Carbon Cycle in a Meromictic Crater Lake: Lake Pavin, France
Résumé
Lake Pavin is a meromictic maar lake located in the French Massif Central. Its maximum depth is 92 m and its mean diameter is 750 m (0.445 km2). Waters below about 60 m are never mixed with overlying waters and are permanently anoxic. DIC (Dissolved Inorganic Carbon) and CH4 concentrations in the deep monimolimnion are 14 and 4 mmol/L respectively. DIC has two origins: biogenic and volcanic.
Data on C species, 13C and 14C isotopes as well as auxiliary data were used to build a carbon mass balance in the lake. Main features of the carbon cycle are:
1. Between 70 and 92 m depth, DIC and methane fluxes coming from the sediment are estimated to 1.5 and 1.0 kmol d−1
respectively.
2. At about 65–70 m depth, a PM14C (percent of modern carbon) minimum suggests the inflow of a mineral water which brought DIC=2.6±0.2 kmol d−1.
3.As suggested by a minimum of δ13C of DIC and very low CH4 concentration at ca. 60 m depth, methane is almost quantitatively transformed mainly into DIC at the redoxcline. The methane escape toward the atmosphere is very low and negligible in the carbon mass balance.
4.The δ13C DIC, DIC vs. O2 plots and some thermal and dissolved oxygen anomalies suggest a fresh water inflow at about 53 m depth; the corresponding input of DIC is estimated to 1.6±0.2 kmol d−1.
5.Between 20 and 50 m depth, mineralization of organic matter produced in the photic zone occurs partially. From a δ13C versus 1/DIC plot, a production of DIC of 0.3±0.2 kmol d−1 can be derived.
6. The maximum of photosynthesis is located within the metalimnion (10–15 m depth). A mean value for the corresponding DIC uptake of 3.5±0.5 kmol d−1 is assumed. During seasonal stratification of the mixolimnion (about April to December), water deeper than 10–12 m are isolated from the atmosphere and superficial inputs as shown by the low PM14C value of 45 % at 10 m depth.
7. In the superficial layer, an input of 1.3±0.2 kmol d−1 of DIC is brought by several small brooks, and an output of 3.0±0.2 kmol d−1
is calculated for the discharge.
8.CO2 exchanges with atmosphere are highly variable seasonally; the lake is strongly under saturated in summer and oversaturated in November. An annual balance is derived from modeling the global lake functioning, leading to a CO2 escape of 4±1 kmol d−1.
Budgets for DIC and alkalinity (alk) are proposed. For this latter, we need to distinguish two kinds of dissolved species:
“conservative” ions (e.g. Na+, K+, Ca2+, Mg2+, Cl−) brought by streams and other water venues “reactive” species (e.g. Fe2+, NH4+, H2PO4−), essentially produced within the sediment and consumed or precipitated at ca. 60 m depth i.e. at the redox interface.
CO2 behaviour is deduced from both DIC and Alk budgets. The dissolved concentration of CO2 in the monimolimnion, associated with those of CH4 and N2, allow assessing the gas outburst natural hazard, which is very low considering the actual conditions.