SOES6047 - Global Climate Cycles SOES 6047 Global Climate Cycles L22: Research Themes: Southern...

SOES6047 - Global Climate Cycles

SOES6047 - Global Climate Cycles SOES 6047

Global Climate Cycles L22:

Research Themes:Southern Ocean history

and climate

SOES 6047Global Climate Cycles

L22:Research Themes:

Southern Ocean history and climate

Dr. Heiko Pä[email protected]. 23638, Rm. 164/34

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22SOES6047 - Global Climate Cycles


Last “themes” lecture:Last “themes” lecture:๏Sapropels and Monsoon changes๏Sapropels give us a detailed insight into the workings of a well confined basin in the past

๏Record stretches from very recent past into the Miocene๏Dramatic changes in Mediterranean water circulation, productivity, and oxygenation

๏Detailed correlation of sapropels to insolation maxima has resulted in an accurate astronomical time calibration for the Neogene, that agrees well with estimates from oceanic sediments. ๏Timing of sapropels give us clues as to phase relationship between orbital forcing and response (wait for Lect. 21!)

๏Sapropels and Monsoon changes๏Sapropels give us a detailed insight into the workings of a well confined basin in the past

๏Record stretches from very recent past into the Miocene๏Dramatic changes in Mediterranean water circulation, productivity, and oxygenation

๏Detailed correlation of sapropels to insolation maxima has resulted in an accurate astronomical time calibration for the Neogene, that agrees well with estimates from oceanic sediments. ๏Timing of sapropels give us clues as to phase relationship between orbital forcing and response (wait for Lect. 21!)

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SOES6047 - Global Climate CyclesObjectives & learning

outcomesObjectives & learning

outcomes๏ learn about the Southern Ocean

๏ appreciate that it occupies a central and crucial role in the mixing of oceanic currents and renewed deep water formation

๏ learn about the mechanisms that connects atmospheric pCO2 concentrations and processes north and south of the polar front

๏ learn that there are conflicting lines of evidence from nutrient utilisation proxies about glacial-interglacial changes in the Southern Ocean

๏ learn about the Southern Ocean

๏ appreciate that it occupies a central and crucial role in the mixing of oceanic currents and renewed deep water formation

๏ learn about the mechanisms that connects atmospheric pCO2 concentrations and processes north and south of the polar front

๏ learn that there are conflicting lines of evidence from nutrient utilisation proxies about glacial-interglacial changes in the Southern Ocean

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Lecture outlineLecture outline๏ General introduction to setting of Southern Ocean

๏ Nutrient concentrations in the Southern Ocean

๏ The role of the Southern ocean as hub between the Pacific, Atlantic, and Indian Ocean

๏ The potential importance of the Southern Ocean for regulating pCO2

๏ Proxy estimates for glacial-interglacial changes of proxies for nutrient utilisation

๏ General introduction to setting of Southern Ocean

๏ Nutrient concentrations in the Southern Ocean

๏ The role of the Southern ocean as hub between the Pacific, Atlantic, and Indian Ocean

๏ The potential importance of the Southern Ocean for regulating pCO2

๏ Proxy estimates for glacial-interglacial changes of proxies for nutrient utilisation

