Outline. Solubility of calcium carbonate Solution of calcium carbonate Effects of solution and precipitation

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1 7 Carbon dioxide

2 Outline 1 Reservoirs of carbon dioxide 2 Relationships in solution 3 Calcium carbonate Solubility of calcium carbonate Solution of calcium carbonate Effects of solution and precipitation 4 Anthropogenic carbon dioxide Atmospheric increase Total quantities involved Sinks for CO 2 Rock weathering Biosphere Ocean Ocean acidification 5 Longer-term issues

3 Importance of CO 2 system Responsible for about 95% of the acid base buffering over the normal range of ph in ordinary seawater Short-term changes in total CO 2 in seawater are due largely to the photosynthetic and respiratory activities of organisms, so a great deal can be learned about biological activity by monitoring this system. To understand precipitation and solution of calcium carbonate in the oceans The influence of the concentration of CO 2 in the atmosphere on the climate of the Earth The reservoir of CO 2 in the oceans is much greater than that in the atmosphere, so small changes in processes affecting the oceanic reservoir could have comparatively large effects on the concentration in the atmosphere.

4 Outline 1 Reservoirs of carbon dioxide 2 Relationships in solution 3 Calcium carbonate Solubility of calcium carbonate Solution of calcium carbonate Effects of solution and precipitation 4 Anthropogenic carbon dioxide Atmospheric increase Total quantities involved Sinks for CO 2 Rock weathering Biosphere Ocean Ocean acidification 5 Longer-term issues

5 Reservoirs of carbon dioxide Both (a) limestone and (b) are largely due to biological activities in seawater. An understanding of the carbon dioxide system in seawater is therefore central to a serious consideration of both natural and human-induced changes in the climate of Earth.

6 Outline 1 Reservoirs of carbon dioxide 2 Relationships in solution 3 Calcium carbonate Solubility of calcium carbonate Solution of calcium carbonate Effects of solution and precipitation 4 Anthropogenic carbon dioxide Atmospheric increase Total quantities involved Sinks for CO 2 Rock weathering Biosphere Ocean Ocean acidification 5 Longer-term issues

7 Hydration rates In acidic solution: CO 2 (f) + H 2 O H 2 CO 3 K 0 = [H 2CO 3 ] [CO 2 (f)] = k CO k H2CO In alkaline solution: CO 2 (f) + OH HCO 3 The overall rates of reactions are ph-dependent, and it happens that there is a minimum in this rate at intermediate values of ph.

8 Half-time to attain equilibrium after some small perturbation of the carbon dioxide system The rates of hydration and dehydration of CO 2 are strongly dependent on temperature and ph. At 20 C it can take nearly 30 seconds to reach halfway towards equilibrium.

9 Carbonic anhydrase The carbonic anhydrases form a family of enzymes that catalyze the rapid interconversion of carbon dioxide and water to bicarbonate and protons (or vice versa). This enzyme is nearly ubiquitous in the respiratory organs of all types of animals. A carbonated drink does not instantly degas when opening the container; however it will rapidly degas in the mouth when it comes in contact with carbonic anhydrase that is contained in saliva.

10 Equilibria in solution

11 Experimental accessibility of the parameters in the CO 2 system It is not technically feasible to measure directly either the concentrations or the activities of HCO 3, CO2 3, or H 2CO 3.

12 Apparent dissociation constants (K*)

13 Development of the algorithms for calcuaiton of the constants K* cannot be predicted from basic principles to an acceptable level of accuracy. In practice, the most accurate approach has been to measure the empirical apparent dissociation constants, K 1 * and K 2 *, at various temperatures and salinities, and to tabulate a set of values for use under the appropriate conditions. Kurt Buch in the 1930s John Lyman in 1937 Mehrbach et al. (1973) and Plath et al. (1980) Box 7.1

14 Calculation of CO 2 species from ph, TCO 2, and ppco 2 It requires only two measurements on an actual sample of seawater. {H + } and ppco 2 {H + } and [TCO 2 ] ppco 2 and [TCO 2 ] where

15 Boric acid equilibria The second most important buffer in seawater after the CO 2 system is composed of the couple, boric acid and borate ion.

16 Alkalinity of seawater Alkalinity - the ability of substances in seawater to combine with hydrogen ions during the titration of seawater with strong acid to the point where essentially all the carbonate species are protonated. HCO %; CO %; B(OH) 4 3.5%; OH 0.2%

17 Total vs. carbonate alkalinity

18 Calculation of the carbonate species from CA and ph Refer to equations (7.32), (7.33), and (7.34) the calculation. Refer to equations (7.36) and (7.37) for the calculation of [TCO 2 ] and ppco 2. where

19 Carbon dioxide system in surface seawater ppco 2 is extremely sensitive to variation in ph.

20 Titration curves of seawater calculated for two conditions

21 Relative importance of carbonate, fluoride, sulfate, hydroxyl, borate in different parts of the ph range

22 Effects of temperature and pressure Temperature - The dissociation constants of the various substances reacting with protons decrease at lower temperatures. Pressure - The dissociation constants increase with increase in pressure.

