NTSITIMEH · Non-traditional stable isotopes to track the impact of methane in Earth history

Over geological times, the evolution of carbon isotope compositions of carbonates (δ13Ccarb) in sedimentary record highlights many positive isotopic excursions (CIEs), reflecting significant perturbations of the carbon cycle in Earth surface environments. Although generally interpreted as a consequence of an increase of organic carbon burial in sediments, the lack of high organic carbon content, as well as the strong spatial and temporal variability, observed in many sedimentary successions recording CIEs challenge this postulate. Among other alternative hypothesis involving regional or local control, the potential influence of methanogenesis, i.e. the biological process of anaerobic organic matter degradation producing methane (CH4), has been raised; its ability to generate similar isotopic signatures has been demonstrated in modern analogue. Although the processes behind CIEs are questioned, providing more information about methanogenesis impact is challenging based on traditional isotopic tool like δ13Ccarb, as its isotopic effect on is similar to that of organic carbon burial increase. Recently, stable isotope compositions of metals used as enzymatic cofactors of CH4-related processes were investigated to explore their potential as biomarkers of methanogenesis. During the last decade, significant advances have been made on using Ni isotopes as tracers of methanogenesis but important challenges remain to better constrains both their potential and limit. In order to improve our understanding of CH4 cycle and its impact on Earth’s surface environments through geological times, we will investigate further the potential of Ni isotopes and its potential couplings with traditional stable isotope in various modern settings and past environments to enhance our ability to track and discriminate the influence of CH4-related processes through Earth’s history.