189−° 196 C and non-condensable gases (CH 4 , N 2 , O 2 , Ar) were
collected with a Toepler pump. The mixture was circulated
on hot CuO at 550 °C for conversion into CO 2 and H 2 O. CO 2
and H 2 O then were then separated from other non-
condensable gas (O 2 , N 2 , Ar) by trapping at −° 196 C. Pure
CO 2 , without NOx, was obtained from the trap at −°140 C. It
was quantified and introduced into a Thermo Fisher Delta V
dual inlet mass spectrometer to measure δ^13 C vs.
PDB. Precision is about ±0.5‰. Trapped water was then
released and introduced into a hot uranium furnace main-
tained at 850 °C to convert quantitatively water-H into H 2. H 2
was then quantified. The yield of H 2 was two times that of
CO 2 within less than 5 %. H 2 was collected with a Toepler
pump and then measured in a Thermo Fisher Delta V dual
inlet mass spectrometer to measure δD vs. SMOW. Precision
is about ±3‰.^14 C analyzes were conducted on AMS
(Accelerator Mass Spectrometer), ARTEMIS program,
LMC14 Saclay.
11.4 Results
11.4.1 Dissolved Inorganic Carbon (DIC)
The vertical profiles (Fig. 11.2) are essentially consistent and
all data is within uncertainty, which is rather low, as pH is at
best obtained with an uncertainty of 0.05 unit. This implies a
10 % error on DIC calculation. For direct DIC measurements
by Assayag et al. ( 2008 ), it is difficult to avoid CO 2 loss as
deep waters degas spontaneously at atmospheric pressure.
DIC increases slightly but steadily in the hypolimnion,
increases sharply with depth within the chemocline, and
continues to increase up to the bottom. The value at 80 m
depth is 14 mmol/L and the total amount in the monimolim-
nion is about 58 10.×^7 moles (corresponding to 2500 t of
CO 2 ).11.4.2 δ^13 Cδ^13 C results on DIC were already presented by Assayag et al.
( 2008 ). The main features are the following (Fig. 11.3): δ^13 C
in epilimnion are seasonally variable and close to 0 ‰. It
decreases steadily with depth in the hypolimnion and reaches
values of about −7 ‰ at 58–59 m depth. It exhibits a strong
increase with depth in the mesolimnion and the value at 68 m
is around +1‰. The increase with depth continues in the
bottom layer and the value at the bottom is between + 3 and
+4‰.
The value in the epilimnion is related to exchange with
atmosphere, photosynthesis and inputs of streams, which
have strongly negative DIC isotopic signatures. Decrease
with depth in the hypolimnion corresponds to organic01020304050607080900510 15DIC (mM)Depth (m)Fig. 11.2 Typical DIC
profile in Lake Pavin
11 Carbon Cycle in a Meromictic Crater Lake: Lake Pavin, France