Transcript Document

UK GEOTRACES: Characterizing nitrate
isotopes in South Atlantic water
masses (40°S).
Oxford 10th-11th September 2012
Robyn Tuerena, Raja Ganeshram,
Anthony Fallick, Andrew Tait, Julie Dougans,
Malcolm Woodward.
Marine Nitrogen cycle
• N Limits on biological production
Sigman et al., 2000; Difiore et al.,2008
• Trace N fluxes
Brandes and Devol, 2002; Codispoti et al., 2001
• Changing productivity patterns
Duce 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 Discovery
JC068, RRS James Cook
Brazil
SACW
AAIW
Agulhas
NADW
AABW
Characterizing nitrate isotopes in South Atlantic water masses (40°S)
Robyn Tuerena, Raja Ganeshram, Anthony Fallick, Andrew Tait, Julie Dougans,
Malcolm Woodward.
Stable isotope biogeochemistry
δ15N
(‰ vs. air) =
𝟏𝟓/𝟏𝟒 (𝐬𝐚𝐦𝐩𝐥𝐞)
𝟏𝟓/𝟏𝟒(𝐫𝐞𝐟𝐞𝐫𝐞𝐧𝐜𝐞)
− 𝟏 x 1000
Spatially and temporally
integrated value!
Sigman et al., 2009
Characterizing nitrate isotopes in South Atlantic water masses (40°S)
Robyn Tuerena, Raja Ganeshram, Anthony Fallick, Andrew Tait, Julie Dougans,
Malcolm Woodward.
Nitrate isotope cycling
1. N2 fixation
2. Remineralization
3. Nitrate utilization
4. Denitrification
N2
ε ~ 25 ‰
4.
~ -1 ‰
1.
NO
3
5‰
4. ε ~0 ‰
3. ε ~5 ‰
PP
2.
ε ~≤ 3 ‰
Characterizing nitrate isotopes in South Atlantic water masses (40°S)
Robyn Tuerena, Raja Ganeshram, Anthony Fallick, Andrew Tait, Julie Dougans,
Malcolm Woodward.
Characterizing water masses
SURFACE
Isotope fractionation
Nitrate utilization, indicator of productivity
Spatial pattern of δ15NNO3 and δ18ONO3
DEEP
Tracer for N2 fixation, denitrification and
partially utilized nitrate
Characterizing nitrate isotopes in South Atlantic water masses (40°S)
Robyn Tuerena, Raja Ganeshram, Anthony Fallick, Andrew Tait, Julie Dougans,
Malcolm Woodward.
Methods
Denitrifier method (Sigman et al., 2001, Casciotti et al., 2002)
NO3- converted to N2O
Isotope ratio mass spectrometry
Characterizing nitrate isotopes in South Atlantic water masses (40°S)
Robyn Tuerena, Raja Ganeshram, Anthony Fallick, Andrew Tait, Julie Dougans,
Malcolm Woodward.
Deep water masses
Station 3
Station 6
Station 10
0
δ15N 0.2 ‰
-1000
δ18O 0.5 ‰
Y Data
Depth
(m)
-2000
-3000
d15N
d18O
-4000
-5000
0
2
4
6
8 10 12 -15
14 -10 -5 0 5
Per mil (‰)
-2 0
10 15
2
4
6
8 10 12
Characterizing nitrate isotopes in South Atlantic water masses (40°S)
Robyn Tuerena, Raja Ganeshram, Anthony Fallick, Andrew Tait, Julie Dougans,
Malcolm Woodward.
Deep water masses
Characterizing nitrate isotopes in South Atlantic water masses (40°S)
Robyn Tuerena, Raja Ganeshram, Anthony Fallick, Andrew Tait, Julie Dougans,
Malcolm Woodward.
Deep water masses
AAIW
AABW 4.9 ± 0.1 ‰
NADW 4.5 ± 0.2 ‰
AAIW 5.4 ± 0.4 ‰
AABW
NADW
AAIW
NADW
AABW
Characterizing nitrate isotopes in South Atlantic water masses (40°S)
Robyn Tuerena, Raja Ganeshram, Anthony Fallick, Andrew Tait, Julie Dougans,
Malcolm Woodward.
Deep water masses
AAIW
Incorporation of NO3- that has been partially
consumed in Antarctic surface waters
NADW
Release of lighter N through
sinking particles?
Depleted source values?
AABW
Formed on shelf, Weddell sea and
entrainment of CDW (~ 4.8 ‰, Sigman 1999)
Characterizing nitrate isotopes in South Atlantic water masses (40°S)
Robyn Tuerena, Raja Ganeshram, Anthony Fallick, Andrew Tait, Julie Dougans,
Malcolm Woodward.
