Hi @agmoore! Great to see you again! Once the organism stopped exchanging carbon with its environment (the death of the tree of which they made the charcoal), it does not matter in what atmosphere it is. So the closing of the cave would not influence samples (such as the charcoal of the paintings) already present. After the closing however, and when the cave is sealed from the environment, its atmosphere can not be assumed to have the same C14 ratio as the outside atmosphere. So samples dating from after the closing cannot directly be carbon dated.
Thanks for sharing, I appreciate it!