Opalescent pools forming at 250 meters depth have recently been discovered in the waters off Greece’s Santorini, the site of the second largest volcanic eruption ever recorded. The series of interconnected, iridescent white pools have high concentrations of carbon dioxide (CO2), and could hold clues to questions about deep sea carbon storage. They may also provide a way of monitoring the volcano for future eruptions.

Said lead author Rich Camilli, of Woods Hole Oceanographic Institution:

“The volcanic eruption at Santorini in 1600 B.C. 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.”

Working in the region in July 2012, Camilli and colleagues from the University of Girona, National and Kapodistrian University of Athens, Institut de Physique du Globe de Paris, and Hellenic Centre for Marine Research (HCMR), used a collection of sophisticated underwater exploration vehicles to locate and characterize the pools, which they call the Kallisti Limnes, from ancient Greek for “most beautiful lakes.”

A previous volcanic crisis in 2011 had led the researchers to begin their investigation at a site of known hydrothermal activity within the Santorini caldera. During an initial survey of a large seafloor fault, the University of Girona’s autonomous underwater vehicle (AUV) Girona 500 identified subsea layers of water with unusual chemical properties.

After the AUV reconnaissance, the researchers sent down HCMR’s Thetis human occupied vehicle.

[caption id=“attachment_78638” align=“aligncenter” width=“720”]uav girona The crew from the R/V Aegaeo ready the University of Girona’s autonomous underwater vehicle (AUV) Girona 500. Researchers used the AUV, equipped with the TETHYS mass spectrometer (shown strapped to hull), to identify subsea layers of water with unusual chemical properties. HCMR’s Thetis sub is shown to the left and the island of Santorini is in the background.
Credit: Photo by David Ribas, University of Girona[/caption]

The submersible’s crew used robotic onboard chemical sensors to track the faint water column chemical signature up along the caldera wall where they discovered the pools within localized depressions of the caldera wall. Last, the researchers sent a smaller remotely operated vehicle (ROV), for sampling the pools’ hydrothermal fluids.

“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,” said Camilli. “In this case, the pools’ increased density isn’t driven by salt - we believe it may be the CO2 itself that makes the water denser and causes it to pool.”

The researchers concluded that the pools have a very low pH, making them quite acidic, and so, devoid of calcifying organisms. But, they believe, silica-based organisms could be the source of the opal in the pool fluids.

Befire the discovery of these CO2-dense pools, the thinking had been that when CO2 is released into the ocean, it disperses into the surrounding water.

“But what we have here,” says Camilli, “is like a ‘black and tan’ - think Guinness and Bass - where the two fluids actually remain separate”, with the denser CO2 water sinking to form the pool.

“Our finding suggests the CO2 may collect in the deepest regions of the crater. It would be interesting to see,” Camilli said, noting it does have implications for carbon capture and storage.

Sub-seafloor storage is becoming more accepted as a potential means of reducing heat-trapping CO2 in the atmosphere, and lessening the acidifying impacts of CO2 in the ocean. But, before fully embracing the concept, society needs to understand the risks involved in the event of release.

Photo: courtesy of WHOI

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