Nitrate isotopes as a constraint on the ocean's fixed N budget Angela Knapp, University of Southern California Peter DiFiore, Princeton University Curtis

Nitrate isotopes as a constraint on the ocean's fixed N budget

Angela Knapp, University of Southern California

Peter DiFiore, Princeton University

Curtis Deutsch, UCLA

Fred Lipschultz, BIOS

Sargasso Sea nitrate N isotopes, GCM experiments,and Atlantic N2 fixation

Daniel Sigman, Princeton University

Talk outline

• Nitrate N isotopes from a Sargasso Sea transect: estimate of “recently” fixed nitrate in Sargasso Sea water column

• MOM3 simulations forced with various Atlantic N2 fixation fields predict recently fixed nitrate in the Sargasso Sea

• Uncertainties: impact of other processes on the N isotopes of nitrate

• Nitrate O isotopes: correct nitrate N isotopes for nitrate assimilation?

N and O isotopes

N : 14N 99.6337% 15N 0.3663%

O : 16O 99.7630% 17O 0.0375% 18O 0.1995%

15N (vs. atm. N2) = (((15N/14N)sample/(15N/14N)air) - 1)*1000‰

18O (vs. VSMOW) = (((18O/16O)sample/(18O/16O)VSMOW) - 1)*1000‰

Nitrate reduction:14NO3

- ⇒ 14NO2-

15NO3- → 15NO2

-

15ε = ((14k/15k) - 1)*1000‰

N16O3- ⇒ N16O2

-

N18O16O2- → N18O16O-

18ε = ((16k/18k) - 1)*1000‰

15ε ~ 15N 3NO - - 15N 3NO - consumed (inst.)18ε ~ 18O 3NO - - 18O 3NO - consumed (inst.)

Kinetic isotope effect

14k

15k

16k

18k

MS 172 Figure 5

0

5

10

15

20

0 0.5 1 1.5 2

[NO3-] (factor of initial value)

newly fixed Nadded

15N~ -2-0‰

water columndenitrification

ε ∼ 20-30‰

sedimentarydenitrification

ε ∼ 0‰

nitrateuptake

ε ∼ 5‰

NO3- 15N

(‰ . )vs air

Complementary constraints from[NO3

-], N* and 15N

N*

Dep

thBATS Validation Cruise 32 (October, 2002):Stations sampled for nitrate and DON isotopes

IncreasingN2 fixation?~2 yr vent. age increase

Nitrate data

1000

800

600

400

200

0

depth (m)

2520151050

[NO3-] (µM)

1086420

15N (‰ vs. air)

108642

δ18

O (‰ vs. VSMOW)

28.0

27.5

27.0

26.5

26.0

25.5

25.0

sigma theta

BATS, 32º N

32º N, 76º W

30º N 29º N 28º N 27º N 26º N 25º N 24º N 23º N 22º N 20º N 19º N cruise

avg

a b c

d e fN2 fix.?

NO3-

assim.?

Nitrate 15N in the Sargasso Sea water column

15N of nitrate (‰ vs. air)

[NO3-] (µM)

•No evidence of southward N2 fixation increase or a southward accumulation of newly fixed nitrate in the thermocline•Basis for average Sargasso Sea profile

1200

1000

800

600

400

200

0

depth (m)

543210-1-2[NO3

-] from N2 fix (µM)

654321NO

3

- 15 (‰ . )N vs air

1.00.80.60.40.20.0-0.2 fraction of NO3

- from N2 fix

a b c

Estimating ‘recently’ fixed nitrate

(15N - 15Nimported)

(15Nnew - 15Nimported)f =

(5.3‰ - 15N)

6.3‰=

From a recently fixed nitrate pool to aN2 fixation rate

Atlantic

Atlantic

S N

S N

“Plan A”

“Plan B”

Deutsch et al. 2007 Atlantic field; 45°N-S; 27.85 Tg N yr-1

Gruber and Sarmiento 1997; 45°N-0°; 28.1 Tg N yr-1

GS ‘97 N. Atl. rate extended to 45°N-45°S; 56.1 Tg N yr-1

MOM4 simulations of the recently fixed nitrate field

Recently fixed NO3-

(mol N m-2)

