1

Iron studies during JGOFS

Philip Boyd

• Fe and JGOFS - historical perspective

• Fe limitation and the Biota

• Biogeochemical cycling of Fe

• Reappraisal - Fe supply and the global C cycle?

IRON and CLIMATE

Marine biota play a key role in climateby regulating atmospheric CO2 levels

One means of regulation is via the BIOLOGICAL PUMP

The PUMP works mostefficiently when all of the available nutrientsare utilised

2

0.5

1.0

1.5

Fe (

mol/kg ice)

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 atmosphericCO2 (Martin, 1990)

Historicalperspective

• S. Ocean Paradox - Gran, Hart

• Trace metal chemistry and contamination (Patterson, Bruland, Martin) - improved techniques

• Shipboard Fe enrichments - Martin, de Baar

• The link to climate - Fe Hypothesis - Martin

• In situ Fe enrichments - SF6 - Watson, Duce, Liss, Martin

0.5

1.0

1.5

Fe (

mol/kg ice)

180

200

220

240

260

280

300

[CO

2] (

ppm

v)

20 40 60 80 100 120 140 160

Age (1000 yr)

Iron

CO2

3

Mahowald et al.,JGR 104,15895-15916 (1999)(g m-2 yr-1) N. Mahowald et al. (1999)

JGOFS Sites in relation to (modelled) dust deposition

Time-series

Field studies/surveys

JGOFS STUDIES IN HNLC POLAR WATERS

There were also major studies conducted in tropical (EQPAC) and subpolar (N PACIFIC) HNLC waters

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Fe limitation and the biota

1) Shipboard Fe enrichments

2) Oceanic surveys - spatial relationships

3) Molecular and other proxies of Fe stress

4) In situ mesoscale Fe additions

Shipboard FeEnrichments

S. Ocean - Martin, de Baar…...EqPac - Coale, Price…..NE Pacific - Martin, Coale…….

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Oceanic surveys - spatial relationships

de Baar et al. (1995) - Polar Front (Atlantic sector)Iron, chlorophyll and CO2 drawdown

500

1000

1500

2000

2500

Fla

vodo

xin

(R.U

.)

1 2

[Fe] (nmol L-1)

Sep 95May 95

Molecular and other proxies of Fe Stress

LaRoche et al. (1996)NE Pacific Flv can substitute for Fd to alleviateiron stress by reducing the cell’sFe requirements

Other proxiesFRRF - biophysical (Falkowski….)Nutrient uptake kinetics (Coale, Timmermans….)

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In situ Fe enrichments - (10 km length-scale)

IronEx I - subducted after 3-4 days, increase in Fv/Fm

IronEx II - chlorophyll 0.3 to > 3 mg m-3, CO2 and DMS

Behrenfeld et al. (1996)

Fe limitation and mesoscale Fe enrichments in the S. Ocean

Largest inventory of unused nutrientsDeep-water formationOther limiting factors? Mixed layers, Si, Krill?

SOIREE Summer, Pacific sectorEisenex - Spring, Atlantic sectorSOFEX - Summer - Pacific sector

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In situ experiments have yielded similar trends

Cellular chlorophyll

Cell size

Increased growth rate, C fixation, Fv/Fm………..

Altered Si:C uptake ratio’sFloristic shifts

DMSP C fixation

Fe-

bin

din

g li

gan

ds

(nM

)

0.0

0.5

1.0

1.5

2.0

2.5

3.0L1L2

A

B

Days0 2 4 6 8 10 12 14 16 18

0

2

4

6

8

10

12

Fe

FeBut there havealso been differenttrends betweentropical and polar HNLC in situexperiments

Iron-binding ligandsand their production

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Fv/Fm

Fe uptake kineticsMesozoo #

Microzoo grazing

Nutrients

Zn, Mn, Cd

Algal sinking rates

Fe stress markers

Bacterial #

POC, PON

DOC

Optics

DMSPp

Bact prod

Phyto #

tau

1ry prod

EXPORT (Th/U, traps)

VERTICAL

PROFILES

Diffusivity

BIOGENIC GASES

OpalFe remineralizationSi production

Pigments

FeL

Redox spps

DICPFe

DFe

C and Nisotopes

Particlesize spectra

Algal respiration

Nitrous oxide

CH4

Fe`, FeLKd

In situ experimentspermit the concurrentexamination of many parameters

permittingthe construction ofFe budgets

and validation datafor models

19 pM

Dissolved FeParticulate Fe

(inorganic and detrital)

SOIREE FeSOIREE Feenriched patchenriched patch(140(140ooE, 61E, 61ooS)S)

OUTpatch

INpatch

Upper mixedUpper mixedlayer 65 mlayer 65 m

Atmospheric flux

Sinking biogenic Fe(from sediment trap data,

mostly diatom aggregates)

Patch areaPatch area56 to 200 km56 to 200 km22

Upwelling

?

0.004 pM d-1 185 pM d-1

?

Sinking non-biogenic Fe

72 pM d-1

Biogenic Fe

Diffusive flux(pre-infusion)

Eddy diffusion(post-infusion)

Four Fe infusions over 13 d(ferrous sulphate solution)

Horizontaldispersion

180 pM d-1

1073 pM 1110 pM

negligible0.1 pM d-1 0.85 pM d-1 0.72 pM d-1

58 pM

11.9 7.3

11.9 3.3

Producers Micro-zooplankton

Meso-zooplankton

?

