The Arctic is widely considered to be the Earth's most sensitive region with respect to climate changes,and acidification of the Arctic Ocean is predicted to be more pronounced than in any other ocean.A 1994 transatlantic section along the longitude~170°W in the western Arctic Ocean has been used as a baseline for estimating the aragonite saturation state(Ω arag)and associated acidification status.Here,we analyze a new~170°W transatlantic section collected in 2010 and compared the results with the previous section measured approximately two decades earlier.The comparison showed a large-scale expansion of corrosive “acidified” water(Ω arag < 1.0)in the western Arctic Ocean relative to earlier observations.Model simulations suggest that the recent significant increases in Pacific winter water inflows via the Bering Strait have contributed to the widespread intrusion of “corrosive acidified” waters into the Arctic Ocean basins.In addition,the biological processes enhanced by the sea ice retreat and the ocean uptake of anthropogenic CO2 have significantly increased the extent of the affected area.