balance, by thetotal differential approach (Anderson, 1976), shows thatmost of the uncertainty in the DIC proportions comes fromthat of the d13CmethanogenicCO2 value (#87%), (the remainingsources of uncertainty deriving from those of the d13CDIC

(12%) and d13CmagmaticCO2 (1%) values). Since thed13CmethanogenicCO2 value relies on a single d13CCH4 data pointand a single d13Corganic matter data point (Camus et al., 1993),a more complete set of measurements (d13CCH4 andd13Corganicmatter) is necessary to improve its precision. Sec-ondly, the calculation of the d13CmethanogenicCO2 uses thed13Corganicmatter assuming that the carbon isotopic composi-tion of the organic matter reflects that of the acetate(which is converted into CH4). To the authors’ knowledge,this assumption has not yet been verified in Lac Pavin.

4.2.3. Physico-chemical and biogeochemical processes in themesolimnion

Methane produced at the bottom of the monimolim-nion or/and in the sediments, diffuses upwards into themesolimnion where it is consumed.

This feature is illustrated in Fig. 8, which plots CH4con-centration versus dissolved Na concentration (used as aconservative tracer), between 58 and 90 m depth, forMay 2003 data. The data are located under the mixing line,showing that part of the CH4 is consumed in the anoxicwater layers, at two distinct depths: some meters belowthe mixolimnion–mesolimnion interface (63 m depth),and around the mesolimnion–monimolimnion limit(71 m depth). In Lac Pavin, CH4 consumption is likely tobe mediated by anaerobic bacteria, at the expense of Feoxides (Lehours et al., 2005). Such a metabolism is calledanaerobic methanotrophy (i.e. bacterial anaerobic CH4 oxi-dation). Only qualitative information about the existence

67.5m65m

62.5m60m58m

85m90m

80m

75m70m

00.5

11.5

22.5

33.5

44.5

5

0.2 0.25 0.3 0.35 0.4 0.45 0.5Na (mM)

CH4

(mM

)

Fig. 8. Methane concentration versus Na concentration. Data for May2003 sampling campaign. The error bars are about 5% of the measuredvalue.


and location of anaerobic methane oxidation (AMO) can betackled in this study, but CH4 oxidation by direct reactionwith dissolved O2 cannot be excluded.

From an isotopic point of view, methanotrophy is asso-ciated with carbon isotope fractionation that enriches theresidual CH4 in 13C, while the produced CO2 is 13C depleted.As the carbon isotope fractionation between CH4 and CO2

ranges from 1.002 to 1.014 (Barker and Fritz, 1981; Cole-man et al., 1981; Whiticar and Faber, 1986; Alperin et al.,1988), this process yields DIC with d13CDIC values rangingfrom !77‰ to !65‰ (assuming an initial d13CCH4 valueof !63.2‰) and consequently leads to a substantial d13Cdecrease (Fig. 7).

Fig. 5 confirms the methanotrophy: the small curvature(concave-down form) of the monimolimnion trend may beexplained by addition of 13C-depleted DIC provided by CH4

oxidation.

4.2.4. Biogeochemical and physico-chemical processes in themixolimnion

In the mixolimnion, the DIC and its d13C are affected byseveral processes: (1) CO2 exchange with the atmosphere,(2) photosynthetic activity, (3) organic matter oxidation,(4) DIC influx from the monimolimnion, (5) methanotro-phy, (6) surface streams and sub-surface spring DIC inputs.

(1) CO2 exchange with the atmosphere. The surfacewater of Lac Pavin was CO2 supersaturated (1.45) in May2003 or CO2 under-saturated (0.40) in July 2004, indicatingthat CO2 is potentially pumped from or degassed into theatmosphere. For the three sampling periods, the d13CDIC

values at the surface water of the lake (ranging between0‰ and 1‰) are close to the calculated d13CDIC value atequilibrium with atmospheric CO2 (#0‰, assuming a car-bon isotope fractionation between CO2(g) and HCO!

3 of8.5‰ and d13CatmosphericCO2 # !7.8‰, Mook et al., 1974).The constancy of this d13CDIC value attests that the CO2 ex-change with the atmosphere may contribute to the DICbalance of Lac Pavin.

