Long-term carbon cycle and paleo-CO2 Flashcards
Organic carbon
Dead buried life, C12 heavy
Inorganic carbon
Primarily CaCO3
- Mineral- calcite
- Rock- limestone
Isotope mass balance
If you mix 2 jars of carbon that have two different isotope ratios, you can calculate the combined isotope ratio (δ13C)
δa+b = δaMa + δbMb
- Where M the proportional mass
Long-term carbon cycle (>100,000 yr)
- Volcanism
- Burial and weathering (oxidation, respiration) of organic carbon
- Burial of inorganic carbon
Volcanism
Transferring crustal carbon to the earth’s surface
- CO2 gas (inorganic carbon) that comes from C that was a combination of organic and inorganic
Burial and weathering (oxidation, respiration) of organic carbon
- Respiration equation- CH2O + O2 → CO2 + H2O (where CH2O is organic carbon)
- CO2 from sedimentary organic C weathering
- Humans accelerate this process for energy
Burial of inorganic carbon
Linked to chemical weathering of silicate rocks (basalt, granite, slates) on land
Silicate weathering on land (related to burial of inorganic carbon)
Rock + water (H2O) + acid (CO2) → H2CO3 carbonic acid → H+ + HCO-3 (bicarbonate ion)
Dissolution or precipitation of CaCO3
- Ca2+ + 2HCO3 ⇋ CaCO3 + CO2 + H2O
- One proton and a bicarbonate will break down rock; acidified water with free protons can break down solid calcium-rich silicate mineral
- Think of CO2 as an acid (need acid and water to weather)
Net effect of weathering on CO2 levels (related to burial of inorganic carbon)
Ca-Mg silicate weathering
- CaSiO3 + H2O + 2CO2 → Ca2+ + 2HCO3 + SiO2
Ca-Mg carbonate precipitation
- Ca2+ + 2HCO-3 → CaCO3 + H2O + CO2
Net result on CO2
- CaSiO3 + CO2 → CaCO3 + SiO2
- removal of CO2 from atmosphere
Carbonate weathering; carbonate precipitation
- Net result: Carbonate weathering and carbonate precipitation all cancel out, no net result on CO2 or effect on longer timescales
How trees affect the long-term carbon cycle- trees and chemical weathering
- Litter decomposing, forming CO2 and organic acids
- Roots secrete protons- charge balance mechanism
- Physical erosion
- Roots also slow down erosion (ex. river banks)- also increases odds for chemical weathering
- Trees recirculate water
- Rate at which plants can accelerate chemical weathering- 3-8x
Charge balance mechanism (roots secreting protons)
Plants make own sugar with CO2, but also need other nutrients- lots of these nutrients in ionic dissolved in water form have a net positive charge
- Plant retains its balance by putting cations (+ charged ions) into soil and spitting out protons
Physical erosion
Roots physically weather rock, increasing odds for chemical weathering- physical weathering itself doesn’t exchange carbon, but making rocks smaller makes them more likely to react chemically
Roots slowing erosion
Trees increase time that un-chemically-weathered soil particle spends in environment, statistically increasing likelihood that it will be chemically weathered
- Slowing down transport of physical soil particles, increasing time
- Breaking apart rock increases surface area, roots on that landscape increases time that chemical weathering can take place → before trees soil particles stayed in soil for less time meaning it was more likely for things to stay unweathered (got to ocean faster)
Trees recirculating water
In a landscape with no trees, rainfall gets into groundwater and goes downhill eventually to ocean; but in a landscape with trees, transpiration will put it back up into the atmosphere so a molecule of water will stay in that region for multiple cycles and could participate in multiple chemical reactions
- Increase odds that groundwater is sucked up into tree, transpires out leaves in water vapor form, and is recirculated back to that parcel of the atmosphere
- Forested systems are wetter- water is necessary to drive chemical weathering, forests make their own rain (rainfall decreases if trees are removed)
Silicate weathering feedback loop
Negative feedback loop
- Water speeds up silicate weathering, also need acid
- Most important negative climate feedback loop on geologic time scale- has kept climate space within pretty reasonable bounds
- CO2 decrease will lessen silicate weathering, which will allow CO2 to increase again
