Worksheet answers

  1. When CO2 gas diffuses through the sea surface and dissolves into seawater it makes the seawater more acidic. Looking at the Bjerrum plot, how would you expect a shift to lower pH to affect the % distribution of DIC into the different forms? Which out of [CO2(aq)], [HCO3-] and [CO32-] (where [ ] denotes ‘concentration of’) would you expect to become more abundant? which less so?

    Bicarbonate ion and dissolved CO2 gas (CO2(aq)) should increase (as a fraction or percentage of the total pool of DIC), as the pH drops below 8.2, while carbonate ion should decrease.

  2. Test out your predictions using the carbon model. Try adding 1000, 2000, 4000 and 8000 Gt of fossil fuel carbon to the atmosphere. What is the maximum impact on surface ocean pH in each case?

    (a) adding 1000 Gt C over a space of 400 years causes a maximum impact of ca. -0.2 units on pH, (b) 2000 Gt C causes pH to drop by a maximum of -0.4 units, 4000 Gt C by -0.7 units, and 8000 Gt C by -1 unit.

  3. What is the value of surface [CO2(aq)] in each case at the time of greatest deviation from its initial value? (NB. Atmospheric CO2 is not coupled to temperature in this model (temperature is constant through all model runs) and therefore atmospheric CO2 is directly proportional to the amount of CO2 gas dissolved in the surface seawater, [CO2(aq)]. Before fossil fuels are added, [CO2(aq)] is 10 μMol kg-1 in the model)

    (a) +10; (b) +20; (c) +50; (d) +110 μMol kg-1.

  4. Likewise what are the greatest changes to surface [CO32-] following the different fossil fuel additions?

    (a) -80; (b) -120; (c) -150; (d) -180 μMol kg-1.

  5. Finally, how does surface [HCO3-] change in each case? Because the model doesn’t plot out [HCO3-] directly you will need to calculate this by difference. { [DIC] = [CO2(aq)] + [HCO3-] + [CO32-] }

    (a) +190; (b) +290; (c) +390; (d) +450 μMol kg-1.