Final answer:
Approximately 3.33% of the present-day concentration of carbonate ions in the surface ocean is consumed in a single year by the uptake of CO2 from fossil fuel sources.
Step-by-step explanation:
The student question asks for the calculation of which fraction of the present-day concentration of carbonate ions (CO32-) in the surface ocean is consumed by the absorption of fossil fuel-derived CO2 in a single year. Given that 8Pg C per year is added to the atmosphere and 30% is absorbed by the ocean, we can calculate the amount of carbon absorbed by the ocean's surface layer. The calculation involves determining the total amount of CO32- in the upper 100 meters of the ocean based on the given surface ocean concentration and then finding what fraction of this is removed by the annual uptake of carbon.
To begin, we must establish the total amount of carbonate ions in the surface ocean:
- Surface area of the ocean: 3 x 1014 m2
- Depth of the surface ocean: 100 m
- Volume of the surface ocean: Surface area x Depth = 3 x 1016 m3
- Concentration of CO32- ions: 2 x 10-4 mol/m3
- Total carbonates in surface ocean: Volume x Concentration = 6 x 1012 mol
Next, we calculate the total carbon absorbed by the ocean from fossil fuels:
- Yearly CO2 from fossil fuels: 8 Pg C (petagrams of carbon)
- Conversion to moles of carbon: 8 x 1015 g C / 12.01 g/mol (Atomic mass of C) = 6.66 x 1014 mol C
- Ocean absorption: 30% of 6.66 x 1014 mol = 1.998 x 1014 mol C/year
Finally, we determine the fraction of CO32- consumed:
Fraction consumed: Absorbed carbon / Total carbonates = 1.998 x 1014 mol / 6 x 1012 mol
= 0.0333 or 3.33%
Thus, approximately 3.33% of the present-day concentration of carbonate ions in the surface ocean is consumed in a year due to fossil fuel CO2 uptake.