Did you know that about three-quarters of Earth is covered by oceans? From space, our planet looks like a pale blue dot. However, researchers in Japan recently suggested that, at one time, our oceans may have been green. They shared their findings in a study published in the journal *Nature*.
So, why did the oceans change color? It comes down to the chemistry of the waters and the evolution of photosynthesis. The rocks we study, known as banded iron formations, help tell the story of our planet’s history.
Banded iron formations date back to the Archean and Paleoproterozoic eons, which were about 3.8 to 1.8 billion years ago. During this time, life was limited to single-celled organisms in the oceans, while the land was a barren landscape filled with gray and brown rocks.
Rainfall eroded iron from the continents, carrying it to the oceans through rivers. Volcanic eruptions also contributed iron from the ocean floor. This element would play a crucial role in the future.
During the Archean eon, Earth’s atmosphere and oceans lacked free oxygen. However, the first organisms were beginning to harness energy from sunlight with anaerobic photosynthesis, which doesn’t require oxygen. As these organisms thrived, they produced oxygen as a byproduct. Initially, this oxygen combined with iron in seawater. It wasn’t until the ocean’s iron was fully oxidized that we saw free oxygen in the atmosphere.
This shift ushered in what scientists call the Great Oxidation Event, a pivotal moment that made complex life possible on Earth. We can see this transition reflected in banded iron formations, which show layers deposited in both oxygen-rich and oxygen-poor conditions.
The idea of green oceans ties back to recent observations in the waters near the Japanese island of Iwo Jima. These waters exhibit a greenish tint due to a form of oxidized iron (Fe(III)). Interestingly, blue-green algae thrive in these environments, indicating high levels of iron in the ocean during the Archean eon.
Genetically engineered modern blue-green algae have shown that they perform better in green water conditions, thanks in part to an additional pigment, phycoerythrobilin (PEB), which works well in green light. In fact, the study’s simulations suggest that early photosynthesis caused enough oxidized iron particles to turn surface waters green.
As for the future, could our oceans change color again? The chemistry of the water and the life that inhabit it determine the colors we see. Researchers believe that if sulfur levels were to rise significantly, oceans could turn a purplish hue, possibly from a dominant presence of purple sulfur bacteria. Intense volcanic activity could lead to such a scenario.
The oceans could also appear red if oxidized iron from land erosion enters the water or if certain algae, known for creating “red tides,” proliferate. This tends to happen in areas with high fertilizer runoff, common near coastal regions.
As our Sun ages, it will eventually become brighter, which could favor the development of more purple, brown, or green hues in our oceans. At some point, the oceans may even evaporate as the Sun expands.
In the grand scheme of things, changes in ocean color are inevitable. Earth’s oceans have undergone many transformations over billions of years, shaped by both geological processes and life itself. What will our oceans look like in the future? Only time will tell.
For more on how these transformations shape our understanding of climate and life on Earth, check out more articles from The Conversation.
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