A giant ‘bullseye’ on the Nullarbor Plain was created by ancient marine life

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Environments across the planet are changing dramatically in response to human population growth and climate change. Some scientists even say that human activity has propelled the Earth into a new geological epoch: the Anthropocene.

Authors


  • Milo Barham

    Lecturer, Earth and Planetary Sciences, Curtin University


  • Matej Lipar

    Research Associate, Physical Geography, ZRC SAZU

Amidst this rapid transformation, some special places, protected by fortuitous geography and geology, are changing so slowly that they retain evidence of Earth’s past over unfathomable time scales.

One such place is the flat, dry expanse of the Nullarbor Plain in southern Australia, where traces of events that happened millions of years ago remain. Using high-resolution satellite imagery, we have begun to map some of these traces.

In new search published today in Earth Surface Processes and Landforms, we report the discovery of an enigmatic “center”-shaped structure more than a kilometer in diameter. We believe these are the remnants of an ancient reef, created by microbes around 14 million years ago when the Nullarbor sat at the bottom of the ocean.

No trees, no water, but not boring

Named the Nullarbor Plain (meaning “treeless”) during settlement, and Oondiri (meaning “waterless”) by some of the region’s First Nations peoples, the area is notoriously dry, flat and barren. The exceptional overall flatness of the plain (the average slope is very, very less than 1°) is one of the first indicators of the stability of the region.

The rocks beneath the Nullarbor Plain are made up of limestone that originally formed in shallow water. seagrass meadows. These rocks can dissolve in rain and weakly acidic groundwater.

Partly because of its dryness, the region has not been intensely dissolved or eroded by rivers or glaciers in the millions of years since it emerged from the ocean. This contrasts sharply with the classic ruggedness of much younger tropical landscapes (like the Hawaiian volcanic islands), which are much wetter and more geologically active.

The plain covers approximately 200,000 km² and, like the curvature of the Earth, the landscape features of the Nullarbor Plain are almost imperceptible to the human eye. Despite this subtlety, the area is not as simple as one might think.

Careful scientific studies and high-resolution satellite data are increasingly revealing the secrets of the Nullarbor Plain’s past.

Mummified marsupials and ancient dunes

Isolated caves dot the Nullarbor Plain. In their dry chambers, remarkably preserved fossils suggest Australia’s extinct animals that would rival the most wonderful zoo menagerie.

Mummified thylacine (Tasmanian tiger) remains and complete thylacoleo (marsupial lion) skeletons from thousands of years ago capture startling snapshots of changing ecosystems.

Older still are the gentle linear ridges that cross the Nullarbor Plain. Recently we have shown that these crests are relics of a long gone landscape. Ancient sand dunes controlled the gentle dissolution of underlying limestone to leave a subtle imprint of windblown patterns millions of years ago.

Target

For our most recent work, we have used landscape data TanDEM-X Digital Elevation Model produced by the German Aerospace Center, which has a resolution of about 12 meters.

Studying these images of the Nullarbor has revealed a previously unnoticed “bull’s-eye” feature: a ring-shaped hill with a central dome, just over a kilometer wide and only a few feet tall. from above.

Initially, we thought we had found the first meteorite impact crater discovered in the Nullarbor Plain. The area is famous for meteorites who can help us understand history of our solar systembut to date, no definitive crater caused by meteorites has been discovered.

However, when we took a closer look at the target, we saw no chemical or high-pressure indicators of an impact.

We discovered the real explanation for the target after cutting and polishing rock samples thin enough to let light through and inspecting them under a microscope. Unlike the limestone seen at hundreds of other sites across the plain, here we saw evidence of tiny microbial organisms holding the sediment together.

Supported by similar “doughnut” structures formed by algae on the Great Barrier Reef, we interpreted the bullseye as an isolated ancient “reef”. This biogenic mound formed on the seabed a long time ago but decayed so slowly after the land was lifted above the waves that it is still recognizable some 14 million years later.

How understanding the past can help the future

Our discoveries add to the region’s growing recognition as an exceptional archive of environmental changes that we need to better understand and protect.

The emergence of the Nullarbor Plain was an important driver of plant and animal evolution. Ancient fossils and even DNA preserved through stable conditions will help us to reconstruct more precisely its vanished ecosystems.

Fuller understanding of how landscapes and ecosystems have been transformed in the past will in turn help us conserve the animals, plants and environments we have today, and minimize the negative impacts of future anthropogenic climate change.

Milo Barham receives funding from the Minerals Research Institute of Western Australia, as well as Iluka Resources Ltd. to investigate the mineral sands, including the margins of the Nullarbor Plain.

Dr Matej Lipar receives funding from the Slovenian Research Agency, the Australian Speleological Federation Karst Conservation Fund and the German Aerospace Center TandemX.

/ Courtesy of The Conversation. This material from the original organization/authors may be ad hoc in nature, edited for clarity, style and length. The views and opinions expressed are those of the authors.

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