Follow the Carbon Trail

Carbon makes the world go around. It is the building block of life on Earth, and in the form of carbon dioxide gas in the atmosphere, it has a powerful impact on the planet’s climate. In the process, carbon also goes around the world and through the oceans—in a complex cycle that, until recently at least, the Earth has kept in balance.

Plants on land and sea take carbon dioxide from the atmosphere and convert it into organic matter eaten by animals. Both decompose, and the carbon is swept into the oceans, where it dissolves and/or sinks and is buried in seafloor sediments. Over millions of years, that carbon becomes incorporated into rocks or hydrocarbons, where it might remain for tens to hundreds of millions of years. Ultimately, volcanoes return some of this carbon in gaseous form to the air, or the rocks are uplifted onto continents and gradually weathered, releasing carbon to soils or the atmosphere to complete the cycle.

The trouble is that we humans have intervened and interfered with the carbon cycle. We have effectively transferred carbon from pools in which it cycles slowly, such as sedimentary rocks, to actively cycling pools, such as the air and ocean. In a short time, we have extracted large amounts of hydrocarbons from the Earth, burned them for fuel, and put an excess of heat-trapping carbon dioxide in the atmosphere.

So a crucial question in the global warming debate is: Can the oceans ramp up correspondingly to absorb this excess CO₂?

The answer will come from addressing key questions about the carbon cycle in the ocean: How much carbon in surface waters is recycled by marine organisms or exchanged back into the atmosphere? How much gets buried on shallow continental margins or exported to the deep ocean? What controls these dynamic processes?

To find out, we must follow the carbon trail. Using innovative techniques, we have searched seafloor sediments for telltale chemical clues that reveal where carbon came from and when. The evidence is demonstrating that carbon charts surprising pathways through the ocean.

Sargasso Sea sediments

In 1998, I joined a cruise aboard Oceanus, a research vessel operated by Woods Hole Oceanographic Institution (WHOI), to the Bermuda Rise, a region north of the island of Bermuda in the Sargasso Sea where sediment accumulates rapidly on the seafloor. Working with WHOI colleagues Nao Ohkouchi, Lloyd Keigwin, and John Hayes, I investigated seafloor sediments we collected for particular organic compounds called alkenones.

Alkenones are lipids with curious characteristics that have proven quite useful for carbon-tracking detectives. First, they are produced almost exclusively by marine algae—especially one species, Emiliania huxleyi. When seawater temperatures are warmer, these algae produce alkenones with fewer double bonds; when temperatures are colder, they make alkenones with more double bonds. They do this to keep their cell membranes fluid, based on the same principle that allows margarine to spread more easily out of the refrigerator compared to butter (whose fats are more “saturated” and contain fewer double bonds). Thus, alkenones preserved in seafloor sediments provide a record of the temperatures of the waters in which they were made.

Next, with scientists at the National Ocean Sciences Accelerator Mass Spectrometry Facility at WHOI, we have developed methods to carbon-date not just conglomerations of carbon-containing material but individual carbon-based compounds, such as alkenones. Using these methods, we could determine where, when, and how the alkenones in Bermuda Rise sediments were made. That provides an important piece in the puzzle we are reconstructing of past ocean and climate conditions on Earth. It offers essential insights into how Earth’s ocean and climate system works and what it is capable of in the future.

A long way from home

The clues from the Bermuda Rise told an unexpected story. First, the top layers of sediments contained organic compounds from marine algae that were thousands of years old. How could carbon that was only recently deposited be so old?

Second, most of the organic material contained alkenones produced in surface waters much colder than the Sargasso Sea. They were made in waters with temperatures like those off the continental shelf of Nova Scotia, where blooms of E. huxleyi are massive enough to supply the amount of alkenones we found on the Bermuda Rise. But how could organic matter produced off Nova Scotia find its way to sediments under the Sargasso Sea more than 1,000 kilometers (621 miles) away?

Since the organic matter could not have sunk down vertically from the Sargasso Sea surface, the conundrum prompted some lateral thinking. My colleagues and I postulated that it came from the continental margins off Nova Scotia and was transported laterally by deep ocean currents.

Our theory is that sediments off Nova Scotia (and elsewhere) are sporadically resuspended by swift currents near the ocean floor—a phenomenon known as “benthic storms,” which, like storms in the atmosphere, can stir up settled material. The resuspended particles are then swept up by a large, powerful deep current called the Deep Western Boundary Current, which flows across the Nova Scotian margin, southward along the continental slope of the United States, and into the South Atlantic.

East of Cape Hatteras, the Deep Western Boundary Current intercepts two deep gyres, hundreds of kilometers in diameter: The Worthington Gyre circulates clockwise and lies just south of the counterclockwise North Recirculating Gyre. We think the two great gyres act like interlocking cogs, drawing Nova Scotian particles from the Deep Western Boundary Current, channeling them eastward between the gyres, and carrying them onto the Bermuda Rise.

Testing the hypothesis

In 2004, an opportunity arose to test our hypothesis directly. To study the Deep Western Boundary Current, WHOI physical oceanographer John Toole had deployed an array of moorings across the continental slope off Cape Cod called Line W. The moorings have a package of instruments that travel up and down the mooring cables daily, taking measurements. With support from the WHOI Ocean and Climate Change Institute, we added optical sensors on the instrument package to monitor the abundance of particles in the water column, and we installed pumps and sediment traps—one near the ocean bottom and one farther up—to collect particles.

The optical sensors revealed “clouds” of particles “blowing” laterally from the continental shelf, down the slope, and out into the deep ocean—persistently in some cases and ephemerally in others. The traps showed similarly complex particle dynamics, as well as evidence for lateral transport of material from the continental shelf to the north.

Together, the findings of this pilot survey were intriguing enough to attract funding from the National Science Foundation (NSF) for further investigations, with WHOI collaborators Liviu Giosan, Jeomshik Hwang, and Steve Manganini, as well as Roger Francois (formerly at WHOI and now at the University of British Columbia). With support from the NSF and WHOI Ocean and Climate Change Institute, we have now deployed two moorings, interspersed between the Line W moorings, on the continental slope at depths of 2,000 and 3,000 meters (1.25 and 1.86 miles). Each has sediment traps—near the seafloor, at an intermediate depth, and in shallower waters.

The traps are positioned to intercept particles raining from overlying surface water, particles being laterally transported eastward off the continental shelf, down the continental slope, and along the seafloor, as well as particles carried southward by the Deep Western Boundary Current. We will be able to analyze the chemistry of the material we collect to determine the composition, age, and source of carbon being exported to the ocean interior.

Although our studies are targeting only one specific region of the ocean, there is growing evidence that lateral carbon transport from the continental margins to the deep ocean is an important component of the oceanic carbon cycle. Observations we are now making offer a unique window into the cycling of carbon in these dynamic regions—shedding light on a heretofore hidden series of chemical transformations and physical phenomena that play an integral role in regulating the planet’s climate and life.