Abstract
The Southern Ocean is one of the rapidly acidifying regions globally, yet direct observational constraints on its carbonate chemistry remain scarce. Here, we combine shipboard measurements of pH and aragonite saturation state (Ωarag) from the summer 2015 Chinese National Antarctic Research Expedition with reconstructed wintertime conditions to characterize acidification in Prydz Bay. We report the first observation-based occurrence of surface aragonite undersaturation (Ωarag < 1.0) in the northern basin—emerging nearly two decades earlier than model projections. Surface Ωarag undersaturation is primarily driven by accelerated uptake of anthropogenic CO2 and shoaling of the aragonite saturation horizon, fueled by persistent upwelling of CO2-rich Circumpolar Deep Water. In the ocean interior, organic matter remineralization, CaCO3 dissolution, and continued anthropogenic CO2 intrusion further lower pH and Ωarag. Our results demonstrate that Prydz Bay is at the forefront of Southern Ocean acidification, highlighting the urgent need to incorporate both anthropogenic and natural biogeochemical feedbacks into high-resolution models to better predict future ecological impacts.
Plain Language Summary
The Southern Ocean is acidifying faster than most of the oceans on Earth, but most of what we know comes from computer models rather than direct observations. In Prydz Bay, East Antarctica, we combine shipboard data collected in summer 2015 with reconstructed winter conditions to track changes in seawater acidity (pH) and aragonite saturation (Ωarag), a key factor for shell-building marine life. We discover that seasonal expansion of low pH and nearly undersaturated aragonite led to the unforeseen early emergence of surface aragonite undersaturation in the northern basin. Strikingly, conditions that models predicted would not occur until around year 2038 have appeared nearly 20 years earlier. This rapid shift is driven by increased uptake of atmospheric CO2 and the rising influence of deep CO2-rich waters. In deeper layers, ongoing CO2 intrusion and the breakdown of organic material make the water even more acidic. These findings reveal that Prydz Bay is on the front lines of ocean acidification, underscoring the urgent need to reduce CO2 emissions and to better understand natural feedbacks in order to protect Antarctic marine ecosystems.
Key Points
- Surface near-undersaturated Ωarag emerged in Prydz Bay during summer, nearly two decades ahead of model predictions
- Enhanced atmospheric CO2 uptake and rapid shoaling of the aragonite saturation horizon dominate surface Ωarag undersaturation
- Persistent upwelling of CO2-rich deep water combined with anthropogenic CO2 penetration shoal the Ωarag saturation horizon
Zhang S., Wu Y., Zhong W., Chen L. & Qi D., 2025. Anthropogenic forcing and upwelling accelerate aragonite undersaturation in the Prydz Bay, East Antarctica. Journal of Geophysical Research: Oceans 130(11): e2025JC023039. doi: 10.1029/2025JC023039. Article (access restricted).
