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

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the solid phase corresponds to a mixing between P-rich HFO
and Fe phosphate. This could be determined in the future by
nano-scale mapping of the Fe/P ratio in SPM samples. The
Fe oxidation at the redox boundary can be mediated by O 2 ,
particularly during mixing episode of the shallow waters
down to the chemocline, but other oxidizing agents such as
NO 3 − and Mn(IV) are also likely involved in more quiet peri-
ods. For instance, the peak of dissolved Mn(II) at the redox
transition zone, interpreted as Mn oxide reductive dissolu-
tion provides evidence for Mn mediated oxidation (Fig.
12.1a; Busigny et al. 2014 ). Moreover, microorganisms may
mediate some of these oxidation processes but their identity
and the exact involved metabolisms have yet to be identified
(Cosmidis et al. 2014 ).
Solid Fe(III) phases sinking in the anoxic zone experience
reductive dissolution in the water column (Fig. 12.9). This

finding was supported by mineralogical study of the P- and

Fe-rich phases (Cosmidis et al. 2014 ), although preservation

of Fe(III) down to the sediments was observed (Fig. 12.5).

This reductive dissolution in the water column is also

required from geochemical modeling in order to reproduce

the very low turbidity values and the variations of Fe isotope

composition in the depth range 60–65 m, just below the peak

of Fe(III) particles formation (Fig. 12.3). The progressive

increase in turbidity and Fe content of suspended particulate

matter below 65 m depth, down to the lake bottom (Fig.

12.3), indicates that an Fe(II) phase precipitates in the moni-

molimnion and increases progressively the particles flow.

Mineralogical analyses have identified well-crystallized viv-

ianite, an Fe(II)-phosphate. Specifically, vivianite was the

dominant Fe-bearing phase detected in sediment traps from

the anoxic zone and in the lake bottom sediments (Fig. 12.5).

25 m

56 m

67 m

86 m

Sediment

Fe (wt%)

0 0.5 11 .5 2 2.5

Phyllosilicates

Fe-(oxyhydr)oxides

Amorphous ferric phosphate

Vivianite (ferrous phosphate)

OXIC

REDOX

TRANSITION

ANOXIC

Fig. 12.5 Iron mineralogy in
sediment traps from the water
column (at 25, 56, 67 and
86 m depth) and sediments
from the lake bottom
(Modified from Cosmidis
et al. 2014 ). The results were
obtained by combining
EXAFS spectra treatments
with bulk Fe content
measurements

Fig. 12.6 Scanning electron microscopy (SEM) images of Fe particles
from Lac Pavin. (a) and (b): Iron phosphate, collected on a filter, from
67 m depth in the water column. Same view in secondary electron mode
(a) and back-scattered electron mode (b). Iron phosphate particles are

made of chain of single cocci-shaped compartments, suggesting micro-

bial control of Fe phosphate precipitation. Scale bar is 1 μm. (c) pyrite

framboid from lake bottom sediment. Scale bar represents 10 μm

V. Busigny et al.

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