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
15N of nitrate (‰ vs. air)
“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
15N of nitrate (‰ vs. air)
“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
nitrification nitrate assimilation
NO3- denitrification
Oxygen atoms in marine nitrate:
a. b.
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
nitrification nitrate assimilation
NO3- denitrification
Oxygen atoms in marine nitrate:
15e = 25‰
15e = 5‰
d15 = -1N ‰
15e = 17‰
d18 = 0O ‰ 18e = 15e
18e = 15e
a. b.
Summary of systematics
Nitrogen atoms in marine nitrate:
nitrification nitrate assimilation
Norg
NO3-
N2 fixation
denitrification
nitrification nitrate assimilation
NO3- denitrification
Oxygen atoms in marine nitrate:
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?
Nitrogen atoms in marine nitrate:
nitrification nitrate assimilation
Norg
NO3-
N2 fixation
denitrification
nitrification nitrate assimilation
NO3- denitrification
Oxygen atoms in marine nitrate:
15e = 25‰
15e = 5‰
d15 = -1N ‰
15e = 17‰
d18 = 0O ‰ 18e = 15e
18e = 15e
a. b.
Cause of the (15,18) minimum?
Nitrogen atoms in marine nitrate:
nitrification nitrate assimilation
Norg
NO3-
N2 fixation
denitrification
nitrification nitrate assimilation
NO3- denitrification
Oxygen atoms in marine nitrate:
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
O of NO3- (‰ . )vs VSMOW
16128415
N of NO3- (‰ . )vs air
[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
N of NO3- (‰ . )vs air
108642018
O of NO3- (‰ . )vs VSMOW
-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
Granger 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
Granger et al., 2004
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
Marine denitrification: 18ε~15ε
J. Granger, unpublished
Marine denitrification: 18ε~15ε
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
-] 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
Figure 5
a. b. c.
Nitrate 15N and 18O in the Sargasso Sea
water column
18O of nitrate (‰ vs. SMOW)
Nitrate 15N and 18O in the Sargasso Sea
water column
18Osal of nitrate (‰ vs. SMOW)
Nitrate 15N and 18O in the Sargasso Sea
water column
15N of nitrate (‰ vs. air) corrected for assimilation with 18O
Nitrate 15N and 18O in the Sargasso Sea
water column
∆(15,18)sal of nitrate
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