The Carboniferous was a period where CO2 levels fell drastically.
Source: Geologic history of seawater: A MAGic approach to carbon chemistry and ocean ventilation
I think the main reasons are tectonic settings and the formation of coal deposits with the arrival of lignite-rich plants that bacteria didn't know yet how to decompose.
My question here is:
Forgetting Carboniferous plants were efficient sequestrating CO2 forming coal deposits after dying, were living Carboniferous plants more efficient sequestering CO2 than present plants?
Meaning, would a Carboniferous forest be more efficient than present forests to sequestrate CO2 nowadays?