Total NO3-

(mol N m-2)

Recent/

Total

data

This study 0.81 3.03 0.27

models

Deutsch 0.77 3.48 0.22

GS I 1.24 3.99 0.31

GS II 1.97 4.80 0.41

data

This study 1.02 16.78 0.06

models

Deutsch 2.30 18.19 0.13

GS I 3.26 19.20 0.17

GS II 5.63 21.71 0.26

0-60

0 m

0-12

00 m

Column inventories

N2 fixation

distribution

Recently fixed NO3- in

North AtlanticRecently fixed NO3

- in South Atlantic

GS I

(North Atlantic only) 63.1% 36.9%

GS II’

(North + South Atlantic) 54.0% 46.0%

GS III’

(South Atlantic only) 44.1% 55.9%

Interhemispheric exchange of recently fixed nitrate

Hemispheric asymmetry:North Atlantic tends to collect recently fixed nitrate

15N enrichment in the nitrate entering the Atlantic?Southern Ocean data

Sigman et al. 1999

Uniform nitrate 18O below 300 min this region


“18O-corrected” 15N of nitrate

18O of nitrate (‰ vs. SMOW)

Low latitude N cycling and the loss of nitrate O isotope signals

The O isotopes may not record the N and O isotope enrichmentof imported nitrate.

High nitrate 18Oin the

shallow Sargasso Sea


“18O-corrected” 15N of nitrate

18O of nitrate (‰ vs. SMOW) 18ε/15ε = 1.0

Decoupling of nitrate N and O isotopes by

simultaneous nitrate assimilation and nitrification

Monterey Bay: Wankel et al., 2007

Conclusions• Nitrate N isotopes indicate 2 µM or more of recently fixed nitrate

in the Sargasso Sea thermocline.

• Coherent spatial trends are not apparent in the region studied. This suggests that gradients in N2 fixation are too weak to imprint nitrate N isotope gradients on the circulating thermocline. We did not sample an adequately wide range in ventilation age to follow the ‘isopycnal’ approach used by Gruber and Sarmiento (1997) for N*.

• We essentially follow a diapycnal approach by our use of MOM3 with high latitude sponge walls. In this context, the isotope data suggest a relatively low rate for N2 fixation in the Atlantic (30 Tg N yr-1 or less, a la Deutsch et al. 1997).

• Uncertainties include the possible impacts of nitrate assimilation inside and outside the Atlantic on the 15N of nitrate in the interior. The nitrate O isotopes were discussed in this context.

• GEOTRACES should provide the opportunity to do this for real.

The End

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.QuickTime™ and a

TIFF (Uncompressed) decompressorare needed to see this picture.

What if N2 fixation aboundswhere denitrification is prevalent?

Deutsch et al. 2004

Low nitrate 15N in the Sargasso Sea thermocline:Consistent with N2 fixation input

Knapp et al. 2005

Conclusions

• Nitrate N and O isotopes have fundamentally different behavior, making them powerful complements.

• Coupled N and O isotope systematics:– Newly produced nitrate: 18O ~ 0‰ vs. SMOW– Nitrate assimilation: 18ε = 15ε (~ 5-10‰)– Denitrification: 18ε = 15ε (20-30‰)

• Application to the eastern North Pacific margin:– 18O:15N anomaly ((15,18) min.) in the thermocline– Interpretations:

• addition of low-15N N from N2 fixation• an active NO3

-/NO2- redox cycle in or near the suboxic

zone

15N enrichment in the nitrate entering the Atlantic?Southern Ocean data

DiFiore et al. 2006

SAMW ~ 7.2‰

Algal nitrate assimilation: 18ε~15ε

Granger et al., 2004

Progressive nitrate consumption 18ε/15ε = 1.0

The cycle and budget of N in the ocean

NO3- N2

N2Norg (NO3-)

