Assessing the efficiency of iron fertilization on

atmospheric CO2 using an intermediate

complexity ecosystem model of the global ocean

Olivier Aumont1 and Laurent Bopp2

1IPSL / LODyC, Paris, France2IPSL / LSCE, Gif s/ Yvette, France

The Ocean in a High CO2 World

Introduction : The HNLC regions

The Iron hypothesisFe

The Iron fertilization experiments

A: IronexI

B: IronexII

C: SOIREE

D: EisenEx

E: SEEDS

F: SOFEX

G: Planktos

H: SERIES

AB

C D

E

F

G

H

Main results

Chlorophyll From a 3 to a 40-fold increase, generally as diatoms

pCO2 A 30 to 90 atm drawdown in surface pCO2

Export Production Contrasting results, generally an increase

DMS An increase

Mitigation of atmospheric CO2

Large scale iron fertilization

Iron fertilization can be used as a means of offsetting the anthropogenic carbone dioxide emission

(Martin et al., 1991)

Previous estimates (modeling studies)

Peng and Broecker, 1991Joos et al., 1991

Box models Southern Ocean Preindustrial: -17 to -59 atmAnthropogenic : -64 to -107 atm

Sarmiento and Orr, 1991 OGCMNutrient restoring

Global Ocean Preindustrial: -3 to -72 atm

Six and Maier-Reimer, 1993 OGCMHAMOCC

Southern Ocean Preindustrial: -34 atmAnthropogenic : -50 atm

Archer et al., 2000 OGCMHAMOCC

Global Ocean Preindustrial: -50 atm

Ganadesikan et al., 2003 OGCMNutrient restoring

Patchy, equatorial Pacific

Low efficiency, < 10% of increase in export production as atmospheric CO2

Questions

Iron fertilization

Can the model simulate the main features of the iron fertilization experiments ?

What is the spatial and temporal variability of the response to fertilization ?

What is the long-term efficiency of the fertilization ?

Outline1. Model description

2. Patchy iron fertilization

3. long-term iron fertilization on the global scale

Tools : Models

OPA PISCES

PO43-

Diatoms

MicroZoo

P.O.M

D.O.M

Si

IronNano-phyto

Meso Zoo

NO3-

NH4+

Small Big

Euphotic Layer (10-200m)

Chlorophyll surface concentrations

Seawifs(98-03)

PISCES

June January

Iron distribution

Annual mean, surface

Annual mean, 1000m

0.1

0.5

1

1.5

3

5

0.1

0.5

1

1.5

3

5

What limits diatoms growth ?

NO3 + NH4 PO4 Fe Si

Iron Fertilization

‘‘ Patchy ’’ Iron Fertilization in the three main HNLC regions

Experimental design

- Iron concentration set to 2 nM in the mixed layer at day 2 and 5

- The model is integrated for 31 days

- Fertilization applied over only one grid box

Iron Fertilization : The Southern Ocean (1)

2. Small response (Chl < 0.7 mg Chl m-3)

3. Moderate response (0.7 < Chl< 2.5 mg Chl m-3)

4. Strong response (2.5 mg Chl m-3 < Chl )

1. Blooming conditions (Chl > 1.5 mg Chl m-3)

0

2.0

4.0

6.0

0

20

40

60

80

Chla

Diatoms relative abundance

The Southern OceanpCO2 (atm)

Export ()

-100

-60

-20

0

0

40

80

120

2. Si limitation, Si initial < 6 umol L

3. Mixed layer depth > 30 m, macronutrient replete

4. Favorable conditions, strongly iron limited

1. Stratification, ice retreat

Why such responses ?

The Southern Ocean : Comparison with data

pCO2 (atm)

Diatoms relative abundance

Chla (mg Chl m-3)

SOFEX South

SOFEX North

SOIREE

Seasonal evolution

January February

July November

Iron Fertilization : The equatorial Pacific

Diatoms relative abundance

Chla (mg Chl m-3)

IRONEX II

pCO2 (atm)

export (%)

Iron Fertilization : everywhere & 50 yr longChanges in Diatoms Relative Abundance

+1

-1

Export Production (GtC/yr)

7

8

-10 0 10 20 30 40 50Years

Fe FertilizationIncrease of Export Production

+50

+5

-5

-50

(gC/m2/yr)

+4

-4

+0.2

+1

-0.2

-1

Changes in Chla (mg Chl m-3)

-10 0 10 20 30 40 50

Years

Fe Fertilization

Atmospheric pCO2(atm)

Carbon Flux(PgC/yr)

1

0

0

Impact on atmospheric pCO2

Preindustrial conditions

-10

-4 atm in 10 yr

Fe Fertilization

0.5

00

-10

7

8

Atmospheric pCO2(atm)

Carbon Flux(PgC/yr)

Export(PgC/yr)

-1.8 atm in 50 yr

-10 0 10 20 30 40 50

-8 atm in 50 yr

>80% due to Southern Ocean

Why such a small efficiency ?Nutrient Limitation of Diatoms Growth

NO3 / NH4 PO4 Fe Si

Control

Fe Fert.

Light limitation

-10

-2

2

-1

1

NO3 (mol/L)NO3 (mol/L)

1

4

10

20

Iron Fertilization : Implications for the Sulfur Cycle

Changes in Surface DMS Concentrations

Export Production

Atmospheric pCO2

Carbon Flux

-8 ppm in 50 yr

-5

-0.5

+0.5

+5

nM

7

8

1

00

-10

DMS Flux(TgS/yr)-15 %

-10 0 10 20 30 40 50

22

26

Conclusions

Patchy Iron fertilization :

The model roughly captures the main features of in situ iron fertilization experiments, except in the North Pacific.

In the Southern Ocean, the response depends highly on the location and the time period of the iron release. Main controlling factors are Si concentrations, the mixed layer depth, and the status of the ecosystem.

The favorable season extends from November to March.

Large-scale Iron fertilization :

Very low efficiency : only 8 ppmv drawdown in atmospheric pCO2 after 50 years.

Iron fertilization should be done continuously to keep the additionally stored CO2 within the ocean.

Possible drawbacks : N2O production, extension of the anoxic regions, changes in the fisheries, possible decrease in DMS production, …

Diatoms relative abundance : vs Data

1

0.8

0.6

0.4

0.2

0

Data from Gregg et al. 2003

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

60°S

40°S

20°S

0°

20°N

40°N

60°N

80°N

Iron Fertilization : The North Pacific

Diatoms relative abundance

Chla (mg Chl m-3)