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Armand, L. K. (2000), ‘An Ocean of Ice – Advances in the Estimation of Past Sea Ice in the Southern Ocean’, GSA Today 10.Berger, W., ‘Global maps of ocean productivity’, Productivity in the ocean: Present and Past. Dahlem Workshop Reports LS44, W. Berger, V. Smetacek, & G. Wefer, eds. (Wiley & Sons, 1989), 429–456.De La Rocha, C. L., et al. (1998), ‘Silicon-isotope composition of diatoms as an indicator of past oceanic change’, Nature 395, 680–683.Elderfield, H. & Rickaby, R. E. M. (2000), ‘Oceanic Cd/P ratio and nutrient utilization in the glacial Southern Ocean’, Nature 405, 305–310.Falkowski, P. G. (1997), ‘Evolution of the nitrogen cycle and its influence on the biological sequestration of CO2 in the ocean’, Nature 387.François, R. et al. (1997), ‘Contribution of Southern Ocean surface-water stratification to low atmospheric CO2 concentrations during the last glacial period’, Nature 389, 929–935.Hodell, D. A., Gersonde, R., & Blum, P., ‘Leg 177 Synthesis: Insights into the Southern Ocean Paleoceanography on tectonic to millenial timescales’, Proc. ODP, Sci. Res. 177, R. Gersonde & P. Hodell, D. A. Blum, eds. (2002), 1–54.Kumar, N., et al. (1995), ‘Increased biological productivity and export production in the glacial Southern Ocean’, Nature 378, 675–680.Marchitto, T. M., Oppo, D. W., & Curry, W. B. (2002), ‘Paired benthic foraminiferal Cd/Ca and Zn/Ca evidence for a greatly increased presence of Southern Ocean Water in the glacial North Atlantic’, Paleoceanography 17, 1038.Open University Course Team, Ocean Circulation (Open University, Pergamon Press, Oxford, 1989).Rosenthal, Y., Dahan, M., & Shemesh, A. (2000), ‘Southern Ocean contributions to glacial-interglacial changes of atmospheric pCO2: An assessment of carbon isotope records in diatoms’, Paleoceanography 15, 65.Sarmiento, J. L. & Toggweiler, J. R. (1984), ‘A new model for the role of the oceans in determining atmospheric pCO2’, Nature 308, 621–624.Schmitz, W. J. (1996), ‘One the World Ocean Circulation: Volume II. The Pacific and Indian Oceans A global update’, Technical Report WHOI-96-08, Woods Hole Oceanographic Institution. 241 pp.Siegenthaler, U. & Sarmiento, J. L. (1993), ‘Atmospheric carbon dioxide and the ocean’, Nature 365, 119–125.Sigman, D. M. et al. (1999), ‘The isotopic composition of diatom-bound nitrogen in Southern Ocean sediments’, Paleoceanography 14, 118–134.Sullivan, C. W., et al. (1993), ‘Distributions of Phytoplankton Blooms in the Southern Ocean’, Science 262, 1832–1837.Takahashi, T., et al. (1997), ‘Global air-sea flux of CO2: An estimate based on measurements of sea-air pCO2 difference’, Proc. Natl. Acad. Sci. USA 97, 8292–8299.Toggweiler, J. R. & Sarmiento, J. L. (1985), ‘Glacial to interglacial changes in atmospheric carbon dioxide: the critical role of ocean surface water in high latitudes’, Geophys. Monogr. Ser. AGU 32.

Some referencesSome references

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IntroductionIntroduction๏ What is the “Southern Ocean”?

๏ legally, water mass poleward of 60°S, approximated by the Polar Front (Antarctic Convergence)

๏ palaeoceanographically, polewards of~50°S, with deep water formation

๏ Why is the SO important (particularly to explain glacial-interglacial ∆CO2) ?๏ Due to ocean circulation patterns,

the SO is the window throughwhich much of the deep oceans“see” the atmosphere (Siegenthaler & Sarmiento, 1993)

๏ SO globally has largest wave height and wind speeds:CO2 exchange at a maximum at around 50°S

๏ SO accounts for >+20% of NET CO2 flux into the ocean (Takahashi, 1997)

๏ SO nutrient limited? At present efficient biological pump, coupled with tendency of cold salty waters to sink

๏ What is the “Southern Ocean”? ๏ legally, water mass poleward of

60°S, approximated by the Polar Front (Antarctic Convergence)

๏ palaeoceanographically, polewards of~50°S, with deep water formation

๏ Why is the SO important (particularly to explain glacial-interglacial ∆CO2) ?๏ Due to ocean circulation patterns,

the SO is the window throughwhich much of the deep oceans“see” the atmosphere (Siegenthaler & Sarmiento, 1993)

๏ SO globally has largest wave height and wind speeds:CO2 exchange at a maximum at around 50°S

๏ SO accounts for >+20% of NET CO2 flux into the ocean (Takahashi, 1997)

๏ SO nutrient limited? At present efficient biological pump, coupled with tendency of cold salty waters to sink

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The Southern OceanThe Southern Ocean๏ Today, the Southern Ocean is the primary site for

intermediate, deep- and bottom-water formation.๏ Almost 2/3 of ocean floor is bathed by Antarctic

Bottom Water (AABW), originating in the Weddell Sea๏ AABW depresses the temperature of ca. 55-60% of

the world’s ocean volume below 2°C๏ The Southern Ocean connects

the three large oceans, and has significant importance in mixing different water masses.