Sinking mesozooplankton biogenic Fe

(predicted from biogenic budget)

0.79 pM d-1

6.5

25.2 pM

Fe budget from SOIREE (Bowie et al. 2001)

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0

100

200

300

400

500

600

0 100 200 300

time from initial release (hour)

fmol

SF

6 l-1

field

model

Dissolved Fe

0

1

2

3

4

0 10 20 30 40 50 60

days from initial iron release

µm

ol m

-3

field

model

pCO2

250

300

350

400

0 10 20 30 40 50 60

µat

m

online CO2model (OUT)model (IN)

Chlorophyll a

0

1

2

3

4

5

1 11 21 31 41 51

µg

Chl

a l-1

> 20µm

< 20µm

total

Numerical modelling of SOIREE (Hannon et al., 2001)

Biogeochemical cycling of Fe

What are the relative contributions of

Atmospheric inputs

Upwelling Martin Dust deposition is dominantin the NE Pacific

de Baar; MeasuresUpwelling is the dominant driver in the S. Ocean

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Remote-sensing (TOMS, SeaWiFs) has enabled us to monitor episodic dust events from source to sink

Other iron supply mechanisms

Ice melt (Sedwick, DiTullio)Offshore transport of Fe in eddies (Crawford, Whitney)Coastal ocean - geomorphology (Bruland, Hutchins)

Upwelling - Cromwell undercurrent (Coale, Chavez)

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THE ‘FERROUS’ WHEEL (Kirchman, 1997)

Evidence that the microbial loop rapidly recycles Fe

Heterotrophic bacteriaHeterotrophic nanoflagellatesViruses ??

DFe (nM)

0.00 0.05 0.10 0.15 0.20 0.25 0.30D

epth

(m

)0

50

100

150

200

250

Spring 61SSummer 61S

(Measures & Vink, 2001)Dissolved Fe profiles:little decrease in DFe duringthe ‘growth season’

Relatively flat profile compared to macro-nutrients

0.5

1.0

1.5

Fe (

mol/kg ice)

180

200

220

240

260

280

300

[CO

2] (

ppm

v)

20 40 60 80 100 120 140 160

Age (1000 yr)

Iron

CO2

IRON AND THE OCEANICC CYCLE

What has been learnt during JGOFS

The Fe Hypothesis

Tenet 1 - Fe would increase phytoplankton growth

Tenet 2 - Fe would increase Csequestration ? Modelling - yes, Field data ??

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0180

xCO

(pp

mv

)2

Age (kyr )BP

200

220

240

260

280

0.0

0.5

1.0

1.5

100 200 300 400

Dus

t (p

pm)

180

xCO

(pp

mv)

2

200

220

240

260

280

0.0

0.5

1.0

1.5

Dus

t (pp

m)

Age (kyr )BP

10 20 30 130 140 150 240 250 260 330 340

Watson et al.(2000)

SOIREECO2 drawdownSi:C uptake ratio’s

Glacialspredicted timing andmagnitudes matchrecords well

Glacial terminationsFe supply - 50%of 80 ppm CO2 change

Modeling

In situ experiments display increases in chlorophyll, but not always increases in export - WHY?

Fig.CourtesyK. Buesseler

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A

Days

0 2 4 6 8 10 120.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7Fe watersHNLC waters

B

0 2 4 6 8 10 12

Fv/

Fm

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Eisenex(Gervais et al. 2002)

Days0 5 10 15 20

Fv/

Fm

0.20

0.25

0.30

0.35

0.40

0.45

0.50

IronEx II

SOIREE

SOFEX

Eisenex

Time-series of Photosynthetic competence provides some clues

Why the exceptional longevity of SOIREE?

INHNLC watershi Silow Felow diatomslow Chl

Fe Patch increased from 50 to 1100 km2

OUTlow Sihi Fe*hi chlahi diatoms

Stretching via horizontal flows

Mixing of water by horizontal diffusion

SOIREE vs. polar blooms

Days0 10 20 30 40 50 60

Ch

loro

ph

yll (

mg

m-3

)

0.0

0.3

0.6

0.9

1.2

1.5

1.8

2.1

2.4

SOIREE APFZ bloom (Abbott et al., 200)BSi max flux (Honjo et al., 2000)

Few data available on the temporal evolution of polar blooms

Mooring data in the vicinity of the APFZ (60S, 170W) from 10 bio-opticalarrays provides excellent coverage

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DOES ‘DILUTION’ OF CELLS PREVENT AGGREGATION

The Future

• Improved global/seasonal coverage of DFe profiles

• Better understanding of Fe biogeochemistry - Fe:C ratios, scavenging, remineralisation length scales

• More data to explore the links between Fe supply, bloom termination and export (and C sequestration)

• Use of SF6 approach for other manipulations - DISCO, FeCycle

• The Fe-phytoplankton link is an example of what can be achieved for other key groups - N Fixers, Calcifiers...

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Conclusions

• During JGOFS in situ mesoscale expts have confirmed the key role that iron plays in the ocean C Cycle

• Expts have placed JGOFS field studies into context - IronEx II and EQPAC

• Iron supply results in similar trends in tropical - polar HNLC waters

• Some divergences remain - Fe-L production