23 The effect of pressure on the ph of seawater The resulting ph is that which a parcel of water would have if it were to sink from the surface to each specified depth.

24 Effect of temperature changes on ppco 2 solubility pump

25 Outline 1 Reservoirs of carbon dioxide 2 Relationships in solution 3 Calcium carbonate Solubility of calcium carbonate Solution of calcium carbonate Effects of solution and precipitation 4 Anthropogenic carbon dioxide Atmospheric increase Total quantities involved Sinks for CO 2 Rock weathering Biosphere Ocean Ocean acidification 5 Longer-term issues

26 Calcite vs. aragonite rhombohederal (calcite) orthorhombic (aragonite) Aragonite is less stable and more soluble under normal conditions than calcite, so in sediments aragonite is much less abundant than calcite.

27 Major agents of CaCO 3 removal 1st: foraminifera ("hole bearers"), 2nd: coccoliths

28 Coral reefs

29 Accumulation rate of CaCO 3 in the modern ocean 32* = 11 (deep-sea) (coral reefs) (slope) *in unit of mol yr 1 much greater than the estimated input of calcium of mol yr 1 The ocean is not in steady state.

30 Degree of saturation of CaCO 3 Solubility product is the equilibrium constant for the reaction in which a solid salt dissolves to give its constituent ions in solution. K sp of CaCO 3 in distilled water at 20 C: for calcite for aragonite Ion product: IP = [Ca 2+ ] [CO 2 3 ] = ( )( ) Degree of saturation: Ω = IP K sp

31 Correction of degree of saturation of CaCO 3 with activity coefficients (Table 6.2 and Table 4.1) IP = [Ca 2+ ] γ Ca [CO 2 3 ] γ CO3 = ( ) ( ) = Ω = =

32 Effect of ion pairing on the carbonate system in seawater (Garrels and Thompson, 1962) up to 90% of the CO 2 3 might be paired with cations, and thus made unavailable to react with and precipitate calcium

33 The corrected values by Garrels and Thompson (1962) free Ca 2+ : 0.91, free CO 2 3 : 0.09 IP K sp = = 2.8 (< directly measured values of 5)

34 K sp * values are sensitive to changes in salinity

35 Why does surface seawater not yield a precipitate of calcium carbonate? Pytkowicz (1973) - Mg 2+ ions effectively delayed or prevented the nucleation of CaCO 3 crystals so that precipitation was not easily initiated in solutions with a Mg 2+ concentration similar to that of seawater. Chave and Suess (1967) - Something (presumably organic matter) in seawater may coat the crystals so that their surfaces become less reactive with the surrounding medium.

36 Solution of calcium carbonate Carbonate sediments (>30% CaCO 3 ) cover about one-half of the deep-ocean floor. The coverage of foram ooze alone is about 35%. lysocline - the depth at which there is the first indication of dissolution of carbonates carbonate compensation depth" (CCD) - the depth where the rain rate of calcium carbonate to the sea floor is exactly compensated by the rate of dissolution of CaCO 3.

37 Why more soluble at depth in the ocean than at the surface? CaCO 3 becomes slightly more soluble as the temperature drops. Increasing pressure increases the K sp * and decreases ph, and in consequence the concentration of CO 2 3 ion is also decreased. Respiration of organisms at depth, living on organic carbon particles falling from the surface, releases carbon dioxide into the water, further decreasing the ph.

38 Relative importance of carbonate, fluoride, sulfate, hydroxyl, borate in different parts of the ph range

39 Peterson s calcite sphere experiment (Peterson, 1966) Calcite spheres were suspended for several months at various depths in the eastern Pacific Ocean.

40 The rate of dissolution is a strong function of the degree of undersaturation R = k(1 Ω) n n 4.54 Keir (1980)

41 Saturation state of calcite and aragonite in the central North Pacific

42 Pacific vs. Atlantic

43 Equilibria between species in carbonate system "A system in equilibrium that is subjected to a stress will react in a way that tends to counteract the stress." - Le Chatelier s principle

44 Effects of solution and precipitation of CaCO 3 on pco 2 and ph

45 Effects of solution and precipitation of CaCO3

46 Outline 1 Reservoirs of carbon dioxide 2 Relationships in solution 3 Calcium carbonate Solubility of calcium carbonate Solution of calcium carbonate Effects of solution and precipitation 4 Anthropogenic carbon dioxide Atmospheric increase Total quantities involved Sinks for CO 2 Rock weathering Biosphere Ocean Ocean acidification 5 Longer-term issues

47 Greenhouse effect John Tyndall - The first measurements of the infrared absorbing capacity of CO 2 and water vapor Glacial episodes might have been caused by a lowered atmospheric concentration of CO 2 (Tyndall 1861). Svante Arrhenius - the first attempt to calculate how changes in the levels of CO 2 in the atmosphere could alter the surface temperature through the greenhouse effect in 1896.

