A multinational group of researchers using undersea vehicles in the waters off Greece’s Santorini has discovered an interconnected series of subsea pools containing high concentrations of carbon dioxide. The pools, which get their distinctive color from opal particles, may hold answers to questions related to deepsea carbon storage as well as provide a means of monitoring the Santorini volcano for future eruptions.

“The volcanic eruption at Santorini in 1600 BC wiped out the Minoan civilization living along the Aegean Sea. Now these never-before-seen pools in the volcano’s crater may help our civilization answer important questions about how carbon dioxide behaves in the ocean,” said Dr Richard Camilli of the Woods Hole Oceanographic Institution, a team member and the lead author of a paper published online in the journal Scientific Reports.

Dr Camilli and co-authors used a series of underwater exploration vehicles to characterize these shallow subsea pools, located within the Santorini volcanic caldera of the Southern Aegean Sea.

These 3 to 16 foot (1 – 5 m) diameter pools, which the scientists call the Kallisti Limnes (translation from Ancient Greek: Most Beautiful Lakes), are located at a depth of about 820 feet (250 m).

“We’ve seen pools within the ocean before, but they’ve always been brine pools where dissolved salt released from geologic formations below the seafloor creates the extra density and separates the brine pool from the surrounding seawater,” Dr Camilli said.

“In this case, the pools’ increased density isn’t driven by salt – we believe it may be the carbon dioxide itself that makes the water denser and causes it to pool.”

“The volcanic complex of Santorini is the most active part of the Hellenic Volcanic Arc. The region is characterized by earthquakes caused by the subduction of the African tectonic plate underneath the Eurasian plate. During subduction, carbon dioxide can be released by magma degassing, or from sedimentary materials such as limestone which undergo alteration while being subjected to enormous pressure and temperature.”

The team found that the pools have a very low pH, making them quite acidic, and therefore, devoid of calcifying organisms. But silica-based organisms could be the source of the opal in the pool fluids.

Until the discovery of the Kallisti Limnes pools, the assumption has been that when carbon dioxide is released into the ocean, it disperses into the surrounding water.

“But what we have here is like a ‘black and tan,’ where the two fluids actually remain separate with the denser carbon dioxide water sinking to form the pool,” Dr Camilli said.

“Our finding suggests carbon dioxide may collect in the deepest regions of the crater. It would be interesting to see.”

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Camilli R. et al. 2015. The Kallisti Limnes, carbon dioxide-accumulating subsea pools. Scientific Reports 5, 12152; doi: 10.1038/srep12152