Methane and hydrogen are key energy sources for microbes, especially in the subsurface, and are fundamental molecules in planetary bodies. Most hydrocarbons can be traced back to life forms, past and present, but water-rock reactions, such as the process called serpentinization, create hydrogen and abiotic methane that form independently of life. These gases likely contribute to the global carbon cycle and the growth of subsurface microbes, but the size of these reservoirs and their impact is not well understood.
A new project entitled Deep Serpentinization, H2, and high-pressure abiotic CH4 (DeepSeep), led by Alberto Vitale Brovarone (IMPMC-CNRS, France/Università di Torino, Italy) seeks to estimate how much deep abiotic hydrogen and methane is produced in subduction zones, where one tectonic plate sinks beneath another. Vitale Brovarone and colleagues will also investigate the role of those gases in fueling subsurface life and the deep carbon cycle. Fellow DCO researchers affiliated with the project include Dimitri Sverjensky (Johns Hopkins University, USA), Isabelle Martinez (Institut de Physique du Globe de Paris, France), Isabelle Daniel (Claude Bernard University, Lyon, France), Simone Tumiati (Milan University, Italy), and Edward Young (University of California, Los Angeles, USA). Vitale Brovarone received a European Research Council (ERC) Consolidator Grant of about $2.7 million over the next five years to support the work.
To study these deep abiotic processes, Vitale Brovarone and his collaborators will collect rocks that formed under high pressure in the mantle but were exhumed by tectonic processes and preserved in mountain belts, like the Alps. Back in the lab, they will analyze the chemistry and composition of these rocks, including tiny bubbles preserved as fluid inclusions. This work will provide the first characterization of deep abiotic hydrocarbons and their signatures from such samples.
Unlike abiotic hydrocarbons from surface environments, the contents of these bubbles may have been protected from contamination by biologically produced compounds. By analyzing “clumped isotopes,” which are molecules with multiple isotopes – atoms with different numbers of neutrons – the researchers aim to identify a pure abiotic signature for these compounds, which then can be used to identify such signatures elsewhere.
The researchers also plan to develop a thermodynamic and geochemical modeling tool using the Deep Earth Water Model and the ENKI portal, and data from their sample analyses. The tool will help to estimate the depth that serpentinization reactions and the hydrocarbon fluids they produce have occurred in the vast subsurface. Then, using molecular dynamics, a type of computer simulation that predicts how molecules behave in a chemical reaction, they can estimate how the abiotic compounds have formed over geological timescales.
“By putting all the analyses and modeling together, we should be able to get an idea of the amounts of these deep gases being produced over a significant part of Earth’s history,” said Vitale Brovarone.
What the researchers discover can have important implications for the understanding of the deep carbon cycle and related geobiological processes throughout history. Currently we know little about how much methane – a potent greenhouse gas – escapes at subduction zones. Abiotic methane is not included in current subduction zone models, and this work may be the first to provide numbers for such models.
The work of DeepSeep may also help quantify the amount of energy available to deep life throughout time. “These kinds of gases are believed to be important molecules for the emergence of life on Earth, and to sustain life in the deep subsurface biosphere,” said Vitale Brovarone. Furthermore, the abiotic signatures that they discover may be used to differentiate between biotic and abiotic carbon compounds on other planetary bodies, like Mars.
ERC Consolidator grants are sizable awards intended to give a boost to mid-career scientists with a track record of excellent research. The funding will support the training of multiple graduate students and postdocs to be based across Europe, and Vitale Brovarone will soon be recruiting new trainees who may be interested in joining the project.
Main image: An example of the rock peridotite that underwent the initial stage of serpentinization. The fluid inclusion trails are rich in hydrogen and methane. Credit: Vitale Brovarone