48 Atmospheric CO 2 concentrations recorded at Mauna Loa Seasonal effects (plant photosynthesis & respiration, fossil fuel burning) Inexorable increase in the concentration of CO 2

49 Latitudinal difference in CO 2 CO 2 Concentration (ppm) Global Stations Carbon Dioxide Concentration Trends Data from Scripps CO 2 Program Last updated March Year SAM S PTB 71 N LJO 33 N MLO 20 N CHR 2 N SPO 90 S PTB = Point Barrow, LJO = La Jolla, MLO = Mauna Loa Observatory, CHR = Christmas Island, SAM = Samoa, and SPO = South Pole Least seasonality at the South Pole smaller land mass in SH Lower in SH human input in NH & oceanic uptake in SH

50 CO 2 in the atmosphere during the last 420,000 years

51 Anthropogenic release of CO2

52 Reservoirs of CO 2 and some current fluxes

53 Rock weathering Dissolution of limestone Feldspar kaolinite

54 CO 2 removal by rock weathering (CO 2 removal by rock weathering) (annual input of HCO 3 ) = (HCO 3 concentration in rivers) (total river flow) Assuming fixation would be proportional to the increased CO 2 concentration of 0.51%(=4.32/830), RW would consume an additional moles or Gt of C per year, only 0.025% of the CO 2 introduced by humans from fossil fuels (8.05 Gt C yr 1 ).

55 Biosphere (land biota) Only that carbon which ends up in tree trunks, or humus, or other storage reservoirs that are not oxidized for at least several years is removed from the atmosphere during the timescale of interest. Values for the change in these large reservoirs are decidedly uncertain, due to the difficulty in making global inventories of forest biomass, and have been subject to considerable debate.

56 Seasonal and interannual variation in atomospheric O 2 In 1988, Ralph Keeling developed a remarkably precise method to measure the concentration of atmospheric oxygen. δ(o 2 /N 2 ) = ( ) (O2 /N 2 ) sample (O 2 /N 2 ) ref.

57 Apportioning the anthropogenic carbon dioxide 53.5% in the atmosphere, 31.3% in the ocean, 15.2% on land

58 Estimation of Oceanic uptake with δ 13 C (Quay et al., 1992) Average net oceanic CO 2 uptak: 2.1 Gt C yr 1

59 Suess effect... refers to the net decrease in the 14 C content of atmospheric CO 2 that has resulted since 1850 due to the burning of radiocarbon "dead" fossil fuels such as coal and petroleum.

60 Estimation from inorganic carbon measurements The oceanic sink accounts for 48% of the total fossil-fuel and cement-manufacturing emissions for the period of (Sabine et al., 2004).

61 Uptake factor UF = [TCO 2] pco 2 As CO 2 is added to a sample of seawater, the ability of the water to take up more CO 2 becomes less and less.

62 Revelle factor The Revelle factor expresses the proportional change in ppco 2 corresponding to a change in the concentration of TCO 2 in seawater

63 Distribution of Revelle factor The capacity for ocean waters to take up anthropogenic CO 2 from the atmosphere is inversely proportional to the value of the Revelle factor.

64 Increase of CO 2 in the atmosphere

65 Simplified schematic of the global C cycle Anthropogenic carbon flux: 8.9 Pg C yr 1 Atmosphere (4) + Land (2.6) + Ocean (2.3) Land: 52 Pg C yr 1 of NPP Ocean: 50 Pg C yr 1 of NPP 13 Pg C yr 1 of BP

66 Effect of changing ph on the distribution of the various carbon species

67 Gradual decrease in ph since LGM

68 [CO 2 3 ] in seawater as related to the partial pressure of CO 2 in the water

69 Time series of ph at Station ALOHA

70 "Naked corals" in the acidified seawater (ph = 7.4)

71 Outline 1 Reservoirs of carbon dioxide 2 Relationships in solution 3 Calcium carbonate Solubility of calcium carbonate Solution of calcium carbonate Effects of solution and precipitation 4 Anthropogenic carbon dioxide Atmospheric increase Total quantities involved Sinks for CO 2 Rock weathering Biosphere Ocean Ocean acidification 5 Longer-term issues