Lake Pavin History, geology, biogeochemistry, and sedimentology of a deep meromictic maar lake

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The lake is located in the youngest volcanic area of the
French Massif Central (De Goër de Hervé 1974 ), 35 km SW
of the city of Clermont-Ferrand, at the geographic location N
45 °29, 7400 E 2°53, 2800 (lake center), and at an altitude of
1197 m a.s.l. It is set in a maar crater, mainly composed of
basaltic, trachyandesitic, granitic and gneissic rocks, formed
from 3450 to 7000 years cal BP (calibrated before present)
according to different authors from^14 C datations ( 3450 ± 110
year BP in Brousse et al. 1969 ; 6600 year BP in Brousse
1969 ; 3450 year BP in Camus et al. 1973 ; 5800 year BP in
Guenet 1986 ; 5800–5900 year BP in Juvigné and Gilot 1986 ;
5990 ± 140 year BP in Juvigné and Gewelt 1987 ; 6000 year
BP in Juvigné et al. 1988 ; Gewelt and Juvigné 1988 ; Guenet
and Reille 1991 ; 6700 ± 110 year cal BP in Boivin et al.
2009 ; 6090 ± 40 year BP i.e. 6970 ± 60 year cal BP in
Chapron et al. 2010 ). Youngest ages seem to be discarded
due artifacts (Delibrias 1979 ; Gewelt and Juvigné 1988 ) and
a consensus would give an age close to 6900 years cal BP.
One of its major features, initially described by Pelletier
( 1968 ), is the presence of a stagnant anoxic deep-water layer
called the “monimolimnion”. The stability of this layer is
favoured by the hollow shape of the basin: with a lake area of
0.445 km^2 and a maximum depth (Dmax.) of 92 m, the aspect
ratio (Dmax./area0.5) is 0.138 (Delebecque 1898 ). This value is
above the limit value of 0.1 that may lead to meromixis (Dussart
1966 ). Hereafter, the terminology of the bottom water layers is
revisited, due to recent insights into this part of the lake.
The profiles of the physico-chemical parameters (dissolved
O 2 , conductivity and dissolved compounds) evidence a zone
with a strong chemical gradient from about 60 to 70 m depth
(Viollier 1995 ; Viollier et al. 1997 ; Michard et al. 1994 , 2003 ).
Thereafter, this layer is named “mesolimnion”, the depth of
the maximum conductivity gradient being the chemocline.
The sharp increase in concentration of dissolved compounds
within the mesolimnion leads to an increase in density of the

bottom water layers and consequently strengthens the stability

of the physical stratification, despite a temperature increase of

about 1 °C (Aeschbach-Hertig et al. 2002 ).

The bottom layer located below 70 m depth is strictly

anoxic, enriched in reduced compounds and not affected by

seasonal vertical mixing. It can be considered at steady state

(Viollier et al. 1997 ; Aeschbach-Hertig et al. 1999 , 2002 ;

Michard et al. 2003 ).

The overlying waters that represent the mixolimnion

(from 0 to ca. 60 m depth) are mainly oxic and affected by

seasonal vertical mixings. Due to seasonal variation in tem-

perature, the mixolimnion waters are expected to overturn

twice a year, in the periods from November to December and

from March to April. However, as pointed by Restituito

( 1987 ), the autumn overturn is much less intense than in the

spring, and may differ depending on the year from 60 to

70 m depth. According to Aeschbach-Hertig et al. ( 2002 ),

some years, the complete seasonal mixing may only concern

the first 30 m.

Ice usually covers the lake surface from late December to

mid March or mid April, inhibiting the water column mixing

during this period.

Two layers with very low dispersivity (Kz) are located

around the metalimnion and the mesolimnion (Aeschbach-

Hertig et al. 2002 ; Bonhomme 2008 ; Bonhomme et al.

2011 ): the first one is only present during the warm period

and situated around 10 m depth, the second one being perma-

nent at about 60–62 m depth.

In the following discussions, the lake will be divided into

five layers (cf. Fig. 11.1):

• The epilimnion between 0 and 5–10 m depth


• The metalimnion, corresponding to the temperature gra-
  dient zone around the thermocline, is present from ca.
  April to November

rainfall

1550

streams

2160

53 m spring

2170/2570

68 m mineral spring

120

evaporation 300/ 700

outlet

5700

monimolimnion

mesolimnion

mixolimnion

Flow

?

bottom layer

epilimnion

metalimnion

hypolimnion

1.3/5.5

volume

2.6/5.2

11.5

1.85

2.73

³ 20

0.1

0.78

0.004

0.05

Kz

4.6

19.7

60 m

Fig. 11.1 Water balance of the lake
In bold within rectangles: flow (md^31 .−);
Within diamond: Kmz(.^21 d−); Within
ellipse: volume (10^6 m^3 ). The dashed arrow
with question mark may correspond to
unrevealed subterranean output, for which the
model is unable to give a value. The
calculated inflow of the sublacustrine input at
53 m depth corresponds in fact to the
difference between this inflow and the
putative hidden output

11 Carbon Cycle in a Meromictic Crater Lake: Lake Pavin, France

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