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What can we learn from the paleo record about past changes in ocean productivity and
controls of atmospheric CO2?
Bob Anderson, Gisela Winckler, Martin Fleisher Lamont-Doherty Earth Observatory
Columbia University
Exploring Ocean Iron Fertilization, WHOI, September, 2007
CO2
Climate
From Brook, 2005Comment onSiegenthaler et al., 2005
Ice core records show tight coupling between CO2 & Climate
War
mer
How did the ocean lower glacial atmospheric CO2 levels?
Plausible mechanisms
1) Increased strength of the biological pump• Increase nutrient inventory (capacity)• Increase nutrient utilization (efficiency; today at ~50%)
2) Increase ocean ALK/DIC ratio ([CO32-])
• Continental weathering• Shelf-basin fractionation (“Coral Reef” hypothesis)• C-org/CaCO3 ratios (“Rain Ratio” hypothesis)
What does “efficiency” of the biological pump mean?
It is the fraction of upwelled nutrients that are utilized and exported to depth as organic matter.
Preformed nutrientsare the master variable to characterize the efficiency of the biological pump.
+ DIC !!Regeneration
Biological pump of Sigman & Boyle, 2000
CO2CO2
pCO2 vs Preformed PO4
Sensitivity of CO2 to preformed nutrients
Marinov et al., Nature, 2006
Princeton Ocean GCM runs with different nutrient utilization scenarios. Constant ocean nutrient inventory
Levitus Nitrate - ColorLevitus Nitrate
From: iridl.ldeo.columbia.edu/SOURCES/.LEVITUS94
Annual average Nitrate Concentration at 20 m
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.
Martin’s “Iron Hypothesis”Dust is inversely correlated with CO2 in Antarctic ice
core records -- is there a causal relationship?
Martin (1990) reasoned that increased dust fluxes relaxed Fe limitation in the glacial Southern Ocean, allowing increased efficiency of the biological pump to draw down atmospheric CO2
Antarctic Ice CoreDust (Fe) - CO2 (anti)correlation
Fe flux from Wolff et al., Nature 2006; CO2 from Brook, Science 2005
CO
2(p
pm)
Fe fl
ux (µ
g m
-2yr
-1)
Questions to ask of the paleo record:
1) Did dust affect Productivity in HNLC regions?
2) Did other sources of Fe have a significant impacton productivity?
3) What caused glacial CO2 to be 80-100 ppm lower?
Questions to ask of the paleo record:
1) Did dust affect Productivity in HNLC regions? (No)
2) Did other sources of Fe have a significant impacton productivity? (I think so)
3) What caused glacial CO2 to be 80-100 ppm lower? (?)
Equatorial Pacific
Search for evidence of dust influence in regions with paired records of dust flux and paleoproductivity.
XVNTR 08VNTR 08
RC17-177
Equatorial Pacific Dust-Climate Correlation
• Dust flux proxy is tightly correlated with climate• Glacial-interglacial amplitude ~2.5X at all sites
Dust flux proxyWest East
Global ice volume proxyWinckler et al., submitted
Equatorial Pacific - Antarctica Correlation
Internally-consistent change in dust flux from at least 3 sources suggests control by global hydrological cycle
Winckler et al., submitted
CEP - No Productivity Response
Proxy records for paleoproductivity and dust flux are uncorrelated over the last 3 glacial cycles
232Th flux (Dust proxy) - Winckler et al., submittedBarite concentration - Paytan, 1995Barite flux (PP proxy) - Anderson et al., in press
0.5
1.0
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0 50 100 150 200 250 300Age (ka)
0.0
0.1
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0.5Barite PP Th-232 Dust
Bar
ite F
lux
(mg
cm-2
ky-1
)
232 T
h Fl
ux (µ
g cm
-2ky
-1)
TT013-PC72 Equator, 140°WP
rodu
ctiv
ity
Dus
t
EEP - No Productivity Response
Productivity shows no response to a 2-fold drop in dust flux over the last deglaciation
232Th flux (Dust proxy) - PC72, Anderson et al., 2006; ODP849, Winckler et al., sub.VNTR08 Barite Flux (PP proxy) - Barite conc. Paytan, 1995
Sediment flux - Pichat et al., 2004
TT013-PC72 Equator, 140°WVNTR08 & ODP 849, Eq., 110°W
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VNTR08-Barite ODP849-Th PC72-Th
232 T
h Fl
ux (µ
g cm
-2ky
-1)
Bar
ite F
lux
(mg
cm-2
ky-1
)
Age (ka)
Pro
duct
ivity
Dus
t
Equatorial Pacific:
Increased glacial dust fluxes had no detectable effect on export production.
Levitus Nitrate - ColorLevitus Nitrate
From: iridl.ldeo.columbia.edu/SOURCES/.LEVITUS94
What about the Southern Ocean?
Here, increased nutrient utilization south of the Antarctic Polar Front has thegreatest potential to affect atmospheric CO2.
