Climate-Related Changes in Southern
Ocean Carbon Fluxes
Bob Anderson [email protected]
Xiaojun Yuan
Lamont-Doherty Earth Observatory
Chris Measures
University of Hawaii
Mary-Elena Carr
Jet Propulsion Laboratory
Outline
•
•
•
•
Why the Southern Ocean?
Paleo evidence for significant past changes
Modern spatial and temporal variability
Strategy to establish sensitivity to
environmental change
Biological Pump
Parameters Sensitive to Global Change that Impact
Ocean Uptake of CO2
• CaCO3/C-org Rain Ratio (last week)
• Regeneration Length Scale (not the topic of this talk)
• Nutrient Utilization (i.e., pump efficiency)
Nutrient Utilization
The true measure of the “Efficiency” of the ocean’s
Biological Pump
(see cover article in last issue of US JGOFS Newsletter by
Sarmiento et al. )
Using nutrients
more efficiently,
lowers atm. CO2 …
and vice versa
Bio Pump Bob
P
N ref
ut or
rie m
nt ed
s
“Efficiency” is
expressed in terms
of nutrient
utilization.
Nutrient Utilization - Southern Ocean is Principal
Area of Potential Impact
1) Upwelling deep waters supply surface with abundant nutrients.
2) "Window" for ventilation of the deep sea (exchange of gases
between the atmosphere and the deep ocean).
Figure from K. Speer as redrawn by T Trull
Annual average Nitrate Concentration at 20 m
Levitus Nitrate - Color
Levitus Nitrate
Only about half of the upwelled nitrate is used by phytoplankton.
Efficiency of the Biological Pump today is low. Potential to alter CO2 is high.
From: iridl.ldeo.columbia.edu/SOURCES/.LEVITUS94
Relevant features of the
Southern Ocean
• ~90% of Global Inventory of Surface Nutrients
• ~50% of Upwelled Nutrients go unused, returning
to depth as Preformed Nutrients
• Here, changes in the efficiency of the Biological
Pump can have their greatest impact on:
• Global inventory of preformed nutrients
• Atmospheric CO2
Outline
•
•
•
•
Why the Southern Ocean?
Paleo evidence for significant past changes
Modern spatial and temporal variability
Strategy to establish sensitivity to
environmental change
Past Changes (Paleoceanographic) in
Southern Ocean Carbon Flux
• Glacial Iron Fertilization (John Martin’s Hypothesis)
• Regional Hot Spots
• Potential Role of Iron
• Surprising changes in phytoplankton taxa
Colder < - Antarctica -> Warmer
Vostok Ice Core (Antarctica): Climate - CO2 Connection
Sigman and Boyle, 2000
Vostok Ice Core: High Dust (Fe) Coincides with low CO2
Martin, Paleoceanography, v. 5, 1990
LGM minus Modern Export Production
(synthesis of published data; all proxies)
High glacial productivity is restricted to the Subantarctic zone
Iron fertilization was not pervasive throughout the Southern Ocean
Kohfeld, LeQuéré, Harrison and Anderson, Science, 2005
LGM minus Modern Export Production
(synthesis of published data; all proxies)
“Hot Spots” - Subantarctic Sites Experienced High Productivity
Kohfeld, LeQuéré, Harrison and Anderson, Science, 2005
Examples from Subantarctic “Hot Spots”
Higher Subantarctic Productivity in LGM supported
by order of magnitude greater C-org burial
C-org Flux (mmol m-2 yr-1)
Cores located
between APF
and SAF
0
10
20
0
20
40
fluxes correct for
focusing
60
Two cores show
reproducible
patterns
Changes
were BIG!
