Southern Ocean "Intermediate Water" Holds the Key to Earth’s Carbon Dioxide Changes

The scientific community has long believed that deep seawater was the main force in sequestering carbon dioxide (CO₂). However, a recent international study has shattered this understanding. A team led by Dr. Tapia and Prof. Sze Ling Ho at the Institute of Oceanography, National Taiwan University, successfully reconstructed 600,000 years of ocean history, pointing out that the long-overlooked "Antarctic Intermediate Water" (AAIW), located 500 to 1,500 meters below the sea surface, played a core role in the major transitions of atmospheric carbon dioxide over the past million years. This achievement has been published in the top international journal, Science Advances.

Climate Turning Point: The Mid-Brunhes Event
Earth's climate undergoes cycles of glacial and interglacial periods, with atmospheric carbon dioxide concentrations typically fluctuating accordingly. However, during the "Mid-Brunhes Event" (MBE) about 424,000 years ago, the interglacial carbon dioxide concentration suddenly jumped by an additional ~35 ppm compared to the previous cycle. In the past, mainstream hypotheses often attributed this to the reorganization of "deep water" in the Southern Ocean, but numerical models have never been able to completely and reasonably explain it. The NTU team's research found that the key mechanism is actually hidden in the shallower intermediate-depth ocean.

Discovering the Key: The Intermediate Water
Using sedimentary cores taken from the South Pacific—one of the world's most data-poor regions for core records—the research team accurately reconstructed the temperature and salinity changes of the Antarctic Intermediate Water over the past 600,000 years. The results revealed significant differences in the ocean state before and after the MBE:

Before the MBE (low CO₂ period): The AAIW exhibited “cold and fresh” characteristics. Not only did this low-temperature fresh water have a strong capacity to absorb CO₂, but the intense ocean stratification at the time acted as a barrier, sequestering carbon in the deep sea for extended periods.

After the MBE (high CO₂ period): The AAIW became warmer and increased in salinity, with its absorption capacity declining accordingly. Weaker stratification made it easier for carbon to be released back into the atmosphere, consistent with the upward trend of carbon dioxide concentrations at that time.

The Chain Reaction of Icebergs and Westerlies
What caused the shift in the properties of the intermediate water? The study points out that the key lies in "Antarctic icebergs" and the "Circumpolar Current." Before the MBE, the number of icebergs released by Antarctica far exceeded that of today, and the strength of the Antarctic Circumpolar Current (ACC) at that time was about 1.3 to 1.5 times greater than present. These icebergs were pushed northward by strong ocean currents, injecting large amounts of cold, fresh water into the AAIW formation region upon melting.

However, after the MBE, as the Southern Hemisphere westerlies shifted southward, limiting the melting of ice shelves and the northward drift of icebergs, the intermediate water became warmer due to a lack of cold, fresh water input. This finding challenges the view that the deep ocean alone is the dominant driver, emphasizing the core position of the intermediate ocean in regulating Earth's carbon cycle.

An Early Warning
Clarifying past carbon dioxide regulation mechanisms is a core task for predicting future climate change. As the current loss of Antarctic ice continues to accelerate, the ocean's ability to sequester carbon may further decline, which in turn could amplify the warming effect under high carbon emission scenarios. This study reveals an unexpected link between carbon dioxide and Antarctic ice melt, providing a crucial new perspective for future climate predictions.

Full text of the study: https://www.science.org/doi/10.1126/sciadv.ady4567