Biogeochemistry of the Earth System QMS Lecture 5 Dr Zanna Chase 16 June 2015

Transcription

1 Biogeochemistry of the Earth System QMS Lecture 5 Dr Zanna Chase 16 June 2015

2 Lecture 5: Inorganic carbon chemistry Outline Inorganic carbon speciation in seawater- conceptual overview Inorganic carbon speciation in seawater- the equations Measurements in the inorganic carbon system Sensitivity of carbon speciation to environmental variables Processes affecting inorganic carbon in the ocean- photosynthesis & respiration, gas exchange, carbonate precipitation and dissolution IMAS 2

3 The ocean contains a large amount of carbon, which can exchange rapidly with the atmosphere (~60 x more C than the atmosphere). IPCC AR5 WG1

4 Some questions to ponder 1. Why does the ocean hold so much carbon? 2. What form/species is the carbon in the ocean? 3. Could the ocean hold more carbon? How much more? 4. Might the ocean hold less carbon? Why would it? 5. What happens to ocean chemistry as carbon is absorbed?

5 Ocean Carbon Inorganic (~99%) Organic (1%) Bicarbonate (HCO3 - ) 90% Carbonate ion (CO3 2- ) 9% Dissolved CO2 (CO2(aq)) 1% Carbonic acid (H2CO3) 0.001% ParDculate organic carbon (organisms) Dissolved organic carbon (decayed organisms) If you can see it, it doesn t matter!

6 Marine Inorganic Carbon System Dickson, A.G., Sabine, C.L., ChrisDan, J.R. (Eds.), Guide to best pracdces for ocean CO 2 measurements. PICES Special PublicaDon 3. IOCCP Report No. 8, 191 pp.

7 Marine Inorganic Carbon System pco 2 w Dickson, A.G., Sabine, C.L., ChrisDan, J.R. (Eds.), Guide to best pracdces for ocean CO 2 measurements. PICES Special PublicaDon 3. IOCCP Report No. 8, 191 pp.

8 Marine Inorganic Carbon System pco 2 a Air- sea flux pco 2 w Dickson, A.G., Sabine, C.L., ChrisDan, J.R. (Eds.), Guide to best pracdces for ocean CO 2 measurements. PICES Special PublicaDon 3. IOCCP Report No. 8, 191 pp.

9 SURFACE OCEAN PCO2- SARMIENTO AND GRUBER CH 8 9

10 Air-Sea CO 2 Flux Reds: flux from sea to air Blues: flux from air to sea flux = k(pco 2w pco 2a ) Takahashi et al 2009 Annual flux represents influence of biology driving undersaturation, upwelling driving supersaturation, and wind-speed facilitating the transfer across the interface

11 Marine Inorganic Carbon System Alkalinity [HCO 3- ]+ 2*[CO 3 2- ] Dickson, A.G., Sabine, C.L., ChrisDan, J.R. (Eds.), Guide to best pracdces for ocean CO 2 measurements. PICES Special PublicaDon 3. IOCCP Report No. 8, 191 pp.

12 Marine Inorganic Carbon System Total alkalinity (TA) = [HCO 3- ] + 2[CO 3 2- ] + [OH - ] + [B(OH) 4- ] - [H + ] ± minor consftuents Dickson, A.G., Sabine, C.L., ChrisDan, J.R. (Eds.), Guide to best pracdces for ocean CO 2 measurements. PICES Special PublicaDon 3. IOCCP Report No. 8, 191 pp.

13 Marine Inorganic Carbon System ph = - log{h + } Dickson, A.G., Sabine, C.L., ChrisDan, J.R. (Eds.), Guide to best pracdces for ocean CO 2 measurements. PICES Special PublicaDon 3. IOCCP Report No. 8, 191 pp.

14 Marine Inorganic Carbon System ΣCO 2 species à DIC Dickson, A.G., Sabine, C.L., ChrisDan, J.R. (Eds.), Guide to best pracdces for ocean CO 2 measurements. PICES Special PublicaDon 3. IOCCP Report No. 8, 191 pp.

15 Marine Inorganic Carbon System pco 2, TA, DIC, ph Any 2 of the 4 parameters can be used to calculate the other 2 Dickson, A.G., Sabine, C.L., ChrisDan, J.R. (Eds.), Guide to best pracdces for ocean CO 2 measurements. PICES Special PublicaDon 3. IOCCP Report No. 8, 191 pp.

16 Inorganic carbon speciation and ph In the following we will step through the equations behind this graph, starting with a simple acid-base system and moving onto the full carbonate chemistry

17 Aquatic Acid-Base Reactions Nomenclature: HBa is the acid (hydrogen ion donor) Ba - is its conjugate base H + is the hydrogen ion K is the (apparent) equilibrium constant Weak acid: one that does not completely dissociate in water

18 Aquatic Acid-Base Reactions continued combining the two equations above (do this!):

19 Aquatic Acid-Base Reactions continued combining the two equations above (do this!): ph = -log 10 [H+] so [H+] = 10 (-ph) Bjerrum plot of the concentration of the species for conditions: K = 10-6 BaT = 10-2 mol/kg

20 Matlab code used to generate Bjerrum plot

21 Question: What is the ph of this solution? ie, what s the ph of a 10-2 mol/kg solution of HBa, given K = 10-6? In order to answer this, we need the electroneutrality constraint: 0=Σ + - Σ - [H + ] = [Ba - ]

22 Some more about the Bjerrum plot at ph < pk, the acid dominates Bjerrum plot for conditions: K = 10-6 BaT = 10-2 mol/kg at ph > pk, the base dominates ph = pk, acid and basic species are equal pk = -log 10 [K]

