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

216

PO 4 3− is clearly released from the sediment, and then migrates
by diffusion, due to concentration gradients.
The global Fe geochemical cycle described above (release
of dissolved Fe(II), diffusion, oxidation-precipitation, Fe(III)
particle drop, and reductive dissolution) represents a specific
case of the “ferrous wheel” (Campbell and Torgersen 1980 ),
in which phosphate is intimately connected with iron. Most
reactions of Fe oxidation, reduction and precipitation in Lac
Pavin are likely bacterially-mediated as evidenced from
high-resolution images of Fe particles (Fig. 12.6; Cosmidis
et al. 2014 ), involving possibly a large diversity of microor-

ganisms evidenced by molecular diversity analyses (Lehours

et al. 2005 , 2007 , 2009 ).

In Lac Pavin, dissolved Fe pool of the monimolimnion is

actively sequestered in the sediment by vivianite precipita-

tion. Thus, the maintenance of a large dissolved Fe pool

requires a ferruginous subsurface spring (Meybeck et al.

1975 ; Jézéquel et al. 2011 ). The reaction of vivianite precipi-

tation can be written as:

32 88

2

4

3

Fe aq PO aq HO 23 Fe PO 42 HO 2 s

+−++

() ()  (). ()^

55

60

65

70

75

80

85

90

0 800 1600

Fe2+ (μM) SRP (μM) δ^56 Fe (‰) FeSPM (μM)

0 200 400 -4 -2^0012

Depth (m)

(A) (B) (C) (D)

Fig. 12.7 Comparison between the results of the geochemical model-
ing (lines) and data measured (symbols) on natural Lac Pavin samples.
(a) Dissolved Fe(II) concentration, (b) dissolved phosphate concentra-

tion (SRP: soluble reactive phosphorus), (c) isotope composition of the

dissolved Fe, (d) Fe concentration of suspended particulate matter

(SPM)

0

0.4

0.8

1.2

1.6

27-dec 15-feb 5-apr 25-may 14-june 2-sep 22-oct11-dec 30-jan

Time

Fe

SPM

at 85m (

μ

M)

Fig. 12.8 Results of the
“pseudo- steady” state
geochemical model. Iron
concentration in suspended
particulate matter (SPM) at
85 m depth in the water
column as a function of time.
From the 11th of April to 11th
of May, diatoms bloom is
represented by an increase of
the particle sedimentation
flux, inducing a sweep of
particles in the water column
(decrease of FeSPM). The
model shows that the system
comes back to equilibrium
state in less than 2 months
after disturbance

V. Busigny et al.

http://www.ebook3000.com