Earth’s Oceans Were Once Green, And Scientists Say They Could Shift Color Again

Japanese scientists say that the evolution of photosynthesis caused Earth's oceans to be formerly green, as per a report. The study suggests that the oceans supported only single-celled organisms and featured environments with barren landscapes of grey, brown, and black rocks. Within a period of 1.5 billion years, the slow changes in ocean chemistry could help explain why blue-green algae developed both types of photosynthetic pigments. The color of the planet's oceans is linked to water chemistry and life influence.

Before Blue: The Green Ocean Era

According to the report, Earth's oceans were once green. The chemistry and evolution of photosynthesis account for this shift. Discovered throughout the Archean and Paleoproterozoic ages, banded iron formations—which were laid between 3.8 and 1.8 billion years ago—were formed when life was limited to one-celled creatures in the oceans; the continents were desolate gray, brown, and black rock and silt terrain.

Green Seas Sparked Life

The first life using sunlight emerged in the Archean eon, when Earth's atmosphere and seas lacked gaseous oxygen. These creatures started the "Great Oxidation Event," a significant ecological turning point permitting advanced life on Earth, using anaerobic photosynthesis. The "bands" of various colours in banded iron formations capture this change with an oscillation between iron deposits devoid of oxygen and red oxidized iron.

The case for green oceans in the Archean eon starts with an observation: waters around the Japanese volcanic island of Iwo Jima have a greenish hue linked to a form of oxidized iron — Fe(III). Blue-green algae thrive in the green waters surrounding the island, and their ancestors evolved alongside other bacteria that use ferrous iron instead of water as the source of electrons for photosynthesis.

Life Changes Ocean Colours

Photosynthetic organisms use pigments (mostly chlorophyll) in their cells to transform carbon dioxide into sugars using the energy of the sun. Genetically engineered modern blue-green algae with phycoerythrobilin (PEB) grow better in green waters, suggesting that pale-green dot worlds viewed from space are excellent candidate planets to harbor early photosynthetic life.

The color of our oceans is linked to water chemistry and the influence of life. Purple oceans could be possible on Earth if the levels of sulfur were high, red oceans could be possible under intense tropical climates, or a type of algae linked to "red tides" could dominate the surface oceans. As the sun ages, changes in the color of our oceans are inevitable, as nothing is permanent at geological timescales.