Vertical Expansion of Aragonite Undersaturated Waters in the Canada Basin of the Arctic Ocean From 2003 to 2019

Abstract The Canada Basin of the Arctic Ocean is considered the region of the world's open ocean most susceptible to the Ocean Acidification (OA). This study examines progression of OA in the Canada Basin, focusing on expansion of surface and subsurface aragonite undersaturated waters (USW). Surface USW thickness increased from 0 m in 2003 to 19 ± 2 m in 2019. This change was due to freshening until 2012, and then due to increased uptake of anthropogenic CO 2 after 2012. In the subsurface layer, USW thickness increased from 94 ± 6 m in 2003 to 136 ± 11 m in 2019. This change is primarily attributed to OA in upstream shelf regions, driven by increased CO 2 uptake and respiration, with some contribution from thickening in the Pacific Winter Water layer. The combined thickening of surface and subsurface USW layers increased the percentage of USW in the 0–250 m water column from 38 ± 3% in 2003 to 62 ± 5% in 2019. Because of the concurrent deepening of the water masses due to the enhanced Beaufort Gyre, the replacement of oversaturated water to USW occurred mostly at the subsurface layer below 190 m. The thickness of the oversaturated layer between surface and subsurface USWs remained almost unchanged. If Beaufort Gyre weakens in the future, it would bring subsurface USW shallower, potentially affecting marine life.