Investigating the role that the Southern Ocean
biological pump plays in determining global ocean
oxygen concentrations and deoxygenation
David P. Keller and Andreas Oschlies
Why is the biological pump in the Southern Ocean so important?
The Southern Ocean plays a key role
global ocean circulation
Why is oxygen important?
The biological pump determines how •There is plenty of oxygen in the Southern Ocean, but in
much organic matter is exported to
the tropics there are areas with little to no oxygen
deeper waters
•In these oxygen minimum zones (OMZs) some marine
organisms cannot survive and biogeochemical cycles
are different from in oxygenated waters
+
»»Denitrification and the production of N2O, a powerful
greenhouse gas, occur in OMZs
»»OMZ biogeochemistry may influence the global climate system
•OMZs are predicted to become larger in the future, but are not fully
understood
Marinov et al., 2006
Figure 2. U.S. JGOFS program schematic of the upper ocean
carbon cycle and the biological processes that drive the
biological pump (i.e., the depicted carbon flux).
Figure 1. A schematic of the overturning/thermohaline circulation in the global ocean,
stressing the importance of the Southern Ocean.
Does the Southern Ocean play a role in global marine
oxygen cycling?
•Polar oceans play an important role in the air-sea flux of oxygen
In combination these process make the Southern Ocean an important driver of
global biogeochemical cycles
•Studies of other biogeochemcial cycles suggest that the Southern Ocean
may play an important role, but to our knowledge no focused studies
have investigated the specific role that the Southern Ocean plays in
global marine O2 cycling
•Previous studies have shown that the efficiency of the biological pump in the
Southern Ocean plays an important role in global C, N, P and Si cycles
Objective
Models
•To investigate how Southern
Ocean biology and the efficiency
of the biological pump influence
global oxygen concentrations and
tropical oxygen minimum zones
University of Victoria Earth System
Climate Model (Eby et. al., 2009;
Keller et al., 2012)
ECCO-BUR Model (Kriest & Oschlies, 2013)
»»1.8° x 3.6° resolution, 19 ocean
levels
»»Transport Matix Method derived ocean
circulation field (Khatiwala, 2007)
»»1° x 1° resolution, 23 ocean levels
•Ocean physics: data assimilated MIT GCM
(Stammer et al., 2004)
»»Includes dynamic, coupled
atmospheric, terrestrial, oceanic,
and sea-ice components
Modelling Experiment
•Shut off biology and the biological
•Ocean physics: Modular Ocean
pump south of 40° S
Model (MOM) 2
•Compare how different models
•N-based marine ecosystem model
respond when run to steady-state
that includes C, N, P, and O cycles
Figure 3. A schematic of the University of Victoria (UVic) Earth System Climate
Model marine ecosystem model component. State variables include phytoplankton
(PO), diazotrophs (PD), zooplankton (Z), detritus (D), nitrate (NO3), phosphate
(PO4), and oxygen (O2). Dissolved inorganic carbon (DIC) and alkalinity (ALK) are
also state variables, but not depicted above. Constant (~Redfield) stoichiometry
(RC:N, RN:P, RO:N, etc.) relates the C, N, and P content of the biological variables and
their exchanges with the inorganic variables.
•Phosphorus-based marine ecosystem model
(Kriest et al., 2012) with benthic burial
»»State variables include PO4, phytoplankton,
zooplankton, detritus, and dissolved organic
phosphate (NPZD-DOP, where N=nutrient)
Results
Basin-Scale Changes in Oxygen
Atlantic
UVic
0
Depth (m)
Control
2000
3000
4000
2000
3000
4000
1200
No SO Biology
2000
3000
4000
5000
80°S
40°S
40°N 80°N
0°
Latitude
40°S
5000
40°S
40°N 80°N
0°
Latitude
UVic
40°S
40°N 80°N
0°
Latitude
Depth (m)
4000
80°S
Depth (m)
Control
Latitude
40°N 80°S
40°S
0°
Latitude
3000
2000
3000
4000
5000
20°N 60°S 40°S 20°S
120
160
Latitude
0°
20°N
O2 in the
upper ocean
organic matter
export
Less organic matter
at depth:
O2 consumption
O2 in the
deep ocean
Oxygen Minimum
Zone size
•The biological pump in the Southern Ocean reduces oxygen concentrations in
the deep ocean (in both the Southern Ocean and other ocean basisn)
•The biological pump in the Southern Ocean affects mid- to low-latitude upper
ocean oxygen concentrations by trapping nutrients that would otherwise fuel
productivity and subsequently reduce oxygen in these regions
360
320
280
240
200
80
40
0
0°
1000
Depth (m)
O2 consumption
No biological pump:
•Overturning/thermohaline circulation transports the imprint of Southern
Ocean oxygen cycling to other ocean basins
1000
Depth (m)
No SO Biology
Figure 9. Simulated percent fraction of the ocean that is
suboxic (O2 < 5 mmol m-3).
