Board Thread:Dark Souls II/@comment-24494913-20140319193521/@comment-24684538-20140319193959

It's never stated that it is the bone dust of the Chosen Undead, simply one interpretation. It could be Gwyn's dust since he link the flame before you, or it could be another undead after you since the Flame will eventually fade and Frampt may have tried to get another undead to link the Flame again.

As for getting across an ocean, Drangleic could have once been Lordran so it may have just been scattered over the land. If Drangleic was not once Lordran then maybe pilgrims worshipping the Flame obtained this dust and brought it with them to Drangleic.