CARBON EATERS: A tiny diatom rests between two larger copepods. By eating the little critters zooplankton prevent the carbon they contain from reaching the ocean floor.
COURTESY OF MARY WILCOX SILVER, UNIVERSITY OF CALIFORNIA, SANTA CRUZ

Most of the action in oceans occurs in the top 300 feet (100 meters). Microscopic plants known as phytoplankton thrive there, capturing carbon as part of photosynthesis. When the plants die, they sink and travel into a "twilight zone" of dim light before reaching the eternal darkness of the ocean's depths. According to a new survey, the biological activity in that zone acts as a gatekeeper, determining whether captured carbon is stored for millennia or quickly recycled to the surface.

Biogeochemist Ken Buesseler of the Woods Hole Oceanographic Institution in Massachusetts and a team of scientists surveyed this understudied twilight world in the Pacific Ocean both off Hawaii and Russia's Kamchatka Peninsula. Deploying new sensors that drift with sometimes strong currents (allowing better measurement of marine snow than sensors placed on the ocean floor or tethered to the surface), the team sampled the flora and fauna and measured the amount of falling carbon material captured to assess the role of the ocean as a true carbon sink. (The ocean currently absorbs roughly half of the greenhouse gases, primarily carbon dioxide, that are released by human activity.) "We were trying to measure the transfer of carbon into the deep ocean," Buesseler says.

During two separate cruises in 2004 and 2005, the scientists were able to take multiple measurements of the flux of carbon burial at the two sites. The data were consistent: Off the coast of Japan and Asia, cooler waters and greater nutrient supply support a thriving ecosystem, including larger phytoplankton incorporating relatively heavy minerals, like silica. As a result, roughly 50 percent of the captured carbon sinks through the so-called twilight zone there—perhaps because it is heavier and therefore descends faster—compared with just 20 percent in the balmier waters off Hawaii, which support smaller life-forms, researchers report this week in Science. "Once you get below 1,000 meters [3,280 feet], you really don't see much change on this last leg," Buesseler says. "Once you get it deep enough, those waters take century or millennia to reventilate."

The researchers can assess how much carbon can be captured and stored in the deep oceans by studying the amount of carbon that gets recycled back to the surface. Currently, that carbon capture is measured by the Martin curve—a set of data from the 1980s that shows how more carbon is trapped the deeper into the ocean it moves. But Buesseler says the Martin curve would underestimate how much carbon gets through in Asia by 50 percent while doubling the actual rate off Hawaii. "It is a good average but it doesn't describe the dynamics of the system where the ocean might be a good carbon sink," he notes. "We've really only scratched the surface."

Already the team has improved its sensor design and plans to take a series of samples in the waters off Bermuda to examine seasonal variations and other fluxes. "We know there are variations in the deep ocean," Buesseler explains. "Much of that seems to be controlled not by changes at the surface but by variations in the twilight zone." He adds: "This twilight zone acts as a gate for the transfer of carbon to the deep ocean." The ability of the ocean to save the globe from climate change may hinge on the behavior of microscopic animals in a dim, watery world.