I can't seem to find the first entry. What prevented anyone from migrating to the New World before the Vikings?
[Answered in comments: It's not specified in the initial post]
If you prevent anyone from coming over the top of the world it won't prevent the Polynesians from colonizing the Americas (Sweet potatoes got to Polynesia from the New World Somehow). Well, it won't unless you have extremely well aimed butterflies, able to use the secondary and tertiary effects of whatever the barrier to colonization is up north to prevent anyone from using the trans-Pacific route.
- About Doug Hoff's Empty America