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 N 2 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 N 2 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 : 14 N % 15 N % O : 16 O % 17 O % 18 O %  15 N (vs. atm. N 2 ) = ( ( ( 15 N/ 14 N) sample / ( 15 N/ 14 N) air ) - 1 ) *1000‰  18 O (vs. VSMOW) = ( ( ( 18 O/ 16 O) sample / ( 18 O/ 16 O) VSMOW ) - 1 ) *1000‰

Kinetic isotope effect 14 k 15 k 16 k 18 k

Complementary constraints from [NO 3 - ], N* and  15 N N*

Depth BATS Validation Cruise 32 (October, 2002): Stations sampled for nitrate and DON isotopes Increasing N 2 fixation? ~2 yr vent. age increase

Nitrate data N 2 fix.? NO 3 - assim.?

Nitrate  15 N in the Sargasso Sea water column  15 N of nitrate (‰ vs. air) [NO 3 - ] (µM) No evidence of southward N 2 fixation increase or a southward accumulation of newly fixed nitrate in the thermocline Basis for average Sargasso Sea profile

Estimating ‘recently’ fixed nitrate (  15 N -  15 N imported ) (  15 N new -  15 N imported ) f = (5.3‰ -  15 N) 6.3‰ =

From a recently fixed nitrate pool to a N 2 fixation rate Atlantic SN SN “Plan A” “Plan B”

Deutsch et al Atlantic field; 45°N-S; 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 NO 3 - (mol N m -2 ) Total NO 3 - (mol N m -2 ) Recent/ Total data This study models Deutsch GS I GS II data This study models Deutsch GS I GS II m m Column inventories

N 2 fixation distribution Recently fixed NO 3 - in North Atlantic Recently fixed NO 3 - 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

15 N enrichment in the nitrate entering the Atlantic? Southern Ocean data Sigman et al. 1999

Uniform nitrate  18 O below 300 m in this region  15 N of nitrate (‰ vs. air) “  18 O-corrected”  15 N of nitrate  18 O 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 enrichment of imported nitrate.

High nitrate  18 O in the shallow Sargasso Sea  15 N of nitrate (‰ vs. air) “  18 O-corrected”  15 N of nitrate  18 O of nitrate (‰ vs. SMOW) 18  / 15  = 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 N 2 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 N 2 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  15 N 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

What if N 2 fixation abounds where denitrification is prevalent? Deutsch et al. 2004

Low nitrate  15 N in the Sargasso Sea thermocline: Consistent with N 2 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:  18 O ~ 0‰ vs. SMOW –Nitrate assimilation: 18  = 15  (~ 5-10‰) –Denitrification: 18  = 15  (20-30‰) Application to the eastern North Pacific margin: –  18 O:  15 N anomaly (  (15,18) min.) in the thermocline –Interpretations: addition of low-  15 N N from N 2 fixation an active NO 3 - /NO 2 - redox cycle in or near the suboxic zone

15 N enrichment in the nitrate entering the Atlantic? Southern Ocean data DiFiore et al SAMW ~ 7.2‰

Algal nitrate assimilation: 18  ~ 15  Granger et al., 2004 Progressive nitrate consumption  18  / 15  = 1.0

The cycle and budget of N in the ocean NO 3 -  N 2 N 2  N org (  NO 3 - )

Complementary constraints from [NO 3 - ], N* and  15 N

N isotopes in the whole ocean N budget Brandes and Devol 2002

Denitritification: 18  ~ 15  Granger et al., in review  18  / 15  = 0.95

Decoupling of nitrate N and O isotopes by simultaneous nitrate assimilation and nitrification Monterey Bay: Wankel et al., 2007

The nitrate O isotopes are useful because of what they don’t record

Summary of systematics

N 2 fixation distribution Recently fixed NO 3 - in North Atlantic Recently fixed NO 3 - 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 [O 2 ]<5 µM van Geen cruise, Nov. 1999

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

Deviations from 1:1 variation in nitrate  18 O and  15 N Southern Baja stationsAll stations  (15,18) =  15 N -  18 O - 5.5‰

Figure 3 a. c. b. d.

