The Tonga eruption buried deep-sea life in the ash

In April 2022, a team of scientists went on a research vessel into the Lao Basin near Tonga to study the animals that live around deep-sea hydrothermal vents. But when they lowered a remotely operated vehicle (ROV) down the vent to search for the critters, they found the seabed, usually a hard basalt surface, covered in sediment. They could see some snails and clams.

„It was like a snow-covered landscape,” he said Roxanne BeinartA marine microbial ecologist at the University of Rhode Island, he was on the trip.

Beinart and his colleagues suspected they were looking at ash from the January 2022 eruption of the Hanga volcano, one of the most powerful eruptions ever recorded. The ash completely altered the ecosystem, killing vulnerable molluscs.

The team recognized that they now have a rare opportunity to document the effects of volcanic eruptions on marine ecosystems. They have published Initial findings Inside Communication Earth and Environment It intends to monitor the recovery of these ecosystems over time.

„This is a real opportunity to understand and study the impacts of a large eruption—we understand where what happened, where the processes are, we know the time scales involved—and to understand the impacts on the seafloor,” he said. Isobel YeoA volcanologist at the UK’s National Oceanographic Centre, was not involved in the study.

Gray Avalanche

Over the course of 24 days, the scientists used an ROV to survey six hydrothermal vent fields and take sediment samples. „You can quickly and easily see that it’s full of glass,” he said, under the microscopes on board. Shawn Arellano, A marine ecologist at Western Washington University co-authored the study. Fine grains of glass are characteristic of volcanic ash.

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To see how deep the ash was, the team improvised a ruler using a long, T-shaped metal rod with the ROV’s claws and added colored markings every 7.6 centimeters (3 inches). They found that ash was up to 1.5 meters (5 feet) deep in places near the volcano. And 96 kilometers (60 miles) away, ash was still 0.5 meters (2 feet) deep at one site.

After the explosion, ash may have fallen from the sky and drowned in water. As it lands on the volcano’s sunken slopes, the researchers hypothesize that it will pick up sediment that thickens and accelerates, much like an ice avalanche would on land. The flow was able to „roar into the deep sea”. Mike ClareA marine geologist at the National Oceanographic Center who was not involved in the study.

The ash had to travel over undulating terrain to reach the hydrothermal vents – a testament to how powerful the currents are. „One of the surprises of the study is that despite this irregular seabed relief, the flows were able to cover this distance,” Clare said.

Buried alive

„That population was wiped out.”

All that ash had a big impact on marine ecosystems.

The scientists were aware of an expedition in 2019 that visited the same hydrothermal vent sites, so they compared footage from that expedition to footage from their ROV. The contradiction was stark. Some of the animals crawling in their videos, such as crabs and crabs, were unaffected by the ash, but the snails and clams they came to study, many of them endangered or vulnerable species, took a hit. „That population was decimated,” Beynard said.

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Using footage from an expedition in 2019, researchers can compare the eruption’s impact on marine life. The two panels on the left were taken before the eruption in 2019, and the two on the right were taken after the eruption in 2022.. Credit: Beynard et al., 2024,, CC BY 4.0

Because the molluscs are less mobile, Beinart and his colleagues suspect they are more affected. Mussels, for example, extend strong filaments that attach to hard surfaces so they can „pull themselves like Spiderman,” Arellano said. The metabolism of these organisms requires a symbiotic relationship with bacteria, and that relationship requires a lot of oxygen. When the ash fell on these animals, their oxygen demand exceeded their ability to escape from under the ash, and they suffocated.

Researchers will return to the region in 2026 to monitor changes in hydrothermal vent sites. „We’re focused on understanding the recovery of these systems and the dynamics of how they might cope,” Beinard said.

Clare noted that scientists know from other areas of volcanic activity that this ecosystem will recover, but how quickly it will happen and how it will work are unknown. „While this study is very valuable and useful, I suspect that its fundamental value will emerge in the coming decade when we start to look back and see how the seafloor is changing,” he said.

– Andrew Chapman (@andrew7chapman), science writer

Quote: Chapman, A. (2024), Tonga eruption buried deep-sea life in ash, EOS, 105, Published on 11 July 2024.
Text © 2024. Authors. CC BY-NC-ND 3.0
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