Beautifully lifeless: the clearest ocean in the world
October 6, 2010 1 Comment
Because of the ability of UV light to shred biological molecules, researchers have speculated that life on Earth may have arisen in some dark corners, possibly under rocks or in the ocean depths where UV waves cannot reach. If that’s the case, we can rest assured that wherever life arose, it couldn’t have been a place containing the “clearest ocean waters on Earth.” That recognition today goes to a patch of water the size of the Mediterranean but lying in the middle of the Pacific, an ocean with a reputation for bounty. This area of the Pacific, however, although boasting waters of a unusual, deep violet color, is as lifeless as it is lovely.
Tough research job, but someone’s gotta do it
Researchers traveling to the area in October 2004 had been attracted to this part of the Pacific—stops included Tahiti, French Polynesia, Easter Island, and Chile—because of satellite images suggesting remarkably low chlorophyll levels in the area. This imaging allows scientists to track chlorophyll abundance in the Earth’s oceans, an important indicator of change on a major scale. When chlorophyll is low, life is scarce.
Indeed, when they traveled to the area, the researchers found some of the clearest ocean waters they had ever seen. According to one scientist on the project, the water was as clear as any of the clearest freshwater anywhere on the planet. He compared it, for example, to the clarity of the water from Lake Vanda, a pristine Antarctic lake lying buried under mountains of ice.
No chlorphyll, little life, but great clarity
But this clarity arises from a lack of chlorophyll and a lack of life. The researchers found that UV could penetrate to extraordinary depths in this area of the Pacific, as deep as 100 meters below the ocean surface. This depth sets a record for UV penetration in ocean waters, and its DNA-destroying properties carry responsibility for the dearth of life in this patch of the Pacific.
The mystery of the organic carbon
The researchers did find some evidence of a food web, however. But the organisms that live there, generally bacterial, must survive in a sort of closed system, constantly recycling the nutrients they need to live, such as nitrogen and phosphorus. Strangely, the team found that this particular part of the ocean was full of dissolved organic carbon, which is of biological origin. How an area so devoid of life could produce so much carbon derived from life remains a mystery. Researchers speculate that the bacteria are so busy addressing their nutrient needs in their limited system that they simply do not get around to degrading the carbon.
Out of the mix
Why would the Pacific harbor this clear, beautiful dead zone? The formation of this odd stretch of sea relies on several factors. The Earth’s oceans are a connected, global system of currents, water running not only at the surface but also at the depths. The thermohaline circulation, also known as the “global conveyor belt,” is one of these currents, driven by changes in the density of water in different parts of the globe. As water warms or becomes more saline, its density changes—warmer water is less dense than cold, and saltier water is more dense. As wind-driven currents move water toward the poles, the water cools and sinks when it gets to the high latitudes. This heavier water then flows back into the ocean basins and eventually wells up again. The oldest waters can take 1600 years to make this trip, but in the meantime, waters mix in the basins, making all of the Earth’s oceans pretty similar as they move their components around the planet.
The clearest ocean water on Earth, however, misses the trip. Its location in the middle of the South Pacific ensures that it doesn’t benefit from global river or ocean circulation. It never cools because it’s in the South Pacific, and thus, the water just stays put, clean and clear and almost completely lifeless.