Ocean acidification is primarily driven by anthropogenic CO₂ and is generally most pronounced in surface waters. However, the Sea of Japan, a semi-enclosed marginal sea characterized by deep-water formation, exhibits enhanced acidification in its deep waters. Using data taken from the Japan Meteorological Agency, a previous study [Chen, C.T.A.*, Lui, H.K.* Hsieh, C.H., Yanagi, T., Kosugi, N., Ishii M. and Gong, G.C. 2017. Deep oceans may acidify faster than anticipated due to global warming, Nature Climate Change; 7:890-894] showed that weakened deep-water ventilation prolonged the residence time of deep waters, leading to declines in dissolved oxygen (DO) and pH between 1965 and 2015. This deoxygenation reflects a reduced oxygen supply resulting from weakened ventilation, coupled with continued oxygen consumption through microbial respiration. Consequently, apparent oxygen utilization (AOU) increased while pH decreased over time. In this study, we extend the observational record to 2023 and show that these long-term trends have persisted, with continued increases in AOU and decreases in DO and pH. Notably, the rates of deoxygenation at depths of 1,000 m and 1,500 m appear to have accelerated since approximately 2010, although the underlying mechanism remains unclear and warrants further investigation.
 

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