Ocean 520 Name:_________________ Chemical Oceanography 30 October 2009 Fall 2009 Points are in parentheses (show all your work) (use back if necessary) MID-TERM #1 – TAKE HOME, OPEN BOOK Do not use more than two hours 1. Doney et al (2008) Ocean Acidification The surface ocean is becoming more acidic due to addition of anthropogenic CO2. Preindustrial surface seawater pH was 8.21. Currently the surface seawater pH is 8.10. a) Calculate how much more [H+] there is now relative to preindustrial times. Give the numerical concentration. (5) b) By what percentage has the [H+] concentration changed since preindustrial times (relative to preindustrial times)? (5) 2. Ocean carbonate reactions Ocean acidification will make calcification by organisms more difficult. a) The concentration of bicarbonate ion (HCO3-) is important to know for solubility reactions. Calculate the concentration of HCO3- in surface seawater if you know the DIC and pH. (10) Assume the apparent equilibrium constants for seawater of K’1 = 10-6.0 and K’2 = 10-9.0 Assume DIC = 2.000 x 10-3 and pH = 8.0b) We have learned the following equilibrium reactions and constants for seawater. 1. CO2(g) + H2O Æ H2CO3 KH’ = 10-1.50 2. H2CO3 Æ H+ + HCO3- K1’ = 10-6.00 3. HCO3- Æ H+ + CO3-2 K2’ = 10-9.00 4. H2O Æ OH- + H+ Kw’ = 10-14.0 5. CaCO3 (aragonite) Æ Ca+2 + CO3-2 Ksp’ = 10-6.19 We can write the solubility of CaCO3 in terms of HCO3- and CO2(g) as follows: CaCO3 + CO2(g) + H2O = Ca2+ + 2 HCO3- Derive the equilibrium constant (K’) for this reaction using the values for K’H, K’1, K’2 and Ks. (10) c) Calculate the equilibrium concentration of HCO3- in equilibrium with aragonite in seawater assuming we know the concentration of Ca2+ in seawater and PCO2 in the atmosphere. (10) Assume that the concentration of Ca2+ = 10.0 x 10-3 M and PCO2 = 380 ppm d) Is surface seawater supersaturated, at equilibrium or undersaturated with respect to aragonite. Explain using the Ω ratio (degree of saturation). (10)3. Gas Exchange Is beer carbonated? a) The pH of your favorite, frosty, carbonated beverage is 5.8 and the HCO3- content is 2.0 x 10-3 mole liter-1. What is the partial pressure of CO2 in the can before you open it? Let K1 = 10-6.0 and KH = 10-1.5. (10) Write a reaction including CO2, HCO3- and H+ b) Calculate the flux of CO2 (in mol s-1 m-2) for your favorite, frosty, carbonated beverage. Assume the surface of the beverage is in equilbrium with the PCO2 of the atmosphere (385 x 10-6 atm.). Let the molecular diffusion coefficient be DCO2= 2 x 10-9 m2 s-1 and let the stagnant boundary layer thickness be Zfilm = 5 x 10-5 m. Which way does the CO2 flux go? (10)4. Solubility Calcite (CaCO3(s)) is the main carbonate mineral in modern marine sediments. Ancient marine rocks contain mostly dolomite (CaMg(CO3)2(s)). The reaction of calcite to dolomite is written as: 2CaCO3(s) + Mg2+ = CaMg(CO3)2(s) + Ca2+ a) Calculate the equilibrium constant for this reaction at 25°C given the following free energies of formation (∆Gf°) and using ∆Gr° = -2.3 RT log K = -5.708 log K at 25°C (10 pts) Species ∆Gf° (kJ mol-1) CaCO3(s) -1128.8 CaMg(CO3)2(s) -2161.7 Mg2+ -454.8 Ca2+ -553.54 b) Write the corresponding equilibrium constant expression ( 5 pts) c) Assuming that seawater contains total concentrations of [Ca] = 1.0 x 10-2 mol/l and [Mg] = 5.3 x 10-2 mol/l what is the stable mineral phase, calcite or dolomite. (10 pts)5. Composition of Rivers The Table below gives the world average composition of river water. Do your calculations using molar concentrations. a) Correct the concentrations of Na, Mg, SO4, K and Ca for seasalt aerosols using Cl as a tracer? What assumptions do you need to make? (5) b) Propose an approach to calculate how much CO2(atm) is consumed by weathering of carbonate rocks (CaCO3) (5) c) Propose an approach to calculate how much CO2(atm) is consumed by weathering of silicate rocks (5) hint: Assume all Si comes from silicate rocks. Calculate the HCO3 that weathering produces. Then calculate the HCO3 that comes from weathering of carbonate rocks. As a check, if all the Ca comes from weathering CaCO3 then Ca should equal 0.5 x HCO3 from carbonate weathering. You will find that it doesn’t. The reason is that some carbonate rocks are MgCO3 or CaMg(CO3)2. d) From your model, what is the total amount of CO2 consumed by weathering (in PgC y-1)? Use a total river flow of 3.7 x 1016 L y-1. (5) e) How does your rate compare with that of Sarmiento and Gruber (2002) as shown in their Figure 1. (5) f) How does the weathering ratio of CaCO3/silicate rocks compare with the outcrop abundance of carbonate sedimentary rocks to silicate rocks. See attached table from Meybeck (1987, American Journal of Science, 287, 401-428). If the weathering ratio differs from the outcrop ratio, suggest why. (5) FromMeybeck (1987) American Journal of Science, 287,