Open Ocean Iron Fertilization Experiments From IronEx-I Through SOFEX: What We Know and What We Still Need to Understand Johnson, K S Chase, Z Elrod, V.

Open Ocean Iron Fertilization Experiments From IronEx-I Through SOFEX: What We Know and What We Still Need to UnderstandJohnson, K SChase, Z Elrod, V Fitzwater, S Plant, J Coale, K Hunter, C Gordon, R MBuesseler, K

Eight open ocean, iron fertilization experiments have now been conducted.

Open ocean iron enrichment experiments that have been conducted to date. All show a strong ecosystem response to iron addition:

IronEx I: Equatorial Pacific, 1993. 3-fold increase in chlorophyll. Patch subducted 4 days into the experiment.

IronEx II: Equatorial Pacific, 1996. 10-fold increase in chlorophyll, 90 µatm draw down in CO2, 5µM drawdown in NO3.

SOIREE: Pacific sector of Southern Ocean, 1999. South of Polar Front. 6-fold increase in integrated chlorophyll, 25 µatm draw down in CO2, 2 µM draw down in NO3.

EisenEx: Atlantic sector of Southern Ocean, 2000. Dispersion into an eddy, 4x increase in chlorophyll.

SEEDS: Western subarctic Pacific Ocean, 2001. 40-fold increase in chl, 13 µM draw down in NO3.

SOFeX North: Pacific sector of Southern Ocean, 2002. North of Polar Front. >10x increase in chlorophyll, >40 µatm draw down in CO2.

SOFeX South: Pacific sector of Southern Ocean, 2002. South of Polar Front. >10x increase in chlorophyll, >40 µatm draw down in CO2.

SERIES:Eastern subarctic Pacific Ocean, 2002. ~10 µM NO3 draw down.

0

500

1000

1500

2000

2500

3000

Tot

al C

hlo

rop

hyl

l a

(ng/

L)

-2 3 8 13 18 23 28

Time Since Initial Fe Addition (days)

SOFeX Southern Patch (0-30 m)

In

Out

PrePhytoplankton BiomassWe have developed a good (but not perfect) understanding of the role of iron in stimulating a shift-up in the ecosystem.

Day of Experiment

0 2 4 6 8 10 12 14 16 18

Phy

topl

ankt

on C

arbo

n (µ

M)

0

2

4

6

8

10

Car

bon

Flu

x (m

mol

m-2

d-1

)

0

10

20

30

40

50

60InOutC Flux

In previous experiments, only IronEx II shows an appreciable carbon export signal. This experiment was not designed to study export (e.g. only 6 234Th obs.) and we do not have a good understanding of bloom termination. Is SOFEX the solution?

50m flux

Weeks post 1st Iron addition

PO

C F

lux

(mM

C m

-2 d

-1)

0

2

4

6

8

10

12

14

16

18

INOUT

1 2 3 4

100m flux

Weeks post 1st Iron addition

PO

C F

lux

(mM

C m

-2 d

-1)

0

2

4

6

8

10

12

14

16

18

1 2 3 4

SOFEX South shows a modest increase in carbon flux. Iron fertilized export is not higher than fluxes measured during AESOPS (10 to 16 mmol/m2/d). Only ~ 3%/d of carbon is exported (=14 mmol C/m2/d /(14 mmol POC/m3 * 30 m mixed layer))

SOFEX North Patch as seen by MODIS on Feb. 5.

The 15 km x 15 km patch created on Jan. 12 to 14 had grown to be 4 km x 230 km and still blooming.

-169.7 -169.5 -169.3 -169.1-54.5

-54.4

-54.3

-54.2

-54.1

-54

0

0.04

0.08

0.12

0.16

0.2

0.24

0.28

0.32

0.36

Fe (nM )N o. Patch R evisit

-169.7 -169.5 -169.3 -169.1-54.5

-54.4

-54.3

-54.2

-54.1

-54

N orth Patch SF6Feb. 8-9

0.032

0.036

0.04

0.044

0.048

0.052

0.056

0.06

0.064

0.068

0.072

-169.7 -169.5 -169.3 -169.1-54.5

-54.4

-54.3

-54.2

-54.1

-54

2

4

6

8

10

12

14

16

C hloro. F luorN o. Patch R evisit

-169.7 -169.5 -169.3 -169.1-54.5

-54.4

-54.3

-54.2

-54.1

-54

6.55

6.65

6.75

6.85

6.95

7.05

O xygen (m l/L)

SOFEX North Patch survey by Revelle on Feb. 8/9

North Patch iron drops ~20x (4/0.2) in 22 days following last addition. Loss rate is ~15%/day.

