Mapping the undersea landscape of the reef

About 160 kilometres off the Queensland coast, the RV Falkor is exploring the deep blue waters of the southern Great Barrier Reef in search of drowned worlds.

The research vessel – operated by the Schmidt Ocean Institute – is nearing the end of a month-long expedition to map undersea features that formed during the last Ice Age, to help us better understand today’s rapidly changing reef environment.

“We’re on the outside the Great Barrier Reef, off the continental shelf, in an area called the Capricorn Channel,” explains Mardi McNeil, a marine geoscientist from the Queensland University of Technology (QUT), speaking to me from the “library” on the Falkor.

McNeil is leading the expedition’s four-person science team, cut down to the bare minimum due to COVID restrictions.

With her are QUT PhD student Haydn Trounce, recent geology graduate Ella Sinclair, and University of Queensland Honours student Ben Houseman. For them, this expedition is constantly exciting. “Most days, you can do a lap of the ship and as far as the eye can see, it’s just the horizon of the water,” says Trounce. “It’s a really cool thing to see.”

This is McNeil’s fourth voyage aboard Falkor, one of the few research vessels to conduct scientific expeditions through the pandemic. It has been in Australian waters since last December and its schedule has been constantly re-arranged. With many voyages cancelled, McNeil’s group has snagged three in a row.

“It’s incredible; this is a decade’s worth of ship time and research,” she says. But it’s been hectic. The current voyage was planned and executed in two and a half weeks. “Normally, you plan a research cruise in about 18 months to two years.”

By the time McNeil returns home for Christmas, she will have spent 110 days at sea since August: nearly four months out of five.

This expedition is the final leg of a three-part adventure to map the undersea landscape along the length of the GBR, from the tip in Torres Strait to the southern edge, several hundred kilometres north of Brisbane.

“Those previous cruises had their own objectives and questions, but they are all interrelated,” McNeil says. “In a sense, this is the final piece of the puzzle.”

The main scientific goal is to explore, discover and map ancient features of the reef from up to 20,000 years ago. At that time, the Earth was in the grip of the Last Glacial Maximum. Much of its water was locked up in glaciers and ice sheets, so coastlines looked very different.

“20,000 years ago, at the height of the last Ice Age, not only was the sea level 120 metres lower, but the coastline was much further out to sea,” McNeil explains. “What is now underwater would have been the coast – low-lying plains, estuaries, rivers… Land that humans at that time could have been walking on.”

When the world warmed again, the meltwater flooded back into the oceans and submerged or “drowned” these coastal features.

The team is searching for everything from reefs, pinnacles and terraces to sand dunes, river channels and deltas. They aim to piece together a map of this long-gone landscape to see how it changed over time, particularly in response to rapid environmental shifts.

To do this, they’re using high-resolution mapping technology to systematically survey the edge of the continent.

“We’re mapping the seafloor using multi-beam sonar,” McNeil says. “That sends down pulses of sound waves that reflect off the seabed and return to the ship, and from that digital signal we’re able to put together an image of the seafloor.”

The Falkor left Brisbane on 22 November and is now moving in tight zigzags over five carefully chosen sections of the reef. By the time it returns to on 22 December, it will have travelled nearly 10,000 kilometres.

The most exciting thing about this kind of research, McNeil says, is watching the screen and seeing the seafloor being “revealed in 3D in real-time before your eyes”.

“It challenges your brain to try and figure out what you’re seeing. What did it look like? What physical processes must have been she in order to form it? What life is growing on these features? How was it in the past when it was a coral reef?”

The team is also sampling sediments: sending down a device – like a box on a chain with jaws – that scoops up seafloor samples for analysis.

This kind of research is crucial to understanding how the reef has changed over time. Its responses to abrupt environmental changes in the past – including natural climate change – will give us much-needed insight into this crucial moment we are facing, when today’s reef is under increasingly severe threats from climate change-induced heatwaves and ocean acidification.

The data can also help scientists predict and understand how our reef may respond to sea level rise and changes to ocean circulation patterns.

While McNeil’s team is focusing on the depths, Trounce spends his days looking up. On behalf of the Reef Restoration and Adaptation Program (RRAP), he is studying aerosols: particles in the air that – if the conditions are right – can seed clouds.

Using an aerosol sampler, he is continuously measuring the atmospheric aerosol particles produced naturally by reefs, which create cloud cover to protect the coral from heat and therefore bleaching. These measurements will be used by RRAP to explore interim technologies to artificially increase this cooling cloud cover.

“We’re in the really early stages of this cloud-brightening idea,” Trounce says. “We have to know what’s here already if we’re going to add more.”

But he’s optimistic. If this project can protect corals from bleaching, it will buy the reef some time while we get our act together to reduce carbon emissions. “What excites me is the possibility of protecting and saving the reef, and keeping it alive for the next generation,” he says.

For three weeks the ship also hosted contemporary artist Taloi Havini. Born in the Autonomous Region of Bougainville in the southwest Pacific Ocean, her work explores human actions over time and space, and includes themes of representation, inheritance, habitats, and epistemologies of Oceania.

Havini was essentially part of the science team while on board, working with the researchers to collect sonar data about the seafloor, as well as taking recordings for her own art practice.

“For us, it was super interesting to see someone else’s completely different perspective,” McNeil says. “We were looking at the same thing – sound – but we were interpreting it and experiencing it completely differently.”

The work Havini creates in response to this voyage will be featured in an exhibition in Venice in March 2021.

The scientific data itself also has a long way to go.

“At the end of the cruise, my 30-terabyte hard drive comes off the ship with me,” McNeil says, “and that data is then available for my colleagues, collaborators and other investigators that are involved in this research.”

The Schmidt Ocean Institute will also make the data accessible to any member of the science community in its archives. It will be added to Geoscience Australia’s public AusSeabed data portal, as well as GEBCO Seabed 2030 project, which aims to map the seafloor across the globe by 2030.

McNeil also hopes their findings will lead to a return mission, with more geological sampling equipment to take samples and cores of these fossil reefs.

“A lot of chemical trace elements and isotopes [are] locked up within the coral skeleton that are a signature of what the ocean conditions were like at the time when those corals were growing,” she says. “So we use those as a chemical archive to interpret climatic history and change over time.”