OUTLINE

Why Iron?

Sources and sinks of iron

The intricacies of Fe chemistry

Fe biogeochemistry – 2 SOLAS case studies

Climate change and Fe biogeochemistry

THE

BIOGEOCHEMISTRY

OF IRON

Philip Boyd

NIWA/Otago

New Zealand

Sept

2011

WHY IRON?

A trace element of

pivotal importance

for phytoplankton

physiology

Images courtesy

B Twining (Bigelow)

Fe:C molar

ratio of 1 x 10-5

“Give me a tanker of iron, and I’ll give you an Ice Age.”

WHY IRON? Fe:C molar ratio of 1 x 10-5

Behrenfeld et al. (1996)

High profile but controversial

Courtesy J. Cullen (WHOI 2007)

0.5

1.0

1.5

Fe (

mol/k

g ic

e)

180

200

220

240

260

280

300

[CO

2]

(ppm

v)

20 40 60 80 100 120 140 160

Age (1000 yr)

Iron

CO2

The Vostok ice core provided tantalising evidence

of the impact of changes in Fe supply on atmospheric

CO2 (Martin, 1990)

Today

IRON & CLIMATE

Marine biota play a key role in climate

by regulating atmospheric CO2 levels

One means of regulation is via the

BIOLOGICAL PUMP

The PUMP works most

efficiently when all of

the available nutrients

are utilised

0.5

1.0

1.5

Fe (

mol/k

g ic

e)

180

200

220

240

260

280

300

[CO

2]

(ppm

v)

20 40 60 80 100 120 140 160

Age (1000 yr)

Iron

CO2

Links between iron and climate

ODP (red) & EPICA (black) dust and iron deposition records correspond

Large-scale changes in dust deposition i.e. most of the Southern Ocean

Suggests a CO2 drawdown of up to 40 ppm from subantarctic iron fertilization

Stop press

Oceanic Inputs (1012 g / y)

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

Mining Industrial Streamload

Iron supply

Mackenzie et al.

(1979)

Most of the ‘Streamload Fe’ is complexed by organic matter in suspended

sediment in estuarine and coastal environments forming Fe-rich sediments

Fe - SOURCES AND SINKS

Dust Rain

Dust and iron supply

Mackie et al. (2008)

Predominant dust source regions and transport routes

Pye (1987)

0

30°S

30°N

Boyle et al. 2005 Sedwick et al. 2005 Bergquist et al. 2006 Sarthou et al. 2007

Blain et al. subm. Bergquist et al. 2006

Jickells et al. 2005

Guieu et al. 2002

Dust supply and profiles

of dissolved iron

Courtesy S. Blain

(CNRS Banyuls)

Global aerosol distributions

Evidence of a dust storm from TOMS Aerosol

Index, April 22 -25 1998 NE Pacific

The Southern Ocean has multiple iron supply mechanisms

Boyd & Ellwood

(2010)

Upwelling of Iron

Cromwell undercurrent (Chavez et al. 1999)

Fe from volcanic eruptions

Kasatochi August 2008

Hamme et al. (2010)

Offshore

movement of

Haida eddies

(containing high iron

levels) in the

Gulf of Alaska

(images Gower,

NASA)

0

1000

2000

3000

4000

5000

6000

0 2 4 6 8 10

Total Dissolvable-Fe conc. (nM)

Dep

t (m

)

C4

C1

C3

C2

C5

C7

C8

C9

C0

C10

KNOT

SEEDS

50ºN

45ºN

140ºE 145ºE 150ºE 155ºE 160ºE 165ºE

KNOT

C9

C10

C8

C7

C5

C2

C1 C3

C4 SEEDS

C0

Vertical profiles of Total dissolvable iron in the S Sea of

Okhotsk (Green), the Oyashio region (Red) and the WSP

(Blue)

Iron transport from the Sea of

Okhotsk to the WSP Nishioka et al. (2007)

Fe supply from recycling

“The picture that emerges is one of an extremely dynamic

trafficking in essential trace metals in sea water”

Morel and Price [2003]

fe ratio = new Fe / (new + regen Fe)

Boyd & Ellwood (2010)

fe ratio’s range from < 0.1 - >0.5

c.f. f ratios

Recycling supplies 40 to 90% of

iron to the biota

Spinning the‘FERROUS’ WHEEL

(Kirchman, 1996)

From Morel and Price [2003]

SINKS FOR Fe

Fe distributions

provide clues

as to the main

sinks for iron

Surface depletion

is indicative of biol.

uptake

Iron supplied is usually rapidly removed by

the marine biota

de Baar et al. (1995) - Polar Front (Atlantic sector)

iron-induced phytoplankton bloom

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 2 4 6 8 10

Fragilariopsis kerguelensis

µ (

d-1

)

Fe dissolved (x10 -9 M )

Km Fediss: 0.44 x 10-9 M

µmax: 0.31 . d-1

Timmermans et al. [2004]

80 µm

Fragilariopsis kerguelensis

Iron uptake kinetics using natural

Antarctic seawater

IRON ACQUISITION STRATEGIES

Boyd & Ellwood (2010

DIFFERENT PHYTOPLANKTON

GROUPS HAVE DIFFERENT

IRON REQUIREMENTS

LOW

HIGH

??

