Much research on subsurface methane has focused on biotic methane, which is produced by methanogenic microbes or from the breakdown of organic matter and petroleum at high temperatures in sedimentary basins. Abiotic methane, on the other hand, forms as a result of magma degassing, high-temperature inorganic reactions or, in potentially larger amounts, by low-temperature chemical reactions between hydrogen and carbon dioxide. As scientists discover more and more sites around the world releasing abiotic methane, often at alkaline springs, it is becoming clear that the gas found at these springs may represent an important portion of the natural global release of methane to the atmosphere.
DCO Deep Energy Community member Giuseppe Etiope (Istituto Nazionale di Geofisica e Vulcanologia, Italy), working with colleagues on site at the Federal Geological Survey Sarajevo, Bosnia and Herzegovina, report a record amount of methane in highly alkaline water collected from one of a series of springs across the Dinaride ophiolite that spans the country. Based on chemical and isotope analysis, the researchers conclude that the methane in these springs formed from abiotic reactions between the carbon in nearby sedimentary rocks and hydrogen gas produced by serpentinization, when water reacts with minerals from the mantle rock peridotite, brought to the surface through plate tectonic processes. In a new paper in the journal Applied Geochemistry , the researchers show that a single hyperalkaline spring can release methane in amounts comparable to that emitted by biotic methane springs in sedimentary basins.
“We discovered a new place of dominantly abiotic methane in a continental serpentinization site,” said Etiope, noting that Bosnia and Herzegovina is the 17th country in the world with this type of abiotic gas occurrence. “The abiotic methane on continents is much more abundant that we thought in the past. It’s not a rare phenomenon.”
In the 1970s, hydrogeological studies in Bosnia and Herzegovina revealed numerous springs with high pH water in the Dinaride ophiolite belt, a piece of mantle from beneath the ocean floor that migrated onto land as a result of mountain-forming processes. When water interacts with the mantle rock peridotite, serpentinization reactions occur that raise the pH of surrounding water and generate hydrogen. Hydrogen then reacts with carbon dioxide in a process called the Sabatier reaction to form methane. The springs are not necessarily the source of the methane, but they do carry the gas to the surface and call attention to the processes occurring below.
The researchers collected water from eight springs in the Dinaride ophiolite belt and analyzed the dissolved gases. A single well, SB1 at Lješljani, had the highest concentration of dissolved methane ever detected, at 2706 micromoles per liter and a pH of 12.8, the highest pH value ever recorded at a serpentinization site. The previous record was 1983 micromoles per liter, sampled at the Coast Range Ophiolite Microbial Observatory in northern California.
The researchers also analyzed the gases for carbon, hydrogen, and helium isotopes, which are atoms of a chemical element containing different numbers of neutrons in the nucleus—a characteristic that affects an atom’s mass, but not its chemical properties. These measurements showed that the methane from six of the wells formed through abiotic processes in the mantle, while two had biotic methane that probably leaked in from nearby coal beds.
These methane-rich springs occur at the boundaries of the peridotite-containing sections of crust, called massifs, because while serpentinization provides the hydrogen for the Sabatier reaction, the carbon dioxide may come from nearby carbon-bearing sedimentary rocks, like limestone. The Sabatier reaction must be catalyzed by metals within specific rocks in the ophiolite, such as chromitite.
Based on water flow rates, the researchers estimate that the methane released by similar springs worldwide could reach up to 1000 tons each year. This number is surely much greater if researchers also consider the gas seeping out from the surrounding rocks. Quantifying that seepage, however, will require extensive areal surveys with appropriate gas sensors.
“We know that methane is exhaling from the rocks around the springs and globally that’s a wide area,” said Etiope. “When you combine all the gas from the area of these exhalations all over the world, it is logical to foresee that a considerable amount of methane enters the atmosphere.”