What controls iron concentrations in the oceans

1
What controls iron concentrations in the oceans?

• See also
• Johnson, Gordon and Coale (1997)
• What controls dissolved iron concentrations in
  the world ocean? Marine Chemistry, 57, 137-161.
• Parekh, Follows, and Boyle (2004)
• Modeling the global ocean iron cycle. Global
  Biogeochemical Cycles, 18, GB1002.

• Lecture 17, Lia Slemons

2
Classification of elements
3
Vertical Profiles of Metals

• Sources dust, continental slope, hydrothermal
• Sinks scavenging sinking, biological uptake
• Speciation metals often reactive with oxygen,
  hydrolysis reactions
• http//www.mbari.org/chemsensor/pteo.htm

Al (nM)
Fe (nM)
4
How would you model iron?
?
Fe2 Fe3
?
?
?
5
Current Biogeochemical Models (e.g. Parekh,
Aumont, Moore)
Biological uptake
Fe2 Fe3
FeL
Complexation
Geochemical sources Dust Continents
Scavenging
Remineralization
Particulate Fe
6
Iron will accept an electron
For a homogeneous system FeT Fe3 FeOH2
Fe(OH)2 Fe(OH)3º Fe(OH)4-
Hydrolysis Reactions Fe3 H2O FeOH2
H log K1 -2.2 FeOH2 H2O Fe(OH)2
H log K2 -3.4 or log b2 -5.6 Fe(OH)2
H2O Fe(OH)3º H log K3
-6.8 Fe(OH)3º H2O Fe(OH)4- H log K4
-9.1
7
Solve for Fe3 in terms of FeT
Hydrolysis Reactions Fe3 H2O FeOH2
H log K1 -2.2 FeOH2 H2O Fe(OH)2
H log K2 -3.4 or log b2 -5.6 Fe(OH)2
H2O Fe(OH)3º H log K3 -6.8 Fe(OH)3º
H2O Fe(OH)4- H log K4 -9.1
8
Let FeT 10-9 and pH 8What is Fe3?
Use mass balance FeT Fe3 FeOH2
Fe(OH)2 Fe(OH)3º Fe(OH)4- FeT Fe3
Fe3 K1/H Fe3 b2/(H)2 Fe3 b2 /
(H)3 Fe3 b4 / (H)4 FeT Fe3 1
K1/H b2/H2 b3/H3 b4/H4 Fe3
FeT/ 1 K1/H b2/H2 b3/H3 b4/H4
Where logb2 -5.6, logb3 -12.4, log b4
-21.5
Cancel all but the major terms Fe3 FeT / 1
K1/H b2/H2 b3/H3 b4/H4 Fe3
FeT /(1 105.8 1010.4 1011.6
1010.5) Fe3 FeT / 1011.6 2-21 Vanishingly
small! How do phytoplankton get iron?
9
In water (heterogeneous system)
Solubility product of iron oxide Fe(OH)3am 3H
Fe3 3 H2O KSO 103.2 Fe3 /
(H)3 Then, from FeT Fe3 1 K1/H
b2/H2 b3/H3 b4/H4 We get FeT
(KSO (H)3 ) 1 K1/H b2/H2 b3/H3
b4/H4
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Example What is the solubility of Fe(OH)3(s) in
terms of the uncharged dissolved species,
Fe(OH)3º?
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Solubility diagram for iron oxide
Fe in deep seawater 0.7 x 10-9M 10-9.15
Solubility minimum at pH 8
12
Current Biogeochemical Models (e.g. Parekh,
Aumont, Moore)
Biological uptake
Fe2 Fe3
FeL
Complexation
Geochemical sources Dust Continents
Scavenging
Remineralization
Particulate Fe
13
Geochemical sources Dust
Aerosol optical depth

• Iron hypothesis (Martin) Vostock ice core

14
All models are wrong, but some are useful. -
George Box, statistician

• Where else might iron come from?

Moore Braucher (2008) Observed dFe (lt300 m)
from open ocean vs. atmospheric transport model
estimates of Luo et al. (2003), symbols indicate
various oceanic basins
15
Why the EUC?
140ºW meridional section
Depth (m)

• Gordon et al. proposed Equatorial Undercurrent
  (EUC) upwelling source of iron (estimate 71-97
  of Fe to eastern equatorial Pacific!)

3ºS
9ºN
0º
Gordon et. al, FeLine in 1990
16
R/V Kilo Moana transect
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Current Biogeochemical Models (e.g. Parekh,
Aumont, Moore)
Biological uptake
Fe2 Fe3
FeL
Complexation
Geochemical sources Dust Continents
Scavenging
Remineralization
Particulate Fe
18
Organic Ligands
Rue and Bruland, 1995)
Organic Ligand Reactions Fe3 Ln-
FeL The ligands (L) can be made by both
bacteria and plankton
FeL K Fe3 Ln-
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Organic Ligands
Rue and Bruland, 1995)
Organic Ligand Reactions Fe3 Ln- FeL K
FeL / Fe3 Ln- 1026.5 for Desferol The
ligands (L) can be made by (an iron
chelating siderophore) both bacteria and plankton
so FeL K Fe3
Ln-
Then FeT Fe FeL
(inorganic species) (organic species)
FeT Fe3 1 K1/H b2/H2 b3/H3
b4/H4 KFeL L Example assuming a ligand
concentration Ln- 0.44nM 10-9.35M and pH
8 FeT Fe3 ( 1 105.8 1010.3
1011.5 1010.4 1017.5) Then Fe
0.0039 FeL 99.996 Rue and Bruland (1995)
analyzed natural seawater and found Fe
0.03 FeL 99.97
In surface SW most Fe is chelated!
20
What constitutes bioavailable iron in
seawater?
Fe(III)' - Fe(OH)2 Fe(OH)30
Fe(OH)4- Fe(II)' - Fe2 FeCO30
organic detritus
21
What form does the iron take?
Wu et al (2001) Science, 293, 847