Ocean's 'Twilight Zone' Plays Important Role in Climate Change
The results of two international research expeditions to the
sinking marine particles is often consumed by animals and bacteria and recycled in the twilight zone -- 100 to 1,000 meters below the surface -- and never reaches the deep ocean.
Using new technology, the researchers found that only 20 percent of the total carbon in the ocean surface made it through the twilight zone off
The twilight zone acts as a "gate," allowing more sinking particles through in some regions and fewer in others, complicating scientists' ability to predict the ocean's role in offsetting the impacts of greenhouse gases. It also adds a new wrinkle to proposals to mitigate climate change by fertilizing the oceans with iron -- to promote blooms of photosynthetic marine plants and transfer more carbon dioxide from the air to the deep ocean.
"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," he said.
Buesseler was a leader of the ambitious project, funded primarily by the US National Science Foundation, called VERTIGO (VERtical Transport In the Global Ocean). More than 40 biologists, chemists, physical oceanographers, and engineers from 14 institutions and seven countries participated in the two VERTIGO cruises in 2004 and 2005 to investigate how marine plants die and sink, or are eaten by animals and converted into sinking fecal pellets.
These sinking particles, often called "marine snow," supply food to organisms deeper down, including bacteria that decompose the particles. In the process, carbon is converted back into dissolved organic and inorganic forms that are re-circulated and reused in the twilight zone and that can make their way to the surface and back into the atmosphere.
The sites off
"This combination of expertise could not be found in any single lab or country," Buesseler said. "We were fortunate to attract such a diverse group of talented scientists willing to unravel the secrets of the twilight zone and its role in the global carbon cycle."
While many studies have investigated the surface of the ocean, little research has been conducted on the carbon cycle below. The VERTIGO team examined a variety of processes to open a new window into the difficult-to-explore twilight zone. They successfully used a wide array of new tools, including an experimental device that overcame a longstanding problem of how to collect marine snow falling
into the twilight zone.
The problem is that particles sink slowly, perhaps 10 to a few hundred meters per day, but they are swept sideways by ocean currents traveling many thousands of meters per day. To collect sinking particles, scientists use cones or tubes that hang beneath buoys or float up from seafloor. That, Buesseler said, "is like putting out a rain gauge in a hurricane."
Buesseler and WHOI engineer Jim Valdes developed Neutrally Buoyant Sediment Traps (NBST) -- free-floating devices that sink to a programmed depth within the twilight zone and neither sink nor rise. They are swept along with the currents for several days, collecting particles, and then programmed to resurface, transmit their position via satellite, and wait for recovery, more than 10 to 20 miles
away from where they were dropped into the ocean.
"It's a bit like finding a needle in a haystack, since they are so small and difficult to spot, especially in rough seas that are common in the open ocean," Valdes said.
On their first scientific mission for VERTIGO in 2004, Buesseler and Valdes could only wait and hope their devices would work. "Seven NBSTs went in the water and all seven came back with their precious cargo -- a first in ocean sciences history," Buesseler said.
Why more carbon reached the depths in the northwest Pacific might be due to many factors. Waters there are full of silica that plankton incorporate to make shells. Do the silica-laden plankton weigh more and thus sink faster, giving bacteria less time to break them down? Do lower water temperatures in the northwest Pacific slow down the breakdown of organic carbon? Do different populations in the food webs at different sites change how organic matter is broken down and marine snow is produced? Scientists will continue to explore these regional differences in the
ability of carbon to reach the deep sea.
In June, Buesseler and colleagues head 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.