The existence of specialized minerals in ancient reefs suggests the Earth's oceans contained very little oxygen 650 million years ago, Australian researcher said on Wednesday.

Fish in the ocean. [File photo]

"This is really interesting, because this is around the time multicellular animals started evolving," University of Melbourne PhD student Ashleigh Hood said, quoted by ABC Science on Wednesday.

Researchers examined rocks taken from Adelaide fold belt in South Australia.

Hood said the Adelaide fold belt is a bunch of rocks that are about 800 to 500 million years old, and the belt was formed when the Earth's plates pushed together.

Dolomite is a cement-like mixture composed of calcium-magnesium carbonate. It does not precipitate from modern seas, but Hood and colleagues found it in their rocks.

These rocks, which would have been in shallow seas, were also found to contain high levels of iron.

"Because the dolomite mineral has so much iron in it, it suggests that iron was present in solution in the Earth's ocean," she said.

"The only time you get iron dissolved in the ocean is when there's no oxygen, because otherwise it would form deposits - it would drop out as iron ore."

Hood said it is very likely that the ocean circulation shut down, or slowed down during these cold times 650 million years ago, and that would have led to low oxygen levels and anoxic conditions.

The research paper is published in Geology journal.

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