Vanished Without a Trace: No Organic Carbon Fingerprint Left by Antarctic Shelf Processes

Ocean currents move water around the globe in a vast circulation system. The Antarctic continental shelf plays an important part in this process. Deep water arriving from the north rises onto the shelf, where extreme cold and the formation of sea ice make it denser. This heavy water then sinks into the deep ocean, helping to form Antarctic bottom water, the coldest, densest and one of the most voluminous water masses in the world's oceans.

As the water crosses the shelf, it encounters abundant marine life and active biological processes. Researchers wanted to know whether these processes change the carbon carried by the water before it sinksinto the deep ocean. The answer matters because rising temperatures and declining sea ice could alter how the Southern Ocean stores and transports carbon.

The southern Weddell Sea shelf is an important place for the formation of Antarctic bottom water. Because it is remote and difficult to reach, studying this region depends on specialised research vessels such as the icebreaker Polarstern, operated by the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI).

The results were striking. Given the intense seasonal blooms of phytoplankton on the Antarctic continental shelf, scientists expected freshly produced “labile” (easy-to-degrade) organic matter to leave a clear molecular imprint on the water sinking into the abyss. Instead, the researchers found almost no detectable change in the amount or composition of the dissolved organic matter carried into the deep ocean. The processes taking place on the shelf had left no trace in this vast reservoir of carbon.

The key to this paradox lies in time. Dense waters remain on the Weddell Sea continental shelf for an unusually long period, between six and twelve years, during which they also circulate underneath the large Filchner–Ronne Ice Shelf. This extended stay in the ice shelf cavern gives microbes enough time to completely break down and “remineralize” any newly produced surface carbon into dissolved inorganic carbon before the water leaves the continental shelf.

“Our study reveals that the molecular properties of dissolved organic matter are conserved during deep-water formation in the southern Weddell Sea,” says Gonzalo Gomez-Saez. “Our findings highlight dissolved organic matter as a stable component of the regional Southern Ocean carbon cycle.”

“Dissolved organic matter is a huge and complex store of carbon in the ocean. By examining its molecules in detail, we can investigate how it changes as water moves through the Antarctic continental shelf,” says Prof. Dr Thorsten Dittmar.

The findings suggest that this large store of carbon is remarkably insensitive to change as the water crosses the Antarctic continental shelf. “Global warming is now reaching the Antarctic shelf seas. If the large reservoir of dissolved organic matter is scarcely affected by processes on the shelf, this suggests that this carbon reservoir could remain largely unaffected by climate change in this region,” says Dr Moritz Holtappels, biogeochemist at AWI.

However, not all forms of ocean carbon necessarily behave in the same way. While dissolved organic carbon remains stable, other forms such as organic particles sinking to the seabed or dissolved inorganic carbon in seawater may respond more strongly to changes in temperature, sea ice cover and marine life.

Researchers will continue to investigate how different forms of carbon respond to environmental change. One effort is Antarctica InSync, an international initiative bringing scientists together to study the rapidly changing Antarctic environment.

As part of this effort, oceanographer and study co-author Prof. Dr Alexander Haumann, also at AWI and LMU Munich, will co-lead a Polarstern expedition to the Weddell Sea in 2028. The team will investigate how carbon moves through the Southern Ocean during the Antarctic winter. These observations will help scientistsunderstand how climate change may affect the ocean's ability to transport and store carbon.

Gonzalo V. Gomez-Saez, Sarah K. Bercovici, Marit Renken, Sahithya P. B. Vemulapalli, Andreas Rogge, Hendrik Grotheer, F. Alexander Haumann, Moritz Holtappels, Thorsten Dittmar: Conserved molecular properties of dissolved organic matter during Weddell Sea Deep Water formation, Nature Communications (2026). DOI: 10.1038/s41467-026-79040-4

• Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI)
• Ludwig-Maximilians-Universität München
• Institute for Chemistry and Biology of the Marine Environment (ICBM) at the University of Oldenburg
• MARUM – Center for Marine Environmental Sciences at the University of Bremen
• University of Bremen's Institute of Environmental Physics
• Max Planck Institute for Marine Microbiology in Bremen
• National Oceanography Centre in Southampton, UK