Summary
4000
5000
0
Difference
low latitude
productivity
40°N
Figure 7. Simulated zonally averaged oxygen concentrations in different
ocean basins and the difference between the experiment and the control
simulations.
0
2000
3000
References:
4000
Eby, M. et al. Lifetime of Anthropogenic Climate Change: Millennial Time Scales of Potential CO 2 and Surface Temperature Perturbations. J. Clim. 22, 2501–
2511 (2009).
5000
60°S 40°S 20°S
Latitude
0°
20°N 60°S 40°S 20°S
Latitude
0°
20°N
Keller, D. P., Oschlies, A. & Eby, M. A new marine ecosystem model for the University of Victoria Earth System Climate Model. Geosci. Model Dev. 5, 1195–1220
(2012).
Stammer, D., K. Ueyoshi, A. Köhl, W. G. Large, S. A. Josey, and C. Wunsch (2004), Estimating air-sea fluxes of heat, freshwater, and momentum through global
ocean data assimilation, J. Geophys. Res., 109, C05023, doi:10.1029/2003JC002082.
Khatiwala, S. A computational framework for simulation of biogeochemical tracers in the ocean. Global Biogeochem. Cycles 21, (2007).
70
60
50
40
30
20
10
0
-10
-20
-30
-40
-50
-60
-70
Figure 6. Simulated annual oxygen concentration differences between the experiment
and control model runs.
0°
Experiment - Control O2 (mmol m )
2000
60°S 40°S 20°S
80
40°S
-3
ECCO-BUR
O2 (mmol m-3)
-60 -40 -20
0
40
20
60
∆ oxygen [experiment - control](mmol O2)
Surface nutrients are
transported to the
north:
Total Ocean Oxygen (Pmol)
Model
Experiment
UVic
ECCO-­‐BUR
Control
257.3
232.2
No SO biology
291.3
277.3
Latitude
-80
surface nutrients
5000
Indian
~ 2000 m
~ 3000 m
No biological pump:
360
~ 300 m
40°N
3000
Experiment - Control O2 (mmol m )
ECCO - BUR
320
4000
280
UVic
Latitude
240
3000
0°
2000
1000
Changes in O2
200
2000
0
Figure 5. Comparison of ECCO-BUR simulated annual net primary productivity (NPP) with and
without Southern Ocean biology and the resulting difference in the amount of carbon exported to the
deep ocean.
160
1000
-150
0
150
300
∆ Export [experiment - control](mmol C m-2 yr-1)
1200
40°S
70
60
50
40
30
20
10
0
-10
-20
-30
-40
-50
-60
-70
600
900
NPP (mg C m-2 d-1)
40°N 80°S
120
1000
70
60
50
40
30
20
10
0
-10
-20
-30
-40
-50
-60
-70
300
Latitude
80
0
-3
0
0°
40
0
80°S
-300
40°S
0
Difference
4000
80°S
percent fraction of suboxic area
Overturning/thermohaline circulation makes Southern
Ocean biogeochemistry globally important
O2 (mmol m-3)
5000
ECCO - BUR with no Southern Ocean Biology
3000
360
The Difference in Exported Carbon at 2000 m
2000
Latitude
320
O2 (mmol m-3)
ECCO - BUR Control Run
Figure 8. Simulated and observed annual global average oxygen
concentrations.
1000
40°N 80°N
0°
280
240
200
160
120
80
40
0
Figure 4. Comparison of UVic simulated annual net primary productivity (NPP) with and without
Southern Ocean biology and the resulting difference in the amount of carbon exported to the deep
ocean.
Oxygen (mmol m-3)
Depth (m)
Depth (m)
1000
Depth (m)
600
900
NPP (mg C m-2 d-1)
5000
0
Difference
300
No SO Biology
0
-150
0
150
300
∆ Export [experiment - control](mmol C m-2 yr-1)
ECCO-BUR
1000
5000
0
-300
UVic
0
Depth (m)
UVic with no Southern Ocean Biology
ECCO-BUR
1000
Control
The Difference in Exported Carbon at 2000 m
Pacific
Depth (m)
UVic Control Run
Suboxic Changes
Global Changes
Depth (m)
Shutdown of the Biological Pump
Experiment - Control O2 (mmol m-3)
Kriest, I., Oschlies, A. & Khatiwala, S. Sensitivity analysis of simple global marine biogeochemical models. Glob. Biogeochem. Cycles 26, GB2029 (2012).
Kriest, I. & Oschlies, A. Swept under the carpet: the effect of organic matter burial in global biogeochemical ocean models. Biogeosci. Discuss. 10, 10859–10911
(2013).