For later -- I haven't read it yet, but am struck by the fact that this paper is from 2015 -- no one bothered to do a study of this kind before then? Hard to believe.
A very quick read through seems to indicate that the authors do not or did not consider copper from the UP (Upper Peninsula) ... perhaps they did not have a UP sample or just believed that could not match any European area artifact ... this would possibility indicate consensus thinking - as ‘everybody knows that copper artifacts across the Atlantic could not possibility have a UP origin’.
They even indicate that some of the artifacts don’t seem to completely match the sample base ...