Voyage to ocean's 'Twilight Zone'

The middle depths get a closer look, in a new effort to probe how ocean's carbon cycle affects climate.

On a mid-January night, the Kilo Moana's floodlit deck glistens under a thin veneer of water drained from sampling bottles and dripping from nets, topped by a fine spray kicked up as the ship rides the swells.

Standing at the stern rail, Brian Guest points to foot-long squids darting just below a foam-mottled surface. The squids are looking to make meals of the fish attracted to the ship's lights.

"That's why I like working night watches," the engineer from the Woods Hole Oceanographic Institution says quietly, as he awaits the return of some sampling equipment pulled up from the ocean's depths. "You get to see so much just looking over the side."

The squids represent more than an evening's diversion, however. They also testify to the ocean's ability to draw carbon from the atmosphere and store it in the water and in its inhabitants.

As human activities pump more heat-trapping carbon dioxide into the atmosphere, researchers are trying to understand how effectively the ocean will continue to function as a long-term "safe-deposit box" for carbon dioxide. Creatures big and small - through their carbon-rich detritus that sinks to the bottom- drive most of this long-term carbon storage.

Scientists on board the Kilo Moana hope to begin the process of determining how much and how fast this material moves into deep waters or onto the ocean floor, as well as how those processes change as the climate changes.

For marine biologists and geochemists, results from the two-year project, dubbed VERTIGO, are expected to yield a wealth of insights into a little-studied layer of ocean: the "twilight zone," which ranges from 1,640 to 3,281 feet deep.

The movement through this region of carbon-rich castoffs from shallower depths is the key driver in the ocean's biological carbon pump, says Ken Buesseler, the project's chief scientist.

For climate researchers, the results are expected to help enhance computer simulations of the ocean carbon cycle and its effect on climate. More accurate simulations not only are needed to improve forecasts of climate change, scientists say, but they also could help gauge the value of controversial proposals to stimulate plankton growth on the ocean's surface. The plankton would then draw more carbon dioxide from the atmosphere for storage in the deep ocean and perhaps help slow global warming.

In June, researchers will return to this site, roughly 65 nautical miles north of Oahu, to begin more detailed studies. Then in 2005, they will move to sites near Japan.

The project comes at a time of increasing interest in the ocean's smallest, most biologically productive inhabitants and the factors that govern their survival. While surface-dwelling phytoplankton use carbon in photosynthesis, they also soak up nutrients such as iron, nitrogen, and phosphorus. So researchers are learning about the large-scale movement of other nutrients, in addition to carbon.

For example, the Southern Ocean, which rings Antarctica, has long been recognized as a rich food source for a range of local marine life. Yet a study published in the journal Nature last month and led by Jorge Sarmiento at Princeton University suggests that the Southern Ocean is the source of basic nutrients for the entire Southern Hemisphere and North Atlantic. Another "hot spot," located in the Northwest Pacific's Sea of Okhotsk, appears to play the same role for the North Pacific.

Indeed, the same processes of falling detritus that draw carbon into the deep ocean do the same for nutrients, which are redistributed by deep-sea circulation and brought back to the surface, Dr. Sarmiento says. Though not part of the VERTIGO collaboration, he suggests that data gathered from VERTIGO may also be used to track the factors that affect nutrient flows from the surface through the "twilight zone" and into the deep ocean.

The link is not lost on the VERTIGO team, which also will be trying to look at the relative speed with which carbon and nutrients move.

If carbon moves more slowly than nutrients, then the ocean system could be more efficient in trapping carbon than previously thought, says Dr. Buesseler, a Woods Hole marine geochemist.

Carbon distribution

Sitting in his stateroom one evening, Buesseler explains that roughly one-third of the carbon dioxide humans introduce from fossil fuels goes into the ocean, a third goes into the atmosphere, and a third goes into land.

Phytoplankton near the ocean surface absorb carbon dioxide through photosynthesis, he adds. But when the organisms and the creatures that eat them die, decomposition returns the majority of that carbon into carbon dioxide in the upper ocean, where it evaporates back into the atmosphere.

It's a "temporary sink" for carbon, whose residence time is measured in months, he says. "If you can get the carbon out of the surface and into deeper waters, [ocean] mixing and physics can keep it there for hundreds to thousands of years."

Skimming the surface

The VERTIGO project traces its pedigree to an international research effort that ran from the late '80s to the late '90s, known as the Joint Global Ocean Flux Study.

The project used satellite measurements and ocean experiments to help unravel the carbon cycle. But the effort focused mostly on the top 100 meters of the ocean, Buesseler says, and measurements of how much carbon hits the 100-meter level and how much hits the sea floor suggested something major was missing.

Roughly 90 percent of the carbon the ocean takes into its surface layer remains there to be exchanged with the atmosphere. On average, the remaining 10 percent works its way down. But only 1 percent of the original surface carbon ever reaches the sea floor.

"That's a huge difference," he says. "And we don't understand the current [carbon] system well enough to know if it could change or if it's changed in the past" and what effect changes may have had.

The VERTIGO team will test two ideas: that the transport of material through the twilight zone is controlled largely by the properties of the material itself - that it naturally clumps into larger and larger particles as it falls or carries its own ballast - or that biology rules, either as bacteria breaks the detritus down or as creatures inhabiting the twilight zone feast on this "marine sediment."

Life aboard the Kilo Moana

In an unusual move, funding agencies have given Buesseler and his colleagues enough money to pay for this seven-day shakedown cruise. But at roughly $20,000 a day, ship time is precious. So sampling and testing operations run 24 hours a day.

Even for visitors, it means at least one turn on watch, helping to deploy and retrieve an array of sampling bottles weighing 600 pounds empty and 1,400 pounds full.

For light sleepers, it means tiny foam ear plugs to deaden the overnight clanking of power winches or the slap of swells on the underside of the ship's superstructure, which in catamaran-like fashion spans two parallel hulls. Each slap sends a thunderous "whang" reverberating throughout the 186-foot ship.

Yet for all the frustrations of bulky gear and semisleepless nights, the researchers and crew try to keep the mood light.

During the long hours of sample-processing, the monotony is broken by Stevie Ray Vaughn's fiery guitar licks and the smell of brewing espresso. A free T-shirt goes to a crew member who, after a three-hour search, was the first to spot an elusive robotic sample collector the team was trying to retrieve from the ocean.

Even marine life cooperates. During the search, whales spout and breach in the distance. A pair of humpback whales appears suddenly and swims just ahead of the slowly moving vessel. Later in the day, a third plays tag just off the stern.

As the cruise winds down, Buesseler reflects on "reentry" ashore.

"It's tough" he says. "We're so self-contained. We don't have to sweat the housekeeping details. Then you're back [on land], and, man, I have to go shopping?"

If that's the case, the Kilo Moana's home port of Honolulu is not a bad place to start.

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