Researchers at National Taiwan University and partner institutions, led by research associate Dr. Raul Tapia and associate professor Sze Ling Ho at the Institute of Oceanography, have uncovered new evidence that Antarctic Intermediate Water (AAIW) — a distinct layer sitting 500–1,500 meters below the ocean surface — played a pivotal role in a major atmospheric CO2 transition that occurred roughly 500,000 years ago. The findings, published in Science Advances, challenge the prevailing view that changes in the deepest layers of the Southern Ocean alone drove this shift, and point instead to intermediate-depth circulation as a previously underestimated regulator of Earth’s carbon cycle.
Background: A Turning Point in Earth’s Climate
Earth underwent cold glacial and warm interglacial periods, accompanied by ~100 parts per million (ppm) of glacial-interglacial CO2 shifts. Around 424,000 years ago, the interglacial atmospheric CO2 levels shot up by ~35 ppm from the previous interglacial period–a shift known as the Mid-Brunhes Event (MBE). Scientists have long debated the cause of this step-change. The leading hypothesis focused on reorganization of bottom-water formation in the Southern Ocean, but climate model studies suggest that deep-water changes alone cannot account for the full CO2 increase. This new study proposes that the story is considerably more complex.
Key Findings
Using sediment cores retrieved from the South Pacific — one of the most data-sparse regions of the global ocean — researchers from the Institute of Oceanography at National Taiwan University and their international partners from Germany and Norway reconstructed the temperature and saltiness of Antarctic Intermediate Water (AAIW) over the past 600,000 years.
Their findings reveal a striking contrast on either side of the MBE (Fig. 1a, b). Before the MBE, AAIW was colder and fresher, enhancing its capacity to absorb more CO2 from the atmosphere. Strong ocean stratification helped lock that carbon away deep in the ocean interior.After the MBE, AAIW became warmer and saltier, reducing its CO2 uptake. Weaker stratification meant carbon could escape back to the atmosphere — coinciding directly with rising interglacial CO2 levels.
The Iceberg Connection
What made AAIW colder and fresher before the MBE? The researchers point to Antarctica’s icebergs (Fig. 1c). Proxy evidence indicates that, prior to the MBE, Antarctica discharged more icebergs than it does today. As these icebergs drifted northward and melted, they delivered large volumes of freshwater into the AAIW formation zone. A stronger Antarctic Circumpolar Current (ACC) — estimated to have been 130–150% more vigorous than today — amplified this effect by transporting icebergs and their meltwater farther north.
Together, these processes cooled and freshened the intermediate water, dramatically boosting its ability to draw down atmospheric CO2. After the MBE, a southward shift in the Southern Westerly Winds reduced upwelling of warm Circumpolar Deep Water onto Antarctic continental shelves, limiting ice-shelf melting and iceberg calving. A weaker ACC transported less freshwater northward. The result: warmer, saltier intermediate water with a diminished appetite for CO2.
Why It Matters
Understanding the mechanisms that controlled atmospheric CO2 concentration in the past is key to predict future change. This study challenges the long-held idea only deep Ocean drove major CO2 shifts, showing that mid-depth waters played a crucial–and largely overlooked–role.
The research also uncovers a surprising link between CO2 and Antarctic ice melt and iceberg activity. In future, as Antarctic ice loss accelerates, the ocean may become less effective at locking away carbon–potentially amplifying future warming under high carbon emission.
Further reading: Tapia, R., Ho, S.L., Nürnberg, D., Meckler, A.N., Iizuka, Y., Tiedemann, R. (2026). “Shifts in Antarctic Intermediate Water properties coincide with atmospheric CO2 rise across the Mid-Brunhes Event.” Science Advances, 10.1126/sciadv.ady4567
Figure 1. (a) Since the MBE, thermocline temperatures (blue line) have steadily warmed. (b) This results in a fourfold weakening of vertical stratification after the MBE. Because colder water can hold more CO2, and its ability to retain it at depth depends on stratification, cooler and more stratified conditions before the MBE favored greater sequestration and retention of CO2 than after. (c) Schematic of the freshwater pathway from Antarctic icebergs to the AAIW formation zone. Iceberg meltwater lowers sea-surface salinity and density, shifting the zone of intermediate-water formation northward and expanding its atmospheric exposure—thereby enhancing CO2 uptake.
