210
ment (Fig. 12.5), representing up to 70 wt% of the total solid
fraction in some sediment samples (Schettler et al. 2007 ).
Imaging at the micro- and nano- meter scales using SEM,
TEM and STXM techniques shows that Fe-phosphates often
encrust microbial bodies (Figs. 12.6a, b), suggesting that
microorganisms are involved in the formation of these phases
(Cosmidis et al. 2014 ). Pyrite (FeS 2 ) is a common phase in
Lac Pavin sediments (Viollier et al. 1997 ; Schettler et al.
2007 ) but is in very low abundance relative to diatoms, iron
phosphates and organic matter (<0.2 wt%; Busigny et al.
2014 ). Fine-grained pyrite occurs in sediments as spherical
aggregates (20–30 μm) of bipyramidal crystals (0.5–1 μm)
termed framboids (Fig. 12.6c). Siderite (FeCO 3 ) occurrence
has also been suggested in lake bottom sediments from bulk
chemical analysis, coupling bulk analyses of Fe and CO 3 con-
centrations (Schettler et al. 2007 ). Preliminary XRD analysis
of the samples from our sediment core collected at the lake
bottom support the presence of siderite.
12.3.3 ModelingThe present geochemical model was limited to first order reac-
tions involving iron and phosphates. It was mostly confined to
the monimolimnion, with a rather thin (5 m thick) oxic zone
replacing the whole mixolimnion (~60 m thick), because
chemical reactions between Fe, P and other elements occur
only in the 3–4 m in the deepest part of the mixolimnion. The
water column was thus represented by two distinct layers: (1)
a 5 m-thick oxidizing layer, representing the bottom part of the
mixolimnion (redox transition zone), where soluble Fe2+ is
oxidized to Fe3+ which is weakly soluble and precipitates as
ferric oxyhydroxides, and phosphate is adsorbed on these fer-
ric particles, and (2) a 30 m-thick reducing layer, representing
the monimolimnion, where ferric oxyhydroxides are reduced
and dissolved, adsorbed phosphate is released, and vivianite, a
ferrous phosphate, is precipitated. Because of the limited
amount of Mn and S relative to large Fe and P concentrations,Table 12.1 Dissolved iron, phosphate (SRP: soluble reactive phosphorus), manganese, sulfate and sulfide concentrations, together with isotopic
composition of the bulk dissolved iron (expressed as δ^56 Fe) in water samples from Lac Pavin collected in July 2007
Sample #Depth SRP Fe(II)a Mn(II)a SO 4 2-b ΣH 2 Sb δ^56 Fe ± 2SDa
(m) (μM) (μM) (μM) (μM) (μM) (‰)
MX-15_10 57.0 1.0 0.0 0.42 14.0 0.0
MX-15_11 57.5 1.0 0.0 0.29 14.2 0.1
MX-15_12 58.0 1.1 0.1 0.31 13.9 0.0
MX-15_13 58.5 1.2 0.0 1.65 14.3 0.0
MX-15_14 59.0 1.1 0.0 2.39 14.7 0.0
MX-15_15 59.5 1.3 0.1 7.49 13.8 0.0
MX-15_16 60.0 1.6 2.1 14.9 13.7 0.6 −2.14 ± 0.04
MX-15_17 60.5 4.9 21 16.1 14.0 0.4 −1.30 ± 0.08
MX-15_18 61.0 5.9 106 17.4 13.8 0.5 −1.34 ± 0.02
MX-15_19 61.5 20 108 21.2 12.7 1.1 −1.25 ± 0.04
MX-15_20 62.0 37 174 19.6 9.3 4.2 −1.25 ± 0.08
MX-15_21 62.5 70 308 16.5 2.4 13.7 −0.51 ± 0.04
MX-15_22 63.0 97 378 15.7 3.6 11.6 −0.40 ± 0.04
MX-15_23 64.0 113 435 16.5 2.4 11.4 −0.33 ± 0.06
MX-15_24 65.0 120 470 16.7 1.3 16.7 −0.26 ± 0.04
MX-15_25 66.0 149 568 18.1 1.4 16.0 −0.15 ± 0.04
MX-15_26 68.0 183 688 19.0 1.4 12.5 −0.23 ± 0.02
MX-15_27 70.0 208 757 20.4 1.2 13.5 −0.11 ± 0.04
MX-15_28 72.0 237 864 21.2 1.8 14.0 −0.05 ± 0.06
MX-15_29 75.0 275 964 22.2 13.6 −0.01 ± 0.08
MX-15_30 80.0 312 1004 23.8 12.3 0.23 ± 0.04
MX-15_31 85.0 321 1074 25.3 11.8 0.20 ± 0.06
MX-15_32 87.0 335 1105 25.1 11.8
MX-15_33 89.0 366 1189 26.4 11.4
MX-15_34 90.0 368 1210 26.5 10.2 0.31 ± 0.08
MX-15_35 90.5 372 1203 27.1 10.5 0.31 ± 0.08
Depth: depth in the water column. The oxic-anoxic interface was at 60 m depth in July 2007
Uncertainties on δ^56 Fe correspond to 2SD (standard deviation)
aData from Busigny et al. ( 2014 )
bData from Bura-Nakić et al. ( 2009 )
V. Busigny et al.