Recently Japanese researchers have proposed that Earth’s ancient oceans were once green, not blue, due to changes in ocean chemistry and early photosynthetic activity, as published in Nature.

About Ancient Oceans

• Geological Marking : Banded iron formations, created between 3.8 and 1.8 billion years ago, record changes in Earth’s early ocean chemistry and atmospheric conditions during the Archean and Paleoproterozoic eons.
• Atmosphere: During the Archean eon, Earth’s atmosphere lacked oxygen, and only anaerobic photosynthetic organisms lived, producing oxygen as a byproduct which initially bonded with dissolved iron in oceans.
• Transition: The accumulation of oxygen eventually led to the Great Oxidation Event, fundamentally transforming Earth’s atmosphere and enabling the rise of complex life.
  • The Archaean eon spanned 1.5 billion years, during which the colour of Earth’s oceans likely shifted gradually and intermittently, influencing evolutionary adaptations like dual pigment usage in algae.

Evidence for Green Oceans

• Observation: Researchers noted that the waters around Japan’s Iwo Jima island appear green due to the presence of oxidized iron (Fe(III)), suggesting a similar ancient ocean condition.
• Evolution: Blue-green algae, although technically bacteria, evolved with pigments like chlorophyll and phycoerythrobilin (PEB), enabling them to thrive in green-hued waters with altered light conditions.
• Simulation: Computer models show that oxygen produced by early photosynthesis caused enough oxidized iron particles in the water to turn the ocean’s surface green before free oxygen accumulated.
• Significant for discovery of suitable planets: Planets appearing pale green from space might be promising candidates for harboring early photosynthetic life forms, offering new directions in the search for extraterrestrial life.
• Implications for marine life: Waters that are deep blue contain less life, while greener waters signal the presence of more phytoplankton.

Reason for Blue color of water

• Selective Absorption of Light: Water absorbs different wavelengths of sunlight to varying degrees.
  • Red, orange, and yellow wavelengths (longer wavelengths) are absorbed more readily by water, while blue and violet wavelengths (shorter wavelengths) penetrate deeper.
  • This leaves blue light to dominate the visible spectrum in clear water.
• Scattering of Blue Light: When sunlight enters water, blue light is scattered in all directions due to its shorter wavelength (Rayleigh scattering).
  • This scattered blue light is what we perceive when looking at large bodies of water, especially in clear conditions.

Future Possibilities of Ocean Changes 

• Purple: If Earth’s oceans became rich in sulfur, perhaps due to intense volcanic activity and low oxygen, purple sulfur bacteria could dominate, turning oceans purple.
• Red: Red oceans could emerge under intense tropical climates, high iron oxidation from decaying rocks, or a dominance of red tide algae fueled by excessive nutrient runoff.
• Aging: As the sun grows older and hotter, increased UV radiation and evaporation may favor purple sulfur bacteria in deep waters, reducing blue hues and leading to more brown, green, or purple coastal oceans.
• Evaporation: Eventually, as the sun expands and Earth’s surface heats up, the oceans will completely evaporate, marking the ultimate end of ocean colour changes.

Conclusion

Over geological timescales, ocean colour is not fixed but evolves with atmospheric, chemical, and biological changes, making the dynamic transformation of oceans inevitable.

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