(2) The photosynthetic activity results in a drawdown ofthe DIC by consumption of dissolved CO2. The preferential12C uptake by organisms with chlorophyll (i.e. phytoplank-ton or algae) induces a relative 13C enrichment of the resid-ual DIC (Atekwana and Krishnamurthy, 1998; Helie et al.,2002) (Fig. 7). This process is pronounced in the epilim-nion, particularly for the July 2004 profile (Figs. 4a, 4band 5). The d13CDIC positive peaks are associated with thepH and dissolved O2 increase in the metalimnion (July2004 profile) and evidence photosynthesis effects in a con-fined zone (Fig. 2b and 2c).

(3) The organic matter oxidation occurs with little or nocarbon isotopic fractionation (Fritz et al., 1978; Petersonand Fry, 1989; Barker and Fritz, 1981). The 13C-depletedCO2 (d13C # !25‰) is incorporated into the DIC pool, thusleading to increase the DIC and decrease the d13CDIC (Fig. 7).This process mainly occurs in the hypolimnion (Figs. 4a, 4band 5).

The depth organization of the d13CDIC profiles may beexplained by the magnitude of biological activity (organicmatter oxidation and photosynthesis), which is modulatedby the seasons. During the spring and summer seasons(such as May 2003 and July 2004), light intensity increases

and consequently the photosynthetic activity becomesmore pronounced. In the epilimnion, the DIC concentra-tions decrease and d13CDIC values increase are then moresignificant than those observed during the fall and winterperiods (November 2002). The accumulated organic mat-ter, during previous spring and summer periods, sinks intothe hypolimnion where it gets partially oxidized into DIC.Therefore, in the hypolimnion, the DIC concentration in-creases and d13CDIC value decreases are more significantfor the profile of November 2002 than those observed forMay 2003 and July 2004 (Figs. 4a and 4b). The two yearlyoverturns (in November–December and March–April) tendto erase the chemical stratification in d13CDIC which isdeveloped by photosynthesis and the organic matter oxi-dation processes between the fall and spring periods. Inter-annual climatic variability, via seasonal mean light,temperature, wind, etc., also controls the magnitude ofphotosynthetic activity. Note that a monthly sampling ofthe water column would help to separate seasonal frominterannual forcing.

(4) DIC influx from the monimolimnion. Assuming thatorganic matter oxidation is the only process occurring inthe lower part of the mixolimnion (i.e. hypolimnion), theintercept line of the mixolimnion trend should be closeto !28‰ (the d13C value of the settling organic matter, Ca-mus et al., 1993). However, this intercept line, close to!11.9‰, does not match the d13C value expected for theorganic matter (#!28‰) and, the data points of the mixo-limnion trend are above the ‘‘organic matter oxidation–photosynthesis (OMO–P) line” (Fig. 5). This gap betweenthe ‘‘OMO–P” line and the mixolimnion trend can be ex-plained by addition of 13C-enriched carbon.

Such a feature has already been described in previousstudies (Oana and Deevey, 1960; Myrbo and Shapley,2006). They concluded that the change of hypolimneticd13CDIC value in several lakes (for example for Connecticut,Minnesota and Montana lakes), was not due solely to POCremineralization but also included a carbon source moreenriched in 13C. They postulated that this source could beCaCO3 dissolution or the addition of 13C-enriched DIC pro-duced by fermentation processes in the anoxic sedimentsin the bottom water layers. Within Lac Pavin, the wateris undersaturated with respect to carbonate minerals, sothe contribution of a 13C-enriched source is due to the13C-enriched DIC diffusing upwards from the monimolim-nion. For each sample of the mixolimnion, the amount ofthe added 13C-enriched DIC can be estimated from carbonchemical (5) and isotope balances (6):

DICmixo ! DICmixoref ' DICmonimo $5(

DICmixod13Cmixo " DICmixorefd13Cmixoref ' DICmonimod13Cmonimo

$6(where DICmixo and d13Cmixo are the measured values in themixolimnion, DICmonimo and d13Cmonimo values refer to theamount of the added 13C-enriched DIC in the mixolimnionand DICmixoref and d13Cmixoref are the theoretical values ifonly organic matter oxidation and photosynthesis pro-cesses were occurring. These theoretical data points wouldlie exactly on the ‘‘organic matter–photosynthesis line”which can be modeled by the following equation (Fig. 5):


d13Cmixoref " $14:2=DICmixoref( ! 28 $7(

The slope of the line joining the data points (1/DICmixoref,d13Cmixoref) and (1/DICmonimo, d13Cmonimo) is written:

c " $d13Cmixoref ! d13Cmonimo(=$1=DICmixoref ! 1=DICmonimo(" $d13Cmixo ! d13Cmonimo(=$1=DICmixo ! 1=DICmonimo( $8(