๏ Today, the Southern Ocean is the primary site for intermediate, deep- and bottom-water formation.๏ Almost 2/3 of ocean floor is bathed by Antarctic

Bottom Water (AABW), originating in the Weddell Sea๏ AABW depresses the temperature of ca. 55-60% of

the world’s ocean volume below 2°C๏ The Southern Ocean connects

the three large oceans, and has significant importance in mixing different water masses.

Reprinted by permission from Macmillan Publishers Ltd: Charles, C.D., Fairbanks, R.G.,Nature Evidence from Southern Ocean sediments for the effect of North Atlantic deep-water flux on climate. (London), v. 355, no. 6359, p. 416-419, Copyright (1992) Not under CC licence

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RECAP G-IG changes in CO2RECAP G-IG changes in CO2

Despite best efforts, still need to explain 110 ppm residual!Despite best efforts, still need to explain 110 ppm residual!

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SOES6047 - Global Climate CyclesOcean Carbon Budget

(present)Ocean Carbon Budget

(present)๏ Total anthropogenic CO2 emissions 7.1 ±1.1 GtC/yr

๏ Partitioning between reservoirs๏ atmospheric storage 3.3±0.2 GtC/yr๏ sum of terrestrial sinks 1.8±2.0 GtC/yr๏ ocean uptake

2.0±0.8 GtC/yr๏ Southern Ocean๏ 0.8±0.4 GtC/yr

๏ slightly different numbers than in figure,but highlight importantrole of Southern Ocean

๏ Total anthropogenic CO2 emissions 7.1 ±1.1 GtC/yr

๏ Partitioning between reservoirs๏ atmospheric storage 3.3±0.2 GtC/yr๏ sum of terrestrial sinks 1.8±2.0 GtC/yr๏ ocean uptake

2.0±0.8 GtC/yr๏ Southern Ocean๏ 0.8±0.4 GtC/yr

๏ slightly different numbers than in figure,but highlight importantrole of Southern Ocean

Courtesy of the University of Michigan: http://sitemaker.umich.edu/section2group4/the_link__human_influences_on_storm_intensity

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SO Primary ProductivitySO Primary Productivity๏ The Southern Ocean is also a region of high primary

production and a region with potential for even higher productivity.

๏ The only oceanic region with large reserves of unutilized nutrients in surface waters (HNLC).

๏ What limits production? Light, clouds, ice, deep mixing, micronutrients (Fe, Mn, Zn), grazing, but not, apparently, macronutrients (N,P)

๏ The Southern Ocean is also a region of high primary production and a region with potential for even higher productivity.

๏ The only oceanic region with large reserves of unutilized nutrients in surface waters (HNLC).

๏ What limits production? Light, clouds, ice, deep mixing, micronutrients (Fe, Mn, Zn), grazing, but not, apparently, macronutrients (N,P)

Courtesy of NASA: SeaWiFS project

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Productivity (II)Productivity (II)๏ Another view of

Primary Production in the Southern Ocean,

๏ This one makes use of colour data from the CZCS satellite

๏ Is Fe input from the continent and seabed features important?

OR is Fe the biolimitingnutrient?

๏ Another view ofPrimary Production in the Southern Ocean,

๏ This one makes use of colour data from the CZCS satellite

๏ Is Fe input from the continent and seabed features important?

OR is Fe the biolimitingnutrient?

From: Sullivan, C.W., Arrigo, K.R., McClain, C.R., Comiso, J.C., Firestone, J., (1993) Distributions of Phytoplankton Blooms in the Southern Ocean, Science, v. 262, p. 1832–1837. Reprinted with permission from AAAS. This figure may be used for non-commercial, classroom purposes only. Any other uses requires the prior written permission from AAAS.

