Massive 'bullseye' in Australian desert is evidence of an ancient reef

A strange doughnut-shaped mound in a desert in southern Australia recently made a surprise appearance in high-resolution satellite images. The odd formation, which from space resembles a big bullseye, is likely the remains of an ancient reef, made by microbes and left over from a time when a vast ocean covered the now-arid environment, new research suggests.

The new study, published July 29 in the journal Earth Surface Processes and Landforms, used data from the TerraSAR-X add-on for Digital Elevation Measurement (TanDEM-X) mission, which ended in 2016 and involved a pair of Earth observation satellites that were launched and maintained by the German Aerospace Center (DLR), Germany's national space agency. Using radar data from the twin satellites, the DLR produced detailed 3D maps of the Earth's landmasses from pole to pole, according to the European Union's European Data Portal.

For the new research, scientists examined DLR maps of the Nullarbor Plain, a flat, remarkably dry landscape that covers about 77,220 square miles (200,000 square kilometers) of southern Australia. A layer of limestone runs beneath the plain's surface substrate; this rock originally formed in shallow marine seagrass meadows that covered the seafloor when the plain was still submerged beneath the ocean, lead author Matej Lipar, a research associate at the Research Centre of the Slovenian Academy of Sciences and Arts (ZRC SAZU) in Ljubljana, and senior author Milo Barham, a senior lecturer at the School of Earth and Planetary Sciences at Curtin University in Perth, Australia, wrote in The Conversation. 

The plain first emerged as the ocean receded some 14 million years ago during the Miocene epoch, and since then, the land has remained relatively unchanged, Lipar and Barham wrote. Unlike wetter, more geologically active regions, the dry plain has not been extensively sculpted by glaciers and rivers flowing over its surface, although strong winds did once sweep sediment onto the landscape, forming sand dunes that have since vanished. 

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"Unlike many parts of the world, large areas of the Nullarbor Plain have remained largely unchanged by weathering and erosion processes over millions of years, making it a unique geological canvas recording ancient history in remarkable ways," Barham said in a statement. Now, the TanDEM-X images have revealed another enigmatic remnant of the Nullarbor Plain's ancient past: a mysterious bullseye-shaped formation that measures about 0.77 miles (1.25 km) wide and just a few yards tall.

"Initially, we thought we had found the first meteorite impact crater to be discovered on the Nullarbor Plain," Lipar and Barham wrote in The Conversation. "However, when we took a closer look at the bullseye we saw none of the chemical or high-pressure indicators of an impact."

For this close inspection, the team collected rock samples from the bullseye that they polished and sliced thinly enough for light to shine through. By placing the rock slices under a microscope, the researchers discovered deposits of microbial boundstone, a type of rock formed from sediments bound together by microscopic organisms, such as algae. 

Similar ring-shaped mounds, made from the calcium carbonate-rich skeletons of green algae, can be found today in the Great Barrier Reef, ABC Science reported. Due to this similarity, the researchers interpreted the newfound bullseye as "an ancient isolated 'reef,'" they wrote in The Conversation. 

"This biogenic mound formed on the seabed long ago but degraded so slowly after the land was lifted above the waves that it is still recognisable roughly 14 million years later," they said.

Originally published on Live Science.