Antarctic Ocean Circulation Changes Linked to Historical Carbon Release

Research led by Dr. Huang Huang of the Laoshan Laboratory in Qingdao has uncovered how significant shifts in ocean circulation around Antarctica played a critical role in historical carbon release. Published in *Nature Geoscience*, the study analyzes data from sediment cores collected from the Southern Ocean, revealing insights into the behavior of Antarctic Bottom Water over the past 32,000 years. The research highlights that around 12,000 years ago, during the last Ice Age, the Southern Ocean's circulation was markedly different, impacting global carbon levels.

Dr. Marcus Gutjahr, a geochemist involved in the study, explains that during the last Ice Age, the Antarctic Bottom Water, which is now known for being cold and dense, did not extend as widely as it does today. Instead, much of the deep Southern Ocean was filled with carbon-rich waters that originated from the Pacific. This carbon-rich deep water had limited interaction with the surface, allowing it to remain isolated and keep atmospheric CO2 levels relatively low.

As Earth warmed and ice sheets began to retreat between approximately 18,000 and 10,000 years ago, the volume of Antarctic Bottom Water increased in two distinct phases, correlating with known warming events in Antarctica. This expansion was linked to reduced sea-ice cover and increased meltwater entering the Southern Ocean, resulting in more vertical mixing. Dr. Gutjahr notes that this mixing brought previously sequestered carbon closer to the surface, allowing it to escape into the atmosphere, significantly contributing to rising global temperatures.

The study challenges previous assumptions that the primary drivers of deep-water circulation changes were located in the North Atlantic. Instead, the findings suggest that the transition from glacial carbon-rich deep-water masses to the newly formed Antarctic Bottom Water was crucial in increasing atmospheric CO2 levels at the end of the last Ice Age. By understanding these historical shifts, researchers hope to better predict future climate scenarios, especially as Antarctic ice shelves continue to melt and influence global ocean dynamics.

Gutjahr emphasizes the importance of tracing historical changes in Antarctic Bottom Water to draw parallels with today’s climate conditions. He stresses that significant warming in deep waters around Antarctica over the past 50 years, which is occurring at a pace faster than much of the rest of the world's oceans, needs to be examined closely to grasp how it affects the ocean's capacity to absorb and release carbon dioxide. By studying paleoclimate data from sediment cores, scientists can gain insights that help improve future climate change projections and assess the potential for further Antarctic Ice Sheet mass loss. The research signifies a pivotal moment in understanding the intricate relationship between ocean circulation, carbon release, and climate change, providing valuable context as we face modern environmental challenges.