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Giant crater-filled reef discovered behind the Great Barrier Reef

The Great Barrier Reef in Whitsundays, Australia The Great Barrier Reef in Whitsundays, Australia Shutterstock
20 Sep
Enormous structure found in the seabed between the Torres Straight and Port Douglas, could unlock the history – and future – of the Great Barrier Reef

They may just look like green blobs, but these computer-generated shapes confirm the existence of an enormous unseen reef system, hidden in the waters behind the Great Barrier Reef.

The extent of the life form was discovered using state-of-the-art 3D LIDAR equipment, which scanned the length of the Great Barrier Reef area to reveal thousands of square miles of ancient reef structures. ‘It was amazing – completely unexpected,’ says Mardi McNeil, marine geoscientist at the Queensland University of Technology. ‘I re-did the calculation several times to make sure it was right.’

Although parts of the structure had been documented by marine scientists in the 1970s and 1980s, it was McNeil’s research team that discovered it was 6,000 square kilometres in total. Three times the initial estimations. ‘The more we looked, the more vast it became,’ she says. ‘It was exciting to realise that we were the first scientists to see the bioherm seafloor in its true extent, and study the implications of its size and complexity.’

bioherms BlighReefLIDAR scan of the discovered structure on the sea floor (Image: McNeil, et al)

It was when the scans were rendered to higher resolutions that the donut shapes, or bioherms, began to appear. These holes, some ten metres deep and 300 metres across, were formed by a comparatively small species of algae that builds up sediment in layers over time. ‘The bioherms near the Great Barrier Reef are made by a living layer of the humble algae Halimeda, which produces the same limestone mineral as corals,’ explains McNeil. ‘They have some similarities with true reefs, though lack the solid rigidity.’

The discovery comes at a crucial time for the Great Barrier Reef, which in the last year has rode out mass bleaching events and continues to endure the long-term pressures of ocean acidification and pollution. Nonetheless, McNeil hopes that the bioherm structures will unlock secrets about the history of the true reef up top. ‘They may have much to tell us about the Great Barrier Reef’s early formation, at the end of the last ice age, just before the continental shelf was flooded by rising sea levels,’ she says. ‘It could show the change in oceanic waters, sea temperatures, and upwelling of nutrient rich waters into what is now the present day Great Barrier Reef lagoon.’

McNeil hopes the find will spur scientists to re-evaluate how much carbon is locked up between the two enormous life forms. Meanwhile, the bioherms’ fossil records could indicate how reef systems have responded to drastic environmental change in the past, hinting at the Great Barrier Reef’s precarious future.

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