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Journey through the ocean's twilight zone, where tiny marine creatures burn through tons of the greenhouse gas carbon dioxide, and one moves from light into shadow.

Here where sunlight dims, 330 to 3,300 feet (100 to 1000 meters) below the sea surface, tiny sea creatures transform carbon into forms bound for deep ocean storage. But until now, it was hard to pin down exactly how much carbon moved through this vast dimension. The creatures living in the twilight zone seemed too voracious, and estimates of their appetite for carbon outmatched the available supply.

A two-week expedition to the North Atlantic investigated how carbon enters the deep ocean.Christian Lindemann

In 2013, researchers with Britain's National Oceanography Centre explored the twilight zone near Ireland from top to bottom, measuring carbon and ocean life at all points. Now, the scientists think they can finally balance the twilight zone's carbon checkbook. The findings were published today (March 19) in the journal Nature. [ Venturing to the Ocean's Twilight Zone ]

"We've really increased our confidence of what is going into this zone, and what is coming out of it," said Richard Lampitt, a biological oceanographer at the center in Southampton, England.

About a quarter of the planet's carbon goes into the oceans, Lampitt said. Most of this carbon that is absorbed by the ocean later returns to the atmosphere (about 90 percent). The rest is recycled within the twilight zone, and just 1 percent falls to the sea floor.

But the carbon that makes it past the bottom of the twilight zone stays trapped in the depths for millennia. Researchers refer to this long-term exile as the Earth's "biological carbon pump."

"This process is responsible for reducing carbon dioxide by about 200 parts per million," before fossil-fuel burning started, Lampitt said. For example, without the biological carbon pump, levels of carbon dioxide would have been closer to 500 parts per million (ppm) instead of 280 ppm about 200 years ago, studies suggest. In 2013, atmospheric carbon dioxide levels crossed 400 parts per million (ppm), a million-year high. (Parts per million denotes the volume of a gas in the air; in this case, for every 1 million air molecules, 400 are carbon dioxide.)

Understanding how carbon sinks through the twilight zone and ends up stored in the deep ocean will help researchers improve climate models and understand the balance of the planet's carbon cycle.

This is a condensed version of a report from LiveScience. Read the full report.

Email Becky Oskin or follow her @beckyoskin. Follow us @livescience, Facebook & Google+.