201about 3 bars at 80 m depth. This is significantly lower than
the hydrostatic pressure and for the present time spontaneous
gas outburst cannot occur in the water column.
It is difficult to estimate the gas exchange at the air-water
interface. It needs a continuous monitoring which is not
available in Lake Pavin up to now. Nevertheless, CO 2 and
CH 4 fluxes were estimated by modeling the whole water col-
umn functioning, as presented in Lopes et al. ( 2011 ). A mean
value of 9 mmol..md−−^21 and ~. 210 × −−^32 mmolmd.−^1 escap-
ing from the lake were derived from the model for CO 2 and
CH 4 respectively. The average CO 2 efflux is then lower than
the global average of 16 mmolm..−−^21 d for freshwaters lakes
(Cole et al. 1994 ). Efflux of CH 4 is very low in Lake Pavin,
regarding the high methane concentration in the monimolim-
nion, but comparable to other meromictic lakes (Borges et al.
2011 ). This fact is explained by an efficient bacterial oxida-
tion at the permanent redoxcline in the water column that
virtually remove all the dissolved CH 4 (Lopes et al. 2011 ).
DIC present in the Lake Pavin has two main origins:
- The DIC resulting of the biochemical processes (OM
mineralization) occurring in the water column and in the
sediment. - The DIC issuing from volcanic influence, brought to the
lake by a mineral water input
A mineral water inflow has been located at 68 ± 2 m depth
(expressed as the depth of influence on water column at the
center of the lake) and the amount of deep originated C is
about 1.7 time the amount of biogenic carbon mineralized as
DIC in the monimolimnion.
Lake Pavin is geochemically studied for about 40 years.
No significant change in its chemical composition has been
detected. This confirms the assumption a steady state of the
monimolimnion where the estimated residence time of dis-
solved carbon species is 40 years. The risk of a spontaneous
gas outburst in the monimolimnion will be thus considered
as negligible for some decades, except if an important land-
slide occurs on the lake shore (Chapron et al. 2010 ) or if a
large amount of gas rich sediment lying on steep slope within
the lake is destabilised. The contribution of gas rich sediment
should be precised in the future as a possible natural hazard.
Acknowledgements A part of this work was supported by the Metanox
program (ANR 2006–2008, PI. G. Fonty) and EC2CO programs
(Methanolac 2008–2009 & Interlac 2009–2010, PI. G. Fonty). Authors
thank the Artemis program and facilities for^14 C determinations. A big
thank you also to Mr and Ms Joubert and their team for their hospitality
at Lake Pavin, as well as to Mairie de Besse for their help. Finally, the
authors thank Alberto Borges for his comments that helped to improve
the manuscript.
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