In this case, I would say it is a natural phenomena dictated by climate rather than by a specific character.
The fact Chinjao could eventually break it means it's not Ragnir's ice because he shouldn't be able to break it.
It could be argued that Haki could break the ice even if it's Ragnir's, but a conservative rationale would suggest if Chinjao could do it then Imu could too, but he didn't, so they can't.