In Antarctica's Weddell Sea, dense water that sinks to become Antarctic Bottom Water carries dissolved organic matter into the deep ocean almost unchanged, even though phytoplankton thrive on the continental shelf above. An international team including Germany's Alfred Wegener Institute (AWI) reported the result in Nature Communications on 10 November 2026. Because the water lingers on the shelf for 6 to 12 years, newly made organic matter is likely broken down by microbes before it can leave.
Why is Antarctic Bottom Water a gateway for carbon into the deep sea?
Antarctic Bottom Water is the coldest and densest water in the world's oceans, and one of the most voluminous water masses. When sea ice forms, most of the salt is expelled from the ice, leaving cold, salty, heavy water behind (Hokkaido University press release, 2022). In the Weddell Sea, deep water from the north rises onto the shelf, becomes dense in this way and sinks into the abyss. Carbon dissolved in the water sinks with it, so the shelf acts as an entry point for carbon heading into the deep ocean.
The paper's abstract notes that Antarctic Bottom Water is shrinking in volume as the climate changes, and that half of it forms in the Weddell Sea. Carbon that reaches the abyss is locked away from the atmosphere for centuries, so less bottom water could affect deep-sea carbon storage. The team set out to test whether the intense biological activity the water meets on the shelf before sinking changes the carbon it takes down.
What did the team measure?
The ocean's largest store of organic carbon is dissolved organic matter, a mixture of tens of thousands of compounds dissolved in seawater. The team examined it in seawater collected on two voyages of AWI's research icebreaker Polarstern.
| Item | Details |
|---|---|
| Samples | Seawater from the southern Weddell Sea collected by Polarstern (2016 and 2021 expeditions) |
| Molecular composition | Instruments at the Institute for Chemistry and Biology of the Marine Environment (ICBM), University of Oldenburg |
| Age of the carbon | AWI's MICADAS (Mini Carbon Dating System) facility |
| Fraction whose molecular properties were studied | Solid-phase extractable dissolved organic matter (SPE-DOM), according to the abstract |
MICADAS dates carbon using radiocarbon. Radiocarbon dating also featured in our story on hidden earthquake layers in the mud of the Kumano Trough. The first author, Dr Gonzalo V. Gomez-Saez, previously worked at AWI and now leads a DFG Emmy Noether research group at Ludwig-Maximilians-Universität München (LMU).
Why do the plankton blooms leave barely a trace?
Dense water in the Weddell Sea stays on the shelf for 6 to 12 years, during which it also circulates in the cavity beneath the Filchner–Ronne Ice Shelf. The team thinks that over this long stay, newly produced surface carbon is broken down by microbes and returned to dissolved inorganic carbon before the water leaves the shelf. This explanation was offered for a result that ran against expectations. With intense seasonal phytoplankton blooms on the shelf, fresh and easily degraded organic matter was expected to leave a molecular mark on the sinking water. When measured, the amount and composition of the dissolved organic matter heading for the deep sea had barely changed.
"Our study reveals that the molecular properties of dissolved organic matter are conserved during deep-water formation in the southern Weddell Sea."
— Gonzalo V. Gomez-Saez (LMU Munich, first author)
From this, the abstract concludes that the dominant signature of solid-phase extractable dissolved organic matter (SPE-DOM) in Weddell Sea Deep Water is already set before dense water from the shelf mixes with warmer water from off the shelf.
How much carbon passes from the Weddell Sea into Antarctic Bottom Water each year?
The abstract estimates that 0.155 ± 0.03 petagrams of carbon are transferred from the Weddell Sea to Antarctic Bottom Water every year. One petagram is 1 billion tonnes, so this comes to about 155 million tonnes. The estimate covers only what enters Antarctic Bottom Water from the Weddell Sea, not the whole ocean. The figure does not appear in AWI's press release and is found only in the abstract.
The abstract is about dissolved organic matter, but it does not make clear whether the 0.155 petagrams refers to dissolved organic carbon alone or includes other forms of carbon. We could not obtain the full paper, so we have not been able to check this. According to the abstract, the molecular properties were studied in the fraction that can be extracted from seawater by solid-phase extraction (SPE-DOM).
What changes when warming reaches the Antarctic shelf?
According to the press release, the answer depends on the form of carbon. Laid out by form, it looks like this.
| Form of carbon | What this study found | Response to warming (press release) |
|---|---|---|
| Dissolved organic matter | Amount and composition barely change across the shelf | May be largely unaffected |
| Organic particles sinking to the seabed | Not studied here | May respond more strongly |
| Dissolved inorganic carbon | New organic matter is likely remineralized into this form | May respond more strongly |
Particles and inorganic carbon may respond more strongly to changes in temperature, sea ice and marine life. On the dissolved organic matter reservoir, Dr Moritz Holtappels, a biogeochemist at AWI, says:
"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."
— Moritz Holtappels (biogeochemist, AWI)
How carbon moves through the Southern Ocean in the Antarctic winter will be studied on a Polarstern expedition to the Weddell Sea planned for 2028. The press release places the voyage within ongoing efforts such as Antarctica InSync, an international initiative studying the rapidly changing Antarctic environment. It will be co-led by Prof. Dr Alexander Haumann, an oceanographer at AWI and LMU Munich and a co-author of the paper.
How does this connect with Japan's Antarctic research?
Antarctic Bottom Water forms in only a few places. A 2022 press release from Hokkaido University lists the Ross Sea, the Weddell Sea and the Adélie Land coast as the three major formation regions. In 2013, a research group including Professor Keiichiro Ohshima of Hokkaido University, in a joint announcement with Tokyo University of Marine Science and Technology and the National Institute of Polar Research, reported the sea off Cape Darnley, 1,200 km east of Japan's Syowa Station, as a fourth formation region. The same release states that water originating from Antarctic Bottom Water makes up 30–40% of all the seawater on Earth.
The new study looked at what one of these formation regions, the Weddell Sea, sends down to the abyss in the form of organic matter. If the long residence time on the shelf is why the signal disappears, the question remains whether regions with different residence times give different results. Neither the abstract nor the press release says whether the organic matter carried by bottom water off Cape Darnley is equally unchanged. Another case where deep-sea communities were linked more closely to water masses than to depth appeared in our story on deep-sea sponges and corals off Alaska.