Where Did Alma Get His Authority?

This article explains that, since there were no members of the tribe of Levi among the Nephites, the Nephites officiated by virtue of the Melchizedek Priesthood rather than the Aaronic. It concludes that Alma received the priesthood before Noah became king and remained righteous enough to retain this authority, although he immersed himself while baptizing Helam as part of the repentance process.