213indicates that Fe(II) and SRP are released at the bottom of
the lake, diffuse through the monimolimnion, and are con-
sumed at the redox boundary (Michard et al. 1994 , 2003 ).
The consumption is attributed to Fe(II) oxidation to Fe(III),
followed by precipitation of a Fe(III)-bearing solid phase
(Fig. 12.9). The oxidation-precipitation process is demon-
strated from suspended particulate matter analyses and tur-
bidity profiles, showing a peak of Fe particles at 50–60 m
depth (Fig. 12.3). This is also suggested by Fe(II)/Fetotal mea-
surements of the solid phase by XANES spectroscopy at dif-
ferent depths (Cosmidis et al. 2014 ). This is finally supported
by the large shift in Fe isotope composition of the dissolved
Fe(II), corresponding to a depletion in heavy isotopes after
Fe(III) precipitate formation (Busigny et al. 2014 ). The solid
phase formed at 50–60 m depth was previously proposed to
be Fe-oxyhydroxides with phosphate adsorption (Michard
et al. 1994 ; Viollier et al. 1995 ) but a recent mineralogical
study also identified the presence of mixed valence
amorphous Fe(II)-Fe(III)-phosphate and possibly Fe(III)-
phosphate in sediment traps placed at 56 m in the water col-
umn (Cosmidis et al. 2014 ). The linear correlation observed
between Fe and P concentrations in SPM provides further
support for Fe phosphates and/or Fe-oxyhydroxides with
strong phosphate adsorption (Fig. 12.4). The Fe/P molar
ratio obtained from bulk SPM analyses (3.07) is significantly
higher than the one expected for pure Fe phosphates (between
1.5 and 2) but also lower than the one of hydrous ferric
oxides (HFO) with adsorbed P (Fe/P ~ 4; e.g. Michard et al.
1994 ; Li et al. 2012 ; Voegelin et al. 2013 ; Cosmidis et al.
2014 ). The intermediate Fe/P ratio in SPM may indicate thatTable 12.2 Iron and phosphorus concentrations and Fe/P molar ratio in Lac Pavin suspended particulate matter
Sample #Deptha Fe P Fe/P Vo lb
(m) (μM) (μM) (molar) (mL)
October 2010 (MX30)
MX30–SPM1 9 0.07 0.15 0.49
MX30-SPM2 11 0.06 0.18 0.32
MX30-SPM3 48 0.41 0.14 2.93
MX30-SPM4 52 0.26 0.11 2.29
MX30-SPM5 54 1.50 0.50 3.03
MX30-SPM6 56 3.37 0.97 3.48
MX30-SPM7 73 1.00 0.32 3.13
MX30-SPM8 85 1.92 0.69 2.79
November 2011 (MX35)
MX35-SPM1 40.2 0.13 0.10 1.33 500
MX35-SPM2 51.7 1.26 0.40 3.16 500
MX35-SPM3 52.6 2.46 0.73 3.35 500
MX35-SPM4 57.7 0.26 0.08 3.22 500
MX35-SPM5 60.3 0.51 0.16 3.15 500
MX35-SPM6 65.2 1.14 0.24 4.67 500
MX35-SPM7 70.2 2.00 0.49 4.12 500
MX35-SPM8 80.2 3.33 1.08 3.08 500
June 2013 (MX41)
MX41-SPM1 0 0.05 0.29 0.19 400
MX41-SPM2 10 0.03 0.29 0.09 500
MX41-SPM3 15 0.03 0.38 0.08 420
MX41-SPM4 50 0.22 0.08 2.95 500
MX41-SPM5 54.2 0.93 0.34 2.73 500
MX41-SPM6 55.1 1.40 0.52 2.71 500
MX41-SPM7 56.2 1.92 0.76 2.54 500
MX41-SPM8 57.2 1.08 0.54 2.00 500
MX41-SPM9 58.3 0.67 0.35 1.91 500
MX41-SPM10 59.3 0.62 0.31 1.99 700
MX41-SPM11 60.2 0.33 0.17 1.91 500
July 2013 (MX42)
MX42-SPM1 57 2.30 0.85 2.72 1000
aDepth: depth in the water column. The oxic-anoxic transition zone is at 50–60 m depth
bVol: volume of water filtered for suspended particulate matter collection
12 Iron Wheel in Lac Pavin