The effectiveness of injection into the ocean of CO2 produced from the use of fossil fuels is investigated using a coupled climate-carbon cycle model. Four fossil fuel emission scenarios are considered, in which emissions peak at 28 Gt C/yr in 2100 (scenario 1), 9 Gt C/yr in 2050 (scenario 2), or decrease to zero by 2100 or 2075 (scenarios 3 and 4). Sequestration sufficient to reduce the net emissions for scenarios 1--3 to that in scenario 4 is considered. Sequestration in scenario 1 results in a CO2 concentration of 857 ppmv by 3100, compared to a peak of 1614 ppmv without sequestration, and slows the accumulation of atmospheric CO2. The supersaturation of the non-polar mixed layer with respect to calcite decreases from 486% pre-industrial to about 250%, while mixed layer pH decreases by 0.66. Only for scenario 3 is sequestration sufficient to limit the peak impact on atmospheric CO2 and mixed layer chemistry to the impacts obtained in scenario 4.