MS 172 Figure 5

0

5

10

15

20

0 0.5 1 1.5 2

[NO3-] (factor of initial value)

newly fixed Nadded

15N~ -2-0‰

water columndenitrification

ε ∼ 20-30‰

sedimentarydenitrification

ε ∼ 0‰

nitrateuptake

ε ∼ 5‰

NO3- 15N

(‰ . )vs air

Complementary constraints from[NO3

-], N* and 15N

N isotopes in the whole ocean N budget

Brandes and Devol 2002

80

60

40

20

0∆

18

O of NO

3

-

(

‰

vs. starting value)

806040200

∆ δ

15

N of NO

3

-

( ‰ vs. starting value)

δ

18

O: δ

15

N = 1.0

δ

18

O: δ

15

N = 0.8

Ochrobacter sp. (n = 6)

P. denitrificans sw (n = 4)

P. denitrificans fw (n = 2)

P. aureofaciens sw (n = 4)

P. aureofaciens fw (n = 1)

P. stutzeri (n = 4)

25

20

15

10

5

0∆

18

O of NO

3

-

(

‰

vs. starting value)

2520151050

∆ δ

15

N of NO

3

-

( ‰ vs. starting value)

1:1

δ

18

O: δ

15

N = 0.6

freshwater

seawater

R. sphaeroides

Denitritification: 18ε~15ε

Granger et al., in review 18ε/15ε = 0.95

Decoupling of nitrate N and O isotopes by

simultaneous nitrate assimilation and nitrification

f = 0.5 ; e = 5‰NO3

- NO3- NO3

-

5‰ 0‰ e = 5‰ 8.5‰ 3.5‰ 5‰ 1.75‰ N H2O 1.5‰ 0‰

NH4+ NO3

-

1.5‰ 1.5‰ 0‰Nitracline

Monterey Bay: Wankel et al., 2007

The nitrate O isotopes are usefulbecause of

what they don’t record

Nitrogen atoms in marine nitrate:

nitrification nitrate assimilation

Norg

NO3-

N2 fixation

denitrification


NO3- denitrification

Oxygen atoms in marine nitrate:

a. b.

The nitrate O isotopes are usefulbecause of

what they don’t record



Norg

NO3-

N2 fixation

denitrification




15e = 25‰

15e = 5‰

d15 = -1N ‰

15e = 17‰

d18 = 0O ‰ 18e = 15e

18e = 15e

a. b.

Summary of systematics



Norg

NO3-

N2 fixation

denitrification




15e = 25‰

15e = 5‰

d15 = -1N ‰

15e = 17‰

d18 = 0O ‰ 18e = 15e

18e = 15e

a. b.

N2 fixation

distribution

Recently fixed NO3- in

North AtlanticRecently fixed NO3

- in South Atlantic

GS I

(North Atlantic only) 65.7% 34.3%

GS II

(North + South Atlantic) 55.0% 45.0%

GS III

(South Atlantic only) 47.1% 52.9%

Hemispheric asymmetry

Stations on the Baja

California margin

Contour every 750 m[O2]<5 µM

van Geen cruise, Nov. 1999

All stations

Red = N (~Point Conception)Blue = S (~S tip of Baja)

12840

18

O of NO3- (‰ . )vs VSMOW

1400

1200

1000

800

600

400

200

0

16128415

N of NO3- (‰ . )vs air

30020010000 2 ( )µM

-30 -20 -10 0*N ( )µM

50403020100

[NO3-] ( )µM

[NO3-]

18O

15

N

a

b

c

d

e

*N [O2 ]

12

10

8

6

4

2

0

16141210864

15 N of NO3- (‰ . )vs air

1:1

12

10

8

6

4

2

0

1614121086415 N of NO3

- (‰ . )vs air

SBB

12

10

8

6

4

2

0

1614121086415 N of NO3

- (‰ . )vs air

a cb

12

10

8

6

4

2

0

161412108615 N of NO3

- (‰ . )vs air deep ENP

1:1

Deviations from 1:1 variation in

nitrate 18O and 15N

Southern Baja stations All stations

(15,18) = 15N - 18O - 5.5‰

Figure 3

a.

c.

b.

d.

