UK GEOTRACES: Characterizing nitrate isotopes in South Atlantic water masses (40°S). UK GEOTRACES: Characterizing nitrate isotopes in South Atlantic water

UK GEOTRACES: Characterizing nitrate isotopes in South Atlantic water masses (40°S).

Robyn Tuerena, Raja Ganeshram,Anthony Fallick, Andrew Tait, Julie Dougans, Malcolm Woodward.

Oxford 10th-11th September 2012

Marine Nitrogen cycle

• N Limits on biological productionSigman et al., 2000; Difiore et al.,2008

• Trace N fluxesBrandes and Devol, 2002; Codispoti et al., 2001

• Changing productivity patternsDuce et al., 2008; Falkowski et al., 1998

Characterizing nitrate isotopes in South Atlantic water masses (40°S)Robyn Tuerena, Raja Ganeshram, Anthony Fallick, Andrew Tait, Julie Dougans, Malcolm Woodward.

Study area

D357, RRS DiscoveryJC068, RRS James Cook

NADW

AAIW

AABW

SACW AgulhasBrazil


δ15N (‰ vs. air) = x 1000

Stable isotope biogeochemistry


Spatially and temporallyintegrated value!

Sigman et al., 2009

N2

PP

Nitrate isotope cycling

1.

3.

4.

2.

ε ~ 25 ‰

ε ~5 ‰

~ -1 ‰

ε ~≤ 3 ‰

1. N2 fixation2. Remineralization3. Nitrate utilization4. Denitrification

4. ε ~0 ‰

NO3-

5‰


SURFACE

Characterizing water masses

Nitrate utilization, indicator of productivity

Tracer for N2 fixation, denitrification andpartially utilized nitrate

Isotope fractionation

Spatial pattern of δ15NNO3 and δ18ONO3

DEEP


Denitrifier method (Sigman et al., 2001, Casciotti et al., 2002)

NO3- converted to N2O

Isotope ratio mass spectrometry

Methods


0 2 4 6 8 10 12 14

Dep

th

(m

)

-5000

-4000

-3000

-2000

-1000

0

d15N d18O

-2 0 2 4 6 8 10 12Y

Dat

aPer mil (‰)

-15 -10 -5 0 5 10 15

Deep water masses


Station 3 Station 6 Station 10

δ15N 0.2 ‰δ18O 0.5 ‰

Deep water masses


Deep water masses

NADW

AAIWAABW

AABWNADW

AAIW

AABW 4.9 ± 0.1 ‰NADW 4.5 ± 0.2 ‰ AAIW 5.4 ± 0.4 ‰


Deep water massesAAIWIncorporation of NO3

- that has been partially consumed in Antarctic surface waters

AABWFormed on shelf, Weddell sea and entrainment of CDW (~ 4.8 ‰, Sigman 1999)

NADWRelease of lighter N through sinking particles?Depleted source values?


Surface water masses

Nitrate utilization enriches δ15N up to 13 ‰ at surface


SAMW



D357

Surface water massesMixing dilutes δ15N values:Agulhas 6-8 ‰SACW 5-7‰


Nitrate Utilization

12

71619

5 µM12 ‰

30 µM5 ‰

17.5 µM6 ‰


15Nnr

-12 -10 -8 -6 -4 -2 0 2

Dep

th (

m)

0

200

400

600

800

1000

1200

1400

Station 2Station 6

15Nnr

-8 -6 -4 -2 0 2D

epth

(m

)

0

200

400

600

800

1000

1200

1400

Station 16Station 19

Mixing

δ15Nnr: Indicates deviations from expected enrichment related to NO3

- concentrations

Agulhas

SACW




Δ15,18 = (δ15N -δ15Nm) -(15ε/18ε)x(δ18O-δ18Om)Sigman et al., (2005)

Deviations from δ15N:δ18O stoichiometry

Conclusions DeepAAIW: partially utilized NO3

- in surface watersNADW: Lighter values may indicate sinking of newly fixed OM

SurfaceWater mass mixing dilutes δ15N enrichment in surface waters

Future workFinish analysis of surface and Western basin samples

Finish particulate analyses

Arctic and Southern Ocean samples


High resolution deep ocean transect

δ15N and δ18O measurements of NO3- give an

integrated signal of N cycling processes over space and time

δ13C and δ15N of particulates can further constrain N cycling

Collaborations

• Aerosol data - Rosie Chance and Alex Baker• Mixing fluxes• Biological productivity in surface waters

AcknowledgementsSample collectionUK GEOTRACES team, Crew of the RRS Discovery and RRS James Cook, Sue Reynolds, Amandine Sabadel

Method developmentSian Henley, Jan Kaiser, Laura Campisi, Karen Casciotti, Matt McIlvin

R.Schlitzer, Ocean Data View, http://www.awi-bremerhaven.de/GEO/ODV, 2012

http://www.awi-bremerhaven.de/GEO/ODV

d15 NO3 (per mil)4 6 8 10 12 14

De

pth

(m

)

0

500

1000

1500

2000

2500

PN (uM)0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

d15 PN1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

NO3 (uM)0 5 10 15 20 25 30 35 40

d15NPN (uM)d15PON Nitrate [uM/L]

Particulates

Below 500 m

Station 3δ15PN increases to ~ 5 ‰ below1000 m

δ15NO3- source waters ~ 6 ‰


2D Graph 3

d15N PN (per mil)

0 1 2 3 4 5 6

Dep

th (m

)

0

50

100

150

200

250

d13C (per mil)-24.5 -24.0 -23.5 -23.0 -22.5 -22.0 -21.5 -21.0 -20.5

POC (uM)0 1 2 3 4 5

d15PN d13POC POC (uM)

Particulates…Surface

Station 9 or 1.5Enrichment in C-13 and N-15 at POC max

Enrichment with depth


Particulates

Documents

UK GEOTRACES: Characterizing nitrate isotopes in South Atlantic water masses (40°S). UK GEOTRACES: Characterizing nitrate isotopes in South Atlantic water