Ocean Sediment, It's A Gas
Methane is an important greenhouse gas, 30 times more potent than CO2, but our knowledge of the methane cycle is woefully inadequate. Sediments on the ocean's floor contain immense quantities of methane and there are enormous fluxes of methane into and out of these sediments. Trapped frozen in ice, there are some 10,000 gigatons of carbon stored under the sea—twice as much carbon as contained in conventional fossil fuel reserves. Some scientists consider the release of this methane the single worst environmental danger we face as a species. A massive release of ocean floor methane could cause real runaway global warming that would have dramatic impact on life. But methane continually leaks from seabeds around the world, contributing to the total amount of carbon injected into the ecosystem. A new report finds that ocean methane concentrations have been underestimated by a factor of 10 to 20 fold.
Estimates of sedimentary methane production vary between 85 and 300 Tg per year, but analytical difficulties have made previous measurements too imprecise to tell what the true amount is. Most previous estimates were calculated from ship board readings, not measurements taken at depth. According to a report in Geophysical Research Letters, Zhang et al., in order to avoid the sampling issues that have plagued other methods, employed an in situ technique to measure methane concentrations in sediments. Motivation for the research and general results are listed in the abstract:
Ocean sediment dissolved CH4 concentrations are of interest for possible climate-driven venting from sea floor hydrate decomposition, for supporting the large-scale microbial anaerobic oxidation of CH4 that holds the oceanic CH4 budget in balance, and for environmental issues of the oil and gas industry. Analyses of CH4 from recovered cores near vent locations typically show a maximum of ∼1 mM, close to the 1 atmosphere equilibrium value. We show from novel in situ measurement with a Raman-based probe that geochemically coherent profiles of dissolved CH4 occur rising to 30 mM (pCH4 = 3 MPa) or an excess pressure ∼3× greater than CO2 in a bottle of champagne. Normalization of the CH4 Raman ν1 peak to the ubiquitous water ν2 bending peak provides a fundamental internal calibration. Very large losses of CH4 and fractions of other gases (CO2, H2S) must typically occur from recovered cores at gas rich sites. The new data are consistent with observations of microbial biomass and observed CH4 oxidation rates at hydrate rich sites and support estimates of a greatly expanded near surface oceanic pore water CH4 reservoir.
In short, they found concentrations as much as 10 to 20 times higher than those determined by shipboard measurements and conclude that production rates are near the high end of past estimates. Researchers have known since the 1960s that there are large natural deposits of frozen ice containing methane lying on the ocean floor, where the high pressure and cold temperature keep them from melting. Oceanographers have pulled up frozen chunks of this material, called methane clathrate or hydrate, from more than 90 locations around the world.
Methane clathrate deep ocean deposit.
Clathrate hydrates are cage-like minerals in which water forms an ice-like cage structure in which gas molecules, such as methane, reside. Huge amounts of methane clathrate exist under the high pressures and cold temperatures of the seafloor, forming a major reservoir of greenhouse gas. Unsurprisingly, some energy companies are investigating ways to tap into this huge reservoir of trapped natural gas.
Other key findings include measurements giving a 30× increase in methane concentration and the conclusion that pore water methane is a massively underestimated carbon reservoir. Note that, when sediment particles get deposited on the seafloor they carry some ocean water with them. Water trapped in this way, scientists call “pore water” because it fills the pores between the sediment grains. Because this water is in contact with methane bearing clathrate ice, it also contains elevated levels of CH4.
In fact, there must be high levels of gas in the water saturating the mud that methane ice is buried in. Otherwise, according to Peter Brewer, ocean chemist at the Monterey Bay Aquarium Research Institute, the hydrate will not be stable. “If you just put a block of methane hydrate on the sea floor it will dissolve really fast,” says Brewer. “The hydrates in sediments have to be in equilibrium with the water around them, which must contain huge amounts of methane. One would guess it is about the same as the amount in the clathrate itself.”
This is why Brewer and his PhD student Xin Zhang, the papers lead author, equipped a remotely piloted submersible with a Raman-based probe to measure methane in place. Raman spectroscopy is well suited to chemical analysis on the ocean floor. The robotic sub can stick its metal probe into the mud next to a hydrate deposit and suck up water, and then a laser beam reveals the amount of methane and other substances in that water. Bottom line: there's a lot more methane down there than is apparent from cores brought to the surface.
A Raman spectroscopy laser on the sea floor.
So are the merchants of doom correct? Could a seafloor eruption of methane cause another episode of rapid warming like that of the PETM or even the more prolonged warming of the recently discovered Middle Eocene Climatic Optimum? Brewer thinks that a mass releases of sea-floor methane is unlikely to cause a blip in today's climate. More likely would be for any released gas to get digested by bacteria or dissolved into sea water, instead of released to the atmosphere. “It's not a scare story,” says Brewer, in an article on Nature.com.
Other than having oceanographers all atwitter, what do these new findings mean? It means that we have discovered science has a long way to go before it can claim to truly understand the carbon cycle. Particularly the sort of mid term sequestration of carbon represented by methane clathrates, falling as it does between the short term cycle of vegetation growth and decomposition, and the long term cycle of geological sequestration. There is more carbon being cycled through Earth's environment than science knows about—10, 20, even 30 times more than previous estimates in this case. It comes from sources undetected and is consumed by processes unknown. But there is a larger lesson here.
What is important about the work done by Zhang et al. is that they devised better instruments and then actually went out and measured the gas they were studying. In other words they did real science—they didn't just muck about with computer models, or even haul samples back to the lab for measurement. They gathered data about methane ice from where it occurs: on the bottom of the ocean. Real experiment, real observations, real science. If only the twits making doomsday predictions based on their software playthings had as much scientific integrity. But then the whole global warming crisis would fizz away, like a clathrate hauled from its ocean larder.
Be safe, enjoy the interglacial and stay skeptical.