23 Summary aquatic acid-base reactions now including water for completeness Reactions Equilibria Species mole fractions HBa = [H + ] + [Ba - ] H 2 O = [H + ] + [OH - ] k a = [H + ][Ba - ]/[HBa] k w = [H + ][OH - ]/[H 2 O] [HBa]/Ba T = [H + ]/([H + ] + k a ) [Ba - ] /Ba T = k a /([H + ] + k a ) Conservation BaT = [HBa]+[Ba - ] Recall: [H 2 O] = = 1 pk w = 14 ph = -log 10 [H+] = 7 Electroneutrality 0=Σ + - Σ - [H + ] = [Ba - ] (ignore water) [H + ] = Ba T k a /([H + ] +k a ) [H + ] 2 + k a [H + ] Ba T k a = 0 (derive this!) solve by finding roots of quadratic equation [H + ] = [Ba - ] + [OH - ] (with water) [H + ] = Ba T k a /([H + ] +k a ) +k w /[H + ] solve by iteration

24 Aquatic acid-base reactions: seawater carbon K 0 CO 2 (g) CO2 Henry s law K 1 K 2 CO 2 +H 2 O HCO3 +H + 2 CO3 +H 2+ recall: HCO 3 - bicarbonate CO 3 2- carbonate K * 1 = [HCO 3 ][ H + ] [CO 2 ] K * 2 = [CO 2 3 ][ H + ] [HCO 3 ] DIC Σ[CO 2 ]+[HCO 3 ]+[CO 3 2 ] CA=[HCO 3 ]+2[CO 3 2 ] 2 equilibrium equations 1 carbon mass balance equation DIC is dissolved inorganic carbon 1 charge mass balance equation (alkalinity) CA is carbonate alkalinity

25 carbon speciation as a function of ph

26 Aquatic Acid Base Reactions seawater carbon cont The equilibrium constants describing these reactions are: K 0 = [CO 2 ]/fco 2 K 1 = [H + ] [HCO 3- ] / [CO 2 ] K 2 = [H + ] [CO 3= ] / [HCO 3- ] The constants are determined experimentally and are T and S (and pressure) dependent.

27 Ulf Ribesell

28 Ulf Ribesell

29 The master variables of the carbonate system in seawater are: DIC, Alk, pco 2 and ph. DIC and Alk are conservative, invariant with T,S,P Any 2 parameters are sufficient to calculate all components. DIC deep ocean value: 2280 umol/kg ALK deep ocean value: 2380 umol/kg

30 For example measure DIC and [H+] (ie ph) can determine the other components as follows: 104 µmol kg -1 (5%) 1818 µmol kg -1 (87%) 272 µmol kg -1 (13%) Conditions: ph = 8.1 and DIC = 2100 µmol kg -1 ; atm pco 2 = 365 µatm at Salinity = 35 and T = 25C

31 Alkalinity: Charge Balance for seawater Ulf Ribesell

32 Ulf Ribesell

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34 Some useful approximations: DIC [HCO 3- ] + [CO 3= ] Alk [HCO3 - ] + 2[CO3 = ] Combine these approximations to get the following approximations (good to ~10%): [CO3 = ] Alk - DIC [HCO3 - ] 2DIC - Alk

35 Another approximation: pco 2 = K 2 [HCO - 3 ]2 K 0 K 1 [CO 2-3 ] [CO3 = ] Alk - DIC [HCO3 - ] 2DIC - Alk temperature dependent: increases with increasing temperature pco 2 K 2 (2 DIC Alk) 2 K 0 K 1 Alk DIC So partial pressure of CO 2 in water increases with increasing temperature, and increasing DIC and decreases with increasing alkalinity

36 Programs to calculate carbon system: - CO2SYS - seacarb - csys - swco2 - CO2calc - ODV - mocsy e.g. given 2 of pco 2, ph, DIC or Alkalinity, as well as pressure, temperature, salinity, calculate full carbon speciation at equilibrium You will get to play with CO2SYS during the lab

37 Vary temperature Alkalinity = 2300 um DIC = 2100 um Salinity = 35 g/kg

38 Vary Salinity Alkalinity = 2300 um DIC = 2100 um Temp = 10C

39 Vary DIC hold all else constant As DIC increases, pco 2 increases and ph decreases DIC (Dissolved Inorganic Carbon) = [HCO 3- ] + [CO 3= ] + [CO 2 ]

40 Vary alkalinity hold all else constant As alkalinity increases, pco 2 decreases and ph increases Alkalinity = charge imbalance of major irons (negative) Alkalinity [HCO3 - ] + 2[CO3 2- ]

41 How and why does carbon vary in the ocean?

42 Summary major effects on the carbonate system CO 2 invasion has no impact on alkalinity Photosynthesis removes DIC and slightly increases alkalinity (sunlit surface water) Respiration adds DIC and slightly removes alkalinity (dark subsurface) Calcification removes alkalinity and DIC in ratio of 2:1 (sunlit surface mostly) DIC = dissolved inorganic carbon = sum of all carbon species Carbonate dissolution adds alkalinity and DIC in a ratio of 2:1 (mostly at depth)

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44 Summary Carbon dioxide reacts with seawater and this is responsible for the large storage of carbon in the ocean The seawater inorganic carbon system is represented by a set of equilibrium acidbase reactions, the total DIC (dissolved inorganic carbon) and the charge balance (alkalinity). These determine seawater ph and the relative abundance of carbon species Gaseous CO 2, pco 2, is the key for exchange of carbon with the atmosphere pco 2 increases with increasing temperature and decreases with increasing alkalinity If any two of DIC, Alkalinity, ph, and pco 2 are measured, the inorganic carbon system can be fully specified Calculations are typically done using software packages, such as co2sys Invasion and evasion of CO 2, organic matter production and remineralisation, and carbonate precipitation and dissolution are important ocean processes affecting the inorganic carbon system