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๏ The Southern Ocean has high surface water concentrations of both P and N, which are used up almost everywhere else

๏ What is actually biolimiting? N can be produced by bio-fixation, P cannot. But what about other elements?

๏ The Southern Ocean has high surface water concentrations of both P and N, which are used up almost everywhere else

๏ What is actually biolimiting? N can be produced by bio-fixation, P cannot. But what about other elements?

Biolimiting nutrients?Biolimiting nutrients?

Courtesy of Ocean Data View: Conkright, M. E., Locarnini, R. A., Garcia, H. E., O'Brien, T. D., Boyer, T. P., Stephens, C., Antonov. J. I., World Ocean Atlas 2001

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Southern Ocean PhosphateSouthern Ocean Phosphate


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Southern Ocean NitrateSouthern Ocean Nitrate


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Iron fertilisation?Iron fertilisation?

© NOAA 1994

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SOES6047 - Global Climate CyclesSouthern Ocean water

massesSouthern Ocean water

masses

Schematic block diagram showing the surface currents and the vertical motion of water masses in the Atlantic Ocean poleward of about 40o S. North Atlantic Deep Water (NADW) becomes Antarctic Circumpolar Water (ACW) and rises to the surface at the Antarctic Divergence. Surface water flowing northwards from the Antarctic Divergence sinks at the Antarctic Polar Frontal Zone (as AAIM, while that flowing southwards may become AABW. The contours show isotherms in oC; this schematic diagram should be compared with the actual temperature and salinity distribution measured in the Drake Passage between 56o and 62o S. From Open University (1989)

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Southern Ocean circulationSouthern Ocean circulation๏ The role of the

Southern Oceanas the “hub” of ocean circulation

๏ complicate, buoyancy drivenupwelling, mixingand downwelling

๏ interaction withpolar frontalsystems

๏ The role of theSouthern Oceanas the “hub” of ocean circulation

๏ complicate, buoyancy drivenupwelling, mixingand downwelling

๏ interaction withpolar frontalsystems

Courtesy WHOI: Schmitz, W.J. (1996), ‘One the World Ocean Circulation: Volume II. The Pacific and Indian Oceans A global update’, Technical Report. WHOI-96-08, Woods Hole Oceanographic Institution.

241 pp.

3D global overturning diagram

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Present-day snapshotPresent-day snapshot

Courtesy of the American Meterological Survey: Speer, K., Rintoul, S.R., Sloyan, B., (2000)The Diabatic Deacon Cell., Journal of Physical Oceanography, v. 30, no. 12, p. 3212–3222.

Schematic Diagram of the overturning circulation in the Southern Ocean

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๏During LGM, equatorward shift of sea-ice edges, fronts, isotherms, and iceberg tracks

๏During LGM, equatorward shift of sea-ice edges, fronts, isotherms, and iceberg tracks

Last Glacial Sea-ice SnapshotLast Glacial Sea-ice Snapshot

Courtesy of GSA: Armand, L.K. (2000), ‘An Ocean of Ice – Advances in the Estimation of Past Sea Ice in the Southern Ocean’, GSA Today, v. 10.

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Southern Ocean nutrient concentrationsSouthern Ocean nutrient concentrations๏ The interaction of ocean

circulation affects the concentration of nutrients suchas silicic acid, and nitrate

๏ Concept of “pre-formed” nutrients:Preformed nutrients are those initially present in seawater at the time of downwelling

๏ The interaction of ocean circulation affects the concentration of nutrients suchas silicic acid, and nitrate

๏ Concept of “pre-formed” nutrients:Preformed nutrients are those initially present in seawater at the time of downwelling

Reprinted by permission from Macmillan Publishers Ltd: Sarmiento, J.L., Gruber, N., Brzezinski, M.A., Dunne, J.P., High-latitude control of thermocline nutrients and low latitude biological productivity. Nature (London), v. 427, no. 6969, p. 56-60. Copyright (2004). Not under CC licence

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Changes in SO propertiesChanges in SO properties๏ The response of the Southern Ocean to changing

environmental conditions due to warming is crucial, due to its role in the carbon cycle

๏ Major processes:๏ Increased stratification after warming?๏ Sea-ice extent ๏ Change in upwelling patterns, nutrient supply from

upwelling (Fe?)

