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 CaCO3 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 CO2 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 CaCO3 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.
Plain Language Summary
In 2014, thriving deep-sea coral reefs were found in the Pacific near Hawaii and the Emperor Seamounts, where conditions were thought too acidic for reef development. To understand how these reefs persist, we studied the carbonate chemistry of this region during two research cruises (2021 and 2022). We found that the depth at which seawater becomes corrosive for coral skeletons (aragonite saturation horizon, ASH) changed far more from year to year than the gradual Pacific trend of 1–2 m yr−1. Human sourced carbon dioxide and natural shifts in ocean water masses do not explain the observed large changes. Instead, local biological and chemical processes, respiration and the dissolution of calcium carbonate, played a major role. When comparing seawater chemistry from 2014 to 2022, the ASH fluctuated even more dramatically than the predicted trend of 1–2 m yr−1. While climate change continues to drive corrosive deep waters closer to the surface, processes that occur on shorter timescales, local respiration and dissolution, can similarly expose deep sea coral reefs to corrosive conditions that can impact their longevity. Therefore, short term and local scale processes can affect long term acidification trends, making gradual shoaling harder to observe in short term data.
Kassem S. F., Shamberger K. E. F., Savoie A. M., Hicks T. L., Feely R. A., Baco A. R. & Roark E. B., 2026. Aragonite saturation horizon variability along North Pacific seamounts and implications for deep-sea coral reefs. Journal of Geophysical Research: Oceans 131(7): e2025JC023926. doi: 10.1029/2025JC023926. Article.
