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The Earth’s Carbon Cycle

The Earth’s Carbon Cycle. Louisa Bradtmiller. Carbon Reservoirs. Size matters. How much carbon a reservoir can exchange with other reservoirs depends on Size Rate of exchange So, if we’re interested in atmospheric CO 2 , there are a few things to keep in mind. Size matters.

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The Earth’s Carbon Cycle

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  1. The Earth’s Carbon Cycle Louisa Bradtmiller

  2. Carbon Reservoirs

  3. Size matters • How much carbon a reservoir can exchange with other reservoirs depends on • Size • Rate of exchange • So, if we’re interested in atmospheric CO2, there are a few things to keep in mind

  4. Size matters • Seems like sediments would be the most likely regulator of atm. CO2, right? • But, they recycle verrrrrrrrry slooooowly • On tectonic timescales, though, they can control climate to some extent • Increased volcanism, seafloor spreading 100Ma • Increased weathering due to Himalayas

  5. Size matters • When our cars/factories emit carbon, it goes into the atmosphere • 5.4 GtC/yr from fossil fuel burning • 1 GtC/yr from deforestation • BUT not all of it stays there! • Approx. 55% stays in the atm. 30% goes into the ocean, and the rest (15%) goes to “greening” (we think).

  6. Atmosphere • We know how much goes into the atmosphere, because we can measure CO2 and CH4 levels. • We know where it comes from, because each source has a different isotopic signature • Fossil fuels are “dead carbon”, -2‰ 13C • Plants average -23‰

  7. Terrestrial Biosphere • Plants take up CO2 during photosynthesis, give it off during respiration (ie. burning, decay) • So, as long as they’re alive (or in early stages of decay) they store carbon • Plants take up 12C preferentially, giving them negative 13C values

  8. Terrestrial Biosphere • How can trees take up MORE carbon? • “Greening” involves the stimulation of increased photosynthesis via increased CO2 levels in the atm. Verified in greenhouse experiments • Another possibility is that anthropogenic N helps to fertilize plants

  9. If you haven’t seen this by now…..

  10. Ocean Carbon • Remember, there are 2 reservoirs: • Surface (upper 100 meters) • Deep (everything else) • Only the surface really “communicates” with the atmosphere- they are in equilibrium • So, is that where the carbon goes?

  11. Ocean Carbon • NOPE. The surface ocean is too small to accommodate that much C, and exchanges too quickly (it would put it right back in the atm. In a few years) • So, it must be going into the deep ocean (thus the conveyor picture)

  12. How I learned to stop worrying and love the bomb • The deep ocean “overturns” on a timescale of about 1500 years. This is long enough to store some serious C • How do we know that, anyway? • Bomb 14C- nuclear tests in the 50’s and 60’s put tons if it into the Atm. Folks (esp. Wally) watched how it was incorporated into the deep ocean

  13. Ocean carbon “pumps” • 3 types: • Solubility • organic, or soft tissue • Carbonate, or inorganic • Solubility refers to the fact that gases dissolve more easily in cold water • So, a colder ocean holds more CO2

  14. Organic Pump

  15. Carbonate pump CO2(gas) CO2 + H2O  H2CO3 H2CO3 H++ HCO3- HCO3- H++ CO32- TCO2 H2O+ CO2 + CO32-2 HCO3-

  16. Carbonate pump • In addition, many organisms make CaCO3 shells. • The solubility of these shells on the seafloor is determined by the concentration of CO32- • So, shells are preserved in shallow water, but not below the lysocline

  17. Carbonate pump • The depth of the lysocline can change with time • Easy to remember: CO2 ~ 1/CO32- • So, during a glacial, there is low CO2 and you preserve more carbonate • Or, the lysocline deepens

  18. Final thoughts • The ocean must be the regulator of modern (glacial-interglacial) carbon changes- it’s the only reservoir that is big enough and exchanges on an appropriate timescale

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