Geochemical fingerprints of the ice-age (Southern) …...Summary of the ice-age ocean and deglaciation 1)Ice-age: Efficient biological pump, low oxygen in the deep sea 2)Ice-age: Locus

Geochemical fingerprints of the ice-age (Southern) Ocean

THE SOUTHERN OCEAN, ITS DYNAMICS, BIOGEOCHEMISTRY AND ROLE IN THE CLIMATE SYSTEM NCAR, Boulder, CO 10 - 13 April 2017

Bob Anderson

Motivation: Ice core records reveal tight coupling between CO2 and climate – Why?

Brook, NATURE|Vol 453|15 May 2008

Take-home messages from this presentation

1) The biological pump was more efficient during the last glacial period, lowering the oxygen concentration in the deep sea.

2) Carbon was released from the deep ocean during deglaciation – via the Southern Ocean

3) Major shifts in the SWW during deglaciation4) Challenge – did the winds drive CO2 release?

Deep-ocean CO2 storage represents a balance between biology and physics

Figure of K Speer redrawn by T Trull

Atmospheric CO2 reflects a global balance between biological drawdown and physical ventilation.

CO2 BiologicalPump

Respiration

CO2

CO2&Nutrients

Preformed Nutrients

Where and How was CO2 stored in the deep ocean?Guiding principle:

“Any biological pump mechanism for lowering ice-age pCO2

atm decreases the dissolved O2 content of the ocean interior”

Sigman et al., 2010, summarizing one of the main points from Broecker, 1982.

C106H175O42N16P + 150 O2 106 CO2 + 78 H2O + 16 HNO3 + H3PO4

How do we assess changes in [O2]?

There is no direct geochemical “proxy”. Therefore:

DO2 constrained indirectly:

Sediment redox state (measure U, Re); depends on:a) Bottom water [O2] (oxygen supply),b) Organic carbon supply (Measure xsBa, opal)

Infer: Bottom water [O2]

Central Equatorial Pacific Illustration

PhD results of Allison Jacobel, LDEO – Geochemical fingerprints of low bottom water oxygen during ice ages

180

220

260

Anta

rctic p

CO

2

(ppm

)

0.2

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103-P

C72

Ba

xs F

lux

(mg c

m-2 k

yr-1

)

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ML1208-1

7P

C

aU

(ppm

)

Age (ka)

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-2.5

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LR

04 B

enth

ic

δ1

8O

(‰

)

ML1208-1

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C

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kton

ic δ

18O

(‰)

Age (ka)

Compelling qualitative evidence for the Pacific Ocean

Earth and Planetary Science Letters 277 (2009) 156–165

Nature Geoscience 5 (2012) 151–155

Earth and Planetary Science Letters 299 (2010) 417–425

Compelling qualitative evidence for the Atlantic Ocean

Nature Geoscience 8 (2015) 40-43 North Atlantic

Nature 530 (2016) 151–155 Southern Ocean

Nature Communications 7 (2016) doi 10.1038/ncomms11539 South Atlantic

Deep N Pacific (>5000m): Magnetic minerals lost from ice-age sediments due to low BWO

Other types of geochemical fingerprints of low BWOKorff et al., 2016, Paleoceanography 31: 600-24

Low-oxygen waters upwelled in the Southern Ocean during the ice ages

Core site in the Amundsen Sea – Modern Ocean OxygenLu et al., 2016, Nature Communications 7: doi 10.1038/ncomms11146

Low-oxygen waters upwelled in the

Southern Ocean during the ice ages

Low I/Ca ratios of planktonic foraminifera (geochemical fingerprint) indicate very low-oxygen water in the ice-age subsurface Amundsen Sea

Lu et al., 2016, Nature Communications 7: doi10.1038/ncomms11146

Low-oxygen waters upwelled in the Southern Ocean during the ice ages

Ice-age O2 concentrations in CDW must have been < ~ 20 µmol/kg for reduction of IO3

- to I-.

Lu et al., 2016, Nature Communications 7: doi 10.1038/ncomms11146

Physical changes in the Southern Ocean proposed to allow low-oxygen conditions

Physical changes in the Southern Ocean proposed to allow low-oxygen conditions

Ice-age expansion of deep overturning cell:a) isolated deep waters, allowing low oxygenb) accompanied by northward shift in upwelling

Watson et al., 2015, Nature Geosci 8: 861-4

Opal (diatom frustules) burial traces shift in locus of upwelling

Dissolved silicic acid section along the prime meridian – WOA09 and ODV

APF


Maximum opal burial in modern sediments south of the APFReflects Si supply to diatoms

Geibert et al., 2005, Glob Biogeochem Cycles 19: GB4001 doi:10.1029/2005GB002465

APF


Core sites spanning modern [Si] gradient used to investigate ice-age conditions


Peak opal burial shifted ~5° Northward during ice ages

Kumar, Anderson et al., 1995. Nature 378: 675-80 with newer results

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Opa

l bur

ial (

mm

ol S

i/m2 /y

r)

Latitude °S

HoloceneGlacial

APF

Competing hypotheses to explain northward shift in opal belt during ice ages

1) Upwelling remained unchanged• expanded sea ice inhibited plankton south of the APF• unused nutrients mixed northward prior to consumption

Charles et al., 1991, Paleoceanography 6: 697-728




Disproven by N isotopes (talks by Adkins and Sigman)Nutrients were utilized efficiently south of the APF




Disproven by N isotopes (talks by Adkins and Sigman)Nutrients were utilized efficiently south of the APF

2) Upwelling center displaced northward(and most upwelled water mixed northward?)

