Periodic Reporting for period 1 - NTSITIMEH (Non-traditional stable isotopes to track the impact of methane in Earth history)
Période du rapport: 2024-02-01 au 2025-07-31
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.
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.
Work performed from the beginning of the project to the end of the period covered
So far, the work carried out has been focused on two of the modern analogues mentioned in the project (i.e. Siders Pond and Dziani Dzaha), which have different physicochemical and biological characteristics and in which the influence of methanogenesis varies.
At Siders Pond, field trips were conducted in order to collect samples. During each field trip, the physico-chemical parameters were monitored (i.e. dissolved [O2], pH, T°C, salinity) and water samples were collected at every 0.5-1m depth (~ 18 samples per sampling day). In the laboratory, water samples were filtered and the filters were then leached to recover the particulate phase. Dissolved and particulate samples were analysed for metal concentrations (Fig. 1). At first, a set of samples will be selected for Ni isotopic analyses. Beyond the framework of this project, Siders Pond is of great interest for a wider scientific community (at and beyond the host institution). Consequently, numerous field trips were conducted in order to build a time series (~ monthly, about 300 samples so far), allowing us to generate a substantial database and sample collection for future collaborations.
At Dziani Dzaha, samples were available from a sediment core recording the first deposits in this system. A set of samples was selected, which included sediments with a variable amount of organic matter, carbonate or sulfide. Several tests of digestion and purification have been performed to assess nickel recovery in the bearing phases, which require adaptations depending on the sample’s mineralogical characterization. Based on these tests, samples have been selected in order to determine the metal concentrations and to analyse the isotopic composition of Ni in the different Ni-bearing phases.
So far, the work carried out has been focused on two of the modern analogues mentioned in the project (i.e. Siders Pond and Dziani Dzaha), which have different physicochemical and biological characteristics and in which the influence of methanogenesis varies.
At Siders Pond, field trips were conducted in order to collect samples. During each field trip, the physico-chemical parameters were monitored (i.e. dissolved [O2], pH, T°C, salinity) and water samples were collected at every 0.5-1m depth (~ 18 samples per sampling day). In the laboratory, water samples were filtered and the filters were then leached to recover the particulate phase. Dissolved and particulate samples were analysed for metal concentrations (Fig. 1). At first, a set of samples will be selected for Ni isotopic analyses. Beyond the framework of this project, Siders Pond is of great interest for a wider scientific community (at and beyond the host institution). Consequently, numerous field trips were conducted in order to build a time series (~ monthly, about 300 samples so far), allowing us to generate a substantial database and sample collection for future collaborations.
At Dziani Dzaha, samples were available from a sediment core recording the first deposits in this system. A set of samples was selected, which included sediments with a variable amount of organic matter, carbonate or sulfide. Several tests of digestion and purification have been performed to assess nickel recovery in the bearing phases, which require adaptations depending on the sample’s mineralogical characterization. Based on these tests, samples have been selected in order to determine the metal concentrations and to analyse the isotopic composition of Ni in the different Ni-bearing phases.
Progress beyond the state of the art and expected potential impact
Given the characteristics of the water column in Siders Pond (Fig. 1), the monitoring of the studied site and the numerous samples collected and analysed will allow us to better understand the biological, physical, and chemical processes controlling the distribution of elements of interest in the water column, and how methanogenesis is expressed in their distribution. This work will help us better understand metal cycles in general, the respective influence of the numerous processes that can impact Ni distribution, and how to better identify its potential influence.
The Dziani Dzaha is a well-known system in which methanogenesis has a significant impact on the carbon cycle. Methanogenesis intensification has been demonstrated over time from this sediment core using stable C and S isotopes. This work will allow us to explore which combination of proxy (traditional and non-traditional isotopes) are the best to discriminate the influence of methanogenesis in both modern and past environments.
Given the characteristics of the water column in Siders Pond (Fig. 1), the monitoring of the studied site and the numerous samples collected and analysed will allow us to better understand the biological, physical, and chemical processes controlling the distribution of elements of interest in the water column, and how methanogenesis is expressed in their distribution. This work will help us better understand metal cycles in general, the respective influence of the numerous processes that can impact Ni distribution, and how to better identify its potential influence.
The Dziani Dzaha is a well-known system in which methanogenesis has a significant impact on the carbon cycle. Methanogenesis intensification has been demonstrated over time from this sediment core using stable C and S isotopes. This work will allow us to explore which combination of proxy (traditional and non-traditional isotopes) are the best to discriminate the influence of methanogenesis in both modern and past environments.