In recent decades, the relentless rise of atmospheric carbon dioxide has catalyzed a profound transformation in the chemistry of the world’s oceans. A groundbreaking study conducted by oceanographers at the University of Hawai‘i at Mānoa reveals new dimensions of this phenomenon, highlighting an accelerated acidification process occurring well below the surface in the North Pacific waters near Hawai‘i. Published in the prestigious Journal of Geophysical Research: Oceans, this research delves deeply into the vertical variability and drivers of subsurface ocean acidification, challenging prevailing assumptions and illuminating critical unknowns in marine carbon dynamics.
The ocean acts as a massive sink for atmospheric CO₂, absorbing roughly a quarter of human-generated emissions annually. While surface waters have long been studied to understand the impacts of acidification—characterized primarily by decreasing pH levels and carbonate ion concentrations—the subsurface layers have remained relatively enigmatic. Leveraging a rare and invaluable dataset collected over 35 years by the Hawai‘i Ocean Time-series (HOT) program, the research team embarked on a detailed analysis of carbon chemistry throughout the entire water column, extending nearly three miles deep at Station ALOHA, a remote site 60 miles north of O‘ahu.
Remarkably, their findings reveal that the intensification of ocean acidification is not confined to surface waters alone. Instead, acidification indicators exhibit even more rapid changes in subsurface layers. This vertical intensification was consistently observed across all measured parameters—pH, partial pressure of CO₂ (pCO₂), and carbonate alkalinity—marking a novel and consequential discovery. The data indicate that these deeper waters, naturally more acidic due to respiration processes and long water mass residence times, are becoming disproportionately altered by increasing anthropogenic carbon inputs and shifts in water mass properties.
Underlying this subsurface acceleration of acidification is a complex interplay of biogeochemical and physical processes. Organic matter derived from plankton and other surface organisms sinks and decomposes throughout the water column, releasing CO₂ which contributes additional acidification beyond what atmospheric equilibrium alone would predict. Furthermore, changes in temperature and salinity in these layers suggest the influence of advected waters sourced from higher latitudes in the North Pacific, where environmental and climatic changes have remodeled water mass characteristics before these currents deliver them to Hawai‘i’s open ocean.
The implications of these findings are profound for marine ecosystems. Many planktonic species and benthic organisms that inhabit subsurface environments rely on stable carbonate chemistry to maintain their physiological processes, including shell and skeleton formation. Enhanced acidification threatens to destabilize these processes, potentially leading to declines in population and shifts in community structure that cascade through the food web. These chemical changes also bear consequences for nutrient cycling and biological productivity, integral components of ocean health and global carbon flux.
Lucie Knor, the lead author and postdoctoral researcher at SOEST, emphasized the unexpected magnitude of this discovery, noting that previous global-scale studies had hinted at subsurface acidification but none had documented such uniform and rapid change across all indicators. The meticulous examination of nearly four decades of data from Station ALOHA not only affirms the continuation of this trend but also highlights regional sources and oceanographic processes that exacerbate the acidification cycle.
Compounding these challenges, the recent era has witnessed an unprecedented frequency of marine heatwaves and severe El Niño events. These anomalies disrupt ocean temperature profiles and circulation patterns, potentially interacting synergistically with acidification processes. The overlap of thermal stress and chemical stressors on marine organisms raises new concerns about ecosystem resilience and adaptability, underscoring the urgency for comprehensive monitoring and mitigation efforts.
A critical aspect of the study is the identification of the role regional-scale circulation and water chemistry transformations play in shaping the subsurface acidification trends observed at Station ALOHA. Christopher Sabine, a co-author and SOEST professor, reveals that the evolving environmental conditions in distant North Pacific source waters propagate downwards through ocean currents, highlighting a connectivity between remote oceanographic events and the local ocean environment surrounding Hawai‘i. This insight reshapes the framework through which ocean acidification is both studied and managed.
Looking ahead, the research team is advancing investigations to isolate and quantify the anthropogenic carbon component within the layered water column, discerning human-driven influences amid complex natural variability. This endeavor is crucial for refining predictive models and informing global climate mitigation strategies. Understanding the vertical distribution and temporal shifts in anthropogenic carbon uptake will enable more accurate assessments of the ocean’s capacity to serve as a carbon sink in a rapidly changing world.
This study’s robust observational approach, grounded in one of the longest continuous oceanographic datasets in existence, exemplifies the critical value of sustained monitoring efforts. Without persistent measurements extending beyond the surface, such comprehensive recognition of subsurface acidification trends would remain elusive. The enduring commitment of programs like HOT lays the foundation for informed ocean stewardship, ensuring that emerging threats to ocean health are detected and addressed in a timely manner.
In an era marked by accelerating environmental change, the revelation that subsurface waters are acidifying more rapidly than surface layers adds a new and alarming dimension to our understanding of the oceanic carbon cycle. It invites a reassessment of marine ecosystem vulnerability and reinforces the necessity for integrated scientific inquiry spanning chemistry, biology, and physical oceanography. These insights bear weighty ramifications for climate models, fisheries management, and conservation efforts not only in Hawai‘i but across global oceanic systems.
Ultimately, this research calls for intensified scientific focus on the vertical stratification of ocean acidification—a complex and dynamic facet of anthropogenic climate influence. As the ocean continues to absorb vast quantities of CO₂, tracking the intricate evolution of its chemistry through the depths will be essential to safeguard marine biodiversity and the broader planetary health that depends on it. The sustained acidification of subsurface waters stands as a clarion warning of the pervasive and multifaceted reach of humanity’s footprint beneath the waves.
Subject of Research: Not applicable
Article Title: Drivers and Variability of Intensified Subsurface Ocean Acidification Trends at Station ALOHA
News Publication Date: 27-Jun-2025
Web References:
- Journal of Geophysical Research: Oceans
- University of Hawai‘i School of Ocean and Earth Science and Technology (SOEST)
References:
Knor, L., Sabine, C., et al. (2025). Drivers and Variability of Intensified Subsurface Ocean Acidification Trends at Station ALOHA. Journal of Geophysical Research: Oceans. DOI: 10.1029/2024JC022251
Image Credits: Carolina Funkey
Keywords: Ocean acidification, subsurface acidification, Pacific Ocean, Station ALOHA, carbon cycle, biogeochemistry, marine heatwaves, El Niño, ocean circulation, Hawai‘i Ocean Time-series, anthropogenic carbon, marine ecosystems