Nitrate isotopes near southern tip of Baja

-3 -2 -1 0 1(15,18) (=

15 - ( N

18 + 5.5‰))O

1600

1400

1200

1000

800

600

400

200

0-10 0*N ( )µM

50403020100[NO3

-] ( )µM

161284015

N or18

O of NO3- (‰ . )vs air or SMOW

[NO3-]

18

O 15

N

ab

c

d

e

*N

(15,18)

Cause of the (15,18) minimum?



Norg

NO3-

N2 fixation

denitrification




15e = 25‰

15e = 5‰

d15 = -1N ‰

15e = 17‰

d18 = 0O ‰ 18e = 15e

18e = 15e

a. b.

Cause of the (15,18) minimum?



Norg

NO3-

N2 fixation

denitrification




15e = 25‰

15e = 5‰

d15 = -1N ‰

15e = 17‰

d18 = 0O ‰ 18e = 15e

18e = 15e

a. b.

NO3- NO2

- …

Quantifying putative processes from the (15,18) anomaly

For 200-800 m:N2 fix. ~ 0.65*denit.N* minimum:-12 µM -24 µM

At 200 m:NO2

- ox. > 0.7*NO2- red.

Bering Sea shelf:O/N isotope decoupling by in situ nitrification18

16

14

12

10

8

6

4

18

O of NO

3-

(‰ . )vs VSMOW

1412108

15 N of NO3- (‰ . )vs air

25

20

15

10

5

0

NO3 - (μ)M

B. BrunelleM.B.: Wankel et al.

Polar Antarcticnitrate isotope fractionation

P. Difiore

Southern Baja station results compared to stations further North

-20 -15 -10 -5 0N* (µM)

1400

1200

1000

800

600

400

200

0

50403020100

[NO3

-] (µM)

-3 -2 -1 0 1(15,18) (=

15 - ( N

18 + 5.5‰))O

1284018


16128415


[NO3-]

18

O

15

N

a

b

c

d

e

*N(15,18)

HOT Station ALOHA

Is the open tropical Pacificthe source of the (15,18) minimum ?

5000

4000

3000

2000

1000

0

depth (m)

642015 N or18 O of NO3

- (‰ . )vs air or SMOW

40200[NO3

-] ( )µM

-3 -2 -1 0 1*N ( )µM

-3 -2 -1 0(15,18) (= 15 - ( N 18 + 5.5‰))O

a b c d e

15 N18 O

ENP vs. HOT

-10 0N* (µM)

28.0

27.5

27.0

26.5

26.0

25.5

25.0

24.5

σθ

( kg m

-3)

16128415


108642018


-3 -2 -1 0 1(15,18) (=

15 - ( N

18 + 5.5‰))O

50403020100[NO3

-] ( )µM

[NO3-]

18

O

15

N

a

b

c

d

e

*N Baja S tip

HOT

(15,18)

N2 fixation = 0.65*denitrification

[NO3-]B - [NO3

-]M

Nitrate 15N

Nitrate (15,18)

observations(200-800 m)

NO2- oxidation > 0.7*NO2

- reduction at 200 m

Assumptions: (1) 15εNiO = 15εNiR, (2) 18ε/15εNiO = 18ε/15εNiR

12

10

8

6

4

2

0

16141210864

15 N of NO3- (‰ vσ. air)

1:1

12

10

8

6

4

2

0

1614121086415 N of NO3

- (‰ vσ. air)

SBB

a cb

12

10

8

6

4

2

0

161412108615 N of NO3

- (‰ . )vs air deep ENP

1:1

Why no anomaly in the

Santa Barbara Basin?

