The Carbon Cycle Chapter 8

1. The Carbon CycleChapter 8 GEO 307 Dr. Garver

2. http://education.jlab.org/itselemental/ele006.htmlhttp://www.rsc.org/periodic-table/video/6/Carbon?videoid=QuW4_bRHbUkhttp://education.jlab.org/itselemental/ele006.htmlhttp://www.rsc.org/periodic-table/video/6/Carbon?videoid=QuW4_bRHbUk

3. Sixth most abundant element • Nearly ten million known carbon compounds and an entire branch of chemistry, known as organic chemistry, is devoted to their study. • Carbon occurs naturally as anthracite (a type of coal), graphite, and diamond. • Many carbon compounds are essential for life as we know it. • Some of the most common carbon compounds are: carbon dioxide (CO2), carbon monoxide (CO), carbon disulfide (CS2), chloroform (CHCl3), carbon tetrachloride (CCl4), methane (CH4), ethylene (C2H4), acetylene (C2H2), benzene (C6H6), ethyl alcohol (C2H5OH) and acetic acid (CH3COOH).

4. Rise in atmospheric carbon dioxide from 1744 to 1992 (increase is exponential)

5. Carbon Cycle

7. Archer Fig. 8.1 50x C of atm Several times more C than atm Fraction of Earth’s C Mostly limestone sedimentary rocks Lots of C!!

8. 1. C exists in a range of forms • Most stable as CO2 and CaCO3 2. Photosynthesis stores energy by producing organic carbon 3. Atmosphere contains only a fraction of carbon 4. Biosphere has several more times carbon than atm • Seasonal signal seen 5, Oceans contains 50x C of atmosphere • Responsible for large changes in CO2 over 100,000 year time scales 6. Weathering of rock consumes CO2 on timescales of millions of years

9. From Archer Fig. 8.1

10. A._________ %______ B._________ %_______ E._______ F.______ C.________ %________ G.________ D.___________ %___________

11. Carbon Sinks (Reservoirs) • Reservoir that uptakes a chemical element from another part of its cycle. • Each year hundreds of billions of tons of carbon, in the form of CO2, are absorbed by oceans, soils, plants.

12. Sinks • (1) biosphere - living and dead material +organic matter in soils (2) atmosphere – CO2 (3) lithosphere - fossil fuels, sedimentary rocks (4) Oceans - dissolved CO2 & calcium carbonate (shells) in marine organisms.

13. Carbon Fluxes

14. Carbon Fluxes • Plants - CO2 from atmosphere. • CO2 enters ocean by diffusion. • Used to produce shells (coral, clams) • Organisms die, shells sink to the ocean floor. • After long periods of time, deposits are physically and chemically altered into sedimentary rocks.

15. Carbon Fluxes • Carbon from ecosystems as CO2 by respiration. • Respiration - both plants and animals -breakdown of carbon-based organic molecules into CO2 • detritus food chain - decomposition of organic matter. • Natural ecosystems store between 20 to 100x more CO2 than agricultural land.

16. Carbon on Earth --- START HERE WEEK 9 • Now let’s back up to the beg. of the chapter and think about Carbon in more detail… • Backbone for constructing the machinery of life & provides the means of storing energy. • Photosynthesis – converts suns energy into carbon biomolecules. • Energy then used by plant, or animal that eats it.

17. Carbon on Earth Over geologic timescales, carbon stores energy on a planetary scale. “Charges up” the biosphere with a store of biomolecules like a giant battery. Extracting energy from fossil fuels harvests ancient energy stores of the biosphere. Rearranges the distribution of carbon among its reservoirs (sinks) on Earth.

18. Carbon ‘Rich, wondrous chemistry’ Carbon chemistry is kept highly organized within living things. Then, after life is finished carbon left in soils and sediments forms itself into humic acidsor kerogen. Produced by biodegradation of dead organic matter.

19. Humic Acids/Humic Substances • Major constituents of soil organic matter • Precursors of fossil fuels Found in peat, coal, streams/lakes, ocean • Created by microbial degradation of dead plant and animal material. • Most stable part of organic matter in soils, can persist for hundreds or even thousands of years.

20. Kerogen • Mixture of organic chemical compounds that make up a portion of the organic matter in sedimentary rocks. • crude oil or natural gas, hydrocarbons • oil shale deposits

21. To understand carbon and all its forms, need to think about oxidation.

22. Reduction-Oxidation Chemistry • ‘Oxidation’- measure of the surplus or deficit of electrons. • Chemical reaction in which atoms have their oxidation number changed. • Interaction between oxygen molecules and all the different substances they may contact, from metal to living tissue. • Most elements have more than one possible oxidation state.