Combining these four Eqs. (5)–(8), we can determine theamount of added 13C-enriched DIC in any sample of themixolimnion:

DICadded " DICmixo ! DICmixoref

" DICmixo ! $c ! 14:2(=$c=DICmonimo ! d13Cmonimo ! 28( $9(

Assuming that this 13C-enriched source is located at thebottom of the monimolimnion (90 m depth), usingd13Cmonimo # 3‰ and DICmonimo # 14 mM, an amount wascalculated of added DIC in the mixolimnion, decreasingfrom about 3 mM (i.e. 60% of the measured DIC, at62.5 m depth) to less than 0.26 mM (i.e. 6% of the mea-sured DIC, above 20 m depth). Note also that these calcu-lated amounts remain practically unchanged, assuming a13C-enriched DIC source located at the top of the mesolim-non (i.e. around 60 m depth): d13Cmonimo # !5‰ andDICmonimo # 1.5 mM. This lack of sensitivity is due to thelarge DIC concentration contrast between the mixolimnionand the monimolimnion. Fig. 9 displays the profile of thecalculated amount of added DIC (from the monimolim-nion) in the mixolimnion.

This DIC influx from the monimolimnion may take placeaccording to two hypotheses: by CO2-rich bubbles comingdirectly from the bottom of the lake or by diffusion. Theneed for a better constraint of the transport of this DIC in-flux is a point that should be underlined.

(5) Methanotrophy. In Fig. 4b, May 2003 and July 2004profiles display a d13CDIC negative peak at the mixolim-nion–mesolimnion interface (between 56 and 60 m depth).This feature can be explained by CH4 oxidation. Due to alow turbulent vertical diffusion coefficient (Kz(mesolimnion) #0.0045 m2/day; Table 4), this interface constitutes a con-fined zone where reaction products accumulate. (Note that

0

10

20

30

40

50

60

70

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 32.2 2.4 2.6 2.8

Amount of added DIC from the monimolimnion tothe mixolimnion (mM)

Dept

h (m

)

November 2002May 2003July 2004

Fig. 9. Amount of added DIC from the monimolimnion to the mixolim-nion versus depth.

90

80

70

60

50

40

30

20

10

0

Atmosphere

Magmatic CO2 (Q90)

Epilimnion

Hypolimnion

Mixolim

nion

Mesolimnion

Dept

h (m

)

CH4

POC

AMO

OMO

MET

DIC

Monimolimnion

Sediments

P

CH4DIC

Subsurfacespring (Q45)

Surfacestreams (Qr)

DIC

DIC

Fig. 10. The biogeochemical processes in Lac Pavin [P: photosynthesis; OMO: organic matter oxidation; MET: methanogenesis; AMO: anaerobic methaneoxidation; the DIC inputs via the sub-surface springs (Q45 and Q90) and the surface streams (Qr)]. The arrows represent the main fluxes of C (particulateorganic carbon, CH4, DIC), mainly the DIC influx from the monimolimnion into the mixolimnion and a CH4 possible leakage into the atmosphere.


this confined zone is also evidenced in Fig. 5 by the pres-ence of the secondary linear trend at the mixolimnion–monimolimnion interface.)

Above this interface, the CO2 produced by the CH4 oxi-dation may lead to an additional upwards DIC transfer,which decreases the d13CDIC values of the mixolimnion.The amounts of added DIC from the monimolimnion calcu-lated above are therefore minimum values.

(6) Surface streams and sub-surface spring DIC inputs.About 10!2 mol s!1 of DIC with low d13CDIC values(!16.5 ± 1.5‰) are added by the surface streams. A similaramount of DIC, 10!2 mol s!1, may be added by the sub-sur-face spring Q45, assuming that the DIC content of this sub-surface spring is similar to that of the surface streams.

Fig. 10 summarizes the carbon biogeochemical pro-cesses that change the DIC and its d13CDIC, in Lac Pavin.

