During the end of the last ice age huge amounts of CO2 which were formerly stored in the deep ocean upwelled in the Southern Ocean and were released to the atmosphere. This was one of the main causes of global warming at that time. However, it is assumed that not all of the stored carbon was released into the atmosphere, but that parts were directly transported northwards by the so-called Antarctic Intermediate and Mode Water into the East Pacific. So far, however, this has not been clearly proven. Using a combination of sediment core analyses and highly developed climate models, an international research team involving the Alfred Wegener Institute has now been able to fill this knowledge gap. This is important to better understand the transport of oceanic CO2 in the present day. The team presents its findings in the scientific journal Nature Communications.
The oceans are the most important sink for anthropogenic atmospheric CO2 during today's global warming. The Antarctic Intermediate Water (AAIW) and the Subantarctic Mode Water (SAMW) play a major part in this. These water masses are formed in Winter in the Southern Ocean, when cold, low-salinity water mixes with nutrient-rich water that floats upwards from the depths. On its way north, this water absorbs CO2 from the atmosphere, sinks and transports the CO2 into deeper water layers. “In a way, these water masses can be thought of as an 'oceanic tunnel' that connects the Southern Ocean with the equatorial ocean regions,” says lead author Cyrus Karas from the Universidad de Santiago de Chile. Together with researchers from Germany, Chile and Denmark, he has investigated how CO2 is absorbed and transported via this tunnel. “To do this, we analyzed sediment cores from the south-eastern Pacific Ocean that cover the last 30,000 years. A period in which atmospheric CO2 vastly rose before the industrial revolution.”
In combination with climate model simulations, the team was able to detect an increased carbon transport. “We found clear traces of carbon that previous studies could not clearly identify,” explains Frank Lamy, marine geologist at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). “This carbon originates from the deep sea of the Southern Ocean and has increasingly travelled with the AAIW and SAMW as far as Chile.” The intermediate and mode water originating from the south is therefore an essential component of the global ocean circulation and increases the export of carbon. The reconstructed current velocities, water temperatures and salinity of the AAIW and SAMW in combination with model simulations also showed that the westerly winds of the southern hemisphere significantly determine the properties of these water masses. These winds are important for the formation and northward transport of AAIW and SAMW, as they enhance upwelling and mixing in the Southern Ocean.
“Our study is the first that extensively reconstructs the evolution of AAIW/SAMW close to its formation area in the Southeast Pacific Ocean at the end of the last ice age,” summarizes Cyrus Karas. “Although the last deglaciation was different than the present climate change, this study helps to better understand the driving mechanisms of AAIW/SAMW and its capacity for transporting oceanic CO2.”
Original publication
Karas, C., Nürnberg, D., Lambert, F. et al. Enhanced deglacial carbon transport by Pacific southern-sourced intermediate and mode water. Nat Commun 16, 5245 (2025). https://doi.org/10.1038/s41467-025-60551-5