Hydrological budget, carbon sources and biogeochemical processes in Lac Pavin (France): Constraints from δ18O of water and δ13C of dissolved inorganic carbon
Résumé
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 bottom water layers (monimolimnion, from 60 to 90 m depth) remain isolated and are never mixed. Hence, they are capable of storing important quantities of dissolved gases, mainly CO2. With the aim of better constraining the water balance and of gaining new insights into the carbon cycle of Lac Pavin, an isotopic approach is used. The δ18OH2O profiles lead the authors to give a new evaluation of the evaporation flow rate (8 L s−1), and to propose and characterize two sub-surface springs. The sub-surface spring located at the bottom of the lake can be deduced from the 1% isotopic difference between the upper water layers (mean δ18OH2O value: −7.3‰) and the bottom water layers (δ18OH2O=-8.4‰). It is argued that this sub-surface spring has isotopic and chemical characteristics similar to those of the magmatic CO2-rich spring (i.e. Fontaine Goyon, δ18OH2O=-9.4‰), and we calculate its flow rate of 1.6 L s−1. The second sub-surface spring is located around 45 m depth, with a composition close to those of the water surface streams (δ18OH2O<-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): 1/3 deriving from methanogenesis (δ13CDIC ≈ +7‰) and 2/3 from the magmatic CO2-rich spring (δ13CDIC ≈ −5‰). Above 80 m depth, the variations in DIC concentrations (ranging from 0.5 to 10 mM) and δ13CDIC values (ranging from −6.5‰ to 4.4‰) are partly explained by the usual methanotrophy, organic matter oxidation, photosynthesis and CO2 equilibrium with atmosphere. The unusually high δ13CDIC values in the upper water layers (ranging from −6‰ to 0‰) compared to the expected δ13CDIC values assuming only organic matter oxidation, demonstrate the leakage of 13C-enriched DIC from the bottom water layers of Lac Pavin (δ13CDIC values ranging from −5‰ to 3‰)