Global Carbon Cycle

1. Global Carbon Cycle

2. Why study the C cycle? • Key element of life – so fundamental • Fossil fuel burning and global warming • Perturbation by humans (atm CO2) • Complex cycle – long & short term cycles; organic and inorganic components • Geological processes operating over millions of years • Biological processes operating on annual time scales • Interactions between long and short term cycles

3. “short”-term “long”-term anthropogenic

4. Combined Inventories Atmosphere 770 Gt C Terr. Systems ~2400 Gt C Oceans ~39,000 Gt C Sed. Rocks 50,000,000 Gt C • Major Inventories • Majority of C tied up in rock cycles – large reservoirs with long residence times • Reservoirs active on short time scales are ocean, atm, & land • Large exchange fluxes to and from atm – atm has short residence time (3 yr); small net fluxes due to biology (most PP is respired) • Problem with adding fossil fuel CO2 to atm – transferring C from long term geologic reservoir to a short term reservoir – may affect short term feedback control mechanisms Fig. 8-3

5. 50.3 Atmospheric CO2 Respiration Air-Sea Exchange Terrestrial primary production and respiration 50 CO2 POC Physical weathering 59.6 - 59.7 Marine primary prod. River transport 60 0.4 Upwelling remineralization Kerogen Land Plants Particle Rain Oceans CO2 POC Deposition Humification Benthic Fluxes Soil humus Uplift of sedimentary rocks 0.1 Recent Sediments 0.1 Carbon Burial Sedimentary Rocks A model – little transfer of biological C to ocean (from land) or sediments (from water)

6. Atmosphere • Most C in atm as CO2 • Some methane and CO • Atm CO2 shows rapid increase in recent time • Beginning with Industrial Revolution • See seasonal variations in recent increase • Uptake in Spring due to plant growth (N hemisphere) • Release in fall from net respiration

7. Northern hemisphere • More land • More terrestrial prod Figs. 1-2 and 1-3

8. Southern amplitude is lower Seasonality offset by 6 mos. Northern hemisphere has more extensive seasonal forests Close tracking between N & S hemispheres

9. Prior to humans, the system showed natural variability (50 – 80 ppm glacial-interglacial) Smaller Holocene changes Interglacial Glacial

10. Holocene changes • Recent high resolution ice core • Natural variability in Holocene is second order change when compared with glacial-interglacial excursions and anthropogenic increase • Allows us to think about a nearly constant pre-industrial interglacial CO2 level of ~280 ppm

11. Recent increases in Atm CO2 • Some due to land use changes (pre-industrial) • Deforestation – two-fold problem • Decrease PS uptake of CO2 • Burn the wood - charcoaling • Mainly due to fossil fuel burning (post-industrial) • Deforestation in tropics may be partially balanced by N hemisphere forest expansion/regrowth

13. IPCC – Intergovernmental Panel on Climate Change Atmos. increase (3.3 PgC/yr) Land use change (1.7 PgC/yr) Residual terrestrial sink (1.9 PgC/yr) Emissions (5.5 PgC/yr) Atmosphere Ocean uptake (2 PgC/yr)

14. Atmosphere ∆pCO2 < 0 (primarily high latitudes) ∆pCO2 > 0 (primarily upwelling regions) CO2 CO2 CO2 Surface Ocean CO2 + CO32- + H2O  2HCO3- CO2 + H2O  Organic Matter + O2 Ca2+ + 2HCO32- CaCO3 + CO2 + H2O CO2 + CO32- + H2O  2HCO3- Sinking particulate organic matter (“biological pump”) Upwelling and vertical mixing Bottom water formation (high latitudes) (“solubility pump”) CO2 + H2O  Organic Matter + O2 HCO3- Deep Ocean CO2 CO2 + H2O  Organic Matter + O2 Ca2+ + 2HCO32- CaCO3 + CO2 + H2O Sediments Oceans are largest “active” reservoir in the carbon cycle – primarily DIC

15. Oceans • Link the “active” or short-term cycles with long-term geological cycles – sink for fossil fuel CO2 • Ocean processes • Biological cycle • Weathering reactions and long term controls • Atm CO2 riverine bicarb  neutralized in ocean  returned to atm or buried in seds • Processes that remove CO2 from atm • Gas exchange – equilibration of sfc ocean with atm • Biological pump • Bottom water formation

