47
layer is not found in the Neckar River sediments, the resuspension and mixing of the
sediments by bioturbation and hydrodynamic forces (e.g, flood) may be responsible
for this feature.
Phosphorus minerals such as vivianite [Fe3(PO,,)2 8H20] and reddingite
[Mn3(PO4)23H2 O] may control PO43" concentrations in anoxic sediments (Nriagu and
Dell 1974; Emerson 1976; Aller 1980a; Nembrini et al. 1982; Holdren and Armstrong
1986):
vivianite:
3 Fe 2+ + 2 PO43" + 8 H20 -----) Fe3(PO4) : 8H20
hydroxylapatite:
5 Ca 2+ + 3 PO43" + OH" ---9. Cas(PO4)3(OH )
reddingite:
3 Mn 2+ + 2 PO43" + 3 H20 "--) Mn3(POa) ,_ 3H20
2d " dif tullOn~
<,>',,,-, / / \ F
7 e
,,<-,,;;;;/~6,: )/-r
.~ --///://. r 6-$3~
,
L 7
; Y/,'~ ~/2 2,1
, V , V / Y/S /,
~d - 411f~llQe
Id -,Jit # v,ltOn
a .
a.
b.
Fig. 4.20a. The various steps in the precipitation processes
b. BSEM-Photomicrograph-Vivianite crystals from Neckar River sediments
(Viv: vivianite. Qz: quartz)