Aragonite Saturation Horizon Variability Along North Pacific Seamounts and Implications for Deep‐Sea Coral Reefs Journal of Geophysical Research: Oceans

The 2014 discovery of living deep‐sea coral reefs along the Northwest Hawaiian Islands (NWHI) and lower Emperor Seamount Chain (ESC), despite the North Pacific's shallow aragonite saturation horizon (ASH) and high CaCO 3 dissolution rates, underscores the need to understand the local seawater chemistry where these reefs persist. We investigated seawater carbonate chemistry using discrete samples along NWHI and ESC from two cruises ∼1 year apart (08/26/21-09/26/21, 09/09/22-10/24/22). Across the two cruises, ASH depth difference ranged from 15 to 77 m. Since the Pacific ASH shoals by 1-2 m yr −1 , this long‐term trend cannot explain the magnitude of ASH change observed. Potential contributions from anthropogenic CO 2 and examining intermediate water mass changes from temperature‐salinity plots did not provide an explanation for the observed changes. Instead, ASH depth variability was primarily governed by localized biogeochemical processes, namely changes in intermediate water respiration and CaCO 3 dissolution. Indicators for dissolution (TA*) and respiration (AOU) suggest changes in ASH depth were driven by changes in dissolution at the northern‐ and southern‐most sites, whereas respiration exerted stronger control at central sites. Combining 2021 and 2022 data with data from 2014 to 2019 revealed high interannual ASH variability, by as much as >200 m. Deep‐sea coral reefs across the NWHI and ESC currently reside close to the ASH depth and likely experience interannual shifts between under‐ and supersaturation. As ocean acidification induced shoaling occurs alongside these interannual fluctuations, the frequency of undersaturation will be an important consideration for deep‐sea coral reef longevity.