The Carbon Cycle-2:
Long Term Processes
Possible C-Cycle Lectures
order? Importance?
1) Long-term global C cycle, weathering reactions
and tectonics
2) Medium/ short-term global C cycle
3) Terrestrial carbon cycle (modern)
4) Oceanic carbon cycle (modern)
5) Methane hydrates: past, present, and future
6) Anthropogenic effects on carbon cycle
Long vs. Short
Short‐Term: “ transfer of carbon between Atm, Oceans, and Life”
Long‐Term: “ regulation of the transfer of carbon to and from rocks”
Short‐Term: “ transfer of carbon between Atm, Oceans, and Life”
Main Reservoirs:
•Atm CO2
•Ocean DIC, OM
•Terrestrial Biota
•Soil Humus/ Carbonate
Main Processes: biological!
•Primary Production
•Remineralization
Long‐Term: “ transfer of carbon to and from rocks”
Main Reservoirs:
•Terrestrial Shale/carbonate
rocks
•Ocean Sediments
•(deep earth rocks*)
• *only source, really..
Main Processes:
•Burial- carbonate and OM
•Weathering reactions &
Kerogen remineralization
•Volcanic inputs/ deep
diagen.
1)
Silicate rocks:
Calamine-Hemimorphite
Feldspar Group
Garnet Group
Mica Group
(Biotite, Muscovite, Phlogopite)
Olivine
Perovskite
Quartz
Serpentine
Clay Minerals
Readings/ References
Hand outs:
• Schlesinger Chpt. 11- general overview C cycle
• Berner, 2004: Long T-Carbon Cycle overview/
introduction
On Website:
•Berner, 1999: GSA today version: “New views on Long-T
Carbon cycle (may be better than handout, actually, as general intro.)
•Berner, 2001: GEOCARB-III – paper on latest model RE longT carbon cycle.
•Sundqust and Visser, 2003 “Geological History of C cycle” –
total tome, but great background ref.
Importance of Long‐T C Cycle:
• Where big masses of C are on planet
• Long‐Term evolution of Atm and Climate:
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•
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Short‐term C cycle can’t store much C, Eg: All terrestrial biomass => CO2 changes Atm concentration by only 25%‐
10X greater changes observed in geological record.
•  Large and Persistent changes in Atm carbon dioxide must come from changes in the fluxes to/from rocks
Long‐T C Cycle:
• C storage in Rocks is the key element
• Essentially the balance between two sub‐
cycles
• 1) Carbonate‐Silicate Rock subcycle
• 2) Long‐Term Organic Carbon Subcycle
1)
Carbonate‐Silicate Sub‐cycle:
first: what are some main rock types?
• Minerals=chemical solid phases
• Rocks=assemblages of minerals
• Three broad types: Igneous, sedimentary, metamorphic (altered forms of first two)
• Igneous rocks: silicates and silicate‐rich
alumino‐silicates (feldspars) • Sedimentary rocks: carbonates and silicate‐
poor alumino‐silicates (clays) CHEMICAL WEATHERING: • major constituents of rocks are rather insoluble (aluminum and iron oxides)
• weathering strips four major cations, Ca2+, Na+, K+, Mg2+
• weathering enriches Al over Si in weathered rock • most of dissolved load from rivers => Sea results directly from dissolution of sedimentary rocks Silicate Rock Weathering: 2CO2 + 3H2O + CaSiO3  Ca2+ + 2HCO3- + H4SiO4
Example reaction from Berner
• 1) CO2 and organic acids from plant photosynthesis react with rocks
• 2) Cations ( e.g., Ca+2) liberated – charge balanced by carbonate anions
• 3) RIVER FLUX TO OCEANS: • Dissolved Ca2+, dissolved HCO3‐ (alkalinity),
silicic acid Once in Oceans: (1) Ca2+ + 2HCO3  CaCO3 + CO2 + H2O
Calcium carbonate precipitation & burial
(2) H4SiO4  SiO2 + 2H2O
Biogenic Silica “precipitation”
• Calcium Carbonate and Opel (biogenic silical ) precipitation occur (both biologically and chemically controlled) • BURIAL of some fraction of both occurs • Over long time scales, must balance inputs
Net effect Decarbonation: CO2 + CaSiO3  CaCO3 + SiO2
Net reaction
• Effect: A CO2 ultimately buried as CaCO3 • Si removed as opel. • Net Removal of CO2 from the atmosphere via silicate weathering
• This RX alone should deplete all Atm CO2 in about 10,000 – 300,000 yrs (latter # if take ocean equilibrium into account)
Key Notes: • Importance of CA and Mg Silicates • Only Silicates w/ Ca and Mg ultimately consume Atm CO2 in this way.
