The Webster–Ashburton Treaty is a decision the 
United Kingdom can enact. It transfers control of Caribou, an American core in Maine, to the 
United States of America. In return, the UK receives +21 prestige, −2 infamy, and +100 relationship with the US. To enact the decision, the UK must be at peace with the United States and of course Great Britain must actually own Caribou. The AI will always do so unless it has less than 0.2 infamy. Furthermore 
Canada loses its cores on Caribou as well as Bangor, Maine, just south of Caribou.
The decision is available from the start of the game.
Caribou will be ceded to USA even if 
New England exists.
Illogically, in the game files the decision is placed in the USA-file in the decision folder instead of the Great Britain-file.
Ingame description
Following the events of the (undeclared) Aroostook War, The Webster-Ashburton Treaty of 1842, between the United Kingdom and The United States of America, delimited with finality the borders between Maine and New Brunswick, regulated the shared use of the Great Lakes, reaffirmed the 49th parallel border and called for a mutual commitment toward ending the Atlantic slave trade.