Where there is very little dissolved [O2], …

Deutsch et al. 2004

J. Granger, unpub.

Marine denitrification: 18ε~15ε

50

40

30

20

10

0

-5.5 -4 -3 -2 -1 0([ln NO3

-]/[NO3-initial])

a15ε = 5‰

15ε = 30‰

5040302010018 O of NO3

- (‰ . )vs initial

50

40

30

20

10

0 b

15ε:18ε = 1 0.9 , 1.1

.Ps aureofaciens ( =3)n . P denitrificans ( =1)n . Ps stutzeri ( =2)n .Ochrobacter sp ( =1)n

Progressive nitrate consumption

Model quantification of putative N2 fixation

-1.5

-1.0

-0.5

0.0

0.5

(15,18)

1.00.80.60.40.20.0N2 / ( / )fixation denitrification F D

=0S =S M

15

10

5

0

18

OB

(‰ . )vs SMOW

20151050 15N

B (‰ . )vs air

(15,18)= -1.3‰= 0‰

Increasing D

Increasing F

, =0S, =S M

NO2- oxidation ~ 0.85*NO2

- reduction at 200 m

Assumptions: (1) complete NO2-/H2O O exchange; (2) 15εNiO = 15εNiR

-3

-2

-1

0

(15,18) (‰)

1.51.00.50.0NO2

- / oxidation NO2

- reduction

(200-800 )observed m

(200 )observed m

Oceanic N Budget

N2 Fixation

Rivers

Atmosphere

Inputs Total

Benthic Denit.

W.C. Denit.

Sedimentation

Outputs Total

Imbalance

Residence time

Flux (TgN/yr)Codispoti and

Christensen 1985

Gruber and

Sarmiento 1997

Brandes and

Devol 2002

25

25

25

75

60

60

21

141

125

42

15

182

85

80

15

180

-66 +2

110-330

25

25

160-380

200-280

75

25

300-380

0 or -200

> 5000 yr ~ 3500 yr < 2000 yr

inpu

tsou

tput

s

N2Norg (NO3-)

NO3- N2

NO3- NO3

-

NO2-

NO3-

NO3-

NO2- NH4

+

Organic nitrogen

VACUOLE

CHLOROPLAST

a

b

c d

Figure 4 -- Granger et al.

80

60

40

20

0

-6-5-4-3-2-10

ln ([NO3

-

]/[NO3

-

initial])

15

ε = 25 ‰

15

ε = 5 ‰

b

80

60

40

20

0

Δδ

15

N of NO

3-

(

‰

vs. starting value)

1101001000

8 6 4 2 0

ln ([NO3

-

])

a

Ochrobacter sp. (n = 6)

P. denitrificans sw (n = 4)

P. denitrificans fw (n = 2)

P. aureofaciens sw (n = 4)

P. aureofaciens fw (n = 1)

P. stutzeri (n = 4)

Open tropical Pacific:source of the minimum ??

12

10

8

6

4

2

0

18

O of NO

3-

(‰ . )vs VSMOW

1614121086415 N of NO3

- (‰ . )vs air

ENP HOT

Quantifying putative NO3-/NO2

- cycling fromthe (15,18) anomaly

Figure 9

Internal ocean cycle(algal assimilation/remineralization)

dominates variations in nitrate concentration

QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.

WOCE data; ODV: R. Schlitzer

Mechanism for isotope fractionationduring algal nitrate assimilation

Needoba et al., 2004


Isotope fractionation during denitrification:two nitrate reduction enzymes

NO3- NO3

-

NO2-

NO3-

a bc

d

NAR

NAPNO2-

e

periplasm

cytosol

bacterial wall

bacterial membrane18ε~15ε

18ε ~ 0.6*15ε

Variation in the isotope fractionation of nitrate assimilation: balanced vs. unbalanced growth

NO3- NO3

- NO2- e~ 0-5‰

e~ 20‰( )NaR

ambient internal

NADH NAD+

The imprint of the N budget is overwhelmed by the N cycle

Trick: Remove the ‘cycle’ component using PO43-

-40

-40

-30

-30

-20

-20

-10

-10

0

0

N*

1400

1200

1000

800

600

400

200

0

d

16

16

14

14

12

12

10

10

8

8

6

6

15 (‰ . )N of nitrate vs air

1400

1200

1000

800

600

400

200

0

e

SBB versus open ETNP

Low NO3- 15N in the Sargasso Sea thermocline:

Consistent with N2 fixation source for excess NO3-

Knapp et al., 2005


Needoba et al. in press

light-limited

Comparing the 15N/14N of internal pool nitrate and medium nitratein a cultured diatom (T. weiss.)

