Australasian Science Magazine

By Stephen Luntz

Malcolm Wallace has made many contributions to geology, but his discovery of the Arkaroola Reef may overshadow them all.

A/Prof Malcolm Wallace says he loved fossils as a child, and wanted to become a palaeontologist. He also liked rocks, however, and at university chose geology. He has brought these interests together for of one of the most significant fossil discoveries in Australian history. If he’s right, this may soon lead to something even bigger.

Wallace was part of the discovery of an enormous reef at Arkaroola in the Flinders Ranges. The reef is ten times the size of something you would find on the Great Barrier Reef, and has been dated to 650 million years ago (AS, Nov/Dec 2008 p.7). A Canadian reef is more than 100 million years older, but Arkaroola dates from a particularly interesting point in the Earth’s evolution.

A little over 700 million years ago the planet experienced the Sturtian Glaciation, when almost the entire planet froze over, with the possible exception of a narrow band of water at the Equator. Eventually the planet emerged, only to experience the same thing happen in the Marinoan Glaciation roughly 50 million years later. Together these events are known as “Snowball Earth”. The Arkaroola Reef dates from between these periods.

Life was almost wiped out by the Sturtian event, yet ecosystems rebounded with such speed they were able to produce a reef 1 km high. Some species thrived on the sunlight in the shallower parts of the ocean while others survived at great depths, probably feeding on what sunk within their reach.

“No one disputes this is a reef,” says Wallace, but that is almost where the agreement ends. For one thing, Wallace and his colleagues have found fossils they believe are primitive animals. If they are right, these are the oldest animals ever found, a truly enormous discovery. So far, however, they have not been able to get their work published, with reviewers expressing skepticism.

“We think these things are primitive metazoans,” Wallace says, referring to the category of animals that includes everything from earthworms to mammals. Others, however, think that what he is referring to are simply large single-celled organisms.

“They’re quite large and have chambers up to 1 cm across,” Wallace says. “The skeptics have not been able to point to any microbes of such size and diversity.”

The problem may be that the reviewers are all on the other side of the world. “All the palaeontologists who have come and looked at these things first-hand have started skeptical and come to believe they’re animals.” On the other hand, those relying on text and photographs have been harder to convince.

The Flinders Ranges have already pushed back the dates for the first animals. The Ediacaran Era is generally considered to be the period in which animals first appeared, and is named for some low hills in the northern part of the Ranges. Wallace notes that Reg Sprigg had similar problems convincing Northern Hemisphere scientists that he really had discovered fossil animals dating to the pre-Cambrian era, partly because they could not see them in situ.

The Arkaroola Reef may also be valuable in what it tells us about the causes of Snowball Earth and the climate between the two events. The region was at a latitude of 15° at the time, and the species appear tropical to Wallace, although some researchers dispute this. The debate could reveal how warm the world became between the massive glaciations.

Wallace has also assisted his student, Jonathan Giddings, in work supporting the theory that the oceans at the time were much more stagnant than they are today. It is possible that organic material falling from the upper layers became trapped in an almost anoxic deep ocean, drawing carbon out of the atmosphere and triggering the second glaciation as a result. However, Wallace says that “we do not know why the ocean’s circulation may have been less than it is today”.

After undergraduate geology at the University of Melbourne, Wallace decided he would like to study the Devonian rocks of the Kimberley. Somewhat counter-intuitively he did this at the University of Tasmania before a post-doc in Adelaide on the debris from the Acraman Crater in the Gawler Ranges. He then returned to the University of Melbourne, where he is now based.

Prior to Arkaroola, Wallace had made his name arguing that south-eastern Australia is more tectonically and seismically active than is generally thought. “The theory is that nothing has happened here for many millions of years, but we found evidence of mountain building

10 million years ago in Victoria. Adelaide is surrounded by fault lines, and there is the Selwyn Fault in Melbourne’s eastern suburbs.”

Wallace is pleased that a federal geological survey is investigating our major cities’ earthquake danger, and suspects that building codes may need an overhaul but adds: “I’m not an engineer”.

Wallace has also proposed that mineral deposits in carbonate rocks are the result of hydrogen sulfide being produced as part of petroleum formation. When liquids carrying lead and zinc meet the gas the metals would “instantly precipitate”, he says.

The idea has been built into prospecting models.