16. Gas exchange • If CO2 were a simple gas, ocean could only take up ~3% of fossil fuel input • Acid-base chemistry enhances ocean uptake • Remember carbonate buffering system? • CO32- + H2O + CO2 2 HCO3- • Buffering rxn drives CO2 to bicarb • Surface waters reach equilibrium with atm in about 1 year • Can keep pace with human activity • But surface ocean too small to have capacity to remove it all

17. Biological Pump • PP and calcite ppt consume DIC • Removed from surface ocean via particle flux • Through interactions with carbonate system, this lowers partial pressure (pCO2) in surface ocean which enhances gas exchange (DpCO2< 0) • Transports CO2 to deep ocean in the form of OM or calcite shells • Limitations of biological pump • Availability of other nutrients (N, P, Fe) • More CO2 doesn’t necessarily lead to more PP

18. Bottom water formation • Removes CO2 by physical movement of water away from surface • Solubility pump • CO2 is more soluble in cold water

19. Intermediate and deep water • Can add CO2 through oxidation of OM • Calcite dissolution – excess CO2 from OM oxidation reacts with sinking calcite

20. Upwelling • Intermediate waters are enriched in DIC • Mixing with deep waters, OM oxidation & calcite dissolution, yields some CO2 increase • Upwelling results in excess pCO2 in surface waters (DpCO2 > 0) • Oceans outgas CO2 • High productivity upwelling can still be net CO2 sinks

21. Global oceanic C sources and sinks for atm C - reflect upwelling and deep water formation and high productivity

23. IPCC calculations • Integrate data on ocean flux data • Calculation attempts to assess short-term sinks for excess atm CO2 due to anthropogenic activities

24. Time scales of ocean C cyle • Ocean processes slow relative to rate of fossil fuel burning • Bottom water circulation on timescales of 100’s of years so equilibration with atm is slow • Deep sea seds equilibrate with atm on timescales of 1000’s of years – where the bulk of the ocean’s neutralizing capacity resides • Oceans respond too slowly to take up all excess CO2 – so atm CO2 is increasing • But, oceans have helped! Oceans have taken up 1/3 to ½ of added CO2

25. Atmosphere ∆pCO2 < 0 (primarily high latitudes) ∆pCO2 > 0 (North Pacific and upwelling regions) Equilibration time ~1 yr CO2 CO2 CO2 Surface Ocean CO2 + CO32- + H2O  2HCO3- CO2 + H2O  Organic Matter + O2 Ca2+ + 2HCO32- CaCO3 + CO2 + H2O CO2 + CO32- + H2O  2HCO3- Sinking particulate organic matter (“biological pump”) Upwelling and vertical mixing Bottom water formation (high latitudes) Equilibration time ~500-1000 yr CO2 + H2O  Organic Matter + O2 HCO3- Deep Ocean CO2 CO2 + H2O  Organic Matter + O2 Ca2+ + 2HCO32- CaCO3 + CO2 + H2O Sediments Equilibration time ~103-104 yr

26. Terrestrial systems • Variety of reservoirs that turnover on different timescales • Soil humus – altered remains of plants • Land plant biomass • Methane – source of atm methane • Terrestrial PP ~ = to Marine PP • Terrestrial systems store excess CO2 differently – humus versus bicarb • Imp for understanding system responses to increasing CO2. Increasing CO2: • might increase PP (neg feedback) • might increase rates of decomposition (pos feedback)

28. 50.3 Atmospheric CO2 Respiration Air-Sea Exchange Terrestrial primary production and respiration 50 CO2 POC 59.6 - 59.7 Marine primary prod. River transport Physical weathering 60 Upwelling 0.4 remineralization Kerogen Land Plants Particle Rain Oceans CO2 POC Deposition Humification Benthic Fluxes Soil humus Uplift of sedimentary rocks 0.1 Recent Sediments 0.1 Carbon Burial Sedimentary Rocks Comparable terrestrial & marine PP

29. negative feedback (temperature and CO2 fertilization) positive feedback (temperature enhancement of soil respiration) Terrestrial system responses to rising CO2 and global warming