Iso

top

e e

ffect

of n

itra

te a

ssim

ilatio

n

light/dark

limited byiron or T

Biogeochemical conditionsalong the ENP margin

40

40

30

30

20

20

10

10

0

0

[NO3-] (µM)

SBB Spring '99 SBB Fall '95 SBB Fall '99 SBB Aug. '77 Liu [1979] SBB Aug. '78 Liu [1979] near SBB 30-33 °N 28-30 °N 25-27 °N 22-25 °N

b

5.0

5.0

4.0

4.0

3.0

3.0

2.0

2.0

1.0

1.0

0.0

0.0

[PO43-

] (µM)c

3002001000

[O2] (µM)

1400

1200

1000

800

600

400

200

0

3002001000

a

4000

3000

2000

1000

0

depth (m)

1.00.0

[O2]/[O 2]sat

43210-1

18 (‰ . )O of nitrate vs VSMOW

0.80.40.0[PO4

3-] regenerated/[PO 4

3-] total

8642015 (‰ . )N of nitrate vs air

M. Lehmann and A. Knapp, unpublished

1200

1000

800

600

400

200

0

depth (m)

2520151050

[NO3-] (µM)

8642015 (‰ . )N vs air

8642018 (‰ . )O vs VSMOW

-6 -4 -2 0 215 - (N 18 + 5.5‰)O

30°N 29°N 28°N 27°N 26°N 25°N 24°N 23°N 22°N 20°N 19°N

A. Knapp, unpublished

Sargasso Sea

Stabilizing feedbacks in the ocean N budget

Quantifying putative N2 fixation fromthe (15,18) anomaly

Model quantification of putative N2 fixation

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.00.80.60.40.20.0 / ( / )fixation input denitrification output F D

=0:S 200 fit m 200-800 fit m

= :S M 200 fit m 200-800 fit m

15

10

5

0

20151050 15NB (‰ . )vs air

15 - (N 18 + 5.5‰)O= -1.3‰, -2.5‰

Increasing D

Increasing F

J. Granger, unpublished


J. Granger, unpublished


Nitrification produces NO3- with 18O ~ 0‰

1200

1000

800

600

400

200

0

depth (m)

2520151050

[NO3-] (µM)

8642015 (‰ . )N vs air

8642018 (‰ . )O vs VSMOW

-6 -4 -2 0 215 - (N 18 + 5.5‰)O

30°N 29°N 28°N 27°N 26°N 25°N 24°N 23°N 22°N 20°N 19°N

Subtropical North Atlantic, Bermuda -- Puerto Rico; A. Knapp, unpub.

All nitrateremineralized

1200

1000

800

600

400

200

0

depth (m)

543210-1-2[NO3


654321NO

3

- 15 (‰ . )N vs air

1.00.80.60.40.20.0-0.2 fraction of NO3

- from N2 fix

a b c

Figure 5

a. b. c.

Nitrate 15N and 18O in the Sargasso Sea

water column

18O of nitrate (‰ vs. SMOW)


water column

18Osal of nitrate (‰ vs. SMOW)


water column

15N of nitrate (‰ vs. air) corrected for assimilation with 18O


water column

∆(15,18)sal of nitrate

1200

1000

800

600

400

200

0

depth (m)

543210-1-2[NO3


654321NO

3

- 15 (‰ . )N vs air

1.00.80.60.40.20.0-0.2 fraction of NO3

- from N2 fix

a b c

Documents

Nitrate isotopes as a constraint on the ocean's fixed N budget Angela Knapp, University of Southern California Peter DiFiore, Princeton University Curtis