Marine Chemistry of Iron

Ferric vs. FerrousFe(III) vs. Fe(II) Transition metal - partly filled d or f orbitals.

The most important bioactive trace element. Exceedingly complex chemistry.

Fe3+ is strongly hydrolyzed in seawater forming [Fe(OH)n3-n]

and other complexes. The ratio of complexed Fe(III) to the free form {denoted Fe’} is estimated to be ~1012. Based on thermodynamic calculations, the dominant species might be Fe(OH)3

o.

Free iron (III) {Fe’} is not likely to be important due to its low concentration (maybe as low as 10-22 M in high nutrient waters)

Total Fe concentrations in surface waters range from < 0.1 nM (severely Fe-limited) to 1-5 nM in iron-replete waters.

Boyd & Ellwood, 2010

Surface enrichment of Fe in N. Atlantic is from dust deposition from Africa

Most of the iron is complexed and some specific Fe-binding ligands are now known.

• Siderophores produced by marine bacteria have recently been discovered (referred to as Aquachelins; see work of Butler et al.).

• These siderophore-Fe complexes are photolabile.

Particulate Fe (> 0.4 µm)

Col

loid

al F

e

Tru

e di

ssol

ved

Fe

North Pacific

L1 and L2 are strong and weak Fe-binding ligands, respectively.

Boyd & Ellwood, 2010

Specific Fe-binding ligands are present in the ocean, with L1 being only present near the surface.

Role of Colloids in Marine Fe cycle

A significant fraction of the Fe may be associated with colloids (i.e adsorbed to tiny particles that don’t sink). Wells and Mayer provided data on iron colloids and they found that >50% of the operationally-“dissolved” Fe may be colloidal.

Availability of Fe to phytoplankton may depend on its particle form. Some phytoplankton may not be able to take up Fe colloids, but instead may rely on photoreduction processes to make it available. (but see Nodwell and Price L&O 46: 765)

Photoreduction of Fe(III) to Fe(II) is important.

Light causes reduction of Fe(III):DOM of Fe(III):colloid complexes to yield Fe(II), which is much more soluble than the oxidized form.

Most importantly, this photo-redox cycling increases the residence time of Fe in the photic zone by minimizing formation of particle-associated Fe, which sinks.

The Fe(II) formed will rapidly oxidize back to Fe(III) with O2 or H2O2, but it will form “relatively” available amorphous Fe(III)-oxides.

Sunda, 2012

Fe transport

Light energy

Light energy

Photo

-redu

ction

Photo-reduction of iron is important in maintaining bioavailability in surface waters.

Iron (III)-ligand complex

oxidants

Free reduced Fe

Free oxidized

Fe

Ligand binding

Oxidized ligand

Aeolian transport of Fe is very important (>95% of Fe input to surface waters is from the atmosphere, mainly as dust (Duce and Tindale, 1991).

Aeolian transport continued. Surface water enrichments of Fe are seen in some places – but Fe is rapidly scavenged from the surface water and water just below the mixed layer, due to biological and chemical processes.

Fe transported out of mixed layer is probably scavenged and deposited to sediments rather than being mixed back up to surface. Thus, upwelling of mid-depth waters is a poor source of Fe. This results in low Fe levels in parts of the ocean that limit primary productivity.

Scavenging rates determined with 234Th (mainly Th in the +IV oxidation state) which has a chemistry similar to that of Fe, shows that subsurface (60-100m) removal of Fe likely occurs.

