Abby Ren, Princeton
Daniel Sigman, Princeton
Nele Meckler, Caltech
Rebecca Robinson, URI
Yair Rosenthal, Rutgers
Gerald Haug, ETH
Foraminifera-bound nitrogen isotopes evidence for reduced nitrogen fixation in the Atlantic Ocean during the last ice age
Pleistocene glacial/interglacial cycles
(C,N,P)org NO
3
-,PO4
-3=0
atmosphere
CO2, NO3
-, PO4
-3
Low-Latitude Biological Pump
thermocline
Biological pump
Low Latitude: Change nutrient inventory
High Latitude: Change nutrient
consumption
Low vs. High Latitude
(C,N,P)org NO
3
-,PO4
-3=0
atmosphere
CO2, NO3
-, PO4
-3
Low-Latitude Biological Pump
thermocline
Low-latitude biological pump
~3 kyr ~15-80 kyr
N2
N2
denitrification
N fixation
Broecker, 1982
N isotopes in nature: 14N = 99.64% 15N = 0.36%
δ
15
(‰ . ) N vs air =
(
15
/N
14
)N
sample
(
15
/N
14
)N
air
- 1
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
* 1000
N isotope terms
MS 172 Figure 5
0
5
10
15
20
0 0.5 1 1.5 2
[NO3-] (factor of initial value)
newly fixed Nadded
δ15N~ -2-0‰
water columndenitrification
ε ∼ 20-30‰
sedimentarydenitrification
ε ∼ 0‰
nitrateuptake
ε ∼ 5‰
NO3- δ15N
(‰ . )vs air
Effects of major N fluxes on nitrate δ15N
Lower water column denitrification during LGM
302520151050age (ka)
14
12
10
8
6
4
δ15
(‰ N
. vs
)air
Denitrification increase
Santa BarbaraBasin
Arabian Sea
Chile Margin
Chile: de Pol-Holz et al, 2006SBB: Emmer and Thunell, 2000AS: Altabet et al., 2002
Low δ15N of nitrate in the North Atlantic thermocline: Paleoceanographic utility
[NO3-] (µM)
δ15N of nitrate (‰ vs. air)
Knapp et al., 2005
Planktonic foraminifera
QuickTime™ and a decompressor
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Orbulina universa
photo: H. Spero Montoya et al., 2002
Study sites
10
8
6
4
2
0
Foraminifera-bound
δ15
(‰ N
. vs
)air
-BarbudaAntiqua
LittleBahama
Bank
GreatBahama
Bank
Indonesia South Pacific South Pacific
Atlantic Pacific
Sargasso Sea subsurface
Indonesia Seasubsurface
Subantarctic
37 15.64 º ' S176 40.02 º ' E
36 22.63 º ' S177 .26.75 º ' E
6 46.25 º ' S116 58.47 º ' E
Mode Water
. G ruber . G sacculifer . O universa . G menardii . N dutertrei . G truncatulinoides . H pelagica . G inflata
Thermocline nitrate vs. coretop foraminifera
Ren et al., 2009
8
7
6
5
4
3
2
δ15
(‰ N
. vs
)air
302520151050 ( )age ka
-2
-1
0
δ18
O C
(‰
. vs
)VPDB
6
5
4
3δ15
(‰ N
. vs
)air
8
7
6
5
4
3
2
δ15
(‰ N
. vs
)air
a.
b.
c.
d.
individual species
bulk sediment
mixed species
O. universa G. sacculifer G. ruber
> 355 µm 250~355 µm 125~250 µm
δ18O, age:Schmidt et al., 2004
LGMHolocene
ODP site 999A:
Caribbean Sea
Ren et al., 2009
1000
800
600
400
200
depth (m)
6543210-1δ15 (‰ N . vs )air
100
80
60
40
20
0
. G ruber
. G sacculifer
. O universa
3.1‰
Modernδ15 :N suspended POM nitrate
- Foraminifera boundδ15 :N Holocene LGM :LGM interpretation
POM: Altabet, 1988NO3
- : Knapp et al., 2005
N fixation rate:LGM/Holocene ~ 20%
Ren et al., 2009
Chile: de Pol-Holz et al, 2006SBB: Emmer and Thunell, 2000AS: Altabet et al., 2002Cariaco: Haug et al., 1998
7
6
5
4
3
2
δ15
(‰ N
. vs
)air
302520151050 ( )age ka
14
13
12
11
10
9
8
7
6
5
4
δ15
(‰ N
. vs
)air
Denitrification increase
N fixation increase
Santa BarbaraBasin
Arabian Sea
Cariaco Basin
:Caribbean Sea . O universa . G sacculifer . G ruber
Chile MarginDeglacial increases
in both denitrification
and N fixation
A stable N inventory regulated by N/P ratio?
Gruber and Sarmiento, 1997
~1P:16N
Fix!
Don’t Fix!
Deglacial scenario:N fixation feedback through N/P sensitivity
Summary
• In Caribbean Sea sediments, planktonic foraminiferal 15N/14N decreases from the last ice age to the current interglacial.
• The foraminiferal change is best explained by less N fixation in the Atlantic during the last ice age, leading to higher nitrate 15N/14N in the Caribbean thermocline.
• The reconstructed increase in N fixation at the end of the last ice age is most likely a response to the previously recognized deglacial increase in global denitrification.
• As with our findings regarding the modern Atlantic N fixation rate, this is consistent with a strong P control on N fixation.
• A significant glacial/interglacial change in the nitrate reservoir has not been ruled out. However, our evidence that Atlantic N fixation acts as a negative feedback argues qualitatively for limits to such a change.
