Decadal change in deep-ocean dissolved oxygen in the North Atlantic Ocean and North Pacific Ocean

Declining dissolved oxygen concentrations are documented at upper and mid ocean depths, but less is known about the deep ocean. Long time-series measurements of dissolved oxygen analyzed with Winkler titration over several decades reveal regional differences at six stations in the abyssal North Atlantic Ocean and North Pacific Ocean. A significant decline in dissolved oxygen was evident at two stations in the northeast Pacific Ocean at 4000 – 4200 m depth (Stations PAPA and M). A similar decreasing but insignificant trend was recorded in the Arctic region of the North Atlantic Ocean (HAUSGARTEN). However, there was no significant decrease in dissolved oxygen at two temperate stations in the North Atlantic Ocean (PAP, BATS) and at one tropical station in the central North Pacific Ocean (ALOHA) all at similar depths > 4000 m over similar time periods. Continued long time-series observations will be needed to monitor global deep ocean processes and the impact of changing climate. We compare these rare long-term observations with model estimations from historical (1850-2014) and projected (2015-2100) forcing under a continued high greenhouse gas emission scenario. • Here we examined deep-ocean time-series for long term changes in dissolved oxygen. • Long-term declines in dissolved oxygen were detected at two sites in the Pacific. • No change was found at others sites in the Pacific Ocean and Atlantic Ocean. Dissolved oxygen concentrations in the global ocean have been declining over the past several decades. This decline is most evident in the surface to mid-depths. At depths > 4000 m we report a significant decrease in dissolved oxygen over the past three decades at two long time-series stations in the northeast Pacific Ocean and a non-significant decline in the Arctic region of the North Atlantic Ocean. However, there was no significant decline in dissolved oxygen at three other long time-series stations, one in the central North Pacific Ocean and two in the temperate North Atlantic Ocean over the same period at similar depths. Long term monitoring at such stations is critical to interpreting biogeochemical changes in the deep ocean.