Age (ky)
230Th-normalized
Glacial
80
100
Interglacial
120
140
RC13-254
RC15-93
160
Anderson et al., 1998, 2002
Decoupling of Opal from Organic Carbon
• Subantarctic Glacial C-org fluxes were greater than
expected from opal flux (Kumar et al., 1995)
• Antarctic Glacial C-org/Opal ratios increased when
export declined (Anderson et al., 2002)
• Two hypotheses:
• Fe fertilization increased C/Si of diatoms
• Other taxa contributed to glacial C-org flux
Opal-Carbon Decoupling S. of APF
S. of APF: Corg/Opal increased during glacial
periods of low export production
Anderson
et al.,2002
• Opal and xsBa fluxes indicate low glacial export fluxes
• Corg/Opal increased 4-5X from max to min export periods
• Did Fe fertilization increase C/Si of glacial diatoms?
Then why are export fluxes so low?
• Did other taxa (e.g., phaeocystis) contribute to glacial Corg flux?
Cape Basin Sites: Evidence for Glacial
Importance of “other taxa”
TN057-21
Drift deposit on
the southern
margin of the
Cape Basin
High temporal
resolution
TN057-06
Near crest of
Agulhas Ridge
outside of the
Cape Basin
Lower temporal
resolution
TN057-6
Cape Basin Fluxes
C-org Flux
(mmol m-2 yr-1)
0
2
4
0
Alkenone Flux
(ng cm-2 ka-1)
6
0
0
TN057-06 .
Age (ka)
200 400 600 800
TN057-06 .
Alkenone Flux
(ng cm-2 ka-1)
0
500
1000
1500
0
10
10
10
20
20
20
30
30
30
40
40
40
TN057-21
Glacial
• Subantarctic pattern of Corg fluxes is accentuated in Alkenones
• Alkenones are produced by Coccolothophorids
Derived from Sachs and Anderson, 2003 and unpublished
LGM minus Modern Export Production
(synthesis of published data; all proxies)
Coccolithophorids contributed to Subantarctic “Hot Spots”
Kohfeld, LeQuéré, Harrison and Anderson, Science, 2005
Subantarctic Indian Ocean: Alkenone
concentrations fell dramatically during deglaciation
Alkenone Concentration (ng/g)
MD94-103 45°35’S 86°31’E 3560 m
160
140
120
100
80
60
40
20
0
0
5000
10000
15000
20000
25000
30000
Age (years)
Coccolithophorids contributed to Subantarctic “Hot Spots”
Unpublished data courtesy of Marie-Alexandrine Sicre
Paleo Flux Summary
• Widespread Iron Fertilization apparently did not occur during
the last glacial maximum
• Subantarctic Hot Spots experienced elevated Corg fluxes
during the LGM
• Corg/Opal ratios increased substantially during LGM
• Increased Corg preservation?
• Fe fertilization of diatoms?
• Contributions from other taxa?
• Coccolithophorid abundance increased in Subantarctic
waters
Speculation about cause of
Subantarctic Hot Spots
Increased local sources of Iron fueled high
productivity in Subantarctic regions during
the LGM
High biomass in some areas suggest benthic source of iron
today in regions that were Hot Spots during LGM
SeaWiFS Images prepared for CROZEX Program
Courtesy of Raymond Pollard SOC
Cape Basin Sites
RC11-83
Drift deposit on
the southern
margin of the
Cape Basin
High temporal
resolution
RC11-83 Cape Basin
• Detrital 87Sr/86Sr
(solid black line)
correlates with
δ13C of benthic
foraminifera
• δ13C responds in
part of Corg rain
• Low detrital
87Sr/86Sr source
from S America
Rutberg et al., Paleoceanography, 2005
Patagonian ice
sheet during
glacial times
delivered Ice-
Rafted Debris
(IRD) with low
87Sr/86Sr
to the
Southern Ocean
Modern
ALACE float
tracks show
that currents
would have
carried
Patagonian
IRD into the
S Atlantic
Courtesy of
S. Gille, SIO
Working Hypothesis links Fe to Subantarctic Hot Spots
• Patagonian Ice Sheet fed IRD into Subantarctic waters
• This IRD supplied
• Lithogenic material with low 87Sr/86Sr
• Clay with high chlorite content (not shown)
• Authigenic material with high εNd (not shown)
• Strong signal from S. American lithogenic material in Cape
Basin indicates entire S. Atlantic affected
• Source of Fe to fuel high Subantarctic productivity
• Kerguelan similarly served as a source of Fe to
Subantarctic Indian Ocean
Paleo Summary
• The paleo record informs us that:
• Carbon Fluxes
• Stochiometric ratios (e.g., Corg/Opal)
• Phytoplankton taxa (e.g., coccolithophorids)…
• … in the So. Ocean are sensitive to changing
environmental conditions in COMPLEX WAYS
• Fe is important
• What else?