Acknowledgement
• Princeton University • Consortium of Ocean Leadership
• Sigman Group: Franky Wang, Brigitte Brunelle,
Julie Granger, Peter Difore.
• J. Bernhard and D. McCorkle
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Nitrogen cycle
8
7
6
5
4
3
2
δ15
(‰ N
. vs
)air
302520151050 ( )age ka
-2
-1
0
δ18
O C
(‰
. vs
)VPDB
6
5
4
3δ15
(‰ N
. vs
)air
8
7
6
5
4
3
2
δ15
(‰ N
. vs
)air
a.
b.
c.
d.
individual species
bulk sediment
mixed species
O. universa G. sacculifer G. ruber
> 355 µm 250~355 µm 125~250 µm
Minimal Holocene δ15N
decreasein
bulk sediment
δ18O, age:Schmidt et al., 2004
Ren et al., 2009
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Foraminifera-bound N isotope analysis
•Sieve, pick, crush
•Clean fragments with wet oxidation
•Acid dissolution to release
internal Norg
•Norg NO3- (persulfate oxidation)
•NO3- N2O (denitrifier method)
•N2O isotopic analysis (continuous
flow, purge/trap, gas
chromatography, gas-source magnetic
sector mass spectrometry)
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Carbon inventory
Fig. S1
20
15
10
5
0
N content (µmol/g)
20151050
cleaning time (hours)
7
6
5
4
3
2
1
0
δ15
(‰ N
. vs
)air
( / )N content µmol g δ15 (‰ )N
Fig. S2
7
6
5
4
3
2
1
0
foraminifera-bound N content (µmol/g)
302520151050age (ka)
70
60
50
40
30
20
10
0
bulk sedimentary N content (µmol/g)
O. universa G. sacculifer G. ruber > 355µm 250~355µm 125~250µm bulk N content
Thermocline nitrate/surface sediment comparison
Altabet, 2005
-24.5
-24.0
-23.5
-23.0
-22.5
-22.0
-21.5
δ13
(‰ C
. vs
)VPDB
302520151050 ( )age ka
10
9
8
7
6
5
/ ( )C N weight ratio
/C N δ13 (‰ )C
Downcore changes in δ13Corg and Corg/TN
40
30
20
10
0
TN (µmol/g)
4003002001000Corg (µmol/g)
LGM Holocene
TN=0.0563(±0.0143)*Corg + 21.8695(±4.8225)
R2=0.36
TN=0.0549(±0.0131)*Corg + 14.6769(±4.3894)
R2=0.83
C/N~15 (by weight) terrestrial organics
Evidence for terrestrial and shelf materialin ODP 999A sediments
-22.0
-21.5
-21.0
-20.5
-20.0
-19.5
-19.0
δ13
(‰ C
. vs
)VPDB
30252015105 ( )Age ka
10.0
9.5
9.0
8.5
8.0
7.5
/ ( / )C N g g
45
40
35
30
25
(%)Carbonate content
δ13 (‰ C . vs )VPDB / ( / )C N g g (%)Carbonate content
Fig. S4
Bulk sediment δ15N records from the North Pacific
Kao et al., 2008
Bulk sediment recordsfrom diverse settings:Mean ocean nitrate δ15N did not decrease (much) into the Holocene.
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0δ15
. (‰ N of N Atlantic thermocline nitrate
. vs
)air
1.41.21.00.80.60.40.20.0 fraction of modern N fixation
Holocene
LGM
Estimating N fixation change
Surfacechlorophyll
Surfacenitrate
Incomplete nutrient consumption in the polar ocean
WINDS
Siegenthaler (1983)
THERMOCLINE
35NO3-
232CO2
10Norg66Corg
25NO3-
166CO2
›
Eq. S
Rapid surface-deep exchange in the polar ocean releases deeply sequestered CO2 to the atmosphere
Mechanisms for reducing the Southern Ocean CO2 leak
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4000
3500
3000
2500
2000
1500
1000
500
04 4.5 5 5.5 6 6.5 7
4000
3500
3000
2500
2000
1500
1000
500
020 25 30 35 40
[NO3-] (µmol/kg) δ15NO
3- (‰ vs. air)
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0
5
10
15
20
25
30
4
6
8
10
12
14
16
40 45 50 55 60 65 70 75latitude along ~140°E (°S)
0
5
10
15
20
25
30
4
6
8
10
12
14
16
40 45 50 55 60 65 70 75
[NO3-]
δ15NO3-
RITS '94 - central Pacific
The link between theN isotopes and nitrateconsumption
4.0
3.6
3.2
2.8
δ18 O
. ( ) N pachyderma sin
(‰ . )v PDB
100806040200 ( )age ka
6
5
4
3
2- diatom bound
δ15
(‰ . )N v air
δ18O + age: Ninneman and Charles, 1997
• More complete nitrate consumption in glacial Subantarctic
Subantarctic (E11-2) diatom-bound 15N/14N: Link to glacial/interglacial cycles
Mahowald et al., 2005
modern annual average dust deposition (g m-2 yr-1)
Subantarctic is well situated fordust-driven iron fertilization during the
ice ages
LGM
Antarctic 15N/14N:higher during the last glacial maximum
Subantarctic nutrient drawdown:CO2 ~40 ppm
AA deep water formation “off”:CO2 ~35 ppm
Polar AA nutrient drawdown:More CO2 without complete deepwater formation shutdown
Glacial/interglacial changes in the Southern Ocean inferred from the N isotopes
Low and High latitude connection
Lower rates of N fixation