FY 2025

Seawater carbonate chemistry along the Hawaiian-Emperor Seamount Chain in the North Pacific

Hicks, T.L., K.E.F. Shamberger, E.B. Roark, A. Baco-Taylor, and R.A. Feely

J. Geophys. Res., 130(5), e2024JC021750, doi: 10.1029/2024JC021750, View open access article at Wiley (external link) (2025)

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.

Plain Language Summary. Reef-building deep-sea corals are facing new threats from climate change, including changes in ocean chemistry. These corals are especially vulnerable to increasing ocean acidity (i.e., ocean acidification). In the ocean, the aragonite saturation horizon (ASH) marks the depth below which waters become increasingly corrosive and deep-sea corals' aragonite skeletons may dissolve. As ocean acidification progresses, the ASH is moving shallower, which can endanger the health and survival of these deep-sea reefs. Our study examines water chemistry near deep-sea coral reefs on seamounts in the Hawaiian-Emperor Seamount Chain in the North Pacific from 2014 to 2019. We observed spatial changes in water chemistry, particularly at depths where deep-sea reefs have been found, that include shallower ASH and lower oxygen levels in more northern waters, which could limit where reefs can grow. Additionally, seasonal changes may also shift the ASH depth by over 100 m, affecting local reef conditions. Understanding these chemical conditions and how they can change is crucial for managing and protecting these important deep-sea coral reef ecosystems as climate change and ocean acidification continue.