• Open to experimentation!
Outline
•
•
•
•
Why the Southern Ocean?
Paleo evidence for significant past changes
Modern spatial and temporal variability
Strategy to establish sensitivity to
environmental change
Recommendations for Future Work
• Exploit natural spatial and temporal variability to establish
critical sensitivities of carbon fluxes to changing
environmental parameters
• Spatial variability of Fe supply
• Persistent patterns of biomass
• Temporal variability of hydrographic conditions and
atmospheric forcing
Persistent spatial variability of biomass characterizes the
Southern Ocean
In the Drake Passage, phytoplankton biomass
increases suddenly and dramatically
“Green water” (offshore) – Stn 064
“Shelf” – Stn 057
“Blue water” – Stn 027
Hypothesis: Shelf waters advected into the Drake
Passage provide Fe for phytoplankton growth
Section of Fe, Mn and Al show advection of metalenriched shelf water
Unpublished data of Measures, Mitchell, Barbeau, Azam, Hewes, Holm-Hansen and Zhou
Section off Elephant Island shows enhanced
chlorophyll levels
Unpublished data of Measures, Mitchell, Barbeau, Azam, Hewes, Holm-Hansen and Zhou
Exploit local Fe sources to study impact on C cycle
Kerguelan - France
Crozet - UK
Antarctic Peninsula - well situated for U.S. Study (Palmer Station)
Recommendations for Future Work
• Exploit natural spatial and temporal variability to establish
critical sensitivities of carbon fluxes to changing
environmental parameters
• Spatial variability of Fe supply
• Persistent patterns of biomass
• Temporal variability of hydrographic conditions and
atmospheric forcing….
….e.g., Strong link to ENSO
ENSO - Antarctic Teleconnections
Ø The largest sea ice interannual variability occurs in the western
hemisphere organized as a quasi-stationary wave -- the Antarctic
Dipole.
ØThe Antarctic Dipole is the climate mode in high latitudes linked
to the ENSO variability in the tropics. The ENSO signal is
amplified in the air-sea-ice coupled system and persists seasons
in high latitudes after ENSO matured in the tropics.
Ø The regional circulation associated with the persistent PSA
center at the Bellingshausen Sea and anomalous heat transport
by the regional mean meridional circulation are the two
mechanisms for the formation of the ADP.
ØThe changes of Hadley Cell, jet stream, and Rossby Wave
propagation link the tropical signal with these two high latitude
processes.
Differences between El Nino and La Nina Composites
SAT
SLP El Nino
Composite
Sea Ice
SLP La Nina
Composite
Yuan 2001, 2005
Interannual
variability is large:
Biomass,
Primary Production
& New Production
1) Biomass & PP largest in
Subantarctic, but variability is
larger south of APF.
2) New Production & variability
are both larger South of APF
Biomass & PP from 6 years of
SeaWiFS data;
NP from 10 Years of Topex data.
Interannual variability is regionally coherent
Biomass & PP anomalies relative to mean of 6 years of SeaWiFS data;
PP from Howard-Yoder algorithm as modified by Carr, 2002.
Biological Response?
• Interanual variability is evident
• Not correlated simply to ENSO
• Further analysis required to reveal
significant relationships
• Strategy to design future process studies
Summary
• Southern Ocean is a key region where
changes in nutrient utilization can affect
atmospheric CO2
• Large changes in:
Carbon flux
Elemental stochiometries
Phytoplankton taxa
Occurred in response to past climate change
• Natural spatial and temporal variability force
Southern Ocean ecosystems today
• Exploit this variability to establish sensitivity
of key parameters to global change
Black Finis