Oceans cover nearly three-quarters of Earth, making it appear to be a pale blue dot from space. In a Nature-published study, Japanese researchers, on the other hand, argue persuasively that Earth’s oceans were once green. The reason Earth’s oceans may have looked different in the ancient past is to do with their chemistry and the evolution of photosynthesis. As a geology undergraduate student, I was taught about the importance of a type of rock deposit known as the banded iron formation in recording the planet’s history.
Banded iron formations were deposited in the Archean and Paleoproterozoic eons, roughly between 3.8 and 1.8 billion years ago. At that time, organisms with one cell only existed in the oceans. The continents were a barren landscape of grey, brown and black rocks and sediments.
Iron was dissolved when rain fell on continental rocks, and rivers carried it to the oceans. Volcanoes on the ocean floor were an additional source of iron. This iron will become important later.The Archaean eon was a time when Earth’s atmosphere and ocean were devoid of gaseous oxygen, but also when the first organisms to generate energy from sunlight evolved. These organisms used anaerobic photosynthesis, meaning they can do photosynthesis in the absence of oxygen.
Because oxygen gas is a byproduct of anaerobic photosynthesis, it sparked significant changes. Iron is bound to oxygen gas in seawater. Once iron in seawater was no longer able to neutralize oxygen, oxygen could only exist as a gas in the atmosphere.
