A geomorphological and geophysical investigation of Adele Island has revealed new insights into the long-term evolutionary history not only of Adele Reef but also of Australia’s North West Shelf.
The mid-shelf Adele platform reef sits between the atoll type Scott Reefs, which are located on the very edge of the North West Shelf, and the highly atypical inshore fringing reefs of the Bucaneer Archipelago.
This reef is of scientific interest because it provides an environmental link between the Kimberley’s oceanic and inshore reef systems.
The findings by Curtin University graduate students Tubagus Solihuddin and Giada Bufarale have been published in the Journal Geo-Marine Letters as part of the Western Australian Marine Science Kimberley Marine Research Program.
Geophysical surveys showed that the Adele reef structure did not form exclusively during the Holocene, which is the current period of geological time that started some 11,500 years ago when the glaciers began to retreat. It is in fact made up of layers of different aged reef packages that grew during previous sea level highstands (intervals where sea level lies above the continental shelf edge).
|Rollolith habitat on the southern Adele Reef consisting of both coralline algae (rodoliths) and corals (coraliths)|
By counting these packages back in time, researcher Giada Burfarale was able to calculate that coral growth at the site of Adele reef first began over 400,000 years ago. Interestingly this is also believed to be a time when coral growth first initiated on the Great Barrier Reef.
“Geophysical surveys also revealed a series of 90 metre deep channels around Adele reef that most likely represent ancient channels cut by the Fitzroy, Isdell or Prince Regent rivers which flowed across the continental shelf during sea level lowstands (times when sea levels were at their lowest),” Co-author Curtin University Dr Mick O’Leary said.
“Also, by comparing geophysical data with stratigraphic reef core data from Scott Reefs and the inner fringing reefs it was possible to calculate that the North West Shelf has subsided more that 30 metres over the last 100,000 years, making it one of the more tectonically mobile regions on the Australian continent,” Dr O’Leary said.
The modern Adele Reef is characterised by having one of the more unique reef habitats on the planet. Earlier investigations by the Western Australian Museum reported the discovery of a unique ‘rollolith’ habitat located along the SW side of the island. Rollolith is the common name given to benthic organisms that grow as unattached free-living spherical colonies enabling live tissue to cover the entire colony surface.
|Percussion coring the Adele Island sand spit using an Atlas Copco post driver|
Reef coring by Tubagus Solihuddin and the team found that this is a relatively recent habitat, forming only as the Adele reef platform reached present sea level, less than 1000 years ago. The resulting increase in surface currents are driven by seven metre tides and large waves which wash across the reef platforms at high tides.
“So what we have found is, rather than observing a style of reef geomorphology that gradually transitions between the atoll style oceanic Scott Reefs and the atypical high fringing reefs on the inner Kimberley, the mid-shelf Adele reef is characterised by its own unique structure, habitats and growth history,” Dr O’Leary said. “These findings provide yet more evidence of just how globally unique the Kimberley reefs are.”
Solihuddin, T., Bufarale, G., Blakeway, D., O’Leary, M.J., (2016) Geomorphology and Late Holocene Accretion History of Adele Reef: a Northwest Australian Mid-Shelf Platform Reef, Geo-Marine Letters, 2016, 1–15. DOI: 10.1007/s00367-016-0465-3
The $30 million Kimberley Marine Research Program is funded through major investment supported by $12 million from the Western Australian government’s Kimberley Science and Conservation Strategy co-invested by the WAMSI partners and supported by the Traditional Owners of the Kimberley.