[Utah-astronomy] Why not just the carbon?

At last evening's after-SPOC Advanced Training Session the subject came up about carbon sequestration. It occurred to me that most (if not all) of the talk I've heard on the subject has been about sequestering CO2 (mostly oxygen) and not just the carbon. Have I got that right? If so, why hide away the oxygen with the carbon rather than just to carbon? Any ideas? Thanks, patrick