What were CO2 levels in Carboniferous?
Carboniferous CO2 levels may not have been unlike today's. Evidence for this is primarily from long term geochemical modeling work, as there are few direct proxies of sufficient age.

I can easily cite one source, as it's from my own paper, but please check the reference list for the other datasets. http://dx.doi.org/10.1016/j.chemgeo.2013.10.012