Figure 1 Sed Geo 94
GRAPH READER
Eh in volts
Pe
pH
[H
+
] in ppm
Log PO
2
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Figure 1
pH-pE(Eh)
diagram for Mn species in seawater (1 atm, 25 °C, 35 ppt. salinity, total dissolved sulfur species: 28 mM (activity coefficient, 0.12; Garrels and Christ, 1965, p.l03). The area of interest is within the stability field of water-- below the oxidation of H
2
O to O
2
, and above reduction of H
2
O to H
2
.
Shaded areas
show mineral phase stable at designated concentrations of dissolved Mn.
Lightly shaded areas
indicate region in which mineral dissolves until concentrations observed under some anoxic conditions are reached.
Reduction of SO
4
2-
to H
2
S and HS
-
(concentration independent) is shown by the
long dashes;
reduction of NO
3
to N
2
is indicated by the
line of short dashes
(concentration independent ).
Solubility of Mn species is calculated at approximate oxic (10
- 3
M; Martin and Knauer, 1984) and anoxic (10
-5
M; Jacobs and Emerson, 1982; Jacobs et al., 1987) concentrations. Rhodochrosite "fence":
Modern Ocean
: Total dissolved carbonate species, 2.2 mM (Skirrow,1975) with an activity coefficient of 0.47 (Garrels and Christ, 1965, p.93).
Late Cambrian
: Total dissolved carbonate species, concentration 18 X present (Berner, 1990).