Earth's oceans were once green and could one day turn purple

The deep blue sea may not have always been so blue. A new study suggests that the Earth's oceans may have changed color over time and could change again in the future, depending on the chemical makeup of the water.

Pale green dot

The oceans were likely once a vibrant green color between 3.8 and 1.8 billion years ago, according to a study published in the journal Nature Ecology & Evolution. During this period of Earth's geologic history, called the Archean eon, the oceans and atmosphere were devoid of oxygen. "Life was confined to single cells in the oceans," said New Atlas. "Rainfall eroded iron from continental rocks. Rivers subsequently carried this dissolved iron into the seas, where it was joined by additional ferrous iron ejected from volcanic vents on the ocean floor."

The abundance of iron and the flourishing of microbial life provide evidence for a green ocean. Blue-green algae, also called cyanobacteria, were one of the earliest life forms on Earth and contributed heavily to the oxygenation of the planet. Specifically, they triggered the Great Oxidation Event, when cyanobacteria performed oxygenic photosynthesis for the first time, which "used the energy from the sun and released oxygen," said a release about the study. "Over time, this oxygen accumulated in the atmosphere, dramatically transforming Earth's environment and paving the way for the emergence of our oxygen-breathing ancestors."

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The released oxygen from this photosynthetic process then reacted with the ferrous iron in the ocean to produce ferric iron. Ferric iron has different properties from ferrous iron, producing "rust-like particles" in the ocean that "changed the wavelengths of light that could penetrate the water," said the release. "As the particles preferentially absorbed blue and red light, mainly green light was transmitted, causing the underwater environment to be dominated by green hues."

Watercolor but bigger

Most photosynthetic organisms use the pigment chlorophyll in their cells to transform carbon dioxide into sugars with the energy of the sun. However, "blue-green algae are peculiar because they carry the common chlorophyll pigment but also a second pigment called phycoerythrobilin," Cédric M. John, a professor and the head of data science for the environment and sustainability at Queen Mary University of London, said at The Conversation. While chlorophyll is best for white light, the secondary pigment is effective in green light. The presence of two pigments could be because the "color of the oceans changed gradually during this period and potentially oscillated" and "could change again in the future, depending on the chemistry of the ocean."

The ocean could turn purple, for example, if the "levels of sulfur were high." This could be "linked to intense volcanic activity and low oxygen content in the atmosphere, which would lead to the dominance of purple sulfur bacteria," said John. The aging sun will also "become brighter, leading to increased surface evaporation and intense UV light," and may "favor purple sulfur bacteria living in deep waters without oxygen." In different circumstances, the ocean could even turn red, brown or green again. But any colorful changes will likely be far down the geologic timeline, leaving us with blue seas for now.