Ocean 400 Name:_________________
Chemical Oceanography 17 February 2009
Winter 2009 (show all your work)
Problem Set #6
1) What controls atmospheric CO2.
As water upwells (Vmix) from the deep ocean to the surface, it equilibrates with the
atmosphere. Biological activity in the surface ocean affects the oceanic carbonate system
and the resulting pCO2 in the atmosphere. For this problem, please calculate atmospheric
pCO2, deep water O2, and AOU for the following four cases:
a.) an abiotic ocean
b.) a productive ocean with preformed phosphate [PO4]º = 0.90 µmol / kg
c.) a productive ocean with no surface PO4
d.) a productive ocean with [PO4]º = 0.90 µmol / kg that is dominated by
siliceous organisms (there are no CaCO3 shells, and hence alkalinity is not
affected by particle flux)
Information you’ll need:
[DICd] = 2250 µmol / kg [Alkd] = 2365 µeq / kg
[PO4 deep] = 2.2 µmol / kg Vmix = 300 cm / yr (note units!)
KH’ (CO2) = 10-1.53 mol / kg atm KH’ (O2) = 10-2.77 mol / kg atm
pO2 = 0.2095 atm K1’ = 10-6 mol / kg
K2’ = 10-9 mol / kg Assume 1 kg = 1 L
Assume the ocean is in steady state. What this implies is that river input is equal
to permanent particle burial, so for this problem do not include rivers or sedimentation
since they will cancel and have a net zero influence on the system. You’ll want to start
by making a PO4 balance for the surface box, and then use Redfield ratios to figure out
surface DIC and alkalinity (see hints below). Assume that the pO2, the partial pressure of
oxygen in the surface ocean, stays constant and in equilibrium with the atmosphere
throughout all four cases.
To calculate DIC from the phosphate balance, use the composite Redfield ratio
determined by Broecker and Peng, which takes into account both “soft” and “hard” parts,
P:N:C:Ca = 1:15:131:26. Assume all the Ca in the B flux is due to CaCO3, so you can use
the ratio of P:Ca:CO3:Alk to find BAlk. In part (d), since no CaCO3 shells are forming,
you should use the Redfield ratio, 1 P:106 C.
We highly recommend that you do your calculations in a spreadsheet after
drawing your box model diagram and writing out your equations. Please e-mail Andrea
your spreadsheet file, named “JaneDoePS4.xls” (obviously, insert your own name), and
hand in a paper copy of the diagrams and equations you used and your answers. You
won’t be graded based on the equations in the spreadsheet.
