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Virginia Institute of Marine Science

VIMS researchers re-enter ocean "twilight zone"

Researchers from the Virginia Institute of Marine Science are once again exploring one of Earth’s least visited and most poorly understood habitats—the vast layer of mid-ocean water that oceanographers call the "twilight zone."

VIMS Associate Professor Deborah Steinberg, technician Joe Cope, and graduate student Stephanie Wilson are currently aboard the 274-foot Scripps research vessel Roger Revelle, which is visiting the cold and nutrient-rich waters of the North Pacific as part of the multi-institution VERTIGO project. Their instruments are targeting waters between 1,500 and 3,000 feet deep, where the filtered sunlight provides only twilight-like illumination on even the brightest days.

VERTIGO, for VERtical Transport In the Global Ocean, is a collaborative international venture that includes VIMS, the Woods Hole Oceanographic Institution (WHOI), the University of California, the Lawrence Berkeley National Lab, the University of Tasmania, New Zealand’s National Institute of Water and Atmospheric Research, India’s Physical Research Lab, and the University of Brussels, Belgium.

The current VERTIGO cruise, to a Japanese time-series site off the Kuril Islands, is the last of three cruises planned for the project. The first cruise in January 2004 provided an engineering test of some of the new sampling equipment. A second cruise in summer 2004 took the VERTIGO team to waters north of Hawaii.

The goal of the three-year project is to better understand how organic matter gets from the ocean’s surface to its depths. The fate of this material is important because it represents a potentially huge sink for carbon dioxide—the main greenhouse gas implicated in global warming.

Atmospheric carbon dioxide dissolves into seawater, where tiny marine plants take up the carbon during photosynthesis. When these phytoplankton die or are eaten by tiny marine animals called zooplankton, their carbon-rich tissues begin to sink to the depths. Carbon that reaches the deep sea contributes nothing to global warming and can remain there for thousands of years.

But marine organisms and chemical reactions continually transform this "marine snow" as it sinks. If these transformations are rapid and sinking is slow, the materials may never reach the deep sea, instead being recycled back into the surface ocean and atmosphere.

"The export of surface particles to the deep sea is called the biological pump," says Steinberg. "A main goal of VERTIGO is to determine how that pump runs—to quantify the fraction of sinking particles that reach the depths and to identify the biological and chemical processes that transform them along the way." The information will ultimately be fed into computer models used to predict the likelihood and consequences of global warming.

Scientists have previously used surface measurements and deep-sea cores to estimate the pump's efficiency. But studying the processes that transform particles in the mid-ocean "twilight zone" has proven more difficult.

"The whole difficulty of studying sinking particles is catching them," says WHOI scientist Dr. Ken Buesseler. "There are very few to start with in this part of the world's ocean, and they sink very slowly relative to the movement of the ocean currents." He compares the challenge to deploying a "rain gauge in a hurricane."

The VERTIGO team is tackling these sampling difficulties by employing a whole suite of specially designed oceanographic instruments. These include "neutrally buoyant sediment traps," which were developed by WHOI engineer Jim Valdes and scientific collaborators Buesseler, Steinberg, and Jim Price. The traps are weighted to float at a particular depth in the water column, where they open and close their funnel-like collection bottles at pre-determined times to capture sinking debris. After 3 to 5 days the whole package pops back up to the surface.

"It's quite a balancing act to get these to sink to the depth we need," says Buessler. "If we're off in our calculations by the weight of even a few quarters, the whole package can sink too deep and never return, or never sink at all. During VERTIGO, we have up to seven of these devices floating below and days later popping back up to the surface where they relay their position to the ship via satellite.”

Another high-tech device called a MOCNESS plankton net (for Multiple Opening and Closing Net Environmental Sampling System) is helping Steinberg's team accomplish their role in VERTIGO, which is to identify the types of zooplankton in the twilight zone, estimate their abundance, and quantify their role in particle transformation.

"Traditional plankton nets are continually open and thus sample and mix organisms from the surface to the depths and back," says Steinberg. "The MOCNESS has a set of 10 nets that can be programmed to individually open and close at a particular depth, allowing us to isolate and study the zooplankton from a discrete water level. The zooplankton in the twilight zone have been studied very little, and we don’t know much about how they make a living down there."

VERTIGO is supported primarily by the U.S. National Science Foundation in collaboration with the Antarctic Climate & Ecosystems Cooperative Research Centre, the U.S. Department of Energy, WHOI, the Fund for Scientific Research-Flanders (Belgium), and other national and international sponsors.

For more information and to follow the progress of the current cruise online, visit http://cafethorium.whoi.edu/website/projects/vertigo_project.html.