100 m
wind-mixed surface layer (O2 present)
CO2
photoautotrophic
Fe(II)-oxidation
Fe(OH)3
biomass
-2
90.0 mol Fe(III) m
-2
22.5 mol C m
(Fe:C = 4:1)
2+
CH4
anoxic
deep waters
H2
-2
methanogenesis
5.9 mol C m
sedimentation
400 m
Fe
2+
-
Fe / HCO3
2+
Fe
CH4
1 mm
methanotrophy
CH4 / Fe(OH)3
mid-ocean
ridge
(CH3COO- , H2)
fermentation
annual deposit
Fe-rich layers
SiO2-rich layers
Fe3 O4
Fe(III)reduction
63.8 mol Fe(III) -2m-2
15.9 mol C m
(Fe:C = 4:1)
26.2 mol Fe(III) m-2
0.7 mol C m-2 (as Fe3O4)
(Fe:C = 36:1)
6.6 mol C m-2 (as FeCO3 )
(Fe:C = 4:1)
CH4 + 8Fe(OH)3 Æ 8Fe2+ + CO2 + 16OH- + 6H2O (-ΔGº)
3. Solute Sorbents
OH
OH
Si
OH
OH
OH
OH
>
Fe
O
O
OH
OH
> Fe
OH
OH
P
OH
O
100
>FeO>FeOH2+
>FeOH
>FeOH2+
>FeOH2+
>FeOH2+
>FeOH2+
>FeOH2+
>FeOH2+
50
>FeO>FeO>FeO>FeO>FeO>FeO>FeO-
>FeOH2+
>FeOH
>FeO>FeOH
>FeO>FeOH
>FeOH2+
0
0
2
4
6
8
pH
10
12
14
Phosphate Adsorbed (%)
Available Surface Sites (%)
100
>FeO-
>FeOH2+
0
2
>FeOH
50
0
4
6
8
pH
10
12
14
The Key Paper
Calculated [PO4] concentrations ranging from 0.03 to 0.29 µM, while today’s
average [PO4] is 2.3 µM. Such a reduction could have reduced the rate of
organic carbon burial by up to 90%.
Bjerrum,
Bjerrum, C.J.,
C.J., and
and Canfield,
Canfield, D.E.
D.E. (2002)
(2002) Nature
Nature 417:
417: 159-162
159-162
BUT, w
hat abo
ut silica
?
today’s oceans average < 0.10 mM Si
cristobalite saturation : 0.67 mM Si
amorphous SiO2 saturation : 2.20 mM Si
100
0m
75
0.
67
50
2.2
0
25
mM
m
M
M
Si
Si
Si
0
6
7
8
pH
9
10
100
0m
75
0.
67
50
2.2
0
25
mM
m
M
M
Si
Si
Si
0
6
7
8
pH
9
10
Net Negative Charge
Net Positive Charge
Implications
Recall that today’s ocean [PO44] averages 2.3 µM
In the absence of silica, we calculate an average [PO44] concentration
of 0.14 µM
For cristobalite saturation, we calculate an average [PO44]
concentration of 0.95 µM
For amorphous silica saturation, we calculate an average [PO44]
concentration of 5.25 µM
Inferred concentrations may be up to 50% low due to diagenetic
remobilization
Ni in BIF Through Time
Konhauser et al. (2009)
Komatiites Through Time
Preferential Mobilization of Ni
Paleo-Seawater Ni Concentrations
Methanogens require Ni in their enzymes
1. hydrogenase
2. CO-dehydrogenase
3. methyl-coenzyme
M reductase
4. urease
Hausrath et al. (2007)
A Very Interesting Time…
Klein (2005)
There are many theories for the timing of the rise of
atmospheric O22, the so-called ‘Great Oxidation Event’.
(1) a change in styles of volcanism
peak mantle plume activity at 3.0-2.9 and 2.7 Ga
(H2, CO, CH4, H2S)
(H2O, CO2, SO2)
(2) increased nutrient availability post glaciation
(3) the demise of methanogens
Previous explanations for a methane collapse revolve around the outcompetition of methanogens by sulfate-reducing bacteria.
We suggest instead that the oceanic Ni famine recorded in BIF would
have strongly stifled methanogenesis without the need to invoke
increasing sulfate concentrations.
How do BIF compare with other marine
sediments as proxies, such as black shales?
From Scott et al. (2008)
The timing of the great oxidation event may have
been determined by evolution of the composition of
volcanic extrusions and the nutrients it supplied to
microbial populations.
Unresolved Issues
1. Source of Fe(II) – MOR, seamount, land?
2. Oxidation? – If yes; O2, photosynthesis, UV
3. Primary Fe ppts – how much Fe(II) component?
4. Metabolism used by seafloor microbes?
What’s the solution? - isotopes
- elemental ratios in BIF
- modern analogues