Gas Hydrates, Methanogenesis, and Carbon Cycling at Continental Margins

Dr. Alberto Malinverno, an expert in quantitative marine geology, came from Columbia University to deliver a talk on a unique substance from the bottom of the ocean which could someday be an energy source for humanity… but which could also drastically worsen the effects of anthropogenic climate change. The substance is a clathrate, or gas hydrate – methane molecules trapped within the crystal structure of ice. Clathrates are unstable at surface pressure and temperature conditions, preferring low temperatures and high pressures. Because of these unique stability conditions, clathrates are found at depth in places where the temperatures are cold: in permafrost, and at the ocean floor near continental margins, where organic carbon in the sediments provides the methane.

Gas hydrates are widespread across the globe. They are thought to hold about twenty percent of all organic carbon on Earth. As Malinverno explained, “gas hydrates can dissociate when temperatures rise, releasing large amounts of greenhouse gases” (causing extensive submarine landslides in the process) and potentially leading to runaway global warming. On the other hand, they could also be exploited as an energy resource, much as oil and gas are exploited now. Therefore, for both of these reasons, it is important that clathrates be better studied and understood.

Malinverno has sailed as a logging scientist on several expeditions of the Integrated Ocean Drilling Program (IODP). The IODP began in the 1950s as Project Mohole, a plan to drill into the mantle. The plan failed, but the IODP flourished, now with data spanning much of the world’s oceans and more cruises every year. Malinverno has used these cruises to gather data on clathrates, which are brought up from the ocean floor in drill cores. The methane begins to dissociate even before reaching the surface, which can cause drilling explosions; it also changes the character of the sediment, leaving it “soupy” or “moussy”. Pressurized cores can be taken, which prevent outgassing and preserve the clathrates. Wireline logs can also be used to examine clathrates at depth, as the methane is an electrical insulator and increases the resistivity of the sediment, just like conventional gas.

The source of the methane in gas hydrates is from methanogenic bacteria in the sediments, even at very great depths. This can be determined from Malinverno’s studies because biogenic gas is isotropically light compared to thermogenic gas formed in rocks like the gas produced from natural gas wells. The bacteria need only carbon and water to perform the reaction. As the carbon is converted to methane and carbon dioxide, there is a corresponding decrease in particulate carbon in the sediments. The rate at which this decrease occurs may be dependent on the ambient temperature, or on the age of the carbon – that is, how long the particles have been buried – but the process is still very poorly understood.

Malinverno explained that clathrates tend to form heterogeneous deposits, being preferentially concentrated in sands but not in fine-grained muds, despite the fact that particulate carbon tends to concentrate in muds and be absent from sands. This is probably due to fluid flow, migration, or diffusion of the methane. So long as a clathrate is within its stability zone, the methane cannot come out of solution unless the concentration and solubility of the methane correspond exactly, making migration difficult except within very narrow conditions. As a three-percent supersaturation of gas is necessary to form a gas hydrate, it is likely that the methane migrated from the muds into the pore spaces in the sands first, then formed clathrates. This hypothesis is supported by evidence from the Gulf of Mexico, where Malinverno and his colleagues found sand intervals containing clathrates which were bracketed above and below by fine-grained, clathrate-free zones; beyond were fine-grained intervals with gas hydrates present. This implies that transport of the methane depleted the gas from the sands’ surrounding area, but only to a certain extent.

Malinverno concluded that these findings are still preliminary, and that there is much more work to be done. “Future research will concentrate on microbial methanogenesis and gas hydrate formation,” he said. In this way, this unique and important substance – ice from the ocean floor that burns when set alight – can come to be understood and, perhaps someday, used to benefit humanity.