Twilight zone plays key role in climate change

New study identifies a critical link in the ocean's ability to store carbon dioxide

A study in the April 27th issue of Science sheds new light on the ocean's "twilight zone"—the dim layer from 300 to 3,000 feet deep where little-known processes affect the ocean's ability to absorb and store carbon dioxide accumulating in our atmosphere.

The multi-author study, which includes Virginia Institute of Marine Science Associate Professor Dr. Deborah Steinberg and graduate student Stephanie Wilson, reports the results of two international research expeditions to the Pacific Ocean.

The study shows that carbon dioxide—taken up by photosynthesizing marine plants in the ocean's sunlit surface layer—does not necessarily sink to the depths, where it is stored and prevented from re-entering the atmosphere as a greenhouse gas. Instead, carbon transported to the depths on sinking marine particles is often consumed by animals and bacteria and recycled in the twilight zone—where it can then mix back into the surface ocean and atmosphere.

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 Hawaii, while 50 percent did in the northwest Pacific near Japan.

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—an idea proposed to promote blooms of photosynthetic marine plants that could potentially 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," says lead author Dr. Ken Buesseler of the Woods Hole Oceanographic Institution (WHOI). "We're interested in what happens in the twilight zone, what sinks into it and what sinks out. 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 Earth's climate."

Scientists with the VERTIGO project studied plankton from the ocean's twilight zone. From L: Toru Kobari (Kagoshima University, Japan), marine technician Joe Cope, graduate student Stephanie Wilson, and Dr. Deborah Steinberg with numbered sample buckets.Steinberg and Wilson's role in the project was to identify plankton species living in the twilight zone and to understand differences in the food webs that propel the marine carbon cycle. The pair, along with technician Joe Cope, spent weeks at sea during research expeditions in 2004 and 2005, and countless more hours in their Gloucester Point lab analyzing samples.

Says Steinberg, "Our results show that the composition of the zooplankton community in the twilight zone—copepods, krill, and other such animals— significantly affects the amount of carbon recycled and its chemical form. This helps explain the differences we saw between our sub-tropical and sub-arctic sites, and is important information for the computer models that are used to understand the carbon cycle and predict climate change."

The ambitious 3-year project, led by Buesseler and funded primarily by the US National Science Foundation, is 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 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 and zooplankton that decompose and consume 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 Hawaii and Japan were selected because they had been the focus of long-term ocean observations by co-authors David Karl (University of Hawaii) and Makio Honda (Japan Agency for Marine-Earth Science and Technology).

Mary Silver (University of California, Santa Cruz) provided expertise in studies of marine snow. Thomas Trull (University of Tasmania, Australia), Philip Boyd (University of Otago, New Zealand), and Frank Dehairs (Free University of Brussels, Belgium) all study the Southern Ocean and could provide important perspectives contrasting ocean carbon cycle off Antarctica and in VERTIGO. David Siegel (University of California, Santa Barbara) helped track the ocean currents and pathways of the sinking particles. James Bishop (Lawrence Berkeley National Laboratory and University of California, Berkeley) was funded by the US Department of Energy to deploy new autonomous optical sediment traps designed to follow the hourly changes in sedimentation as well as ship deployed particle sampling systems to quantify the abundance and composition of particles in the twilight zone.

"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 100 meters per day, but they are swept sideways by ocean currents traveling many 1000s 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 Bermuda to examine seasonal changes in the rain of carbon through the twilight zone. They will repeatedly sample at a single site, using a new and improved NBST called the Twilight Zone Explorer.

"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.

—by Stephanie Murphy of WHOI; adapted by David Malmquist of VIMS