Weddell Sea deep water carries stable organic carbon, study finds

Deep water arriving from the north rises onto the shelf, where extreme cold and sea-ice formation make it denser. This heavy water sinks into the deep ocean and helps form Antarctic bottom water, the coldest, densest and one of the world’s most voluminous water masses.

Abundant marine life and active biological processes affect the carbon carried by that water. Understanding those effects matters because rising temperatures and declining sea ice could alter how the Southern Ocean stores and transports carbon. Reaching the remote, difficult-to-access Weddell Sea shelf also requires specialized vessels such as Polarstern.

How did researchers detect subtle changes?

The team combined complementary methods:

• Advanced instruments at the University of Oldenburg examined the molecular makeup and structure of complex organic mixtures.
• AWI’s MICADAS, or Mini Carbon Dating System, facility helped determine the age of the carbon.

The analyses found that intense biological activity and major changes in water masses left almost no detectable trace in the amount or composition of dissolved organic matter entering the deep ocean. The molecular properties of the deep-water reservoir remained largely conserved.

What explains the missing fingerprint?

The key was time. Dense waters remain on the Weddell Sea continental shelf for an unusually long 6 to 12 years while circulating beneath the large Filchner–Ronne Ice Shelf.

That residence period outlasted the five-year gap between the 2016 and 2021 expeditions. Microbes had enough time to completely break down and “remineralize” newly produced surface carbon before the water left the shelf. Remineralization here means converting that organic material into dissolved inorganic carbon.

This conversion helps explain why fresh, easily degraded carbon left no distinct molecular trace, even as the deeper organic-carbon pool remained stable.

What does this mean in a warming ocean?

Holtappels, an AWI biogeochemist, said: “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.”

The result does not mean every carbon form behaves the same way. Organic particles sinking toward the seabed and dissolved inorganic carbon in seawater may respond more strongly to changes in temperature, sea-ice cover and marine life. Because the study covered two expeditions in one deep-water formation region, it does not establish identical stability across the Southern Ocean.

Researchers will continue tracking how different carbon forms respond to environmental change. Antarctica InSync, an international initiative, brings scientists together to study the rapidly changing Antarctic environment.

The paper, “Conserved molecular properties of dissolved organic matter during Weddell Sea Deep Water formation,” appeared in Nature Communications in 2026. Its DOI is 10.1038/s41467-026-79040-4.