4.3. Specificity of Lac Pavin

The weighted average d13CDIC value of Lac Pavin (#0‰)is higher compared to those previously reported for otherlakes (Oana and Deevey, 1960; Quay et al., 1986; Kusakabeet al., 1989; Giggenbach, 1990; Sano et al., 1990; Tanyilekeet al., 1996; Miyajima et al., 1995; Wachniew and Rozan-ski, 1997; Herczeg et al., 2003; Bade et al., 2004; Myrboand Shapley, 2006). In most lakes, the low weighted aver-age d13CDIC values (ranging approximately between !30‰and !20‰, Quay et al., 1986; Miyajima et al., 1995; Badeet al., 2004) are explained by the dominant contributionof DIC produced by organic matter oxidation. In hard-water lakes, which contain carbonated sediments, theweighted average d13CDIC values are higher (rangingapproximately between !2‰ and 2‰, Oana and Deevey,1960; Wachniew and Rozanski, 1997; Herczeg et al.,2003), due to the contribution of carbonate dissolution tothe DIC. Finally, for lakes (especially for Lake Nyos andMonoum) for which the magmatic CO2 inputs were shownto dominate the DIC, the weighted average d13CDIC valuesranged between !3‰ and !5.5‰ (Kusakabe et al., 1989;Giggenbach, 1990; Sano et al., 1990; Tanyileke et al., 1996).

The high weighted average d13CDIC value (#0‰) of LacPavin, above that of magmatic CO2 (#!5‰), indicates thedominant contribution of methanogenic 13C-enriched DICand implies that the oxidation of the 13C-depleted CH4 isincomplete. The rate of CH4 oxidation is probably lowerthan that of production; this leads to the supposition thata certain amount of CH4 may escape upwards to the mixo-limnion or may accumulate within the monimolimnion.Since the CH4 concentrations remained stable, within themonimolimnion, over the last 10 years (i.e. no CH4 accu-mulation), a CH4 leakage upwards to the mixolimnionand into the atmosphere must probably occurs.

5. Conclusions

This new data set (CH4, DIC, d13CDIC, d18O) collectedalong depth profiles, during three different periods, en-ables better constraint on the hydrodynamics and addsnew insights into the carbon biogeochemical cycle of LacPavin.

Regarding the hydrodynamics, d18O proved to be apowerful tool: (i) to confirm the meromictic character ofthe lake, (ii) to evaluate the evaporation term of the waterbalance and (iii) to specify the nature of the two sub-surface springs located in the mixolimnion and in themonimolimnion. The main water input, located in themixolimnion (Q45), shows compositions close to those ofsurface water streams. The secondary water input, locatedin the monimolimnion (Q90), shows similar isotopic andchemical characteristics to those of the Fontaine Goyonspring. Its flow rate was estimated to be of about1.6 L s!1.

Regarding the carbon cycle, the d13CDIC, DIC and CH4

profiles allowed evaluation of the relative contributionsof the CO2 sources (magmatic versus biogenic) and identi-fication of the various biogeochemical processes takingplace in the water column.

In the monimolimnion, the DIC results from magmaticCO2 provided by a Fontaine Goyon-type water input (forabout 2/3) and from methanogenesis most likely via ace-tate fermentation (for about 1/3). These relative contribu-tions agree with previous determinations by Aeschbach-Hertig et al. (1999, 2002) and Olive and Boulègue (2004),but suffer from large uncertainties, showing that the pres-ent state of knowledge must be improved, particularlywith regard to the methanogenic pathway.

In the mesolimnion and the mixolimnion, the followingexpected processes were identified: CO2 exchange with theatmosphere, photosynthesis, organic matter oxidation andmethanotrophy.

The most interesting features of this study are thedetection of a DIC leakage from the monimolimnion intothe mixolimnion, and the lower rate of CH4 oxidationcompared to that of CH4 production, potentially sug-gesting a leak of CH4 into the atmosphere (underinvestigation).

As Lac Pavin can also be viewed as a natural analogue ofa geological reservoir subjected to natural CO2 injection,this study shows that the d13CDIC–DIC tool turns out to beefficient for monitoring the contributions of the CO2

sources and sinks. In particular, it is shown that thed13CDIC–DIC tool can be used to detect CO2 leakage be-tween geological permeable layers. This feature appearspromising for the monitoring of experimental sites sub-jected to artificial CO2 injection for geological sequestra-tion purposes.

Acknowledgements

We are grateful to Michel Girard, for the maintenance ofthe AP2003 mass spectrometer. We particularly thankMarc Evrard, Dominique Lavergne for chemical analysisand Nicole Vassard, for help during this study. A specialthank you to Anne Catherine Lehours for her commentson microbiology. We are grateful to the two anonymousreviewers for their constructive comments and to Dr. Mar-tin Novak for the handling of the manuscript. This studywas supported by the Centre de Recherches sur le StockageGéologique du CO2; Institut de Physique du Globe de Paris-Total-Schlumberger ADEME partnership.


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Hydrological budget, carbon sources and biogeochemical processes in Lac Pavin (France): Constraints...

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