30. Controls on atm CO2 • Break down overall cycle to components • Look at effects on particular components

32. Short-term biological cycle • Years to decades • Does not include calcite ppt/dissolution • Does not include anthropogenic inputs • PS versus respiration nearly balanced – little loss • Some transport of org C from land to oceans • Most gets oxidized in the ocean • Small amount of marine OM buried in seds • Leaves behind some O2 in atm • Short-term cycles process a lot of CO2 - 30-50% of atm CO2 consumed per year

33. O2 Net productivity CO2 Organic matter

34. O2 Net productivity CO2 Organic matter

35. O2 Net productivity CO2 O2 Uplift and kerogen oxidation CO2 Organic matter

36. O2 Net productivity CO2 O2 Uplift and kerogen oxidation CO2 Organic matter

37. Long term org C cycles • Millions of years • Components include: OM in sediments, fossil fuels, atm O2 versus CO2 • Burial of OM from Short-term cycle • Inc P and T; most ends up as kerogen, • Some winds up in fossil fuels (oil, coal) • OM in shale is largest reservoir on earth (long t) • Removal balanced by kerogen oxidation/weathering • Affects atm O2 • Net burial leaves O2 in the atm

38. Also linked to pyrite burial/oxidation which requires OM as an intermediate to catalyze the sulfate reduction Produced by bacterial sulfate reduction - linked to carbon oxidation O2 in atm controlled by a balance between pyrite and OM burial in seds and later oxidation on land Without this balance atm O2 would increase to 150% of present Levels and depletion of atm CO2 in < 10,000 years (see text)

39. Long-term inorg C cycle • 100’s of millions of years • Balance between weathering and plate tectonics • Weathering silicate rocks consumes CO2, transferred to the ocean as bicarb, removal of bicarb by organisms & calcite, burial in seds, subduction, vulcanism (also affects other cations – Ca, Mg, Na) • Cycle is a balance between weathering (takes up CO2) and tectonics (releases CO2) • Plate tectonics – more vigorous then more CO2 release • Climate sensitivity (weathering)

40. CO2 removal Bicarbonate transport CaCO3 ppt. CaCO3 (“regenerates” CO2) (“regenerates” CO2) Figs. 8-17

41. Short-term Long term (organic) Long term (inorganic/tectonic)

42. Link with short term C cycle In surface oceans “adds” back CO2 Onset of modern plate tectonics “turns this on”

43. Increase in surface temperature due to increase in solar luminosity Drop in CO2 by increased weathering at higher temp Decrease greenhouse – increase ppt of carbonates?

44. Bob Berner’s calculations of changes in CO2 over the Phanerozoic

45. Bob Berner’s calculations of changes in CO2 over the Phanerozoic “Hot” houses

46. Bob Berner’s calculations of changes in CO2 over the Phanerozoic “Hot” houses “Ice” houses

47. Fig. 8-18

48. Effect of humans • Pre-industrial • Steady state on decadal to century timescales • Ocean a net source of CO2 • Neutralizes river bicarb and oxidation of OM from rivers • Burial of org C • That which escaped oxidation and marine OM • Humans • Oceans a sink for CO2 • Increase sediment and nutrient load to rivers/ocean • Eutrophication, hypoxia, denitrification

49. Fig. 10-16 A portion of the biogeochemical cycles of inorganic carbon (Cing) and organic carbon (Corg), nutrient N and P, and suspended solids (SS) in the land–ocean system. (a) Geological, long-term system; (b) one possible situation today. In (b), the fluxes of organic and inorganic carbon and suspended solids to the seafloor are increased over their pristine geological values in (a). These increases are due to human activities. Notice the net heterotrophic nature of the ocean giving rise to a net flux of carbon dioxide to the atmosphere prior to human interference in the carbon cycle. Now more carbon dioxide enters the ocean because of the burning of fossil fuels and deforestation practices (see Chapter 12). Fluxes are in millions of tons of C, N, P, and suspended solids per year. (After Wollast and Mackenzie, 1989.)

50. CO2 All fluxes are millions of tons of C per year 460 (net) 100 (net; approx. 50,000 (r) - 50,100(pp)) 360 200 oxidation 400 Corg(terr.) Riverine inputs The Ocean 400 Cing rxn. (1) Corg(marine +terr) Burial in sediments Cing 200 140 rxn. (1)