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Marine snow measured in the twilight zone

Researchers measuring the amount of carbon that passes through the ocean's "twilight zone," found striking differences from two areas in the Pacific Ocean that could have a large effect on calculations of the amount of carbon stored in the deep ocean. An international team of researchers led by Ken Buesseler developed a new tool to accurately measure the carbon in the twilight zone, and used the tool to study the twilight zone near Hawaii and in the northwest Pacific Ocean.

The twilight zone is the layer between the sun-lit surface ocean where photosynthesis occurs and the deep ocean. The zone receives much than 1 percent of the light that the surface gets and it lies between 300 and 1,600 feet below the ocean surface. Not much is known about this area.

Buesseler and colleagues chose to study this zone to find out how much carbon travels through it. Plankton grows in the bright, top layer. The plankton and dead matter float down through the twilight zone -- Buesseler calls it "marine snow" because it just falls through the ocean until it is eaten or drops on the ocean floor. This "snow" is full of carbon.

Carbon is the reason the researchers are studying this area. Ken Buesseler explains in his own words. "It is an important component of the oceanic food web, and we're interested in how much carbon is exported into the deep ocean. The oceans are taking up about half the carbon dioxide released due to fossil fuel burning. Carbon dioxide is a greenhouse gas, so the more that is in the atmosphere, the warmer our climate becomes. Unless the carbon that gets into the ocean goes all the way down into the deep ocean and is stored there, the oceans will have little impact on the atmosphere and the climate."

To get better information, the team developed a new way to measure the marine rain. Ocean researchers used to use cones underneath buoys or anchored to the bottom to collect data much like a stable rain gauge. Buesseler's team built a more accurate particle detector that sinks down to the ocean depth it is programmed to and collects particles as it travels with the currents for days.

Using this new measuring device, the team measured the carbon passing through the twilight zone in Hawaii. Only 20 percent of the sinking carbon survived to the 1,600 feet depth. The other 80 percent was consumed by plankton. In contrast, when they took measurements in the northwest Pacific, 50 percent of the carbon made it to 1,600 feet in the northwest Pacific.

Buesseler says that they don't really have an explanation for the big difference. "It may have to do with the lower temperatures in the northern Pacific that slow down the bacterial breakdown of organic carbon," he says. The research team will continue to look for answers to understand how much carbon is stored in the deep ocean.

This research appears in the 27 April issue of the journal Science.