Scavenging Intensity

Depth (m)

mixed layerFe Fe

Fe

Fe

mixing

recycling

Scavenging export

deposition

1% light level

Sohm et al. 2011

N2 fixation enzymes require lots of iron. Summary table of geographic distribution of N2 fixation in relation to nutrient status

Dissolved Fe

Fe-poor

Fe-poor

Fe-poor

Fe-replete

Fe-replete; P limited

Fe-replete

Fe-replete

Relatively low N2 fixation

The historical record of atmospheric CO2 and Fe deposition as measured in an ice core (probably from Greenland). Taken from Millero (1996)

High Fe, low CO2

Low Fe, high CO2

Depth in ice core (m)

Fe inputs to the ocean are connected with atmospheric CO2 and probably climate

Large-scale Iron Fertilization Experiments

Brainchild of John H. Martin of the Moss Landing LaboratoryFe-Ex I

Fe-Ex II

Sooiree

EisenEx

Equatorial Pacific

Southern Ocean

Fe(II) SF6 mixture released and the water mass tracked lagrangian style Many other Fe-Fertilization experiments have now been conducted

Annual average mixed layer nitrate concentration (µM) Boyd et al 2007

Changes in Chl a and primary productivity in Fe fertilized patch during IronEx I

IRONEX I conducted in 1993 at 5o S, 90o W, south of the Galapagos Islands

Based on Fe-Ex I (taken from Millero)

Fe-Ex I produced a relatively small response

Changes in nitrate and chlorophyll a profiles after Fe fertilization during Iron Ex II (1996)

From Coale et al., 1996

Days after Fe addition

From Millero, 1996

IRONEX II conducted at 3o S, 104o W

FluorNO3

-

pCO2

CO2 drawdown during IronEx II

The fCO2 is plotted against SF6, the tracer used to tag the Fe-fertilized water mass. The higher the SF6, the closer to the center of the patch. The overall decline in SF6 over time was due to outgassing and vertical mixing.

From Coale et al, 1996

Do results of Fe fertilization experiments Do results of Fe fertilization experiments represent what would happen with natural Fe represent what would happen with natural Fe supply? How are they different? supply? How are they different?

Were the chemical and biological responses Were the chemical and biological responses observed representative of what would be observed representative of what would be expected with natural inputs of Fe? expected with natural inputs of Fe?

Is Fe fertilization a workable strategy to Is Fe fertilization a workable strategy to increase primary production (and associated increase primary production (and associated fisheries yield), and to draw COfisheries yield), and to draw CO22 out of the out of the

atmosphere (to mitigate global warming)?atmosphere (to mitigate global warming)?

Low pCO2

Natural iron fertilization on the Kerguelen Plateau – in the Fe-starved Southern Ocean

Blain et al., 2007 Nature 446

Evidence for Fe and vitamin B12 Co-limitation of primary production in the Ross Sea, Antarctica

Bertrand et al. 2007 L&O 53:

Vitamin B12 (cyanocobalamin) contains the trace element cobalt (Co).

B12 is not produced by algae but it is by bacteria

End

From Millero, 1996

Photo-reduction

Fig 9.25 in 3rd Edition

Nitrate supported growth in phytoplankton requires more Fe than ammonium supported growth because nitrate reductase contains Fe!

Wells (1997) suggested that laminations of diatom tests in equatorial sediments may have originated from blooms of diatoms produced by changes in the Fe concentration of the Equatorial undercurrent. This may have been caused by tectonic activity near the source waters of this current, near Indonesia.

Cobalt (Co)

Present in cyanocobalamin (vitamin B12), a methyl carrier in biochemistry.

Present at only 4-50 pM in North Pacific. Could be biolimiting.

A required growth factor for some species. Uptake may be enhanced by organic complexation (as with Fe).

Recent evidence for a cobalt binding ligand in seawater, similar to that of Cu and Zn ligands.

Prymnesiophytes have a higher Co requirement than diatoms. Required for production of methylated compounds?

Fe-starved HNLC areas Fe-replete areas

Fe:C ratios in phytoplankton and exported particles.

Boyd et al 2007

Values are generally higher in Fe-replete areas

Changes in Fe concentrations in a mesoscale eddy over time

Eddy just formed

12 months later

Typical “mature water mass” Fe profile

Boyd & Ellwood, 2010

>0.4 µm

Multiple sources of new iron to the southern ocean

Boyd & Ellwood 2010

Dust

Island wake

Iceberg

Sea ice

Fe-rich sediments

Bathym

etric

upwell

ing

Island wake

Dust

Eddys & sediments