Throughout most of human history, our planet has earned its “blue planet” designation from oceanic domination of its surface. A recent research publication in Nature Geology & Evolution disproves traditional beliefs about ancient oceans by showing they were green in color and may become purple in upcoming geological periods.
Nagoya University researchers identified the origin of this green ocean color as the Archean eon, which spanned from 3.8 to 1.8 billion years ago. Rainwater during this time period dissolved iron from rocks in continental regions before rivers transported it to the ocean. The underwater volcanic vents served as another source of ferrous iron that entered the ocean waters. The oxygen-depleted water contained high iron concentrations, which produced a green-colored ocean environment.
The Great Oxidation Event began during an atmosphere-altering period when cyanobacteria started producing oxygen. The microorganisms from that time differed from contemporary plants because they did not use chlorophyll as their sole photosynthetic mechanism. The microbes employed phycoerythrobilin (PEB) as their light-absorbing pigment because it captured green light, which was the dominant underwater wavelength at that time. Genetic research shows cyanobacteria survived in the green ocean environment through the development of phycoerythrin proteins.
The rising oxygen levels in the ocean caused iron to create insoluble ferric compounds that settled as rust. The reaction between iron and oxygen in the water cleared the green hue and led to the formation of modern blue oceans. The modern site of Iwo Jima displays green water because of iron oxidation, which reveals traces of ancient ocean conditions.
Scientists predict that oceans will experience another color transformation in the future. Computer models indicate that sulfur-rich oceans resulting from volcanic events would display purple coloration, while tropical conditions producing red iron oxide would transform the seas into red bodies of water. These distant changes in Earth’s chemistry demonstrate both the active nature of its chemical processes and how living systems and environmental elements develop in unison.
Will future generations observe a purple planet instead of the current blue planet? The planet’s evolving geochemical processes together with time will determine the future ocean color.
