19111.4.4 Methane
Presence of methane in the deep waters of the lake was
known for a long time, but the first precise measurements are
those from Schmid ( 1997 ). In the mesolimnion, a large
increase with depth is observed, from about 0 mmolL−^1 at
60 m to 15 .mmolL−^1 at 68 m. In the bottom layer, a slower
increase is observed: from less than 2 mmolL−^1 at 70 m to
more than 4 mmolL−^1 at 87 m. The pattern is more scattered
near the sediment interface. Thus, an amount of more than
107 moles of methane, i.e. 160 tons, is stored in the bottom of
the lake. Analysis between June 2006 (Fig. 11.5) and July
2007 (not shown) agree well with the previous published
data (Aeschbach-Hertig et al. 1999 ).
In the mixolimnion, except near the surface, the CH 4 con-
tent is lower than 5 μmolL−^1 and a minimum value close to
the detection limit ( 3 nmolL−^1 ) was observed at 45 m depth
in June 2006.2090807060Depth (m)^5040302010030 40 50 60(^14) C PMC
70 80 90 100
June 2007
July 2009
Fig. 11.4 14 C of DIC values
(Percentage of Modern Carbon,
PMC) vs. depth, in June 2007 and
July 2009. The minimum PMC
value ca. 65 m depth is interpreted
as a signature of a mineral water
type inflow, which bring old
carbon. PMC is abnormally low in
the uppers layers (43.8 % at 10 m
depth), reflecting exchanges
between monimo- and
mixolimnion. Plankton growing in
surface waters is more or less^14 C
depleted depending on the grow
depth
-2.5 -1.5 -0.5 0.5 1.52.5 3. 54 .5
0
10
20
30
40
50
60
70
80
90
0246810121416
Depth (m)
Methane (log [CH 4 ] μM)
O2 (mg/L)
log CH4 μM
DissolvedOxygen (mg/L)
O 2 (mg/L)
log(CH 4 μmol L-1)
-2.5 -1.5 -0.5 0.5 1.5 2.5 3.5 4.5
Fig. 11.5 Methane (log
[CH 4 ] in μM) and
dissolved oxygen
(mgL−^1 ) profiles in
Lake Pavin (June 2006).
The dashed horizontal
grey line corresponds to
the depth of maximum
of conductivity gradient
(d(C25)/dz in
μScm m−−^11 ), i.e. the
chemocline depth,
occurring at 62 m in
June 2006
11 Carbon Cycle in a Meromictic Crater Lake: Lake Pavin, France
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