Two Decades of Ocean Acidification in the Surface Waters of the Beaufort Gyre, Arctic Ocean: Effects of Sea Ice Melt and Retreat From 1997–2016

Anthropogenic CO2 uptake drives ocean acidification and so decreases the calcium carbonate (CaCO3) saturation state (Ω). Undersaturation of surface water with respect to aragonite‐type CaCO3 was first reported for 2008 in the Canada Basin, preceding other open ocean basins. This study reveals interannual variation of Ω in the surface Canada Basin before and after 2008. A rapid decrease of Ω occurred during 2003–2007 at a rate of −0.09 year−1, 10 times faster than other open oceans. This was due to melting and retreat of sea ice, which diluted surface water and enhanced air‐sea CO2 exchange. After 2007, Ω did not further decrease, despite increasing atmospheric CO2 and continued sea ice retreat. A weakened dilution effect from sea ice melt and stabilized air‐sea CO2 disequilibrium state is the main reason for this stabilization of Ω. Aragonite undersaturation has been observed for the last 11 years, and aragonite‐shelled organisms may be threatened. Plain Language Summary Anthropogenic CO2 absorbed by the ocean results in decreases in pH and the calcium carbonate saturation state of seawater (Ω). This is ocean acidification (OA). In the Arctic Ocean, the most susceptible region in the world to OA, melting sea ice and anthropogenic CO2 absorption promote both OA and the decline of Ω. This study used observations collected over the last two decades to estimate Ω interannual variation in the surface waters of the Beaufort Gyre and to understand the effects of different factors: atmospheric CO2, air‐sea CO2 disequilibrium, seawater temperature, and dilution by sea ice melt. The results showed that Ω decreased rapidly from 2003 to 2007 at a rate ~10 times faster than other open ocean basins, resulting in undersaturation of aragonite‐type calcium carbonate. After 2007 Ω was stable, rather than continuously decreasing with increasing atmospheric CO2. The rapid decrease and then stabilization of Ω before and after 2007 are related to strong and then weakened dilution effects from sea ice melting. In addition, enhanced and then stabilized air‐sea CO2 exchange conditions contributed to variation of Ω. Aragonite undersaturation has now been observed for the last 11 years which may threaten shelled organisms residing in the Canada Basin. Key Points Aragonite saturation state (Ω) of surface water declined rapidly at −0.09 year−1 from 2003 to 2007, 10 times faster than other open oceans Aragonite undersaturation has been observed for the last 11 years, beginning in 2006