Seawater Carbonate Chemistry Along the Hawaiian‐Emperor Seamount Chain in the North Pacific

Abstract Below the aragonite saturation horizon (ASH), the aragonitic skeletons of deep‐sea reef building corals are more susceptible to dissolution. Ocean acidification is causing the ASH to shallow worldwide, threatening the health and future of deep‐sea coral reefs. Deep‐sea reefs in the North Pacific already exist at or below the ASH, making them particularly vulnerable to future ocean acidification. Here we analyze multiple years (2014–2019) of seawater chemistry data from the Hawaiian‐Emperor Seamount Chain (HESC), focusing particularly on intermediate depths (300–800 m) where deep‐sea reefs have been found. Intermediate water masses were identified across the HESC based on characteristic temperature, salinity, and density ranges. We then characterize the corresponding carbonate chemistry of each water mass. North Pacific Intermediate Water (NPIW) dominates at intermediate depths for most of our sites. However, the influence of Pacific Subpolar Intermediate Water (PSIW) increases north of 29°N. PSIW has a shallower ASH and lower oxygen conditions than NPIW. The increasing influence of PSIW may thus play a role in restricting reef development, partially explaining why deep‐sea reefs have not been found on seamounts north of Koko (34.8°N) in this region. In addition, topographic induced upwelling and temporal variability (seasonal, annual) have the potential to shift the ASH by >100 m depth. As ocean acidification progresses, chronic exposure to corrosive waters may negatively affect reef development and persistence. Characterizing the current carbonate chemistry conditions and variability is critical for informed decision making and management efforts to preserve these valuable ecosystems under future climate change.