演講主題	The Ocean's Hidden Reservoir: How Antarctic Intermediate Water Locks Away Atmospheric CO2
演講日期	2026年8月4日(星期二)
演講時間	自中午12:00～至下午13:00
主講者	國立台灣大學海洋研究所　Raúl Tapia　助理研究員
演講地點	綜合一館GH1 414教室
主辦單位	C3.地球科學研究所
備註	Why did atmospheric CO2 remain relatively stable during some of Earth’s warmest climates, while rising rapidly during others? The Southern Ocean—and in particular Antarctic Intermediate Water (AAIW), the largest modern oceanic reservoir of anthropogenic CO2—may hold part of the answer. Yet the processes that control how effectively AAIW stores carbon over different timescales remain poorly understood. In this talk, we will explore how changes in upper-ocean structure influenced the ability of AAIW to isolate carbon from the atmosphere. Using a 600,000-year record from the South Pacific, a major region of AAIW formation, we will follow how interactions between the upper ocean and intermediate waters shaped carbon storage across successive climate cycles. We begin with a major reorganization around the Mid-Brunhes Event (~430 ka). Before this transition, a colder, fresher AAIW and a strongly stratified upper ocean appear to have favored long-term carbon storage. Afterwards, warmer, saltier intermediate waters and weaker stratification likely reduced this capacity by both lowering CO2 solubility and shortening the isolation of AAIW from the atmosphere. We then zoom in on Marine Isotope Stage 11, one of the warmest interglacials of the past 500,000 years. Despite conditions that should have favored unusually high atmospheric CO2, concentrations remained close to pre-industrial levels. Was the Southern Ocean acting as a carbon buffer? We examine evidence that enhanced upper-ocean stratification allowed AAIW to retain substantially more carbon than today, supported by changes in subsurface carbonate chemistry and simple carbon-cycle modelling suggesting an additional 10–20 ppm of atmospheric CO2 was sequestered. Together, these records invite us to reconsider AAIW not as a passive water mass, but as a dynamic regulator of atmospheric carbon whose efficiency depends on upper-ocean stratification. As the modern Southern Ocean continues to warm and stratify, these past examples provide a valuable opportunity to discuss how one of Earth’s largest carbon reservoirs may respond to future climate change.