• Key reason is the existence of seawater reactions removing carbon to sediments as minerals
• Mg carbonates (?), or exchange of Mg for Ca in Hydrothermal systems. • Ie: K and N do not form carbonates‐ so while they have similar weathering rx, but no net CO2 removal. Carbonate weathering: no direct effect (1) CaCO3 + CO2 + H2O  Ca2+ + 2HCO3
ON LAND: carbonate precipitation & burial
(1) Ca2+ + 2HCO3  CaCO3 + CO2 + H2O
IN OCEAN: carbonate precipitation & burial
• Carbonate weathering Rx in rocks is exact reverse of Carbonate ppt rx in oceans • No net CO2 effect
• However: important as flux pathway, and to keep track of weathering rates and isotopic balance. •
CO2 Source? CO2 + CaSiO3  CaCO3 + SiO2
Net reaction
• This RX alone should deplete all Atm CO2 in about 10,000 – 300,000 yrs (latter # if take ocean equilibrium into account)‐
• => Must be sources Reverse: Decarbonation CaCO3 + SiO2  CO2 + CaSiO3
decarbonation
• Volcanism, Metamorphism, Diageneis • => Over geologic time, balances
2) The Organic Sub‐cycle:
1) CO2 + H2O  CH2O + O2
OM (Kerogen) Burial
• RX 1‐ same as basic photosynthesis‐ BUT in this case only applies to OM ultimately buried and preserved in marine sediments (~ >1m). • Current world: • only occurs in ocean
• <0.1 % of primary prod is buried
• Mostly occurs on margins
• Mostly near river deltas, >1m in anoxic seds
2) CO2 + H2O  CH2O + O2
“Geo-respiration” (kerogen weathering)
• RX 2‐ same as basic respiration‐ BUT in this case only applies to OM weathered from continental rocks (shales, etc)
• “Kerogen”‐
• Operational: for OM you can’t get out of old rocks. • Insoluble black precipitate, left over after all extractions
• Mostly aliphatic stuff 2) CO2 + H2O  CH2O + O2
“Geo-respiration” (kerogen weathering)
• How does RX 2 actually happen?
• 1. Direct oxidation of Kerogen at weathering
• Mechanistically, somewhat mysterious
• Likely has linked abiotic (O2, Hv) and biological component‐
• (Possibly some fancy microbial free radical adaptations..
• 2. Indirect
• Intermediate products of anoxic bacteria (e.g., CH4, acetate)‐ end up coming out of sediments or subduction zones to surface O idi d i
b k CO2
2) CO2 + H2O  CH2O + O2
“Geo-respiration” (kerogen weathering)
• How does RX 2 actually happen?
• 2. Indirect
• Intermediate products of anoxic bacteria (e.g., CH4, acetate)‐ end up coming out of sediments or subduction zones to surface • Oxidized in ocean or atm back to CO2
• Eg:
• 2CH2O  CO2 + CH4
• CH4 + 2O2  CO2 + 2H2O
3) Modeling long‐T carbon cycle
• Define: SURFICIAL SYSTEM: oceans + atmosphere + biosphere + soils • Assume can’t store much (C reservoir can’t change significantly)‐ so therefore fluxes in must = fluxes out
Apply two balances:
assuming Quasi‐steady state of millions of years
Carbon
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dMc/dt = Fwc + Fwg + Fmc + Fmg – Fbc ‐ Fbg
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Wc= weathering ca and mg silicates
Wg = weathing organics
Mg = degassing flux Bc = Burial flux carbonates
Bg = Burial flux organics
A second constraint: Isotopes
Carbon isotopes
d(cMc)/dt = wcFwc + wgFwg + mcFmc + mgFmg
– bcFbc ‐ bgFbg
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Where F= fraction of 13C Wc= weathering ca and mg silicates
Wg = weathing organics
Mg = degassing flux Bc = Burial flux carbonates
Bg = Burial flux organics
Result: prediction of Atm Co2 and O2 over last 500 MY