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

209

proxy of small-sized Fe particles (formed at the peak of tur-
bidity) but when settling particles collapse and form larger
aggregates, the turbidity signal is decreased. Another inter-
esting feature is that Fe/P molar ratio in SPM is roughly con-
stant (around ~3; Fig. 12.3a) although some variations are
observed in the redox transition zone. A new dataset for
SPM collected in October 2010, November 2011 and June
2013 is presented in Fig. 12.4 (see data in Table 12.2). It
shows that, below 50 m depth in the water column, particu-
late Fe and P concentrations are linearly correlated, with a
slope of 3.07 (i.e. Fe/P molar ratio), slightly lower than the
slope found for the dissolved fraction (3.45 in Fig. 12.2).

Figure 12.3b shows three different profiles of turbidity mea-
sured in June 2011, September 2011 and June 2013, selected to
represent some of the variability observed in the lake. One or
several peaks of turbidity, with various magnitudes, are gener-
ally observed from the surface of the lake down to ~25 m.
These peaks represent living organisms, such as diatoms, green
algae and/or cyanobacteria (Amblard, 1984 , 1986 , 1988 ;
Stebich et al. 2005 ). Another peak of turbidity is observed at
~50–60 m depth, and corresponds to a peak of particulate iron
(Fig. 12.3). The intensity of this peak is usually between 4 and
6 NTU (Nephelometric Turbidity Unit), but an occasional col-
lapse (e.g. down to 1.5 NTU in December 2012) or increase (up
to 20 NTU in October 2010) were also observed.

12.3.2 Mineralogy of Particulate Matter

in the Water Column and Bottom

Sediments

SEM and XRD analyses show that the particles collected in

the water column and sediments of Lac Pavin mostly contain

diatom frustules (Michard et al. 1994 ; Viollier et al. 1997 ;

Cosmidis et al. 2014 ), consistently with early studies of phy-

toplankton diversity in Lac Pavin (e.g., Devaux 1980 ).

Organic matter also represents a significant portion of the par-

ticulate matter, with concentrations ranging from 26 to 34

wt% in sediment traps (Cosmidis et al. 2014 ). Organic matter

content decreases strongly in sediments, down to ~5–10 wt%,

due to biological degradation by Fe(III) reduction and metha-

nogenesis metabolisms (Restituito 1984 ; Lehours et al. 2009 ;

Biderre-Petit et al. 2011 ; Cosmidis et al. 2014 ). Detailed min-

eralogy of Fe-bearing particles collected using sediment traps

set up at different depths in the water column of Lac Pavin is

presented in Fig. 12.5. It shows that Fe-bearing particles in

the shallower oxygenated zone are mostly detrital phyllosili-

cates and Fe(III)-oxyhydroxides. Poorly crystalline ferric/fer-

rous phosphates (Cosmidis et al. 2014 ), possibly with some

Fe-oxyhydroxides (Viollier et al. 1997 ), precipitate at the

redox transition zone. In the anoxic zone, a ferrous phosphate

(vivianite, Fe2+ 3 (PO 4 ) 2 .8(H 2 O)) precipitates increasingly with

depth and is the most abundant Fe-bearing phase in the sedi-

0102030

50

55

60

65

70

75

80

85

90

95

0 246810

Depth (m)

O 2 (mg/L)

abc

0 400 800 1200

0246810

O 2 (mg/L)

-2 -1 0

0246810

O 2 (mg/L)

SO 4 2-, H 2 S, Mn2+ (μM) SRP, Fe2+ (μM) δ^56 Fe (‰)

Fe2+

SRP

O 2 O 2 O 2

δ^56 Fe

SO 4 2-

H 2 S

Mn2+

Fig. 12.1 Concentration profiles of selected dissolved chemical spe-
cies (a and b) and Fe isotope composition (c) in Lac Pavin water sam-
ples collected in July 2007. The oxygen (O 2 ) concentration is reported

in all diagrams for highlighting the redox transition zone (between 50

and 60 m) and fully anoxic conditions (below 60 m). The bottom of the

lake is at 92 m depth. SRP corresponds to soluble reactive phosphorus

12 Iron Wheel in Lac Pavin