A Very Brief Introduction to the Ocean Carbon Cycle Mick - MIT
Transcript of A Very Brief Introduction to the Ocean Carbon Cycle Mick - MIT
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A Very Brief Introduction to the Ocean Carbon Cycle
Mick Follows
Dec 2004
http://ocean.mit.edu/~mick/Docs/carbon-intro.pdf
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Ice core records from Vostock, Antarctica (Petit et al., 1999)
● What controls glacial-interglacial variation of atmospheric CO2?● What is the connection to climate change?
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The Global Carbon Cycle
Gruber & Sarmiento (2002)
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Observing ocean biogeochemical properties
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What is the distribution of carbon in the ocean?
WOCE-JGOFS Global Ocean Survey (1990's)
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Observed Atlantic Dissolved Inorganic Carbon, DIC
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Observed Pacific Dissolved Inorganic Carbon, DIC
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● Background concentration of DIC ~ 2000 micromol kg-1
● Why so much carbon in the ocean?
Carbonate chemistry.
dissolved inorganic carbon
DIC = [CO2*]+[HCO3-]+[CO3
2-]
[CO2*] : [HCO3-] : [CO3
2-]
1 100 10
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What sets the distribution of carbon in the ocean?
- Combination of transport by circulation, chemical and biological processes.
1. Transport - relationship to S2. “Solubility Pump” - relationship to T3. “Biological Pumps” - relationship to PO
4
3-
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Global circulation
Macdonald and Wunsch (1996) inversion
Volume flux of warm & cold water
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Southern Ocean Speer et al. (2000)
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Atlantic, salinity (psu)
AAIW
NADW
AABW
ventilated thermocline
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Pacific, salinity (psu)
AAIW NPIW
AABW
ventilated thermocline
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DIC and S distributions show some common structures:
- Common ocean transport processes
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DIC and S distributions show some strong differences:
- DIC affected by biological processes but S is not
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Observed Atlantic Dissolved Inorganic Carbon, DIC
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Relationship of DIC at equilibrium with overlying atmos pCO2=280ppmv
Cooler waters hold more carbon at equilibrium
mol m-3 = micromol kg-1 / 103
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“Solubility Pump”
- cooler water holds more DIC at equilibrium for given atmos pCO2
- cooler waters are denser and form deep waters- creates vertical gradient of DIC in the ocean- air-sea heat fluxes drive air-sea carbon fluxes
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Atlantic
Vertical gradients of DIC and T generally inversely related:
Solubility Pump
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Pacific
Vertical gradients of DIC and T generally inversely related
Solubility pump
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Air-sea flux of CO2 inversely related to air-sea heat flux
red – ocean gaining heat red – ocean gaining carbon
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But DIC in the deep ocean exceeds the equilibrium concentration at that temperature... why?
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Photosynthesis occurs in sunlight region (upper 150m)Requires light and nutrients, creates oxygen
Respiration; remineralization of organic matter, consumes oxygen
Biological (soft tissue) Pump
Phytoplankton
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photosynthesis
106 CO2 + 16 NO3- + H2PO4
- + 122 H2O + X hν ⇔ C106H263O110N16P + 138 O2
respiration
Redfield Ratio – oceanic organic matter has “fixed”ratio of carbon and nutrients
Biological (soft tissue) Pump
Phytoplankton
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Distribution of chlorophyll in surface ocean observed from space (SeaWiFS, 1997-2000 mean)
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Lots of carbon around so productivity is limited by availability of other nutrients (e.g. PO
4
3-) and light
Supply of nutrients to sunlit surface ocean largely dependent on wind driven upwelling
Modelled upwelling at 50m
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Organic carbon flux in sinking particles
Martin et al. (1987)
“Soft tissue pump”
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DIC and PO4
- common biological processes
- soft tissue pump
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DIC and PO4
- common biological processes
- soft tissue pump
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Could change in ocean carbon cycle cause glacial-interglacial changes in atmospheric pCO2?
Vostock (Petit et al., 1999)
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Could changes in the oceans have reduced atmospheric pCO2 by >80ppmv at the Last Glacial Maximum?
Changes in solubility pump:
Cool deep ocean by 2oC -20ppmv Cool upper ocean (thermocline) by 5oC -20ppmv
Changes in biological pumps:
Extreme enhancement of biological export efficiency -40ppmv
Changes to circulation and mixing?
STILL AN OPEN QUESTION
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