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Carbon Dioxide, Alkalinity and Ph Carbon Dioxide, Alkalinity and pH OCN 623 – Chemical Oceanography 15 March 2018 Reading: Libes, Chapter 15, pp. 383 – 389 (Remainder of chapter will be used with the classes “Global Carbon Dioxide” and “Biogenic production, carbonate saturation and sediment distributions”) Student Learning Outcomes (SLOs) At the completion of today’s section, students should be able to: 1. Identify the chemical species involved in the marine CO2/carbonate system 2. Explain CO2 dissolution in seawater and subsequent reactions 3. Explain the concepts of pH, alkalinity, and dissolved inorganic carbon, and write the equations defining these quantities 4. Explain the relationship between carbonate dissolution/precipitation and pCO2. Why is it important to understand the CO2 system? • CO2 controls the fraction of inbound radiation that remains trapped in the atmosphere (greenhouse effect), which in turn strongly influences planetary climate • CO2 is the raw material used to build organic matter • CO2 controls the pH of the oceans • Distribution of CO2 species affects preservation of CaCO3 deposited on the sea floor Ocean Carbon Cycle in a Global Context Land sink 2.4 Atmosphere [590 + 204] 70.6 20 1.5 70 22.2 Land-use change 0.4 aa)++ 2.0 CaCO formation gh– 7.2 0.5 3 (bb+)+ 0.2 NPP d–, g– 50 PIC zooplankton DIC Fossil fuel River fluxes 38.5 c(c+) and cement [POC+ DOC] Respiration [900 + 20] emissions [3] [25] Mixed [>5000 – 330] necton layer 0.3 Interior 0.1 ocean (ih+)+ l+ m– 10 f+ 1.9 e? Coastal Export 2 Transport/mixing a+ decreased buffer capacity ocean j+, k? b+ decreased solubility in warm ocean c+ enhanced recycling in warm ocean PIC POC DOC 1.4 d- carbon overconsumption n– [700] e? DOM recycling/export f+ increased denitrification Remineralization g- increased nitrogen fixation DIC h- reduced calcification 0.1 <0.1 i+ reduced particle ballast [37,100 + 115] j+ increased stratification k? increased Southern Ocean winds l+ reduced deep water formation m- reduced upwelling Deep ocean n- increased carbonate dissolution Sabine and Tanhua 2009 CO2 Speciation xCO 2 Perturbations to one species leads to a pCO2 redistribution of all the other species fCO2 k [CO ] + H O K0’ 2 2 fCO2 [CO2]*{ { 1% [H2CO3] K1’ Using the thermodynamic There are many forms constants (Kx), any two - + of dissolved inorganic 88% [HCO3 ] + H measured parameters can be carbon in water K2’ used to calculate the 2- + concentration of all the species 11% [CO3 ] + H Individual dissolved species can not be measured directly CO2 Speciation xCO2 There are four pCO2 measurable carbon parameters in water fCO2 k [CO ] + H O K0’ 2 2 [CO ]* { fCO2 2 { 1% [H2CO3] K1’ - + 88% [HCO3 ] + H K2’ 2- + 11% [CO3 ] + H Measurable parameter: CO2 fugacity Takahashi climatological annual mean air-sea CO2 flux for reference year 2000 Based on ~3 million measurements since 1970 and NCEP/DOE/AMIP II reanalysis. Global flux is 1.4 ±0.7 Pg C/yr Takahashi et al., Deep Sea Res. II, 2009 The global mean air-sea CO2 flux for the period from 1982 to 2009 gives an average contemporary net uptake of 1.47±.23 Pg C yr-1 Surface observations have large variability over a wide range of time and space scales making it very difficult to properly isolate the anthropogenic increases. Uptake of 2 Pg C yr-1 only requires a DpCO2 of 8ppm. CO2 Speciation xCO2 GENERAL DEFINITION: There are four The acid-buffering capacity pCO2 measurable carbon of seawater parameters in water fCO 2 Carbonate Alkalinity Total Alkalinity (TA) k represents ability of [CO ] + H O K0’ 2 2 ≡ -2 - { CA 2[CO3 ] + [HCO3 ] seawater to resist pH fCO2 [CO2]* change upon addition of { [H2CO3] K1’ 1% - -2 acid Typically, HCO3 and CO3 are present at ~1000x conc of 88% - + Remember the concept of [HCO3 ] + H other proton acceptors K2’ a “buffer” (from basic 11% [CO 2-] + H+ chemistry): a substance 3 Hence: CA nearly equals TA that resists pH change upon addition of acid or Measurable parameter: base Total Alkalinity - 2- - 2- 3- - + TA = [HCO3 ] + 2*[CO3 ] + [B(OH)4 ] + [HPO4 ] + 2*[PO4 ]… + [OH ] - [H ] Surface TA/Talk/Alk/AT Distribution is Very Similar to Salinity Shallow Indian Ocean Data (depth < 100 m) CO2 Speciation xCO2 There are four pCO2 measurable carbon parameters in water fCO2 Inorganic Carbon is stored k in the ocean as Total CO 2 K0’ [CO2] + H2O (a.k.a. DIC) [CO ]* { fCO2 2 { K1’ 1% [H2CO3] DIC and TA are state variables, meaning they - + 88% [HCO3 ] + H are not a function of K2’ temperature or pressure 2- + 11% [CO3 ] + H Measurable parameter: Total CO2 - 2- TCO2 = [CO2]* + [H2CO3] + [HCO3 ] + [CO3 ] TCO2/DIC/CT: Surface Distribution is Similar to Nutrient Distributions Shallow Indian Ocean Data (depth < 