A "hidden ocean" of meltwater has been found off the coast of Antarctica

This is not a real "underground ocean" under the mainland. In this case, the "hidden ocean" is a figurative name: it refers to meltwater that has mixed with seawater and is hidden deep in the coastal ocean off the West Antarctic Peninsula.

The discovery is important for climate models. If glacial meltwater goes deeper than thought, it could have a stronger influence on ocean mixing, heat transport and the state of marine ecosystems.

Details

The West Antarctic Peninsula is one of the regions of Antarctica where the climate is changing particularly fast. Glaciers there are retreating, ice shelves are melting, and more freshwater is being released into the ocean.

It used to be more commonly thought that meltwater was mostly held near the surface. That makes sense: fresh water is lighter than salty seawater, so it can form the top layer.

But a new study shows that the picture is more complicated. Scientists studied water in three areas off the West Antarctic Peninsula: Cierva Cove, Paradise Bay and off Petermann Island. They analysed salinity, chemical composition and oxygen and hydrogen isotopes - natural 'markers' that tell us where the water came from.

These analyses showed that at depths of more than 50 metres, there are consistent signs of presalinisation. And the chemical data point to glacial meltwater, not melting sea ice or any other source of fresh water.

The proportion of meltwater at these depths was small, about 0.5-2 per cent. But for the ocean, even such percentages are important. Freshwater changes the density of seawater, which means it affects how the layers of the ocean mix with each other.

Scientists speculate that meltwater can enter the deep in several ways. For example, water coming out from under a glacier rises up as a light stream, but can then spread out to the side at depths where its density matches the surrounding water. Then currents and mixing carry it further along the coast.

Why it matters

The ocean off Antarctica looks like a complex system of layers. If fresh water is added to it, these layers can become more stable and less able to mix.

This has implications for heat. In Antarctica, warmer deep water can approach glaciers and ice shelves from below, accelerating their melting. If meltwater is trapped not only at the top but also at depth, it can change how heat circulates near the coast.

This is also important for ocean life. The mixing of the water determines how nutrients rise to the surface. And phytoplankton, krill, fish, penguins and other animals in the Antarctic food chain depend on it.

Background

Antarctic melting is usually represented through visible processes: icebergs breaking off, shrinking sea ice, retreating glaciers. But some of the change is happening invisibly - within the water column itself.

The new study shows just such a hidden process. Meltwater doesn't just spread out over the surface, but can go deep and be stored there as a subsurface reservoir.

The authors emphasise that this is not yet a complete picture of the entire Antarctic. The work is based on data from three coastal areas, so observations in different seasons and years are needed. This will help to understand how often such hidden layers of meltwater form and how they change with further warming.

Source

Research: Aaron Micallef et al, "Glacial meltwater is the primary source of subsurface freshening off the Western Antarctic Peninsula", Frontiers in Marine Science, 2026.