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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 ObservatoryColumbia 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 nutrients are 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 (

ppm

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flux

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g m

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r-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 proxy West East

Global ice volume proxy Winckler 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

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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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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 zone

Iron fertilization was not pervasive throughout the Southern OceanKohfeld, 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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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

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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!

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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-

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Alkenone Flux Sachs & Anderson, 2003

Uranium Sachs & Anderson, 2005

Nd Piotrowski et al., 2005

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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 CaCO3 compensation, 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

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Hughen Atm. 14C from Cariaco Basin. Hughen et al., 2006

CO2 from Monnin et al., EPSL, 2004

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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