Recently, Sifan Gu, a tenure-track Associate Professor of School of Oceanography, Shanghai Jiao Tong University published a research article titled “Reduced Antarctic Bottom Water overturning rate during the early last deglaciation inferred from radiocarbon records” in Nature Communications.

Combining radiocarbon records with model simulations, we find that Antarctic Bottom Water (AABW) overturning was reduced during the Heinrich Stadial 1 (HS1, ~17.5-14.7ka), challenging the conventional paradigm of an intensified AABW during this period.

The rapid CO2 rise during the early deglaciation is often linked to enhanced ventilation by intensified Antarctic Bottom Water (AABW) overturning. The recorded radiocarbon ventilation seesaw during the early deglaciation, which describes improved Southern Ocean and reduced North Atlantic abyssal radiocarbon ventilation, has been interpreted as intensified AABW and reduced North Atlantic Deep Water convections. However, abyssal radiocarbon records also reflect changes in surface reservoir ages and interior water mass mixing. Using isotope-enabled simulations, we show that this seesaw results from weakened AABW overturning and decreased Southern Ocean surface reservoir age. With AABW occupying abyssal ocean, weakened AABW overturning increases transit time, with the magnitude increasing northward. This transit time increase outpaced the declining  induced Southern Ocean surface reservoir age decrease in abyssal North Atlantic, but not in abyssal Southern Ocean, producing radiocarbon ventilation seesaw. Our results suggest sluggish deep water overturning from both poles during the early deglaciation.

This work is supported by National Key R&D Program of China, Science and Technology Innovation Project of Laoshan Laboratory, Chinese National Science Foundation, and Shanghai Frontiers Science Center of Polar Science.

Link to the article: https://www.nature.com/articles/s41467-025-62958-6

Figure. 1 Simulated deglacial evolutions in C-iTRACE compared with proxy records. (A) North Atlantic Deep Water (NADW) (black) and Antarctic Bottom Water (AABW) (red) overturning rates (unit: Sv) in C-iTRACE. (B) Simulated abyssal (below 3000m average) radiocarbon ventilation age (unit: yr) on the Iberian Margin (solid line) compared with radiocarbon records (dots). (C) Simulated abyssal (60°W-20°E, south of 60°S, below 3000m average) radiocarbon ventilation age (unit: yr) in the Southern Ocean (solid line) compared with radiocarbon records (dots).  (D) Simulated Southern Ocean surface reservoir age (Rage) (unit: yr) compared with records (dots).  (E) Simulated ideal age (iage) of the deep Iberian Margin (blue) and deep Southern Ocean (red) (unit: yr). (F) Sum of the Southern Ocean surface reservoir age anomaly and ideal age (dash) anomaly compared with simulated radiocarbon ventilation age anomaly (solid) of the abyssal Iberian Margin (blue) and abyssal Southern Ocean (red) (unit: yr). (G) Simulated AABW% indicated by the idealized dye tracer (Methods) in the abyssal Iberian Margin (blue) and deep Southern Ocean (red).