SF6 drops ~70x (100/1.4) in same time frame. Loss rate is ~21%/day. Gas exchange ~11%/d in South Patch. Horizontal diffusion ~21%-11% = 10%/d.

Iron loss rates (15%/d) are ~10%/d by diffusion and ~5%/d by bio. uptake and scavenging.

SF6 (fM)

0 25 50 75 100 125 150 175

Fe

(nM

)

0

1

2

3

4

5

Fe = 0.55 + 0.026 SF6

R2 = 0.54

No. PatchJan. 17/18

No. PatchFeb. 7/8

SF6 (fM)

0 1 2 3 4 5

Fe

(nM

)

0.0

0.1

0.2

0.3

0.4

0.5Fe = -.05 + 0.11 SF6

R2 = 0.41

( )InitialValues

Year Day

25 30 35 40 45 50

SF

6 (f

M)

0

50

100

150

200

250

300

SF6 Loss by

Gas Exchange

RevelleMelville

Polar Star

SF6 loss in South Patch is dominated by gas exchange (11%/d mean). Mixing losses of SF6 and, presumably, iron are quite small (3 to 7%/day).

What happens in the South Patch? We add iron as Fe(II), which our analytical method doesn’t measure. In warm water, Fe(II) is rapidly (minutes) oxidized to Fe(III), which we do measure. Oxidation rate is slow in So. Patch (days).

Time

1/26/02 0:00 1/27/02 0:00

Iron

(nM

)

0.0

0.1

0.2

0.3

0.4

0.5

0.6End So. PatchFert. 1

SF6 (fM)

0 50 100 150 200

Fe

(nM

)

0.0

0.4

0.8

1.2

Fe = 0.25 + 0.000 SF6

R2 = 0.00

So. PatchJan. 26/27

0 50 100 150 200

Fe

(nM

)

0.0

0.4

0.8

1.2

Fe = 0.25 + 0.000 SF6

R2 = 0.00

So. PatchJan. 26/27

So. PatchJan. 30/Feb. 1

0 50 100 150 200

Fe

(nM

)

0.0

0.4

0.8

1.2

Fe = .43 + 0.004 SF6

R2 = 0.24

SF6 (fM)

0 50 100 150 200

Fe

(nM

)

0.0

0.4

0.8

1.2

Fe = .25 + 0.005 SF6

R2 = 0.61

So. PatchFeb. 5

After 1st Fe addition - +0.7 nM total

After 2nd Fe addition - +1.4 nM total

After 4th Fe addition - +2.8 nM total

Low temperatures (-0.6°C) in South Patch reduced Fe(II) oxidation rate and iron losses were likely even lower than in N. Patch. What is impact on biota?

0 4 8 12 16

<0.

4 µ

m Ir

on (

nM)

0.0

1.0

2.0

3.0

4.0IronEx II

FIA FeAAS Fe

Time (d)

0 4 8 12 16

SF

6 (f

M)

0

50

100

150

200

SF6 = 257 e-0.48*t R2 = 0.98

Fe = 2 e-0.6*t

In contrast, Fe loss during IronEX II was very rapid, due to extreme rates of horizontal mixing: SF6 lost at 48%/d! Is that why IronEx II had export?

Conclusions:• Major loss of iron in SOFEX North Patch was mixing & not uptake or scavenging.• Phytoplankton were probably never iron limited in the North Patch. Would large amounts of export have occurred?

• Iron oxidation rate in SOFEX South Patch was very slow (days). Iron was likely lost at a much slower rate than in North Patch.

• Were phytoplankton iron limited in the South Patch? Concentrations were probably not extremely low over the 28 day observing period.

• Observing export might require an experiment longer than 28 days or warmer water.

Thanks to REVELLE, MELVILLE & POLAR STAR scientists & special thanks to NSF & MBARI for support