Fe chemistry is complex!!

Algal

Cell

[Fe ] [Fe ]

[Fe(III)L] organic complexes

? ?

? ? photoreduction

sidero-

phores

PFe

Fe2O3

Fe(OH)3

FeOOH colloids

photoreduction

[Fe(OH) ] [Fe(OH) ] 2+

+

2 3+ 2+

[FeCO ] 0

3

[FeOH ] +

[Fe(II)L]?

(Courtesy of de Baar)

PHOTOCHEMISTRY

INTERACTIONS

WITH THE BIOTA

Forms of Iron

in Seawater

• Dissolved Colloidal Particulate

• Dissolved – two oxidation states - Fe(II) and Fe(III)

• Mainly present in non-reactive forms in the upper ocean

• Colloidal – poorly understood

• Particulate – biogenic, lithogenic, detrital

• Lithogenic iron present as oxyhydroxides, silicates and

aluminosilicates

• Fe silicates and aluminosilicates are mainly very unreactive

- contribute little to dissolved Fe in seawater and their fate

is governed by coagulation and settling processes

OH

OH

N

O

NN

NN

NN

NH2

OH OOH

O

O

HN

NH2HN

O

O

O

COOH

COOHHOHO COCH

H

H

H

H

H

H

H

(CH2)2 N

OH

C

O

CH3

CH2

NH

O

C

CH2

OH O

CH3CN(CH2)2

CH2

NH

O

CO2HHO C

C

CH2

C.

B.

A.

NH

O

HO OH

NH

O

NH2

O

O

HOO

N

O

HN O

HNOH

OHOON

NHO

O

N NH2

NH

HO

HO

H

H

H

Fe II and III - the redox cycle of Fe

The relative proportions of Fe(II) and Fe(III) in DFe in surface seawater

depend on the relative rates of reduction and oxidation

In well-oxygenated seawater Fe(III) is the thermodynamically stable form

Fe(II) is relatively soluble in seawater, Fe(III) is less so

Fe(II) is primarily produced photochemically and biologically, and is

rapidly reoxidised to dissolved Fe(III)

Fe(III) may be converted to Fe(II) by reductive processes including

photochemical, enzymatic, within micro-environments, & the formation of Fe(II)

organic complexes

Fe(II) Fe(III)

oxidation

reduction

Why is DFe so

unreactive?

• More than 99.999% of dissolved iron in

seawater is organically bound (Rue & Bruland,

1995)

• Fe(III) forms extremely strong organic complexes

• Upon addition of 7 nmol L-1 Fe (III) to seawater, >

50% was complexed to a strong ligand within 2

minutes (Wu and Luther, 1995)

• These organic complexes are referred to as iron-

binding ligands

OH

OH

N

O

NN

NN

NN

NH2

OH OOH

O

O

HN

NH2HN

O

O

O

COOH

COOHHOHO COCH

H

H

H

H

H

H

H

(CH2)2 N

OH

C

O

CH3

CH2

NH

O

C

CH2

OH O

CH3CN(CH2)2

CH2

NH

O

CO2HHO C

C

CH2

C.

B.

A.

NH

O

HO OH

NH

O

NH2

O

O

HOO

N

O

HN O

HNOH

OHOON

NHO

O

N NH2

NH

HO

HO

H

H

H

WHAT IS A LIGAND ?

The binding strength is measured electrochemically

and referred to as the

Conditional stability constant

So far two classes of iron-binding ligands

L1 or strong binding class [K FeL1, Fe(III)1 = 1013 L mol-1]

L2 or weaker ligand class [K FeL2, Fe(III)1 = 1011.5 L mol-1]

Ligands are molecules

characterised by high

binding strength

L1 are probably siderophores

L2 are released during particle breakdown (Boyd et al. 2010)

Who produces Ligands?

Lab studies show that marine bacteria produce siderophores with similar

conditional stability constants as L1 class ligands.

SIDEROPHORES are high affinity Fe(III) binding agents which form

the basis of iron-transport systems.

Microbes release siderophores externally, they bind and solubilise Fe

present in minerals, adsorbed onto particles or bound within existing complexes.

Field techniques are insufficiently sensitive to resolve the nature of FeL’s.

Case Study I

MESOSCALE IRON ENRICHMENTS

Boyd et al.

