Why Earth’s Ancient Oceans Were Green—and May Turn Purple
Earth’s oceans were once green—and could turn purple again, say scientists. Discover how ocean color reveals the planet’s past and future.
Why Earth’s Ancient Oceans Were Green—and May Turn Purple
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Published April 15, 2025
From space, our planet appears as a calm, blue orb—its oceans covering more than 70% of its surface. But new research suggests this wasn’t always the case. In a compelling study published in Nature, Japanese scientists propose that Earth’s early oceans were not blue at all, but a vivid shade of green—and that one day, they could even turn purple.
This shift in ocean color is more than an aesthetic curiosity; it reflects deep transformations in ocean chemistry and life on Earth. By looking back billions of years into the planet’s history, scientists are uncovering a story of iron, light, and life itself.
The Chemistry Behind a Green Ocean
Long before animals roamed the land and plants blanketed the continents, Earth was a very different place. During the Archean and Paleoproterozoic eons—between 3.8 and 1.8 billion years ago—the atmosphere lacked oxygen, and the oceans teemed with dissolved iron. These conditions created an environment where single-celled organisms could evolve without the need for oxygen.
In this iron-rich world, early microbes performed a primitive form of photosynthesis—anaerobic photosynthesis—which didn’t rely on oxygen. Instead, they used iron as the electron donor. Over time, these microscopic organisms began producing oxygen as a byproduct, triggering the planet’s first major ecological transformation: the Great Oxidation Event.
The evidence lies in striking geological formations known as banded iron formations. These layered rocks are the fossilized footprints of this transformation, alternating between oxidized and unoxidized iron. Their red and grey bands represent the gradual infusion of oxygen into Earth’s oceans and atmosphere, laying the foundation for more complex life.
Iwo Jima: A Window into the Ancient Ocean
The researchers drew inspiration from a curious natural laboratory: the waters surrounding Iwo Jima, a volcanic island in Japan. These waters appear green due to high concentrations of oxidized iron—Fe(III). Interestingly, blue-green algae thrive in this environment. Despite the name, these are not true algae, but rather ancient photosynthetic bacteria called cyanobacteria.
Modern cyanobacteria carry two important pigments: chlorophyll, which we associate with green plants, and a lesser-known pigment called phycoerythrobilin (PEB). While chlorophyll captures red and blue light, PEB is more efficient in green light. This dual-pigment strategy may have evolved precisely because early oceans were green—an evolutionary advantage for harvesting sunlight in a spectrum shaped by iron-rich waters.
Could Earth’s Oceans Turn Purple Again?
As fascinating as the past may be, scientists are also pondering the future. Could ocean color change again? The answer, surprisingly, is yes. Earth’s oceans are dynamic, responding to shifts in climate, chemistry, and biological activity.
If volcanic activity increases and sulfur levels rise, purple sulfur bacteria—organisms that thrive in low-oxygen, high-sulfur conditions—could dominate. This would give our oceans a purplish tint. Such scenarios aren’t purely hypothetical; they echo conditions seen in Earth’s distant past and in certain sulfur-rich environments today.
Other possibilities include red oceans caused by “red tide” algae blooms or iron-rich runoff from land, particularly in warmer, more eroded climates. Already, coastal regions near agricultural and industrial zones experience such changes periodically.
What Ocean Color Tells Us About Life—Here and Beyond
The color of Earth’s oceans is not just a reflection of water depth or clarity—it is a powerful indicator of the underlying chemistry and biology. It can even serve as a clue in the search for life beyond Earth. If a distant exoplanet appears as a green or purple dot, scientists might infer the presence of primitive photosynthetic life, much like Earth in its early chapters.
Moreover, as our sun ages and grows brighter, Earth’s climate will shift dramatically. Rising temperatures and UV radiation could once again favor sulfur-dwelling bacteria, altering not only ocean ecosystems but their very hue. Eventually, oceans may evaporate entirely, leaving behind only geological records of the waters that once covered the Earth.
Conclusion: Oceans as Time Machines
In the grand timeline of Earth’s 4.5-billion-year history, the blue oceans we know today are a relatively recent phenomenon. The green seas of the Archaean era remind us that life, light, and chemistry are intricately linked. And as we look toward the distant future—or the possibility of life on other worlds—understanding ocean color may prove to be a powerful lens into the evolution of planets and the biology they support.
What we see when we look at the ocean is more than just surface beauty—it’s a reflection of Earth’s long, unfolding story. And that story, scientists say, is still being written in every ripple, current, and changing shade.
Disclaimer
This article is intended for educational and informational purposes only. It does not substitute professional scientific advice. For in-depth insights, consult peer-reviewed research or speak with an environmental or geoscience expert.
source : live science