LGM minus Modern Export Production(synthesis of published data; all proxies)
High glacial productivity is restricted to the Subantarctic zoneIron fertilization was not pervasive throughout the Southern Ocean
Kohfeld, LeQuéré, Harrison and Anderson, Science, 2005
Sites around the Southern Ocean with detailed records showing glacial productivity < interglacial
Nutrient utilization south of the APF has the greatest potential to impact global inventory of preformed nutrients. Marinov et al., (2006)
SW Pacific - Two Cores & Three ProxiesConsistently show glacial productivity < Holocene
Anderson et al., 2002
S Atlantic Productivity anti-correlated with dust
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ED
C F
e Fl
ux (µ
g m
-2y-1
)
Opa
l Flu
x (g
cm
-2ky
-1)
ED
C F
e Fl
ux (µ
g m
-2y-1
)
xsB
aFl
ux (m
g cm
-2ky
-1)
Age (ka)
Dia
tom
Pro
duct
ivity
Pro
xyE
xpor
t Pro
duct
ion
Pro
xy
Opal & Ba fluxes: Anderson et al., 2002 - EPICA Dome C Fe flux: Wolff et al., 2006
S Atlantic core RC13-259 - 54°S, 5°W
Site is downwind of the Patagonian dust source
If dust-borne Fe stimulated nutrient utilization in the glacial Southern Ocean, then it should have been evident here.
Southern Ocean (South of APF):
Any iron fertilization by increased glacial dust fluxes was more than offset by other factors that reduced export production.
Did Fe have any impact on glacial productivity in the Southern Ocean?
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 Spot”
Higher Subantarctic Productivity in LGM supported by order of magnitude greater C-org burial
Anderson et al., 1998, 2002
0
20
40
60
80
100
120
140
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0 10 20
Age
(ky)
RC13-254 RC15-93
Interglacial
Glacial
C-org Flux (mmol m-2 yr-1)
Patterns reproduced in
two cores
Changes were BIG!
Why such different behavior among cores downwind of Patagonia?
Blue = Lower glacial productivity; Red = Higher glacial productivityContours = Summer Nitrate µM; ample nutrients N of APF
Why such different behavior among cores?
Is the APF (convergence) a barrier to supply of essential factor?
Patagonian ice sheet during glacial times
delivered Ice-Rafted Debris
(IRD) 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
Icebergs as a source of Fe -
location matters!
QuickTime™ and aTIFF (Uncompressed) decompressor
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QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
Alaskan photos from John Crusius
Antarctic Subpolar
APF would have been a barrier to icebergs, IRD, and any Fe released from IRD
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Evidence for Patagonian Fe fertilization?
1) YES - Isotopic and mineralogical data; Diekmann, Walter, Kuhn, & others at AWI;
2) Nd isotopes in Cape Basin (highlighted star)
TN57-21 eps Nd & productivity Cape Basin: Nd isotopes correlate with productivity proxies…May reflect Fe supply.
TN57-21-
-9.0
-8.5
-8.0
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-6.0
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Depth (cm)
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Depth (cm)
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eNdU auth
230 T
h-n
Alk
enon
eFl
ux(n
g/cm
2 /ka)
Aut
hige
nic
U (d
pm/g
)
εNd
εNd
MIS 4MIS 2 MIS 3
Alkenone FluxSachs & Anderson, 2003
UraniumSachs & Anderson, 2005
εNdPiotrowski et al., 2005
S A
mer
ica
S A
fric
a
LGM minus Modern Export Production(synthesis of published data; all proxies)
Hypothesis- “Hot Spots” reflect Fe from Patagonia & KerguelanCurrent work on S Pacific shows no hot spots; supports local Fe fertilization
Kohfeld, LeQuéré, Harrison and Anderson, Science, 2005
Ongoing
Summary:No evidence for Fe fertilization of HNLC regions (EqPac &
So. Ocean) by increased glacial dust fluxes.
Subantarctic: Hot spots of high productivity may have been fertilized by local sources of Fe; not dust, maybe icebergs.
Impact of Subantarctic on CO2 minor because disconnected from main inventory of preformed nutrients.
Increased ocean stratification, with feedbacks from CaCO3compensation, lowered glacial atm. CO2
(Marchitto et al., Science, 2007)
What caused lower glacial CO2?Increased ocean stratification was a primary factor.
Indirect evidence from 14C of benthic forams at 700m in N Pacific.Accelerated overturning of deep waters brought CO2 to the atm.,and 14C-depleted DIC, both to intermediate depths and to the atm.
Marchitto et al, Science, 2007
More direct evidence: Deglacial increase in So Ocean upwelling coincided with rise in CO2 and drop in ∆14C of Atm. CO2
Incr
ease
d U
pwel
ling
Hughen Atm. 14C from CariacoBasin. Hughen et al., 2006
CO2 from Monnin et al., EPSL, 2004
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1
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7
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100
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500F(opal)
Hughen06
IntCal04
IntCal Atm. 14C from Reimer et al., 2004
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0 5 10 15 20 25 30180
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Dom
e C
CO
2(p
pm)
Opa
l M
AR
(g c
m-2
kyr-1
)
Age (ka)
ACR
ACR
Opa
l M
AR
(g c
m-2
kyr-1
)
Incr
ease
d U
pwel
ling
∆14C
of A
tm. C
O2
(‰)
Opal flux from TN057-13PC 53.2°S, 5.1°E
Deglacial increase in upwelling is evident at sites all around the Southern Ocean
Red star = TN057-13