Context and overall objectives of the project
Methane (CH4) is a potent greenhouse gas that has the capability to significantly impact the Earth’s climate. Recent works estimated CH4 emissions from natural, impacted and human-made aquatic ecosystems at almost half of the total global emissions. Improve our understanding of CH4 emissions variability from natural environments, as well as the biological processes and environmental parameters regulating them, is thus required to better assess methane sources and their potential impact on Earth’s environments. One approach to support research trying to predict the impact of CH4 increases on Earth’s surface environments is to explore further their impact in the past.
Over geological times, higher atmospheric CH4 concentrations are suggested, especially before the onset of the Great Oxidation Event (GOE) during the Paleoproterozoic. Yet, the impact of CH4-related processes on Earth’s surface environments remains debated, especially after the GOE as it would require massive emissions from a partially oxygenated ocean. Biosphere models for oceanic CH4 emissions during Archean and Proterozoic, however, only consider turbulent diffusive transport, which favors efficient CH4 oxidation. A better consideration of other gas transport, as well as of the impact of shallow or continental environments would be beneficial for a better assessment of the magnitude of CH4 fluxes to the atmosphere. Episodes of methanogenesis intensification, at least restricted in time and space, may therefore have occurred and had a significant impact on Earth’s surface biogeosphere.
To explore further this possibility, we can investigate the evolution of carbon biogeochemical cycle within the sedimentary archives in which the isotopic expression of methanogenesis activity could be preserved. Over geological times, the evolution of carbon isotopic compositions of carbonates (δ13Ccarb) is punctuated by many positive isotopic excursions. One emblematic example is the Lomagundi-Jatuli Event (LJE, 2.3-2.1 Ga), which highlights the remaining questions this project intends to address. The LJE represents the highest positive carbon isotopic excursion in terms of duration and intensity ever observed in Earth’s history, reflecting a significant perturbation of the carbon cycle in surface environments. Although classically interpreted as a consequence of an increase of organic carbon burial in sediments, the strong spatial and temporal variability observed and the lack of high organic carbon content in many sedimentary successions challenge this postulate. Alternative hypothesis involving regional or local control have then emerged, and the processes behind this major isotopic event are still questioned. Among other, the potential influence of methanogenesis has been raised; its ability to generate similar isotopic signatures has been demonstrated in modern analogues. Its potential impact in shallow and/or continental environments could have had a significant impact in terms of climate regulation and surface environment disturbances. Better constrain the potential impact of methanogenesis in the genesis of such isotopic events could thus improve our understanding of CH4 cycle over Earth’s history.
It is, however, challenging to discriminate methanogenesis influence based on traditional isotopic tool like δ13C as its isotopic effect on is similar to that of organic carbon burial increase, which highlights the necessity to find other proxies for additional constraints. Microbial enzymes involved in methanogenesis pathways require trace metal elements. Nickel (Ni) is an essential enzymatic cofactor for this reaction and its bioavailability has an impact on the intensity of methanogenesis activity. On this basis, stable isotope composition of Ni was investigated to explore its potential as biomarker of methanogenesis. Although significant Ni isotopic fractionation has been demonstrated during methanogenesis from cell growth cultures experiments, modelling and experimental works illustrated that Ni adsorption on Fe- and Mn-oxide minerals can generate at least a similar Ni isotopic fractionation, and further investigations are required to better constrain both its potential and limit.
In order to improve both our capability to track and evaluate the impact of CH4-related processes on Earth’s surface environments through geological times, this project intends to further explore the potential of Ni isotopes as biomarkers in various modern settings considered as analogue of past environments, as well as potential coupling with other enzymatic metals cofactors exerting control on the methanogenic activity and traditional stable isotopes sensitive to methanogenic activity. A selection of rock samples from sedimentary successions recording positives carbon isotopic excursions will be then investigated applying the above-mentioned coupling to better constrain the role of CH4-related processes in the origin of these isotopic perturbations.
