Astronomer Carl Sagan famously had Voyager-1 turn briefly and snap a picture of the earth — a ‘pale blue dot — from 3.7 billion miles away.

Back in the Archean eon (3.8-1.9 billion years ago), that image would have been a ‘pale green dot’ — there was little or no atmosphere on earth, and the oceans were green, as a group of Japanese scientists have deduced.

There were only single-celled organisms in the oceans, making food from the iron dissolved in the water — a process that released oxygen and led to ‘the great oxidation event’ about 2.4 billion years ago.

Iron deposits from this period, known as banded iron formations, show layers of oxidised and unoxidised iron, recording this key environmental shift.

Japanese researchers studying the greenish waters around the volcanic island of Iwo Jima found similarities to ancient oceans. The green of the water is due to oxidised iron and supports blue-green algae — primitive bacteria that can use both green and white light for photosynthesis.

In the future, sulphur from volcanic activity could see purple bacteria proliferating. And if oxidised iron enters the oceans, you may have rolling waves of red.

An unprepossessing sight, for sure!

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Published on June 15, 2025