Revolutionary Study Reveals Earth’s Oceans Were Once Green: Insights from Volcanic Waters

Earth is a remarkable place, with oceans covering nearly three-quarters of its surface. Interestingly, recent research from Japan suggests that these vast waters might not have always been blue. According to a study in Nature, scientists present evidence that ancient oceans could have appeared green.

So, what changed the color of our oceans? The answer lies in ocean chemistry and the history of photosynthesis. During the Archaean eon, about 3.8 to 1.8 billion years ago, Earth’s atmosphere and oceans were devoid of oxygen. Life was simple, consisting mainly of single-celled organisms. The continents at this time were mostly barren, filled with dull rocks.

Rainfall would wear down these rocks, releasing iron into rivers, which carried it to the oceans. Volcanic activity also contributed iron to the water. Initially, this iron existed in a dissolved form that lacked oxygen.

Organisms evolved to harness energy from sunlight using anaerobic photosynthesis. This type of photosynthesis did not require oxygen and produced it as a byproduct. The oxygen released started binding with the dissolved iron, creating oxidized iron in the oceans. Eventually, this shift led to the "Great Oxidation Event," a crucial moment in Earth’s history that allows for complex life to flourish by introducing significant amounts of oxygen to the atmosphere and oceans.

The new study’s argument for green oceans stems from observations at Iwo Jima, Japan, where the waters take on a greenish tint due to oxidized iron. Remarkably, blue-green algae thrive in these waters. Though called blue-green, these bacteria are not true algae, but they play a significant role in photosynthesis. They contain different pigments, including chlorophyll, which is common in plants, and phycoerythrobilin (PEB), which works better in green light conditions.

Research shows that genetically modified blue-green algae with this special pigment grow more efficiently in green waters compared to those with only chlorophyll. This suggests that early oceans, rich in iron, would have favored organisms that could utilize these conditions effectively.

As photosynthesis progressed, Earth’s oceans would have gradually changed color as oxygen levels increased and iron oxidized. The Archaean period lasted approximately 1.5 billion years — a significant time where ocean chemistry transformed slowly, potentially alternating between green and blue hues.

Now, could our oceans change color again? Yes, it’s theoretically possible. For instance, purple oceans could arise if sulfur content increases due to intense volcanic activity. Similarly, red oceans might occur in humid climates if red algae dominate a region, often spurred by nutrient runoff from land.

As the sun ages, it will become hotter, leading to greater evaporation and possibly favoring colors associated with certain bacteria that thrive in low-oxygen conditions. Thus, the hues we see in the oceans could evolve yet again, making the oceans a dynamic canvas of colors.

In the grand scheme, ocean color reflects not just water chemistry but also the intricacies of life itself. The transformations witnessed over billions of years remind us that Earth’s history is filled with remarkable changes, and with them, new possibilities for the future.

For more insight into Earth’s oceans and their potential transformations, check out this research in Nature: Matsuo et al, 2025.

Sources: Nature

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