(2007)

SOLAS has conducted several major

Iron biogeochemical studies

50.1

50.12

50.14

50.16

50.18

50.2

50.22

-144.83 -144.80 -144.77 -144.74 -144.71

Long

Lat

-4.00 -2.00 0.00 2.00

-8.00

-6.00

-4.00

-2.00

0.00

2.00

4.00

C67

C68

C69

Iron is added to

the ocean along with

the tracer SF6

An initial area of

70 km2 is enriched

with Iron The formation of

a coherent

patch of high

SF6 is used to

track the

enriched waters

for weeks

The resulting bloom was large

enough to be viewed from space

The bloom as it develops

provides a laboratory to study

concurrent changes in physical,

chemical and biological signals

including climate-reactive gases

SERIES

10 15 20 25 30 35

mm

ol m

-2

0

100

200

300

400

500

Y A

xis

2

0

20

40

60

80

100

120

140

160

Days

DIFFERENT MICROBES

DOMINATE EACH PHASE OF

THE SERIES BLOOM

[POC]

Blooms and gas production /consumption Different groups influence gas concentrations in seawater

Direct Greenhouse

Stratospheric ozone

Tropospheric ozone

Sulfate

Fossil fuel soot

Biomass burning

Indirect tropospheric (CCN)

Solar

-2

-1

0

1

2

3

Glo

bal m

ean r

adia

tive f

orc

ing /

W m

-2

INDIRECT GREENHOUSE

DIRECT TROPOSPHERIC AEROSOLCO2

CH4

N2O

Halocarbon

CCN

Comparison of radiative forcing IPCC (1996)

CO2 and bloom development

Phytoplankton fix carbon

A small but significant

proportion of this C

settles out of the surface

ocean

To restore equilibrium

CO2 is drawn down into

the ocean

-40 -35 -30 -25 -20 -15 -10 -5 0 5

-10

-5

0

5

10

15

20

25

270

280

290

300

310

320

330

340

350

fCO2 (D18)

CO2 drawdown over

> 1000 km2

SERIES bloom

Chlorophyll (mg m-3)

Bloom

day 19

day 24

BLOOM DECLINE

CAN TAKE PLACE

VERY RAPIDLY

Boyd et al.

[2004]

7% of the

algal bloom

C was exported

to 125 m depth

Case Study II

Fe Biogeochemistry in HNLC waters

using the SF6 tracer – FeCycle

Boyd et al.

[2005]

Croot et al.

(2007)

20 40 60 80 100 120 140 160 180 200 220 240

-60

-40

-20

-100

1

3

5

10

25

50

100

200

300

400

500

600

700

800

900

SF6

Log SF6

-0.3

0.3

0.6

0.9

1.2

1.5

1.8

2.1

2.4

2.7

20 40 60 80 100 120 140 160 180 200 220 240

-60

-40

-20

Boyd et al. (2005)

Vertical & lateral iron supply terms from SF6

hours

Vertical

Diffusivity

PFe Export

Aeolian

Lateral

advection Biological

recycling

Mixed layer

Kilometers-10 -5 0 5 10 15

-10

-5

0

5

10

15

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Fluxes measured during FeCycle

Iron uptake & recycling

Boyd & Ellwood (2010

A

Fe uptake (nmol l-1 d-1)

0.00 0.05 0.10 0.15 0.20

Dep

th (

m)

0

10

20

30

40

50

total

> 20 µm5-20 µm2-5 µm0.2 - 2 µm

B

0.00 0.05 0.10 0.15 0.200

10

20

30

40

50

C

0.00 0.05 0.10 0.15 0.200

10

20

30

40

50

D

0.00 0.05 0.10 0.15 0.200

10

20

30

40

50

McKay et al.

(2005)

Vertical

Diffusivity (g)

15+3

PFe Export (h)

216+27 to 548+128

Aeolian

500 (a)

5 to 50 DFe (b) [9 µmol m-2]

450 to 495 PFe (c) [34 µmol m-2]

>1976 (f) 0 (d)

2453 to 4055 (e)

Lateral

advection Biological

recycling

Mixed layer

Boyd et al. (2005) i.e. a fe ratio of < 0.1

Iron biogeochemistry & climate change

Jickells et al. (2005)

ODP (red) & EPICA (black) dust and iron deposition records correspond

Large-scale changes in deposition i.e. most of the Southern Ocean

Suggests a CO2 drawdown of < 40 ppm from subantarctic iron fertilization

Stop press

How will these multiple supply mechanisms be altered?

Boyd & Ellwood

(2010)

Is a rapidly emerging field (SOLAS; GEOTRACES)

Adding iron to the oceans continues to attract commercial interest

and remains controversial

The impact of climate change on iron supply has many unknowns

New approaches such as iron stable isotopes hold great promise

IRON BIOGEOCHEMISTRY

Conclusions