Nitrate isotopes near southern tip of Baja

Cause of the  (15,18) minimum?

NO 3 -  NO 2 -  …

Quantifying putative processes from the  (15,18) anomaly For m: N 2 fix. ~ 0.65*denit. N* minimum: -12 µM  -24 µM At 200 m: NO 2 - ox. > 0.7*NO 2 - red.

Bering Sea shelf: O/N isotope decoupling by in situ nitrification B. Brunelle M.B.: Wankel et al.

Polar Antarctic nitrate isotope fractionation P. Difiore

Southern Baja station results compared to stations further North

HOT Station ALOHA Is the open tropical Pacific the source of the  (15,18) minimum ?

ENP vs. HOT

N 2 fixation = 0.65*denitrification [NO 3 - ] B - [NO 3 - ] M Nitrate  15 N Nitrate  (15,18) observations ( m)

NO 2 - oxidation > 0.7*NO 2 - reduction at 200 m Assumptions: (1) 15  NiO = 15  NiR, (2) 18  / 15  NiO = 18  / 15  NiR

Why no anomaly in the Santa Barbara Basin?

Where there is very little dissolved [O 2 ], … Deutsch et al. 2004

J. Granger, unpub. Marine denitrification: 18  ~ 15  Progressive nitrate consumption

Model quantification of putative N 2 fixation

NO 2 - oxidation ~ 0.85*NO 2 - reduction at 200 m Assumptions: (1) complete NO 2 - /H 2 O O exchange; (2) 15  NiO = 15  NiR

Oceanic N Budget N 2 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 or -200 > 5000 yr~ 3500 yr< 2000 yr inputs outputs N 2  N org (  NO 3 - ) NO 3 -  N 2

NO 3 - NO 2 - NO 3 - NO 2 - NH 4 + Organic nitrogen VACUOLE CHLOROPLAST a b c d Figure 4 -- Granger et al.

Open tropical Pacific: source of the  minimum ??

Quantifying putative NO 3 - /NO 2 - cycling from the  (15,18) anomaly

Figure 9

Internal ocean cycle (algal assimilation/remineralization) dominates variations in nitrate concentration WOCE data; ODV: R. Schlitzer

Mechanism for isotope fractionation during algal nitrate assimilation Needoba et al., 2004 Granger et al., 2004

Isotope fractionation during denitrification: two nitrate reduction enzymes 18  ~ 15  18  ~ 0.6* 15 

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

The imprint of the N budget is overwhelmed by the N cycle Trick: Remove the ‘cycle’ component using PO 4 3-

SBB versus open ETNP

Low NO 3 -  15 N in the Sargasso Sea thermocline: Consistent with N 2 fixation source for excess NO 3 - Knapp et al., 2005

Granger et al., 2004

Needoba et al. in press light-limited Comparing the 15 N/ 14 N of internal pool nitrate and medium nitrate in a cultured diatom (T. weiss.) Isotope effect of nitrate assimilation light/dark limited by iron or T

Biogeochemical conditions along the ENP margin

M. Lehmann and A. Knapp, unpublished

A. Knapp, unpublished Sargasso Sea

Stabilizing feedbacks in the ocean N budget

Quantifying putative N 2 fixation from the  (15,18) anomaly

Model quantification of putative N 2 fixation

J. Granger, unpublished Marine denitrification: 18  ~ 15 

J. Granger, unpublished Marine denitrification: 18  ~ 15 

Nitrification produces NO 3 - with  18 O ~ 0‰ Subtropical North Atlantic, Bermuda -- Puerto Rico; A. Knapp, unpub. All nitrate remineralized

Figure 5 a.b.c.

Nitrate  15 N and  18 O in the Sargasso Sea water column  18 O of nitrate (‰ vs. SMOW)

Nitrate  15 N and  18 O in the Sargasso Sea water column  18 O sal of nitrate (‰ vs. SMOW)

Nitrate  15 N and  18 O in the Sargasso Sea water column  15 N of nitrate (‰ vs. air) corrected for assimilation with  18 O

Nitrate  15 N and  18 O in the Sargasso Sea water column ∆(15,18) sal of nitrate