Oceans once turned green, they could change again

CÉDRIC M JOHN

Nearly three-fourths of Earth is covered by oceans, making the planet look like a pale blue dot from space. But Japanese researchers have made a compelling case that Earth’s oceans were once green, in a study published in Nature.

The reason Earth’s oceans may have looked different in the ancient past is due to 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. Life back then was confined to one-cell organisms in the oceans. The continents were a barren landscape of grey, brown and black rocks and sediments.

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 can do photosynthesis in the absence of oxygen. It triggered important changes as a byproduct of anaerobic photosynthesis is oxygen gas. Oxygen gas bound to iron in seawater. Oxygen only existed as a gas in the atmosphere once the seawater iron could neutralise no more oxygen.

Eventually, early photosynthesis led to the “great oxidation event”, a major ecological turning point that made complex life on Earth possible. It marked the transition from a largely oxygen-free Earth to one with large amounts of oxygen in the ocean and atmosphere.

The “bands” of different colours in banded iron formations record this shift with an alternation between deposits of iron deposited in the absence of oxygen and red oxidised iron.

The recent paper’s case for green oceans in the Archaean Eon starts with an observation: waters around the Japanese volcanic island of Iwo Jima have a greenish hue linked to a form of oxidised iron. Blue-green algae thrive in the green waters surrounding the island.

In their paper, the researchers found genetically-engineered modern blue-green algae with PEB grow better in green waters. Although chlorophyll is great for photosynthesis in the spectra of light visible to us, PEB seems to be superior in green-light conditions.

Before the rise of photosynthesis and oxygen, Earth’s oceans contained dissolved reduced iron (iron deposited in the absence of oxygen). Oxygen released by the rise of photosynthesis in the Archean Eon then led to oxidised iron in seawater. The paper’s computer simulations also found oxygen released by early photosynthesis led to a high enough concentration of oxidised iron particles to turn the surface water green.

The changes in ocean chemistry were gradual. The Archaean period lasted 1.5 billion years. This is more than half of Earth’s history. By comparison, the entire history of the rise and evolution of complex life represents about an eighth of Earth’s history. The lesson from the recent Japanese paper is that the colour of our oceans are linked to water chemistry and the influence of life. We can imagine different ocean colours without borrowing too much from science fiction.