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Global terrestrial carbon estimation

Global terrestrial carbon estimation. map ecosystem extents assume C storage (t/ha) in: vegetation litter soils. LGM terrestrial biosphere: ~750-1500 Gt smaller? (~35-70% smaller?). (Crowley, 1995). Glacial atmospheric CO 2 lowering must be due to greater storage in ocean.

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Global terrestrial carbon estimation

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  1. Global terrestrial carbon estimation • map ecosystem extents • assume C storage (t/ha) in: • vegetation • litter • soils LGM terrestrial biosphere: ~750-1500 Gt smaller? (~35-70% smaller?) (Crowley, 1995)

  2. Glacial atmospheric CO2 lowering must be due to greater storage in ocean Modern surface pCO2 (Takahashi et al., 2002) • at equilibrium, atmospheric pCO2 determined by Henry’s Law • pCO2 = [CO2] / K0 • need mechanisms to lower [CO2] or raise K0 (solubility)

  3. dissolved inorganic carbon (DIC): • SCO2 = [CO2]+ [HCO3-] + [CO32-] • ~1% ~90% ~10% • where CO2 CO2(aq) + H2CO3 • Therefore we can lower [CO2] by: • decreasing DIC • shifting DIC equilibrium to right • cooling (slightly influences K1 & K2) • freshening (slightly influences K1 & K2) • alkalinity:DIC change

  4. Temperature & salinity (effects on K values only) • LGM temperature (colder) • CO2more soluble in cold waters (K0) • DIC also shifts away from CO2 ([CO2]) • could account for -30 ppm • LGM salinity (saltier) • CO2lesssoluble in salty waters (K0) • DIC also shifts toward CO2 ([CO2]) • could result in +10 ppmv (Takahashi et al., 2002)

  5. What else determines the speciation of DIC (at constant T, S)? Electroneutrality In any solution, the sum of cation charges must balance the sum of anion charges Conservative alkalinity Excess of conservative cations over conservative anions (conservative: no [ ] change with pH, T, or P) Alk = S(conserv. cation charges) - S(conserv. anion charges) = ([Na+] + 2[Mg2+] + 2[Ca2+] + [K+]…) - ([Cl-] + 2[SO42-]…)  2350 meq/kg

  6. The conservative alkalinity excess positive charge is balanced primarily by three non-conservative acid-base systems: DIC, boron, and water Titration alkalinity Moles of H+ equivalent to the excess of proton acceptors (bases) over proton donors (acids) Alk  [HCO3-] + 2[CO32-] + [B(OH)4-] + [OH-] – [H+] carbonate alk borate alk water alk DIC therefore shifts to right as conservative alkalinity increases, providing more negative charges

  7. DIC speciation and pH H+ OH- • pH and DIC systems “move together” in terms of charge • DIC buffers pH changes • add strong acid: CO2 forms, consuming H+, hindering pH drop

  8. Conservative alkalinity and DIC together • increase Alk/DIC: DIC shifts to right (pCO2 drops) • decrease Alk/DIC: DIC shifts to left (pCO2 rises) • add Alk/DIC at 1/1: very little change in DIC speciation CaCO3 Dissolution: Alk:DIC 2:1 Precipitation: Alk:DIC 2:1 Organic matter Respiration: Alk:DIC Photosynthesis: Alk:DIC CO2 gas Invasion: Alk:DIC Evasion: Alk:DIC Increase Decrease Lesser increase Lesser decrease

  9. Carbonate system parameters • carbonate system can be reduced to four interdependent, measurable parameters: • DIC • alkalinity • pCO2 • pH • full characterization requires measurement of only two

  10. Some useful approximations DIC  [HCO3-] + [CO32-] Alk  carbonate alk = [HCO3-] + 2[CO32-] Therefore: [HCO3-]  2DIC – Alk [CO32-]  Alk – DIC And since: pCO2 = K2[HCO3-]2 / K0K1[CO32-] It follows that: pCO2  K2(2DIC – Alk)2 / K0K1(Alk – DIC) Using average surface water values: 1% increase in DIC gives ~10% increase in pCO2 1% increase in Alk gives ~10% decrease in pCO2

  11. Role of seafloor CaCO3: • Carbonate • compensation • CaCO3 dissolves with: • low T • high P • low [CO32-] DIC removed DIC added supersaturation under- saturation

  12. Carbonate compensation • say, DIC added to deep ocean at start of glaciation • deep ocean equilibrium shifts to left and CO32- drops • seafloor CaCO3 dissolves, releasing Alk:SCO2 in 2:1 ratio • pushes equilibrium back to right until CO32- recovers • since initial SCO2 addition was simple rearrangement within ocean, whole ocean has net Alk:SCO2 gain • at sea surface, this shifts equilibrium to right (CO2 drops)

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