Geochemical fingerprints indicate increased nutrient utilization (efficiency of the biological pump) throughout the Southern Ocean – and low oxygen in the deep sea.

Maximum upwelling south of the modern APF coincided with deglacial rise in atmospheric CO2 - Winds invoked

SUMMARY OF EVIDENCE:

Opal burial traces upwelling:• Diatoms use available Si, • Deglacial increase in opal burial traces southward shift in upwelling and supply of nutrients

• Peak opal burial exceeds anywhere in modern ocean(No modern analog during So Ocean reorganization)

Modified fromAnderson et al., 2009

HS1

TN057-13 53.17°S

Geochemical fingerprints of CO2 ventilation are consistent with interpretation of opal flux

Martínez-Botí et al. Nature 518, 219-222 (2015) doi:10.1038/nature14155

Atlantic Southern Ocean Eastern Equatorial Pacific

Sea-airDpCO2Opal flux

Planktonicd13C

Atm pCO2Atm d13CO2

Deglacial So Ocean upwelling injected nutrient-rich waters into the thermocline (AAIW)

Poggemann et al. 2017. Earth and Planetary Science Letters 463: 118-26



Cdw (nutrient tracer) increases abruptly during HS1 at 850m (AAIW) but not below 1300m (UNADW), reflecting nutrient injection in the Southern Ocean



Pattern of AAIW nutrient injection (Cdw fingerprint) matches the opal tracer of upwelling in the Southern Ocean

What physical forcing was responsible?

Wind?

Buoyancy flux?

Watson et al., 2015, Nature Geosci 8: 861-4

Evidence for shifting winds

WAIS Divide Ice Core

Excess deuterium (dln, a geochemical fingerprint of moisture source conditions) changes abruptly with each NH climate oscillation –

implicating shift in winds with each NH abrupt climate change.

Markle et al., 2017, Nature Geosci 10: 36-40

Greenlandwarming

Greenlandcooling

Composite records


Patagonian GlaciersRapid retreat 18-16 ka

(2013) Scientific Reports 3: 2118; DOI:10.1038/srep02118

See also Denton et al., 1999, Geografiska Annaler Series a-Physical Geography 81A: 107-53

Hall et al., 2013, Quat. Sci. Rev., 62: 49-55. Composite records


New Zealand GlaciersRapid retreat 18-16 kaRequired rapid warming

Inferred southward SWWExpansion of the

subtropical gyre

Putnam et al., 2013, Earth and Planetary Science Letters, 32: 98-110.

Linked to contemporary changes south of Australia


Subtropical species spread S of Australia during Heinrich Stadials –local warming and displacement of currents


Tristan da Cunha:Fossil evidence in bog sediments for displacement of storm tracks

Ljung et al., 2015, Quaternary Science Reviews 123: 193-214


Aukland Island:Leaf wax isotopes in bog

sediments trace large shift in moisture source (winds)

Jon Nichols, LDEOUnpublished δD

P, ‰

VS

MO

W

Megg's Hill Peatland, 51°SC27-basedC29-basedC31-basedFeb. 2015 peatland waterFeb. 2015 precip. median

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Age, cal. ka

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Opa

l Flu

x, g

cm−2

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TN057-13-4PC, 53.2°S

Summary of the ice-age ocean and deglaciation

1) Ice-age: Efficient biological pump, low oxygen in the deep sea

2) Ice-age: Locus of So Ocean upwelling located north of its present position.

3) Deglaciation: Upwelling shifted southReleased CO2

Injected nutrients into AAIW (thermocline) 4) Deglaciation: Winds shifted south5) Challenge: Did the winds play a role in forcing

So Ocean changes?

Redox state: Exploit trace elements (uranium) precipitated under anoxic conditions

[O2] [U]

Ua precipitation

Seaw

ater

Dep

th in

por

e w

ater

€

Ua =FUdMAR

=DS (∂[U ]/∂z)

MAR

€

∂z

~Constant

€

∂[U ]

Variable∂z decreases, and Ua increasesas [O2] decreases orCorg flux increases

Lowering [O2] or increasing C-org rain raises Ua

[O2] [U]

Ua precipitation

Seaw

ater

Dep

th in

por

e w

ater

€

Ua =FUdMAR

=DS (∂[U ]/∂z)

MAR

€

∂z

~Constant

€

∂[U ]

Variable∂z decreases, and Ua increases as:

[O2] decreases orCorg flux increases

Plausible scenario for the ice-age Pacific Ocean But not based on any quantitative O2 estimates

Black = Modern observationsOrange = Plausible LGM Jaccard et al., 2009

Carbon must have been transferred to the deep ocean during the ice ages

The deep ocean is:

1) The only C reservoir large enough to accommodate 200 GtC from the atmosphere during each peak ice age...

2) ...and a much larger inventory of carbon released from the terrestrial biosphere.

3) The only large C reservoir capable of exchanging carbon with the atmosphere as rapidly as indicated by the ice cores.

Documents

Geochemical fingerprints of the ice-age (Southern) …...Summary of the ice-age ocean and deglaciation 1)Ice-age: Efficient biological pump, low oxygen in the deep sea 2)Ice-age: Locus