Southern Ocean mystery: Satellites reveal a strange glow

For years, scientists have puzzled over a strange patch of the Southern Ocean that glowed turquoise in satellite images. This region, often obscured by clouds and sea ice, stood out for its brightness – but the usual suspects didn’t fully explain it.

Coccolithophores, the microscopic algae known for their shiny calcium carbonate shells, are typically responsible for such optical effects. But they thrive in warmer waters, and this area was far too cold.

Bright signals in the Southern Ocean

Back in the early 2000s, researchers from Bigelow Laboratory for Ocean Sciences identified a band around Antarctica called the Great Calcite Belt.

The band was full of coccolithophores and known for its high levels of particulate inorganic carbon. Satellite data showed their shells were clearly responsible for the bright signals in that zone.

Yet farther south, another bright area appeared – just as reflective, but without an obvious cause. For years, scientists couldn’t explain it. Cloud cover, rough seas, and ice made it hard to observe directly.

Now, researchers have identified the likely source: a different kind of plankton altogether. Silica-rich diatoms, which build dense, reflective structures known as frustules, may be lighting up this part of the ocean.

“This work takes a broad brush to understand the biological and geochemical dynamics of this far-flung body of water in ways that haven’t been previously possible,” said study lead author Barney Balch.

From the ship to the Southern Ocean

That discovery came from a research cruise aboard the R/V Roger Revelle. The team sailed from Hawaii down to 60°S latitude, stopping at key points to gather data on water color, calcification rates, photosynthesis, and concentrations of both inorganic carbon and silica.

“Satellites only see the top several meters of the ocean, but we were able to drill down with multiple measurements at multiple depths,” said Balch. “We’ve never had such a complete suite of integrated measurements through the water column in this part of the ocean.”

The scientists also tracked the movement of eddies – spiraling currents that bring water from deeper regions up toward the surface.

Along the way, they measured both calcium carbonate and silica. These minerals both reflect light and also play a big role in how carbon gets stored in the ocean.

Explaining the reflective glow

By combining biogeochemical data, optical measurements, and microscopic imaging, the team mapped how the plankton community changes moving south. In the warmer, layered waters of the subtropics, dinoflagellates dominated.

In the Great Calcite Belt, coccolithophores took over. But in the cold, silica-rich waters south of the Polar Front, diatoms became the main players.

“This combination of complementary methods provides a ‘smoking gun” that the high levels of reflectance scientists have observed in satellite images south of the calcite belt can be explained by frustules,” said Balch.

These frustules, built by diatoms, reflect light in much the same way as coccolithophore shells – though it takes many more of them to produce the same optical effect. Their dense concentration in this region helps explain the brightness satellites have picked up for years.

Surprise discovery in cold waters

Even more unexpectedly, the researchers also found signs of coccolithophores in these colder waters. While not dominant, they were present.

“Surprisingly, though, the team also observed small concentrations of inorganic carbon, some amount of calcification happening – a first – and visual evidence of coccolithophores in the far southern waters,” Balch said.

That discovery suggests coccolithophores might tolerate colder conditions more than previously thought. It’s also possible that eddies act as a delivery system, bringing small populations northward into the Great Calcite Belt.

Rethinking carbon and satellites

These findings matter far beyond the Southern Ocean. This region is one of the planet’s most important carbon sinks. Understanding which organisms live where – and how they appear in satellite imagery – helps scientists build better climate models and more accurate ocean-monitoring tools.

Right now, satellite algorithms often struggle to distinguish between coccolithophores and diatoms. That could lead to misreadings of carbon activity and marine biology in key areas.

“We’re expanding our view of where coccolithophores live and finally beginning to understand the patterns we see in satellite images of this part of the ocean we rarely get to go to,” Balch said. “There’s nothing like measuring something multiple ways to tell a more complete story.”

The full study was published in the journal Global Biogeochemical Cycles.

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