Surface water masses
Nitrate utilization
enriches δ15N up to
13 ‰ at surface
SAMW
Characterizing nitrate isotopes in South Atlantic water masses (40°S)
Robyn Tuerena, Raja Ganeshram, Anthony Fallick, Andrew Tait, Julie Dougans,
Malcolm Woodward.
Surface water masses
D357
Characterizing nitrate isotopes in South Atlantic water masses (40°S)
Robyn Tuerena, Raja Ganeshram, Anthony Fallick, Andrew Tait, Julie Dougans,
Malcolm Woodward.
Surface water masses
Mixing dilutes δ15N values:
Agulhas 6-8 ‰
SACW
5-7‰
Characterizing nitrate isotopes in South Atlantic water masses (40°S)
Robyn Tuerena, Raja Ganeshram, Anthony Fallick, Andrew Tait, Julie Dougans,
Malcolm Woodward.
Nitrate Utilization
2 1
19 16
5 µM
12 ‰
7
30 µM
5‰
17.5 µM
6‰
Characterizing nitrate isotopes in South Atlantic water masses (40°S)
Robyn Tuerena, Raja Ganeshram, Anthony Fallick, Andrew Tait, Julie Dougans,
Malcolm Woodward.
Mixing
0
0
200
200
400
Agulhas
400
Depth (m)
Depth (m)
δ15Nnr:
600
Indicates deviations from
800 related
expected enrichment
to NO3- concentrations
600
800
1000
1000
1200
1200
1400
Station 2
Station 6
-12 -10 -8 -6 -4 -2 0
15Nnr
SACW
Station 16
Station 19
1400
2
-8
-6
-4
-2
15Nnr
Characterizing nitrate isotopes in South Atlantic water masses (40°S)
Robyn Tuerena, Raja Ganeshram, Anthony Fallick, Andrew Tait, Julie Dougans,
Malcolm Woodward.
0
2
Surface water masses
Δ15,18 = (δ15N -δ15Nm) -(15ε/18ε)x(δ18O-δ18Om)
Sigman et al., (2005)
Deviations from δ15N:δ18O stoichiometry
Characterizing nitrate isotopes in South Atlantic water masses (40°S)
Robyn Tuerena, Raja Ganeshram, Anthony Fallick, Andrew Tait, Julie Dougans,
Malcolm Woodward.
Conclusions
High resolution deep ocean transect
Deep
AAIW: partially utilized NO3- in surface waters
15N and δ18O measurements of NO - give an
δ
3
NADW: Lighter values may indicate sinking
of newly fixed OM
integrated signal of N cycling processes over
Surface
space and time
Water mass mixing dilutes δ15N enrichment in surface waters
δ13C and δ15N of particulates can further
Future
constrainwork
N cycling
Finish analysis of surface and Western basin samples
Finish particulate analyses
Arctic and Southern Ocean samples
Characterizing nitrate isotopes in South Atlantic water masses (40°S)
Robyn Tuerena, Raja Ganeshram, Anthony Fallick, Andrew Tait, Julie Dougans,
Malcolm Woodward.
Collaborations
• Aerosol data - Rosie Chance and Alex Baker
• Mixing fluxes
• Biological productivity in surface waters
Acknowledgements
Sample collection
UK GEOTRACES team, Crew of the RRS Discovery and RRS James
Cook, Sue Reynolds, Amandine Sabadel
Method development
Sian Henley, Jan Kaiser, Laura Campisi, Karen Casciotti, Matt McIlvin
R.Schlitzer, Ocean Data View,
http://www.awi-bremerhaven.de/GEO/ODV, 2012
Particulates
d15 PN
1.0
Below 500 m
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
PN (uM)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0
Station 3
δ15PN increases to ~ 5 ‰ below
1000 m
1000
Depth (m)
δ15NO3- source waters ~ 6 ‰
500
1500
2000
d15N
PN (uM)
d15PON
Nitrate [uM/L]
2500
4
6
8
10
12
14
d15 NO3 (per mil)
0
5
10
15
20
25
30
NO3 (uM)
Characterizing nitrate isotopes in South Atlantic water masses (40°S)
Robyn Tuerena, Raja Ganeshram, Anthony Fallick, Andrew Tait, Julie Dougans,
Malcolm Woodward.
35
40
2D Graph 3
Particulates
Particulates…
POC (uM)
0
Surface
1
2
3
4
5
d13C (per mil)
-24.5 -24.0 -23.5 -23.0 -22.5 -22.0 -21.5 -21.0 -20.5
0
Station 9 or 1.5
Enrichment in C-13 and N-15
at POC max
100
Depth (m)
Enrichment with depth
50
150
200
d15PN
d13POC
POC (uM)
250
0
1
2
3
4
d15N PN (per mil)
Characterizing nitrate isotopes in South Atlantic water masses (40°S)
Robyn Tuerena, Raja Ganeshram, Anthony Fallick, Andrew Tait, Julie Dougans,
Malcolm Woodward.
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