100 m) CO2 Speciation xCO2 There are four pCO2 measurable carbon parameters in water fCO2 {H+} = Hydrogen ion activity k [CO ] + H O K0’ 2 2 There are 5 different pH { fCO2 [CO2]* + + - scales. The most common { {H }sws = {H }f + [HSO4 ] + [HF] [H CO ] K1’ are pH and pH 1% 2 3 sws T + + - {H }T = {H }f + [HSO4 ] - + pH of seawater is slightly 88% [HCO3 ] + H basic; acidification is a K2’ 2- + process, not a state 11% [CO3 ] + H Measurable parameter: pH + pH = -log10{H } Surface pH distribution reflects combined patterns of alkalinity and TCO2 pH Light gray = warm water corals Dark gray = cold water corals Feely, Doney and Cooley, Oceanography (2009) Group Task How does seawater pH change when atmospheric CO2 is added to the ocean? What are the reactions?? Group Task How does seawater pH change when atmospheric CO2 is added to the ocean? What are the reactions?? Answer: The pH decreases because of the release of hydrogen ions: CO2(g) → CO2(aq) - + CO2(aq) + H2O → HCO3 + H - -2 + The limited amount of CO 2- available HCO3 ← CO3 + H 3 means that not all of the H+ produced by the middle reaction can be consumed Take home message: Gas Each of the 4 measurable Exchange carbon parameters can tell us something different about the basic processes operating in the Ocean pCOpCO22 pHpH TCOTCO22 Ocean TAlkTAlk Physics Biology Question: Gas Rising CO How will ocean carbon system respond 2 Exchange Levels to changes in ocean processes? Changing Climate pCOpCO22 pHpH Changing Biology? TCOTCO22 Ocean TAlkTAlk Physics Biology Equations for CO2 Speciation The equilibrium of gaseous and aqueous CO2: CO2(g) ↔ CO2(aq) Subsequent hydration and dissociation reactions: − {H +}[HCO ] K * = 3 ↔ ↔ - + 1 CO2(aq) + H2O H2CO3 HCO3 + H [CO2 ] + 2− - ↔ -2 + {H }[CO3 ] HCO3 CO3 + H K2* = − [HCO3 ] + Asterisk (*) indicates a Hint: When you add a CO2 species to the system, follow the H . Thus, the “stoichiometric” constant following is a reasonable approximation when pH is between 7.5 and 8.5: -2 - CO2(aq) + CO3 + H2O ↔ 2HCO3 Distribution of CO2 Species at CO2(aq) Different pH Values CO2(g) ↔ CO2(aq) - + CO2(aq) + H2O ↔ HCO3 + H - -2 + HCO3 ↔ CO3 + H Seawater values shown ---freshwater curves are shifted left aob.oxfordjournals.org Effects of Pressure on Carbonate Speciation 1 atm 1000 atm CO2(g) ↔ CO2(aq) + − -5.89 -5.55 {H }[HCO ] K1* 10 10 - + K * = 3 CO2(aq) + H2O ↔ HCO3 + H 1 [CO2 ] K * 2 10-9.13 10-8.93 + 2− HCO - ↔ CO -2 + H+ {H }[CO3 ] 3 3 K2* = − [HCO3 ] As you raise a sample from depth: - Ks’ decrease - Reactions shift to left - pH increases Group Task Why is raising a sample of seawater from depth to the surface like opening a can of soda??? What exactly is happening? How does the pH change? Hint: What happens to the dissolved CO2? Group Task Why is raising a sample of seawater from depth to the surface like opening a can of soda??? Answer: In both cases there is: 1) An aqueous solution containing a large amount of dissolved CO2 2) Pressure is released, causing the CO2/carbonate reactions to shift to the left (due to decreased Ks) 3) CO2 gas is released and pH increases - -2 + HCO3 ← CO3 + H - + CO2(aq) + H2O ← HCO3 + H CO2(g) ← CO2(aq) CaCO3 Precipitation/Dissolution A tricky subject when discussing “CO2” (or, more properly, pCO2) 2+ 2- 2- Ca + CO3 → CaCO3 (consumption of CO3 ) Does this reduce the CO2 (pCO2) level of the seawater? CO2(g) ↔ CO2(aq) 2- CO + H O ↔ HCO - + H+ No! Lost CO3 will be replaced: 2(aq) 2 3 - 2- + - -2 + HCO3 → CO3 + H HCO3 ↔ CO3 + H But this H+ release causes: Hint: when pH is between 7.5 and 8.5: - + HCO + H → CO + H O -2 - 3 2 2 CO2(aq) + CO3 + H2O ↔ 2HCO3 Thus, CaCO3 precipitation causes a decrease in DIC and TA, but an increase in pCO2 Chemical Reactions of Carbonate Species in Seawater Atmosphere [Ca2+] is one of the 6 major ions in seawater CO2 To a first order it is considered conservative K1 K2 aq – + 2– + with salinity (~10.3 mmol/kg at salinity of 35) CO2 HCO3 + H CO3 + H (CO2 + H2O) 2- Therefore, [CO3 ] primarily controls the Ca2+ saturation state of the waters with respect to CaCO3 aragonite, calcite, magnesian calcite calcification 2+ 2- Ksp = solubility product of a solid Ωa = [Ca ][CO3 ] Ksp’ = apparent solubility product Ksp’ Ωa > 1 ~ Supersaturated Solubility of CaCO3 increases with decreasing temperature and increasing pressure. CaCO3 + CO2 + H2O Increasing Solubility from left to right: dissolution dolomite < calcite < aragonite < high-magnesian calcite 2+ - Ca + 2HCO3 Student Learning Outcomes (SLOs) At the completion of today’s section, students should be able to: 1. Identify the chemical species involved in the marine CO2/carbonate system 2. Explain CO2 dissolution in seawater and subsequent reactions 3.
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