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Lost City Expedition: Science

Methane and Hydrogen Formation From Rocks – Energy Sources for Life

One of the distinct geological characteristics of Lost City is that it lies upon basement rocks that originated in the Earth’s mantle and were brought to the surface through faulting. These rocks are called peridotites and consist of minerals that are rich in magnesium and iron. The most common is the mineral olivine, which consists of magnesium, ferrous iron and silica. Below temperatures of about 425°C (about 800° F), olivine is unstable in the presence of seawater and reacts to form the hydrous Mg-rich silicate mineral serpentine and an iron-oxide called magnetite. We refer to this process of hydrating mantle rocks as “serpentinization”. Serpentinization causes important changes to the physical state of the rocks and the chemical composition of the system, and produces important nutrients for microbial activity.

During the formation of magnetite, part of the ferrous (Fe2+) iron in olivine is oxidized to ferric iron (Fe3+) to form magnetite. This change in the valency of iron consumes oxygen from the fluid and leads to a state that chemists call reducing conditions. As a consequence of the formation of magnetite, hydrogen gas (H2) is produced from the reduction of seawater during serpentinization. Seawater also contains carbonate ions (HCO3- or CO22-) and sulfate ions (SO42-) which can become reduced to form methane (CH4) and hydrogen sulfide (H2S) during the serpentinization process. The presence of the reduced species H2, CH4 and H2S in the fluids that seep out of the rocks provide important energy sources for different microbial species that seem to thrive around the Lost City structures.

In recent years, many scientists that have studied the mid-ocean ridge system have detected the presence of elevated concentrations of methane (and in some cases hydrogen) in the water column using a CTD package similar to the one that we will be using. These chemical anomalies have been found in a number of areas, particularly along the Mid-Atlantic Ridge. In many cases, the high methane concentrations can directly be linked to the presence of mantle rocks exposed on the seafloor that are undergoing serpentinization reactions. One of the goals of our expedition to Lost City will be to combine studies of the mineralogy and chemistry of the rocks with studies of the vent fluids, water column samples, and microbiology to better understand the links between the inorganic processes in the subsurface with biological activity around the vent structures.