190
matter degradation (OM with δ^13 C≈− 28 ‰). The increase
in the monimolimnion can be associated with input of
deep CO 2 and/or methanogenesis. δ^13 C of deep C can be
derived from measurements in Fontaine Goyon spring
water (−4‰), which is a CO 2 − rich mineral water emerg-
ing at 1.5 km NE from the lake. Such mineral waters are
very common in the French Massif Central and have δ^13 C
in the − 2 to −4‰ range (Demont 1981 ; Matthews et al.
1987 ). Methanogenesis of organic matter at −28‰ yields
equivalent molar amounts of methane and CO 2. δ^13 C of
methane was measured at −63 ‰ in 1987 (Camus et al.
1993 ) and in this work (about −60‰ in the monimolim-
nion). If we consider that acetoclastic methanogenesis is
the main reaction leading to the formation of methane, we
can derive from the isotopic balance that δ^13 C of the pro-
duced CO 2 is about + 7 ‰.
From the δ^13 C value at the bottom (+3.50‰), a simple
calculation will indicate that biogenic CO 2 represents about
70 % of the total CO 2 input.
The δD of CH 4 is about −276‰ (mean value of 2 sam-
ples from 78.3 to 86.5 m depth in Oct. 2006), which
excludes a hydrothermal origin for the methane (cf. e.g.
Whiticar 1999 ).
11.4.3 14 C of DICAs the time scale of lake processes is less than 1 year,^14 C
(half life » 5700 years) is not suitable as a chronometer.
However, it can be useful as a tracer of mixing of “volcanic”
C (^14 C= 0 ) and C from superficial waters in a basaltic area
(^14 C» 100 percent of modern carbon or PMC). Actually,
water from the main tributary of the lake presents a^14 C value
of 105 PMC.
A first value in the monimolimnion in 1998 at 80 m depth
was 36 .. 00 ± 8 PMC (discussed in Assayag et al. 2008 ).
More recently, Olive and Boulègue ( 2004 ) found 31 and 33
PMC in waters from the bottom of the lake. At first glance,
this suggests a proportion of volcanic C of about 65–69 %,
which is in contradistinction with the result derived from the
δ^13 C value.
A recent profile in the lake (June 2007) indicates a mini-
mum of^14 C at 65–68 m depth (Fig. 11.4) corresponding to an
input of “old” carbon. This result suggests a direct input of
volcanic carbon at intermediate and bottom depths whereas
biogenic DIC would be released essentially at the bottom.
An attempt to quantify these inputs is presented in the
Sect. 11.5.3.2.Fig. 11.3 Example of
δ^13 C of DIC profiles in
Lake Pavin (Assayag
et al. 2008 )
D. Jézéquel et al.