Being 100 miles North of Siberia, one would expect that the island might have a little snow...but that's not what makes or breaks a "guano" island.
The Guano Treaty gives U.S. explorers the right to temporarily extend U.S. sovereignty to any uninhabited, unclaimed island...so long as they mine it. The predominant type of such mining was once for bat guano, but other mining may also qualify for this temporary extension of American territorial sovereignty.