A Review of H2, CH4, and Hydrocarbon Formation in Experimental Serpentinization Using Network Analysis

Abstract

The origin of methane and light hydrocarbons (HCs) in natural fluids from serpentinization has commonly been attributed to the abiotic reduction of oxidized carbon by H 2 through Fischer-Tropsch-type (FTT) reactions. Multiple experimental serpentinization studies attempted to identify the parameters that control the abiotic production of H 2 , CH 4 , and light HC. H 2 is systematically and significantly formed in experiments, indicating that its production during serpentinization is well established. However, the large variance in concentration (eight orders of magnitude) is difficult to address because of the large number of parameters that vary from one experiment to another. CH 4 and light HC production is much lower and also highly variable, leading to a vivid debate on potential role of metal catalysts and organic contamination. We have built a dataset that includes experimental setups, conditions, reactants, and products from 30 peer-reviewed articles reporting on experimental serpentinization and performed dimensionality reduction and network analysis to achieve an unbiased reading of the literature and fuel the debate. Our analysis distinguishes four experimental communities that highlights usual experimental protocols and the conditions tested so far. As expected, H 2 production is mainly controlled by T and P though a strong variability remains within a given P-T range. Accessory metal-bearing phases seem to favor H 2 production, while their role as catalyst or reactant is hampered by the lack of mineralogical characterization. CH 4 and light HC concentrations are highly variable, uncorrelated to each other, and much lower than concentrations of potential reactants (H 2 , initial carbon). Accessory phases proposed as FTT catalysts do not enhance CH 4 production, confirming the inefficiency of this reaction. CH 4 only displays a positive correlation with temperature suggesting a kinetic/thermal control on its forming reaction. The carbon budget of some experiments indicates contamination in agreement with available labeled 13 C studies. Salts in initial solutions are possible sources of organic contaminants. Natural systems certainly exploit longer reaction time or other reactional paths to form the observed CH 4 and HC. The reducing potential of serpentinization can also produce intermediate metastable carbon phases in liquid or solid as observed in natural samples that should be targeted in future experiments.

Dataset for H2, CH4 and organic compounds formation during experimental serpentinization

“…Kularatne et al, 2018;Michiels et al, 2018;Brunet, 2019). We hope that this dataset and the analysis by Barbier et al (2020) stimulate complementary experiments that could fill the identified gaps; this would allow editing future versions of this dataset in a few years and potentially help people better understand the fate of carbon under highly reducing conditions such as the one produced during serpentinization.…”

“…In our recent paper (Barbier et al, 2020), we used network analysis and machine-learning algorithms to analyse a processed version of this dataset. We found that, as previously known, pressure and temperature are the two most important parameters that govern the production of H 2 and OC.…”

“…Huang et al, 2019;McCollom et al, 2020). More information and the detailed analytical methods can be found in Barbier et al (2020). The dataset can also be extended to other reactions under similar conditions using different solid reactants, such as mine-tailing products, to produce H 2 , CH 4 and other carbon species (e.g.…”

Editorial: Deep Carbon Science

“…The relevance of the frequently invoked Fischer-Tropsch-type (FTT) reactions to produce methane from the abiotic reduction of oxidized carbon by H 2 is questioned. Barbier et al (2020) follow the forms and movement of carbon through the near ubiquitous, extensive serpentinization process, operating beneath most past and present seabeds.…”

On geological hydrogen

“…Nature, 436, 499-503. https://doi.org/10.1038/ nature03980. Barbier, S., Huang , F., Andreani , M., Tao , R., Hao, J., Eleish, A., Prabhu, A., Minhas, O., Fontaine, K., Fox, P., & Daniel, I.A. (2020) Conrad, R., & Seiler, W. (1979).…”