Researchers have discovered evidence of a massive distributed methane tank formed by chemical reactions deep inside the seabed.
Abiotic methane – created in reactions that do not involve organic substances or living beings – has long been known to be buried in the seabed and released via deep-sea valves, but the origin of the gas in this underwater environment is not fully understood.
"Identifying an abiotic source of deep-sea methane has been a problem we've been wrestling with for many years," said marine geochemist Jeffrey Seewald of the Woods Hole Oceanographic Institution (WHOI).
"Here is a source of chemical energy created by geology."
In a new study, Seewald and other researchers from WHOI analyzed rock samples from the Earth's upper mantle and lower ocean crust collected throughout the ocean: a total of 160 pebbles, derived from many ocean ridges, along with underwater zones – such as Mariana's forearm – and elevated portions of oceanic crust that called ophiolites.
In almost all deep-sea sites that were taken in, spectroscopy and microscopy techniques revealed that the stones contained methane pockets, often together with hydrogen.
When it comes to how methane is produced, scientists say it happens that seawater, which is slowly moving through deep oceanic crust, gets trapped inside hot, rock-forming mineral called olivine – the primary component of the Earth's upper mantle.
Over time, the mineral begins to cool. When it does, the water stored in the "liquid inclusions" inside the rock passes through a chemical reaction called serpentinization, which ends up yielding both methane and hydrogen.
Once formed, the researchers explain methane and hydrogen can remain sealed inside the rock "over geological time scales until extracted by dissolution or cracking of the olivine value".
It is important, and not just for methane on earth.
We know that methane is found elsewhere in the solar system – for example on Mars, and many other distant worlds as well – and the new finds help explain how it can remain there, even in the absence of liquid water or hydrothermal activity.
"Because fluid inclusions can be formed in olive-rich rocks that interact with water on celestial bodies elsewhere in our solar system, their formation can have important consequences for maintaining microbial life outside the Earth," the authors write in their paper, noting that any venting or evacuation of these fuel sources from the rocks can potentially sustain life forms with nothing else to eat.
"Current release of trapped volatiles with these mechanisms can provide enough H2 [hydrogen] and CH4 [methane] to provide microbial ecosystems with electron donors in natural environments where H2 or CH4 formation would otherwise not be favorable. "
Back on earth, it is possible that this chemical production and release cycle itself has been an important factor in the survival of terrestrial, marine-dwelling organisms since ancient times.
In fact, the researchers say that the process "probably occurred since the onset of plate tectonics" and "may have supported microbial ecosystems within different geological environments".
That said, the team acknowledges that their explanation for how this massive methane distribution came about is somewhat speculative. The origin of the trapped fluids cannot be unambiguously determined, they note, but notes that their discovery of other chemicals inside the rocks is "compatible with a seawater-like source fluid".
Although they are not 100 percent correct in the origins of this mysterious methane, the other big takeaway is just how gargantuan the container they have discovered can end up.
While quantification of the degree of buried oceanic methane may not have been the primary goal of the study, based on the strike rate in the analyzed rock samples, the team estimates that the oceanic deposits would totally exceed the amount of methane in the Earth's atmosphere before the industrial age.
"Extrapolating our results globally suggests that inclusions may represent one of the largest sources of abiotic CH4 on earth," the researchers conclude.
The results are reported in PNAS.