The ocean appears blue to our eyes today, but has it always been that color throughout history? The blue color we see is caused by the water absorbing colors in the red end of the visible light spectrum. But the composition and optics of seawater have changed over time, leading some to wonder – what color was the ocean before it became blue?
In this article, we’ll explore the science behind the color of the ocean and how it has changed over Earth’s history. Factors like the evolution of photosynthetic life and changes in ocean chemistry have impacted the ocean’s color. By looking at these influences, we can piece together the story of the ocean’s changing hue.
What Makes the Ocean Blue?
The ocean appears blue predominantly due to the water absorbing colors in the red end of the visible light spectrum. Here’s how this works:
Absorption of Longer Wavelengths
Visible light from the sun contains all the colors of the rainbow. But when this light enters the ocean, much of the red, orange, and yellow light gets absorbed by the water molecules within the first few meters. Shorter wavelengths at the blue/violet end of the spectrum travel deeper into the water column before being absorbed.
Scattering of Shorter Wavelengths
The blue and violet light waves are also scattered preferentially as they pass through water. This scattering reflects the shorter wavelengths back out, allowing blue light to reach our eyes. The scattered light that returns to the surface creates the blue color we see.
Water Composition
The specific absorption and scattering properties of water are due to its molecular structure. The bonds between hydrogen and oxygen absorb light efficiently at the red end of the spectrum. Shorter blue wavelengths have frequencies that can pass through these bonds without being absorbed as much.
So in summary, water molecules in the ocean absorb long red wavelengths while scattering short blue ones, giving the sea its characteristic blue color. But changing compounds in seawater can alter this process.
When Did the Ocean Become Blue?
Evidence suggests the ocean has been blue for most of Earth’s history. But scientists think the intensity and hue of the color has evolved over billions of years:
Hadean Eon – Gray Oceans (4.6 – 4 billion years ago)
In the hellish conditions of the Hadean eon following Earth’s formation, the young planet likely had gray oceans. With high concentrations of iron, silicates, and other compounds from volcanic activity, the seas would have absorbed more of the light spectrum and appeared more gray than blue.
Archean Eon – Cyan Seas (4 – 2.5 billion years ago)
By the Archean, the ocean may have obtained a cyan tint. Iron concentrations decreased, allowing the ocean to absorb less red light. But the seas still contained minimal oxygen and organic molecules to produce a vivid blue. Cyanobacteria evolved during this eon, emitting the first oxygen into the oceans.
Proterozoic Eon – Teal Oceans (2.5 billion – 542 million years ago)
Increased oxygen levels from ancient cyanobacteria likely turned the Archean oceans a teal color by the Proterozoic Eon. But the seas contained less oxygen than today, along with dissolved organic molecules that absorbed more light. These conditions prevented the ocean from becoming a deep blue.
Phanerozoic Eon – Blue Seas Emerge (542 million years ago – present)
By the start of the Phanerozoic Eon around 540 million years ago, the ocean probably started to obtain a familiar blue hue. As oxygen levels climbed to near modern levels, most dissolved organics oxidized away. This left cleaner water to selectively absorb the long wavelengths, creating a vibrant blue color recognizable in today’s oceans.
When Did the Ocean Become Its Current Specific Blue?
The ocean has likely been predominantly blue since the start of the Phanerozoic Eon. But the exact shade and intensity of blue has fluctuated over the last half billion years due to changes in marine organisms, seawater chemistry, and climate:
Role of Phytoplankton
Phytoplankton like algae contain pigments such as chlorophyll that absorb light. When phytoplankton concentrations are high, they can absorb more blue light and shift the oceans towards green or gray. During events like algae blooms, the water can become darker and more green temporarily.
Changes in Seawater Chemistry
Higher concentrations of dissolved elements and sediments can alter the absorption and scattering properties of the oceans. In times of high iron input, more blue light was absorbed, reducing the ocean’s blueness. Clay-rich sediments shifted the color towards green. While clear, low-sediment seas enhanced the blue color.
Impact of Climate Change
Climate shifts have cascaded into changes in ocean circulation, plankton communities, runoff patterns, and ice cover – all impacting the blue color:
- Colder periods often increased plankton, reducing blueness.
- Warm intervals like the Cretaceous boosted blue due to less iron and runoff.
- Ice ages saw clay-rich sediments carried to sea by glaciers, greying the waters.
In essence, the specific blue wavelength reflected back has constantly shifted with Earth’s climate and marine ecosystem makeup over time.
How Has the Blue Color Changed in Recent Centuries?
The blue hue of the ocean today is probably quite similar to prior centuries before industrialization. But human activities over the last 250 years have noticeably impacted the color in some regions:
Increased Sediment Runoff
Soil erosion from agriculture, mining, and land disturbances has led more sediments to enter the ocean, absorbing light and reducing the blue color, particularly near coastlines.
Nutrient Pollution
Fertilizer runoff and sewage dumping have increased nutrients like nitrogen and phosphorus. This boosts phytoplankton growth, absorbing more blue light and shifting the color towards green.
Oil Spills
Oil from spills and leakages floats on the ocean surface, forming films that reflect more light. This alters the blue color to a more silvery sheen and greenish tones.
However, away from coasts and human activity, the open ocean maintains its blue hue similar to previous history. But new satellite measurements show subtle changes are occurring in ocean color globally due to climate change impacts.
How Could Climate Change Impact Ocean Color in the Future?
Projected impacts from climate change could noticeably shift the shade of blue in our oceans in the coming decades:
Changes in Plankton
Rising sea temperatures will alter plankton species compositions and growth rates. Some climate models predict more frequent algal blooms, absorbing more blue light.
Increased Ice Melt
More freshwater input to the sea from melting ice sheets and glaciers could carry more dissolved organic carbon. Higher runoff could also increase sediments. Both factors could make the oceans less blue.
Ocean Acidification
Increased CO2 absorption is making oceans more acidic, changing the chemistry of seawater. How this will interact with light scattering is still uncertain, but it could potentially impact ocean color.
Less Oxygen in Water
Warmer oceans are projected to hold less dissolved oxygen. Since oxygen influences light absorption, decreasing concentrations could reduce blueness.
| Period | Approximate Age | Likely Ocean Color |
|---|---|---|
| Hadean Eon | 4.6 – 4 billion years ago | Gray |
| Archean Eon | 4 – 2.5 billion years ago | Cyan |
| Proterozoic Eon | 2.5 billion – 542 million years ago | Teal |
| Phanerozoic Eon | 542 million years ago – present | Blue |
Conclusion
While the ocean has likely been blue for roughly the past 500 million years, the exact shade has fluctuated across eras, periods, and climate shifts. Factors like ocean chemistry, plankton, and runoff have caused the color to shift on gradients between blue, green, gray, and teal tones. Recent human activities are also impacting ocean color in local regions. Moving forward, climate change effects on factors like plankton growth, ice melt, ocean acidity, and oxygen content could alter the blue color we attribute to our seas. Gaining a better understanding of how these mechanisms interact will be key to predicting the ocean’s future hue.