๏ Changes in major biogeochemical fluxes due to changes in both primary and export production?

๏ The response of the Southern Ocean to changing environmental conditions due to warming is crucial, due to its role in the carbon cycle

๏ Major processes:๏ Increased stratification after warming?๏ Sea-ice extent ๏ Change in upwelling patterns, nutrient supply from

upwelling (Fe?)

๏ Changes in major biogeochemical fluxes due to changes in both primary and export production?

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Nutrient changes and CO2Nutrient changes and CO2๏ Upwelling of deep, nutrient-rich water in the Southern

Ocean results in nearly 1/3 of TOTAL marine productivity (Berger, 1989)

๏ 2/3 of silica supplied to annually to the ocean is removed as hard parts of planktonic siliceous micro-organisms in the Southern Ocean

๏ -> high accumulation rates of biogenic opal, and formation of a circum-Antarctic biogenic silica belt (between the Polar Frontal Zone and the northern seasonally sea-ice covered Antarctic zone of the A counter current)

๏ Due to productivity, and importance for air-sea gas exchange, most palaeogeochemical models of pCO2 are highly sensitive to changes in nutrient utilisation and/or alkalinity of Antarctic surface waters

๏ Hence, variations in the export of organic matter from the Southern Ocean has been associated with drawdown of pCO2

๏ Upwelling of deep, nutrient-rich water in the Southern Ocean results in nearly 1/3 of TOTAL marine productivity (Berger, 1989)

๏ 2/3 of silica supplied to annually to the ocean is removed as hard parts of planktonic siliceous micro-organisms in the Southern Ocean

๏ -> high accumulation rates of biogenic opal, and formation of a circum-Antarctic biogenic silica belt (between the Polar Frontal Zone and the northern seasonally sea-ice covered Antarctic zone of the A counter current)

๏ Due to productivity, and importance for air-sea gas exchange, most palaeogeochemical models of pCO2 are highly sensitive to changes in nutrient utilisation and/or alkalinity of Antarctic surface waters

๏ Hence, variations in the export of organic matter from the Southern Ocean has been associated with drawdown of pCO2

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Evidence from the LGM Evidence from the LGM ๏ During the LGM,

palaeorecords from the Southern Ocean suggestthat the gradient between middepth and deep d13C values was dramatically higher thantoday

๏ separates low-CO2

middepth waters abovefrom high-CO2 deep water below

๏ During the LGM, palaeorecords from the Southern Ocean suggestthat the gradient between middepth and deep d13C values was dramatically higher thantoday

๏ separates low-CO2

middepth waters abovefrom high-CO2 deep water below

Courtesy of IODP: Hodell, D.A., Gersonde, R., & Blum, P., ‘Leg 177 Synthesis: Insights into the Southern Ocean Paleoceanography on tectonic to millenial timescales’, Proc. ODP, Sci. Res. 177, R.Gersonde & P.Hodell, D. A.Blum, eds. (2002), p. 1–54.

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Evidence from the LGM Evidence from the LGM

Courtesy of IODP: Hodell, D.A., Gersonde, R., & Blum, P., ‘Leg 177 Synthesis: Insights into the Southern Ocean Paleoceanography on tectonic to millenial timescales’, Proc. ODP, Sci. Res. 177, R.Gersonde & P.Hodell, D. A.Blum, eds. (2002), p. 1–54.

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๏ COMPLICATED ...๏ Due to its potential importance for glacial interglacial

changes in pCO2, many attempts have been made to quantify this change, for both north and south of the polar front

๏ Examples: δ15N isotope systematics (Kumar, 1995;Sigman 1999) find littleglacial-interglacial change in nutrient utilisation north of polar front, but a glacial-interglacial decrease on the Antarctic side๏ supported by Cd/Ca data

(Elderfield & Rickaby, 2000) ->

๏ δ30Si (de la Rocha et al., 1998) and Cd/Ca suggest INCREASE in nutrient utilisation from glacial to interglacial south of polar front, opposite to δ15N

๏ COMPLICATED ...๏ Due to its potential importance for glacial interglacial

changes in pCO2, many attempts have been made to quantify this change, for both north and south of the polar front

