Ancient oceans filled with sulphur, not oxygen, scientist finds

The ocean to Australia's north was once filled with toxic sulphur and contained bacteria that may have made the water look purple, new research indicates.

By examining some of the world's best preserved rocks, once under water and now part of land formations near the Gulf of Carpentaria, an Australian scientist has gained a rare glimpse of what the oceans were like 1.6 billion years ago.

Dr Jochen Brocks, of the Australian National University, found the sea was inhabited by purple and green sulphur-loving organisms which depended on light to live.

Scientists have always assumed that as the earth's atmosphere became more oxygenated, so too did the oceans.

But they could not explain why, if that was the case, complex forms of life did not become abundant earlier than about 600 million years ago.

Dr Brocks' research, published in the latest edition of Nature, provides the first evidence, independent of isotopic findings, that the oceans remained oxygen-starved and sulphur-choked, even while oxygen in the Earth's atmosphere was increasing.

In an ancient rock sample from the McArthur Basin in northern Australia, Dr Brocks found molecular remains of green and purple coloured pigments that were used by the bacteria in photosynthesis.

Photosynthesis is the process by which plants, algae, and some bacteria harness the energy of sunlight to produce food.

Dr Brocks said the presence of the green and purple sulphur bacteria showed the ocean at that time was filled with sulphur close to the surface, where light could penetrate and keep the bacteria alive.

"If indeed the oceans were (full of sulphur) during this middle period of earth's history, it would rewrite much of what we've believed about a fifth of the planet's history," Dr Brocks said.

"It would seem that this is the reason why the world was ruled by bacteria for such a long time."

The new research provides significant clues to the mystery of the other oceans of Earth's "middle age".

"Understanding the world's oceans at this time is an important factor in determining why complex forms of life, such as modern algae and animal life, arose so late in the planet's history," Dr Brocks said.

The first animal embryos, still mere clusters of cells, only appeared about 600 million years ago.

When the oxygenless and toxic waters finally retreated about 800 million years ago, complex eukaryotes, such as algae, finally conquered the world's open oceans, Dr Brocks said.

Their development culminated in the Cambrian Explosion 542 million years ago and marked the sudden appearance of most groups of the animals we know today.

Dr Brocks, from ANU's Research School of Earth Sciences, collaborated with colleagues from Harvard and MIT in the US and the University of Aberdeen in the UK.

- AAP