23. p. 86 Reduction-Oxidation Chemistry

24. ‘Oxidation’is a measure of the surplus or deficit of electrons. • Oxygen atoms each want to steal 2 electrons. • Gives it its most stable electronic configuration. • CO2 – oxygen takes 4 electrons (+4) • CH4 – hydrogen atoms each give 1 electron to carbon (-4) • CH2O – C gains 2 from H but give 2 to O • C has many oxidation states so we group them into families.

25. 1. Inorganic (oxidized) carbon - Most of the C on Earth • Most stable form; • CaCO3 (Limestone) • Dissolved in oceans 2. Organic Carbon (reduced & intermediate) - Hydrocarbons • Oil • Natural gas (methane is the main component) • 3. Carbohydrates (life is comprised of C in the intermediate state) • Sugars

26. Life on Earth is based on the ‘nifty trick’of harvesting energy from the sun and storing it by creating organic carbonfrom inorganic (oxidized) carbon. CO2 + H2O + energy CH2O + O2 Photosynthesis – forward direction of eqn (plants) Respiration – backward direction of eqn (consumers)

27. Photosynthesis • Serves 2 purposes in the biosphere: • uses carbon in the oxidation state • stores energy from the sun in the form of organic carbon • Nearly all of the organic carbon produced by photosynthesis is respired sooner or later. • Peat deposits may hold organic carbon for thousands of years.

28. But, on geologic scales, only organic carbon that ends up in ocean sediments escapes degradation back to CO2 • Earth has built up sizable pool of carbon in the reduced form of ocean sediments and sedimentary rocks (former ocean sed. on land).

29. Fig. 8.1 • CO2 in atm – tiniest fraction of all C on Earth • Atm contains 700 gigatons of C • Gton = 1 billion (109) metric tons • Other sinks have more C • All ‘breathe’ CO2, causing atm CO2 to vary naturally. • Timescales – 1 yr. to millions.

30. The Land Breathes 2 forms of carbon associated with the landscape; • Terrestrial biosphere (plants/animals) -similar in size to atm • Soil Carbon Pool -more C than in living biosphere -highly varied place to place

31. Living Carbon • Annual cycle

32. The Ocean Breathes • Larger sink than land surface or atm. • Dissolved inorganic C • not only dead, it’s oxidized. • CO2, H2CO3 (carbonic acid) • Dissolved organic carbon • detritus • Living C • Fish, algae • Flux – similar to land • C is released from water in some areas and dissolves in others

33. Even though ocean and land fluxes are similar… • Oceans don’t effect the CO2 levels of atm as quickly as biosphere can. • Atm absorbs CO2 from oceans until the rate in and out balances. • It takes hundreds of years for this equilibrium, because the oceans circulate so slowly. • Glacial/interglacial cycle are an example of how oceans effect atm CO2 • Ice age every 150 million years • Interglacial last 10,000 years • Very different timescale than atm/land flux!!

34. Ice core data shows changes in CO2 • Glacial intervals 180 – 200 ppm CO2 • Interglacial 260 – 280 ppm (preindustrial) • No one is sure why the CO2 cycles up and down, but the oceans are thought to be the source.

35. The Rocks Breathe • Sedimentary rock carbon pool largest • Limestone (CaCO3) + some organic C (Kerogen) • Fossil Fuels • Flux is very small!! • But sink is HUGE! • 500x atm/land combined

36. Urey Reaction • CaSiO3 + CO2 CaCO3 + SiO2 • Silicate rocks are weathered to produce sedimentary rocks. Flux is extremely small • But, if degassing stopped CO2 would be gone from atm in a few hundred thousand years. Takes CO2 out of atm Puts CO2 back into atm

37. The Carbonate-Silicate Cycle • Also known as the Urey Reaction • Rainfall acts to "wash" CO2 out of the atmosphere in the form of carbonic acid: • CO2 (gas) + H2O (liquid) --> H2CO3 (carbonic acid) • The carbonic acid weathers the rocks on the Earth's surface, releasing ions of calcium (Ca++) and bicarbonate (HCO-3) into the oceans.

38. These combine into calcium carbonate (CaCO3) on the sea floor either through • formation of carbonate rocks • living organisms making carbonate shells • The calcium carbonate is eventually subducted down into the Earth (via plate tectonics), where high temps and pressures convert it back to CO2 gas. • CO2 gas gets outgassed from volcanos back into the atmosphere.

40. Take Home Points • Most stable form of C is oxidized as CO2 or CaCO3 • Photosynthesis stores energy by producing organic C • There is less C in atm than any other sink. • Land and ocean sinks effect atm C, seasonally for land, 100,000 year scale for oceans. • Flux from sed. rock sink is small (weathering) though size of this sink is the largest.