11:30 AM - 11:45 AM
[BCG04-10] Nitrogen isotope compositions of ca. 2.0 Ga Zaonega Formation in Russia: their constraints on Paleoproterozoic oceanic environments and microbial activities
★Invited Papers
Keywords:stepwise combustion analysis, Paleoproterozoic, Nitrogen isotope
Sedimentary rocks of organic-rich detrital sediments from 1.98 Ga Zaonega Formation, Russia are deposited at about the same time as the Lomagundi Jatuli Event that was one of the largest positive carbonate carbon isotope (δ13C) excursion in earth history and are also known to contain shungite (solidified petroleum-like material). It is a key stratum when considering the carbon cycle between the atmosphere, hydrosphere, and biosphere in the early Proterozoic era. The carbon isotope variability recorded in the Zaonega Formation has been linked to the nitrogen isotope variability, but it may be biased to the analytical sample and needs to be reassessed. The nitrogen isotope composition of sediments is more linked to microbial ecology than carbon isotopes, and more detailed nitrogen isotope analysis can constrain the early Proterozoic microbial ecosystems.
In this study, using stepwise combustion nitrogen isotope analysis for sedimentary rocks from Zaonega Formation, released amounts of carbon, nitrogen and argon, together with nitrogen isotope ratio (δ15N) and 40Ar / 36Ar ratio (Ar ratio) were calculated. By these results, host phase-dependent differences of nitrogen was estimated to presume the marine environment including the biological nitrogen cycle at that time. The analysis was performed on the greywacke and black shale samples. As a preliminary analysis, both greywacke and black shale samples were combusted from 400 ℃ to 800 ℃ in 50 ℃ steps, and finally at 1200 ℃ in oxygen atmosphere (fine step). From the results, it was clarified that major release of nitrogen and argon, or δ15N gaps were observed at combustion temperatures of 400 ℃, 500 ℃, 600 ℃, 700 ℃ and 1200 ℃. Based on this preliminary analysis, a simplified version of released profiles and isotope compositions of N and Ar were measured in 9 samples of black shale after that (rough step).
Both greywacke and black shale samples showed similar releasing profiles in N and Ar. Major release of nitrogen and carbon were observed in the same temperature range (mainly 650-700 ℃) with δ15N value of less than ~+4 ‰ in both samples, suggesting that the host phase of nitrogen was the organic matter. On the other hand, a heavy δ15N value of up to ~+12 ‰ was obtained at the lower and higher temperature range than that of an organic matter. In addition, a higher Ar ratio was obtained in the same temperature range, suggesting that this nitrogen was hosted in clay minerals. The δ15N value, up to ~+12 ‰, obtained in this study is the highest value reported in previous studies of the Zaonega Formation. Thus, the nitrogen in clay mineral have a high δ15N value. This result suggests that the biological nitrogen cycle (BNC) in the oxidative environment may have been predominant in the land or shallow waters where clay is formed.
Nine black shale samples, evenly sampled from the bottom to the top of the core sample, were analyzed by rough step and δ15N values were profiled to determine host phases of nitrogen and their sources in each sample. As a result, the δ15N values of organic matter and clay minerals were clearly separated in the range of ~0 to ~+5 ‰ and of ~+1 to ~+12 ‰, respectively. It shows that the nitrogen isotope ratio in the seawater nitrogen reservoir vary and became rich in 15N over time. It can be interpreted that seawater gradually changed from a reducing environment to an oxidizing environment.
These profiles suggested that oxygen concentrations could be increased on land and ocean surfaces, which lead to the gradual development of the BNC in an oxidative environment. Such an environment may cause an increase in the supply of nutrient elements due to high weathering rate, in primary production in the photic zone and in variety of ecosystem due to the development of the chemocline. This study contributes to the elucidation of the peculiar marine environment of the Paleoproterozoic era.
In this study, using stepwise combustion nitrogen isotope analysis for sedimentary rocks from Zaonega Formation, released amounts of carbon, nitrogen and argon, together with nitrogen isotope ratio (δ15N) and 40Ar / 36Ar ratio (Ar ratio) were calculated. By these results, host phase-dependent differences of nitrogen was estimated to presume the marine environment including the biological nitrogen cycle at that time. The analysis was performed on the greywacke and black shale samples. As a preliminary analysis, both greywacke and black shale samples were combusted from 400 ℃ to 800 ℃ in 50 ℃ steps, and finally at 1200 ℃ in oxygen atmosphere (fine step). From the results, it was clarified that major release of nitrogen and argon, or δ15N gaps were observed at combustion temperatures of 400 ℃, 500 ℃, 600 ℃, 700 ℃ and 1200 ℃. Based on this preliminary analysis, a simplified version of released profiles and isotope compositions of N and Ar were measured in 9 samples of black shale after that (rough step).
Both greywacke and black shale samples showed similar releasing profiles in N and Ar. Major release of nitrogen and carbon were observed in the same temperature range (mainly 650-700 ℃) with δ15N value of less than ~+4 ‰ in both samples, suggesting that the host phase of nitrogen was the organic matter. On the other hand, a heavy δ15N value of up to ~+12 ‰ was obtained at the lower and higher temperature range than that of an organic matter. In addition, a higher Ar ratio was obtained in the same temperature range, suggesting that this nitrogen was hosted in clay minerals. The δ15N value, up to ~+12 ‰, obtained in this study is the highest value reported in previous studies of the Zaonega Formation. Thus, the nitrogen in clay mineral have a high δ15N value. This result suggests that the biological nitrogen cycle (BNC) in the oxidative environment may have been predominant in the land or shallow waters where clay is formed.
Nine black shale samples, evenly sampled from the bottom to the top of the core sample, were analyzed by rough step and δ15N values were profiled to determine host phases of nitrogen and their sources in each sample. As a result, the δ15N values of organic matter and clay minerals were clearly separated in the range of ~0 to ~+5 ‰ and of ~+1 to ~+12 ‰, respectively. It shows that the nitrogen isotope ratio in the seawater nitrogen reservoir vary and became rich in 15N over time. It can be interpreted that seawater gradually changed from a reducing environment to an oxidizing environment.
These profiles suggested that oxygen concentrations could be increased on land and ocean surfaces, which lead to the gradual development of the BNC in an oxidative environment. Such an environment may cause an increase in the supply of nutrient elements due to high weathering rate, in primary production in the photic zone and in variety of ecosystem due to the development of the chemocline. This study contributes to the elucidation of the peculiar marine environment of the Paleoproterozoic era.