Ocean's Twilight Zone Has Role in Climate Change


Photo credit: a_chiriboga

Paging Rod Serling: The dim layer of ocean known as the "twilight zone" has placed a major crimp in our plans to send carbon dioxide to a watery grave, where we hope to trap it from reentering the atmosphere as a greenhouse gas.

The results of two international research expeditions to the Pacific Ocean, published in the April 27 issue of the journal Science, reveal that carbon dioxide—taken up by photosynthesizing marine plants near the sunlit ocean surface—does not necessarily sink to the inscrutable depths below. In fact, it seldom even gets there. Any carbon transported downwards on sinking marine particles (called "marine snow") is often gobbled up by animals and bacteria, after which it's converted back into dissolved organic and inorganic forms that are recirculated and reused... in the twilight zone.Once cruising in the twilight zone, the carbon makes a break for it to the surface. And before you can say "Adios, suckers," it's back in the atmosphere—and back to heating up the planet.

Using new technology, the researchers discovered that only 20 percent of the total carbon in the ocean surface made it through the twilight zone off the coast of Hawaii; it fared a little better in the northwest Pacific near Japan, with 50 percent of the carbon making it through.

Imagine a stretch of border fencing that is reinforced in some areas, but weaker in certain spots. Similarly, the twilight zone allows more sinking particles through in some regions, but fewer in others, which makes it difficult for the scientists to predict how the ocean might offset the impacts of greenhouse gases. And what of proposals to mitigate climate change by "seeding" the oceans with iron—promoting blooms of photosynthetic marine plants—to sequester atmospheric carbon dioxide to the deep ocean? That's not going to work as long as the ocean's twilight zone is enabling carbon's escape-artist antics.

"The twilight zone is a critical link between the surface and the deep ocean," said Ken Buesseler, a biogeochemist at Woods Hole Oceanographic Institution (WHOI) and lead author of the new study in Science, co-authored by 17 other scientists. "We're interested in what happens in the twilight zone, what sinks into it and what actually sinks out of it. Unless the carbon that gets into the ocean goes all the way down into the deep ocean and is stored there, the carbon can still make its way back into the atmosphere. Without long-term carbon storage at depth, the ocean can do little to stem the increase in atmospheric levels of carbon dioxide, a greenhouse gas that impacts the earth's climate."

Why more carbon reached the depths in the northwest Pacific has the researchers stymied, and although theories abound, it's a question they are keen to nose out the answer to.

"Only with continued observations and new techniques can we hope to understand this often overlooked layer in the ocean that is as important to the global carbon cycle as the sunlit surface layer where atmospheric carbon dioxide first enters the ocean," Buesseler said. :: Newswise

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