Abby Ren, Princeton

Daniel Sigman, Princeton

Nele Meckler, Caltech

Rebecca Robinson, URI

Yair Rosenthal, Rutgers

Gerald Haug, ETH

Foraminifera-bound nitrogen isotopes evidence for reduced nitrogen fixation in the Atlantic Ocean during the last ice age

Pleistocene glacial/interglacial cycles

(C,N,P)org NO

3

-,PO4

-3=0

atmosphere

CO2, NO3

-, PO4

-3

Low-Latitude Biological Pump

thermocline

Biological pump

Low Latitude: Change nutrient inventory

High Latitude: Change nutrient

consumption

Low vs. High Latitude

(C,N,P)org NO

3

-,PO4

-3=0

atmosphere

CO2, NO3

-, PO4

-3

Low-Latitude Biological Pump

thermocline

Low-latitude biological pump

~3 kyr ~15-80 kyr

N2

N2

denitrification

N fixation

Broecker, 1982

N isotopes in nature: 14N = 99.64% 15N = 0.36%

δ

15

(‰ . ) N vs air =

(

15

/N

14

)N

sample

(

15

/N

14

)N

air

- 1

⎛

⎝

⎜

⎜

⎞

⎠

⎟

⎟

* 1000

N isotope terms

MS 172 Figure 5

0

5

10

15

20

0 0.5 1 1.5 2

[NO3-] (factor of initial value)

newly fixed Nadded

δ15N~ -2-0‰

water columndenitrification

ε ∼ 20-30‰

sedimentarydenitrification

ε ∼ 0‰

nitrateuptake

ε ∼ 5‰

NO3- δ15N

(‰ . )vs air

Effects of major N fluxes on nitrate δ15N

Lower water column denitrification during LGM

302520151050age (ka)

14

12

10

8

6

4

δ15

(‰ N

. vs

)air

Denitrification increase

Santa BarbaraBasin

Arabian Sea

Chile Margin

Chile: de Pol-Holz et al, 2006SBB: Emmer and Thunell, 2000AS: Altabet et al., 2002

Low δ15N of nitrate in the North Atlantic thermocline: Paleoceanographic utility

[NO3-] (µM)

δ15N of nitrate (‰ vs. air)

Knapp et al., 2005

Planktonic foraminifera

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Orbulina universa

photo: H. Spero Montoya et al., 2002

Study sites

10

8

6

4

2

0

Foraminifera-bound

δ15

(‰ N

. vs

)air

-BarbudaAntiqua

LittleBahama

Bank

GreatBahama

Bank

Indonesia South Pacific South Pacific

Atlantic Pacific

Sargasso Sea subsurface

Indonesia Seasubsurface

Subantarctic

37 15.64 º ' S176 40.02 º ' E

36 22.63 º ' S177 .26.75 º ' E

6 46.25 º ' S116 58.47 º ' E

Mode Water

. G ruber . G sacculifer . O universa . G menardii . N dutertrei . G truncatulinoides . H pelagica . G inflata

Thermocline nitrate vs. coretop foraminifera

Ren et al., 2009

8

7

6

5

4

3

2

δ15

(‰ N

. vs

)air

302520151050 ( )age ka

-2

-1

0

δ18

O C

(‰

. vs

)VPDB

6

5

4

3δ15

(‰ N

. vs

)air

8

7

6

5

4

3

2

δ15

(‰ N

. vs

)air

a.

b.

c.

d.

individual species

bulk sediment

mixed species

O. universa G. sacculifer G. ruber

> 355 µm 250~355 µm 125~250 µm

δ18O, age:Schmidt et al., 2004

LGMHolocene

ODP site 999A:

Caribbean Sea

Ren et al., 2009

1000

800

600

400

200

depth (m)

6543210-1δ15 (‰ N . vs )air

100

80

60

40

20

0

. G ruber

. G sacculifer

. O universa

3.1‰

Modernδ15 :N suspended POM nitrate

- Foraminifera boundδ15 :N Holocene LGM :LGM interpretation

POM: Altabet, 1988NO3

- : Knapp et al., 2005

N fixation rate:LGM/Holocene ~ 20%

Ren et al., 2009

Chile: de Pol-Holz et al, 2006SBB: Emmer and Thunell, 2000AS: Altabet et al., 2002Cariaco: Haug et al., 1998

7

6

5

4

3

2

δ15

(‰ N

. vs

)air

302520151050 ( )age ka

14

13

12

11

10

9

8

7

6

5

4

δ15

(‰ N

. vs

)air

Denitrification increase

N fixation increase

Santa BarbaraBasin

Arabian Sea

Cariaco Basin

:Caribbean Sea . O universa . G sacculifer . G ruber

Chile MarginDeglacial increases

in both denitrification

and N fixation

A stable N inventory regulated by N/P ratio?

Gruber and Sarmiento, 1997

~1P:16N

Fix!

Don’t Fix!

Deglacial scenario:N fixation feedback through N/P sensitivity

Summary

• In Caribbean Sea sediments, planktonic foraminiferal 15N/14N decreases from the last ice age to the current interglacial.

• The foraminiferal change is best explained by less N fixation in the Atlantic during the last ice age, leading to higher nitrate 15N/14N in the Caribbean thermocline.

• The reconstructed increase in N fixation at the end of the last ice age is most likely a response to the previously recognized deglacial increase in global denitrification.

• As with our findings regarding the modern Atlantic N fixation rate, this is consistent with a strong P control on N fixation.

• A significant glacial/interglacial change in the nitrate reservoir has not been ruled out. However, our evidence that Atlantic N fixation acts as a negative feedback argues qualitatively for limits to such a change.

