May 12, 2003

Seafloor sediments hold clues to runaway global warming

By Kasey White

Scientists from UCSC and other institutions around the world arrived in Rio de Janeiro last week after spending two months at sea on the research ship JOIDES Resolution near an ancient submarine mountain chain off Africa, known as the Walvis Ridge.

There they studied evidence of a massive release of methane that caused extreme global warming 55 million years ago.

This extraordinary episode of global warming, often referred to as the Paleocene-Eocene Thermal Maximum, is unique in Earth's history in terms of magnitude and rate of warming, as well as in the manner in which it began.

Scientists think that roughly 2,000 gigatons (2 trillion tons) of methane, a potent greenhouse gas, was released to the ocean and atmosphere 55 million years ago, triggering a runaway process of global warming.

The scientists on Ocean Drilling Program (ODP) Leg 208 set out to test this hypothesis about the origin of the Paleocene-Eocene Thermal Maximum.

Geochemists speculate that methane escaped from submarine clathrates, ice crystals that trap methane and are distributed in sediments on the outer edges of continental margins worldwide. For reasons that remain unknown, the clathrates suddenly began to decompose on a massive scale, increasing the amount of methane in the atmosphere and ocean. This decomposition process appears to have lasted for a period of 40,000 years, ultimately warming the planet by more than 5 degrees Celsius.

Sediments far below the seafloor hold evidence of the methane release and its consequences.

"The rapid release of methane 55 million years ago should have left a distinct signature in the form of calcite-poor or clay-rich layers, the distribution and thickness of which would be controlled by the total mass of methane released," said James Zachos, cochief scientist of ODP Leg 208 and professor of Earth sciences at UCSC.

This is because the rapid release of such a large mass of methane and its subsequent oxidation to carbon dioxide would have significantly altered ocean chemistry. The added carbon dioxide would have increased the overall acidity or corrosiveness of seawater. This, in turn, would increase the dissolution of calcite shells of microplankton, which are the dominant component of seafloor sediments, leaving behind only nonsoluble clays. The dissolution of calcite would initiate in the deepest parts of the ocean and rapidly spread upwards as additional carbon dioxide entered the ocean. The overall extent and duration of dissolution would ultimately be controlled by the total mass of methane released.

"By establishing the vertical extent of carbonate dissolution, as well as the duration of dissolution, it should be possible to determine the total amount of carbon dioxide that was added to the ocean during the event," Zachos said.

Retrieving the sediments proved to be a formidable challenge. To accurately reconstruct past climate, technicians needed to retrieve the sediments without deformation of layers and other structures.

"Imagine the difficulties of retrieving a layer of sediment about 1 meter thick with the consistency of mud from 200 to 300 meters below the seafloor at a water depth of 4,800 meters, without disturbing the centimeter-scale layering," said Dick Kroon, cochief scientist from Vrije Universiteit Amsterdam.

Technicians used a device known as the Advanced Piston Corer (APC) to obtain the cores. The extreme stress placed on the system, however, quickly began to take its toll on the APC. In the very first hole, the steel core barrel literally snapped in half from the enormous stresses applied while attempting to extract it from the sediment. The lower half of the barrel could not be retrieved, and the hole had to be abandoned. On several other occasions, the steel rods that secure the core barrels came back to the surface twisted or bent.

"At this point, we were a little concerned about whether we could achieve our objectives," Zachos said.

Determined, the drilling crew made the necessary repairs, adjusted their drilling strategy, and pushed on. Their efforts ultimately paid off; the thin boundary layer was recovered at five sites in water depths between 2,500 and 4,800 meters. Remarkably, at each site the layer was fully intact and in perfect condition.

Initial examination of the sediments provided immediate insight into the scale of calcite dissolution during this event. Micah Nicolo, a graduate student at Rice University, remarked, "When the cores were opened in the ship's lab, we were stunned by the change in colors of the sediment, from bright white carbonate to deep red clays."

Each core, regardless of depth, yielded a sequence of carbonate-rich sediment dissected by a distinct, dark clay layer, varying in thickness from 100 to 50 centimeters. The base of each clay layer, regardless of depth, contained essentially no calcite--indicating dissolution of calcite sediment throughout the ocean.

Ellen Thomas of Wesleyan University studied additional impacts of the methane release.

"The extent of dissolution may explain why so many bottom-dwelling organisms that precipitate calcite shells became extinct at that time," she said.

The scale of carbonate dissolution recorded in these cores is significant. It is suggestive of a much larger flux of methane, possibly double original estimates. It may also point toward an additional source of greenhouse gas.

"It far exceeds what has been estimated by models assuming a release of 2,000 gigatons of methane," Kroon said.

The initial results also suggest that the deposition of carbonate shells on the deeper reaches of the seafloor did not resume for at least 50,000 years, and that the total recovery time to a "normal state" took as long as 100,000 years. This result suggests that full recovery from these extreme events takes considerable time.

The cores recovered on this leg may also provide insight into the ultimate cause of the thermal maximum. Toward the end of the Paleocene epoch, the planet was slowly warming due to rising levels of carbon dioxide emitted from volcanoes.

"Several of us suspect that the melting of clathrates and rapid release of methane was initiated by gradual warming that pushed the climate system across a physical threshold," Zachos said.

Once started, the release of methane and resultant warming fueled the release of more methane, a positive feedback effect. This phenomenon is a concern for future global warming, Zachos said.

Studies of the sediment cores from Walvis Ridge and others recently recovered in the Atlantic and Pacific Oceans will allow scientists to test these and other ideas about the origin of the Paleocene-Eocene Thermal Maximum.

ODP is an international partnership of scientists and research institutions organized to study the evolution and structure of the Earth. It is funded principally by the U.S. National Science Foundation, with substantial contributions from its international partners. The Joint Oceanographic Institutions (JOI) manages the program. Texas A&M University is responsible for science operations, and Lamont-Doherty Earth Observatory of Columbia University is responsible for logging services.

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