A large volcanic eruption in 1459
Until recently, this eruption was unknown, but recent historical evidence suggests that the volcano Kuwae, in Vanuatu erupted about this time, in a huge explosion that destroyed an island.
Other ice cores in Greenland and Antarctica show this eruption, but because they generally have less accurate dating than the DSS core, the precision of the date was not so high.
The historical records, and even tree-ring records have suggested a date of 1453 for the eruption, a date somewhat too early for the DSS evidence.
Volcanic fallout does take a while to reach the polar icecap, but even when this is accounted for and potential errors in dating the ice core are included, the earliest ice-core date for this eruption is 1456.
So this makes for an interesting mystery – are the tree-ring or ice-core records being confused by a second eruption, or something else? The matter is of more than academic interest because large events like this are used to tie dating in records from a range of sources.
Also, volcanic eruptions are a known significant "forcing" of the climate system, and establishing a volcanic record helps in modelling the climate response.