๏ Examples: δ15N isotope systematics (Kumar, 1995;Sigman 1999) find littleglacial-interglacial change in nutrient utilisation north of polar front, but a glacial-interglacial decrease on the Antarctic side๏ supported by Cd/Ca data

(Elderfield & Rickaby, 2000) ->

๏ δ30Si (de la Rocha et al., 1998) and Cd/Ca suggest INCREASE in nutrient utilisation from glacial to interglacial south of polar front, opposite to δ15N

Conflicting paleonutrient proxies

Conflicting paleonutrient proxies

Reprinted by permission from Macmillan Publishers Ltd: Elderfield, H. & Rickaby, R. E.M. ‘Oceanic Cd/P ratio and nutrient utilization in the glacial Southern Ocean’, Nature, v. 405, p. 305–310. Copyright (2000). Not under CC licence.

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๏ To reconcile the conflicting proxy evidence for higher, lower, or equal nutrient utilisation in the Southern Ocean between glacial-interglacial time, Elderfield & Rickaby (2000)suggest sea-ice is the answer:

๏ If sea-ice cover in SO during glacial times is high, then higher PO4 (less utilisation) south of polar front could be reducing productivity ..... can also explain δ15N signal

๏ Their interpretation: increased glacial sea-ice cover acts as a barrier to air-sea exchange, preventing loss of CO2 from surface oceans:“Sea-ice hindered productivity in the Southern Ocean, leading to high surface water nutrients with concomitent enrichment in δ15N or organic matter, but, ironically, trapped the excess carbon in the deep ocean, thus lowering atmospheric CO2 at glacial times

๏ To reconcile the conflicting proxy evidence for higher, lower, or equal nutrient utilisation in the Southern Ocean between glacial-interglacial time, Elderfield & Rickaby (2000)suggest sea-ice is the answer:

๏ If sea-ice cover in SO during glacial times is high, then higher PO4 (less utilisation) south of polar front could be reducing productivity ..... can also explain δ15N signal

๏ Their interpretation: increased glacial sea-ice cover acts as a barrier to air-sea exchange, preventing loss of CO2 from surface oceans:“Sea-ice hindered productivity in the Southern Ocean, leading to high surface water nutrients with concomitent enrichment in δ15N or organic matter, but, ironically, trapped the excess carbon in the deep ocean, thus lowering atmospheric CO2 at glacial times

How to re-concile data ....How to re-concile data ....

Reprinted by permission from Macmillan Publishers Ltd: Elderfield, H. & Rickaby, R. E.M. ‘Oceanic Cd/P ratio and nutrient utilization in the glacial Southern Ocean’, Nature, v. 405, p. 305–310. Copyright (2000). Not under CC licence.

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Key point summaryKey point summary

๏The Southern Ocean occupies a central role as the “mixing tap” of oceanic currents between the Atlantic, Pacific and Indian Ocean

๏ It plays a central role in deepwater formation, and the upwelling and distribution of nutrients to the world’s oceans

๏ It acts as the main connection between deep waters and the atmosphere, and has vigorous air-sea gas exchange

๏Therefore, it has a potentially large role in explaining the glacial-interglacial “remaining” pCO2 difference of ~110ppm

๏ two main theories for glacial-interglacial change:palaeoproductivity and/or stratification

๏Proxy records disagree๏Some researchers think they can explain and reconcile

the evidence by invoking dramatic changes in sea-ice cover

๏The Southern Ocean occupies a central role as the “mixing tap” of oceanic currents between the Atlantic, Pacific and Indian Ocean

๏ It plays a central role in deepwater formation, and the upwelling and distribution of nutrients to the world’s oceans

๏ It acts as the main connection between deep waters and the atmosphere, and has vigorous air-sea gas exchange

๏Therefore, it has a potentially large role in explaining the glacial-interglacial “remaining” pCO2 difference of ~110ppm

๏ two main theories for glacial-interglacial change:palaeoproductivity and/or stratification

๏Proxy records disagree๏Some researchers think they can explain and reconcile

the evidence by invoking dramatic changes in sea-ice cover

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