Hydrological budget, carbon sources and biogeochemical processes in Lac Pavin (France): Constraints from δ18O of water and δ13C of dissolved inorganic carbon

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 delta O-18(H2O) 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 delta O-18(H2O) Value: -7.3 parts per thousand) and the bottom water layers (delta O-18(H2O) = -8.4 parts per thousand). 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, delta O-18(H2O) = -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 (delta O-18(H2O) < -7 6 parts per thousand). Methane (4 mM) and dissolved inorganic carbon concentrations (approximate to 14 mM) allow the reestimation of the relative DIC contributions in the bottom of the lake (90 m depth): 1/3 deriving from methanogenesis (delta C-13(DIC) approximate to +7%) and 2/3 from the magmatic CO2-rich spring (delta C-13(DIC) approximate to -5 parts per thousand). Above 80 m depth, the variations in DIC concentrations (ranging from 0.5 to 10 mM) and delta C-13(DIC) values (ranging from -6.5 parts per thousand to 4.4 parts per thousand) are partly explained by the usual methanotrophy, organic matter oxidation, photosynthesis and CO2 equilibrium with atmosphere. The unusually high delta C-13(DIC) values in the upper water layers (ranging from -6 parts per thousand to 0 parts per thousand) compared to the expected delta C-13(DIC) values assuming only organic matter oxidation, demonstrate the leakage of 13C-enriched DIC from the bottom water layers of Lac Pavin (delta C-13(DIC) values ranging from -5 parts per thousand to 3 parts per thousand.). (c) 2008 Elsevier Ltd. All rights reserved.