Life Erupting From Volcanic Mud Soup

In 2022, Tonga's Hunga Volcano erupted with devastating impact on Tongan island communities and the surrounding marine environment. It was the most powerful eruption on the planet in more than 100 years.

In the wake of the eruption, the seafloor around this submarine volcano, located 65 km north of Tonga's main island, was blanketed with volcanic ash. This ash is so fine that, when disturbed, it can behave more like soup than sediment. It may even be liquefied and remobilised by earthquakes, continually resurfacing the seafloor.

The ash is flour-like, and these fine volcanic deposits are easily reworked by seafloor currents. When undisturbed, it settles as soft, uncompacted mud and silt into low-lying areas of the seafloor. It also settles into depressions where thick ponds of mud may form. Scattered boulders, launched from the volcano by the eruption and rafted on seafloor sediment flows, sit on top of these mud plains like islands in a dark grey sea.

However, what might look barren is, in fact, slowly coming back to life. This is what the science team onboard CSIRO research vessel (RV) Investigator has crossed the Pacific Ocean to investigate.

Putting seafloor mud under the microscope

Led by Associate Professor Rebecca Carey from the University of Tasmania , an international team of researchers, including a member of the Tonga Geological Services, is mapping the erupted volcano structure and studying life on the seafloor . To do this, they're diving into the extensive scientific tool kit onboard RV Investigator.

Researchers have deployed a multicorer , equipment that looks a bit like a lunar lander, to take short, one-metre samples of the seafloor sediments and examine the life they contain. To go deeper into the mud, they've been deploying the ship's giant piston corer. This impressive piece of equipment lives in a cradle that runs the entire length of the ship's back deck and can collect sediment cores up to 24 metres long.

A much smaller benthic (seafloor) sled has also been used by the science team to collect biological specimens from the seafloor.

The physical sampling has been accompanied by visual surveys of the seafloor using an underwater camera system called the deep towed camera or DTC. The DTC is an incredible tool for ocean exploration and can capture high resolution photos and video of the seafloor at depths to 4000 metres.

Sediment cores and deep towed camera videos reveal two types of communities: those that cling to hard surfaces, and opportunistic species that thrive within the mud.

Here, the scattered boulders become vital.

A non-rolling stone gathers sea moss

Sitting on top of the muddy soup-like deposits, boulders serve as island havens and footholds for recolonisation. Bryozoans, commonly called 'moss animals' (as their colonies resemble moss) and bottom-dwelling species exploit these hard surfaces, forming miniature reefs. These scattered boulders are the stepping stones of recovery, providing the first toeholds for life in the sea of mud.

However, while more than three years have passed since the eruption , marine life on the desert-like volcanic mud landscape remains scarce. It is perhaps limited by the challenge of growing on a seafloor that is too soft to provide a firm footing or is constantly resurfacing, leading to cycles of burial and renewal.

Digging a little deeper reveals a different story for some tiny marine pioneers though.

Mud soup for you

In the top few centimetres of mud, scientists have seen signs of recolonisation. Here they have found populations of microscopic, single-celled organisms called porcelaneous Miliolid foraminifera, or 'forams', that thrive in soft, muddy sediment. Keeping them company are tube-like species of agglutinated (glued together) benthic foraminifera. These make tiny, armoured shells out of the fine-grained sediment and sit vertically in the mud, with their pseudopodia (arm-like structures) protruding above the surface.

As they say, life always finds a way.

Importantly, the delicate shell remains of floating and free-swimming microfossils, including planktonic foraminifera and pteropod molluscs, are seen in greater abundance in the top few centimetres of the mud. This suggests that the sediment soup is starting to settle and become more stable and habitable as it compacts.

Mele Manu, Senior Geologist with the Tonga Geological Services, said the voyage would provide Tonga with critical new data on the seafloor changes and hazards following the 2022 eruption, strengthening their capacity for monitoring and risk preparedness.

"The research also enhances our understanding of ecosystem recovery and the return of marine life, supporting Tonga's biodiversity and livelihoods that depend on them," Ms Manu said.

Importantly, the observations and data collected on the voyage will help increase understanding of how long the marine ecosystem takes to recover following large-scale disturbances. This knowledge is vital for Tonga and other Pacific Island communities who rely on the ocean for their food security and prosperity, helping to support Pacific resilience against the impacts of natural disasters.

This research is supported by a grant of sea time on RV Investigator from the CSIRO Marine National Facility (MNF). Supported by the National Collaborative Research Infrastructure Strategy (NCRIS) , the MNF is one of Australia's most productive pieces of research infrastructure, delivering research to help ensure the security, protection and prosperity of our marine environment.

CSIRO scientists are also working in the Kingdom of Tonga as part of an ACIAR-funded soil management project which aims to improve farming systems resilience in Tonga through improved soil management.