Acknowledgement

• Princeton University • Consortium of Ocean Leadership

• Sigman Group: Franky Wang, Brigitte Brunelle,

Julie Granger, Peter Difore.

• J. Bernhard and D. McCorkle

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Nitrogen cycle

8

7

6

5

4

3

2

δ15

(‰ N

. vs

)air

302520151050 ( )age ka

-2

-1

0

δ18

O C

(‰

. vs

)VPDB

6

5

4

3δ15

(‰ N

. vs

)air

8

7

6

5

4

3

2

δ15

(‰ N

. vs

)air

a.

b.

c.

d.

individual species

bulk sediment

mixed species

O. universa G. sacculifer G. ruber

> 355 µm 250~355 µm 125~250 µm

Minimal Holocene δ15N

decreasein

bulk sediment

δ18O, age:Schmidt et al., 2004

Ren et al., 2009

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Foraminifera-bound N isotope analysis

•Sieve, pick, crush

•Clean fragments with wet oxidation

•Acid dissolution to release

internal Norg

•Norg NO3- (persulfate oxidation)

•NO3- N2O (denitrifier method)

•N2O isotopic analysis (continuous

flow, purge/trap, gas

chromatography, gas-source magnetic

sector mass spectrometry)

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Carbon inventory

Fig. S1

20

15

10

5

0

N content (µmol/g)

20151050

cleaning time (hours)

7

6

5

4

3

2

1

0

δ15

(‰ N

. vs

)air

( / )N content µmol g δ15 (‰ )N

Fig. S2

7

6

5

4

3

2

1

0

foraminifera-bound N content (µmol/g)

302520151050age (ka)

70

60

50

40

30

20

10

0

bulk sedimentary N content (µmol/g)

O. universa G. sacculifer G. ruber > 355µm 250~355µm 125~250µm bulk N content

Thermocline nitrate/surface sediment comparison

Altabet, 2005

-24.5

-24.0

-23.5

-23.0

-22.5

-22.0

-21.5

δ13

(‰ C

. vs

)VPDB

302520151050 ( )age ka

10

9

8

7

6

5

/ ( )C N weight ratio

/C N δ13 (‰ )C

Downcore changes in δ13Corg and Corg/TN

40

30

20

10

0

TN (µmol/g)

4003002001000Corg (µmol/g)

LGM Holocene

TN=0.0563(±0.0143)*Corg + 21.8695(±4.8225)

R2=0.36

TN=0.0549(±0.0131)*Corg + 14.6769(±4.3894)

R2=0.83

C/N~15 (by weight) terrestrial organics

Evidence for terrestrial and shelf materialin ODP 999A sediments

-22.0

-21.5

-21.0

-20.5

-20.0

-19.5

-19.0

δ13

(‰ C

. vs

)VPDB

30252015105 ( )Age ka

10.0

9.5

9.0

8.5

8.0

7.5

/ ( / )C N g g

45

40

35

30

25

(%)Carbonate content

δ13 (‰ C . vs )VPDB / ( / )C N g g (%)Carbonate content

Fig. S4

Bulk sediment δ15N records from the North Pacific

Kao et al., 2008

Bulk sediment recordsfrom diverse settings:Mean ocean nitrate δ15N did not decrease (much) into the Holocene.

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0δ15

. (‰ N of N Atlantic thermocline nitrate

. vs

)air

1.41.21.00.80.60.40.20.0 fraction of modern N fixation

Holocene

LGM

Estimating N fixation change

Surfacechlorophyll

Surfacenitrate

Incomplete nutrient consumption in the polar ocean

WINDS

Siegenthaler (1983)

THERMOCLINE

35NO3-

232CO2

10Norg66Corg

25NO3-

166CO2

›

Eq. S

Rapid surface-deep exchange in the polar ocean releases deeply sequestered CO2 to the atmosphere

Mechanisms for reducing the Southern Ocean CO2 leak

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4000

3500

3000

2500

2000

1500

1000

500

04 4.5 5 5.5 6 6.5 7

4000

3500

3000

2500

2000

1500

1000

500

020 25 30 35 40

[NO3-] (µmol/kg) δ15NO

3- (‰ vs. air)

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0

5

10

15

20

25

30

4

6

8

10

12

14

16

40 45 50 55 60 65 70 75latitude along ~140°E (°S)

0

5

10

15

20

25

30

4

6

8

10

12

14

16

40 45 50 55 60 65 70 75

[NO3-]

δ15NO3-

RITS '94 - central Pacific

The link between theN isotopes and nitrateconsumption

4.0

3.6

3.2

2.8

δ18 O

. ( ) N pachyderma sin

(‰ . )v PDB

100806040200 ( )age ka

6

5

4

3

2- diatom bound

δ15

(‰ . )N v air

δ18O + age: Ninneman and Charles, 1997

• More complete nitrate consumption in glacial Subantarctic

Subantarctic (E11-2) diatom-bound 15N/14N: Link to glacial/interglacial cycles

Mahowald et al., 2005

modern annual average dust deposition (g m-2 yr-1)

Subantarctic is well situated fordust-driven iron fertilization during the

ice ages

LGM

Antarctic 15N/14N:higher during the last glacial maximum

Subantarctic nutrient drawdown:CO2 ~40 ppm

AA deep water formation “off”:CO2 ~35 ppm

Polar AA nutrient drawdown:More CO2 without complete deepwater formation shutdown

Glacial/interglacial changes in the Southern Ocean inferred from the N isotopes

Low and High latitude connection

Lower rates of N fixation