Fossil Records Show Methane in Seafloor Sediments Released During Periods of Rapid Climate Warming
Scientists have found new evidence indicating that during periods of rapid climate warming methane gas has been released periodically from the seafloor in intense eruptions. In a study published in the current issue of the journal Science, Kai-Uwe Hinrichs and colleagues Laura Hmelo and Sean Sylva of the Woods Hole Oceanographic Institution (WHOI) provide a direct link between methane reservoirs in coastal marine sediments and the global carbon cycle, an indicator of global warming and cooling.
Molecular fossils from methane-consuming bacteria found in sediments in the Santa Barbara Basin off California deposited during the last glacial period, (70,000 to 12,000 years ago), indicate that large quantities of methane were emitted repeatedly from the seafloor during warmer phases of the last ice age. Methane, one of the major greenhouse gases, is stored on the seafloor as an ice-like solid known as methane hydrate.
Previous evidence for such massive eruptions was based on isotopic properties of calcite shells of foraminifera, microscopic marine animals commonly called forams. Because a variety of factors could lead to very similar signals in their shells, that evidence has remained controversial.
The preserved molecular remnants found by the WHOI team result from bacteria that fed exclusively on methane and indicate that large quantities of this powerful greenhouse gas were present in coastal waters off California. The team studied samples that were deposited between 44,000 and 37,000 years ago.
"For the first time, we are able to clearly establish a connection between distinct isotopic depletions in forams and high concentrations of methane in the fossil record," Hinrichs, an assistant scientist in the Institution's Geology and Geophysics Department, says. "The large amounts of methane presumably released during one event about 44,000 years ago suggest a mechanism different from those underlying the emissions at warmer periods, i.e. slow decomposition of methane hydrate triggered by warming of bottom waters. The sudden release of these enormous quantities of methane was probably caused by landslides and melting of the methane hydrate."
Since there was already indirect evidence of methane eruptions in the Santa Barbara Basin area, Hinrichs and colleagues looked for fossil remnants of bacteria that would have flourished only under high concentrations of methane. In a 44,000-year-old sediment sample, a distinct type of biomarker representing bacterial communities that oxidize methane in the absence of oxygen provided evidence for an abrupt, catastrophic release of methane, presumably trapped as hydrate below the sea floor.
The WHOI team's data, from sediment cores taken by the Ocean Drilling Program off southern California, show that substantial quantities of methane were released at least several times during the past 60,000 years, leading to periodic fluctuations in the levels of methane in deep waters in the Santa Barbara Basin.
The researchers say increased bottom water temperatures could mobilize or release significant amounts of methane hydrate in shallow waters. According to some current estimates, there are about 10,000 billion tons of methane stored beneath the ocean and on continents. In comparison, the contribution of humans to the atmosphere's inventory of greenhouse gases by fossil-fuel burning amounts to about 200 billion tons of carbon in the form of carbon dioxide. If even a small portion of the stored methane were to escape into the atmosphere, the resulting greenhouse warming would be catastrophic.
"It was a surprise to find this sort of evidence," says Hinrichs, who was originally looking for evidence indicating mechanisms other than methane. "Although this research tells us something about the amount of methane consumed by bacteria in the ocean, it doesn't tell us anything about methane emissions into the atmosphere because neither forams nor methane biomarkers record the portion of methane that escaped out of the ocean. But one thing is for sure, our results clearly show that relatively minor environmental changes can have a major impact on sensitive coastal regions with yet unknown consequences for climate and biota."
Hinrichs plans to look for similar evidence elsewhere to determine whether this process, as a driver of climate variation, happened simultaneously at other locations around the world. This work, Hinrichs says, is just the beginning of better understanding of the role of methane in the carbon cycle and ultimately on climate on geologic time scales.
"We have a very poor understanding of the biogeochemical mechanisms that control production, destruction and accumulation of methane in sediments underlying the ocean," he notes. "We need to understand the big picture of what drives methane and the carbon cycle and the actual impact of methane emissions from hydrates on climate."
WHOI is a private, independent marine research and engineering, and higher education organization located in Falmouth, MA. Its primary mission is to understand the oceans and their interaction with the Earth as a whole, and to communicate a basic understanding of the ocean's role in the changing global environment. Established in 1930 on a recommendation from the National Academy of Sciences, the Institution is organized into five departments, interdisciplinary institutes and a marine policy center, and conducts a joint graduate education program with the Massachusetts Institute of Technology.