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

217

with corresponding solubility product of ~10−36 at 5 °C
(Al-Borno and Tomson 1994 ), the temperature of Lac Pavin
deep waters. Another sink of the dissolved Fe pool to the
sediment is driven by the precipitation of pyrite (Fig. 12.6).
The mechanism of pyrite formation in Lac Pavin remains
unclear but it is likely that pyrite (Fe(II)S(0)S(−II)) or its pre-
cursor (e.g. mackinawite, FeS) is formed just below the
redox boundary between 60 and 65 m depth, where sulfate is
reduced into sulfide, and sulfide reacts with dissolved Fe(II)
(Fig. 12.9; Bura-Nakic et al. 2009 ). This hypothesis was pro-
posed to explain the strong similarity between Fe isotope
compositions of sedimentary pyrites and dissolved Fe(II) at
the redox boundary (Busigny et al. 2014 ). The solubility of
pyrite in water at ambient temperature is very low. Although
various reaction pathways can be considered (e.g. Butler
et al. 2005 ), the equilibrium solubility product is for instance
10 −7.2 for the reaction (Rickard and Luther 2007 ):

Fe aq HSaq SFeSsaH q

2

2

0

2 2

++++ +

() ()  () ()^

The last authigenic Fe-bearing solid phase, which has been

identified in Lac Pavin sediments, is siderite (FeCO 3 ;

Schettler et al. 2007 ). Siderite precipitation can be written as:

Fe aq CO aq FeCOs

2

3

2

3

+−+→

() () ()^

Siderite solubility product (Ksp) is strongly dependent on

temperature and varies over five orders of magnitude from

25 to 250 °C (Bénézeth et al. 2009 ). The siderite Ksp is not

well-known for temperature of ~5 °C but is near 10−10, thus

indicating a very low solubility at high Fe(II) concentration

such as those measured in Lac Pavin. Although siderite may

precipitate during early diagenetic process, at the sediment

water interface, when Fe(III) is reduced to Fe(II) and highly

reactive organic matter is oxidized to CO 2 by anaerobic

Benthic Fe(II)aq and Paq flux

Fe(II)

aq

diffusion

Fe(II)aq oxidation Fe(III)aq precipitation

Fe(III)(OH) 3

Fe(III)PO 4

Fe(II)Fe(III)PO 4

reductive dissolutionFe particles sinking

complete reductive dissolution

Fe(II)aq + Paq

H 2 Saq

+

Fe(II)aq

via O 2 , Mn(IV)...

Fe(II) 3 PO 4 .8(H 2 O)

FeS

SO 4 2-aq

Sediments:

diatoms (SiO 2 ), organic matter (Corg),

vivianite (Fe(II) 3 PO 4 .8(H 2 O)), pyrite (Fe(II)S 2 ),

detrital components (e.g. clays)

± siderite (Fe(II)CO 3 )

REDOX TRANSITION ZONE

ANOXIC ZONE

Fig. 12.9 Schematic model illustrating the Fe biogeochemical cycle in
Lac Pavin (see main text for description). The so-called “ferrous wheel”
is highlighted in black and bold. Only the processes in the anoxic zone
(monimolimnion) and the redox transition zone (identified by the light
grey area) are represented, while the oxic zone (mixolimnion) is not

illustrated here. The chemical reactions in grey correspond to additional

interactions with sulfur and phosphate cycles (beside the “iron wheel”).

Solid phases are reported in italic. The subscript “aq” stands for aque-

ous chemical species (i.e. dissolved)

12 Iron Wheel in Lac Pavin