JpGU-AGU Joint Meeting 2020

Presentation information

[J] Oral

B (Biogeosciences ) » B-BG Biogeosciences & Geosphere-Biosphere Interactions

[B-BG02] Interaction between Life, Water, Mineral, and Atmosphere

convener:Yuichiro Ueno(Department of Earth and Planetary Sciences, Tokyo Institute of Technology), Takeshi Kakegawa(Graduate School of Science, Tohoku University), Ken Takai(Extremobiosphere Research Center, Japan Agency for Marine-Earth Science & Technology), Yohey Suzuki(Graduate School of Science, The University of Tokyo)

[BBG02-05] Heterogeneity in nitrogen source for microbial activity recorded in 2.0 Ga Zaonega Formation, NW Russia

*Akizumi Ishida1, Kouhei Sasaki1, Ko Hashizume2, Takeshi Kakegawa1, Aivo Lepland3 (1.Department of Earth Science, Graduate school of Science, Tohoku University, 2.Faculty of Science, Ibaraki University, Japan., 3.Geological Survey of Norway, Norway)

Keywords:stepwise combustion method, nitrogen isotope, Paleoproterozoic are

We performed stepwise combustion method on the samples from 2.0 Ga Zaonega Formation (ZF), NW Russia to understand the heterogeneity in nitrogen in the sedimentary rocks via nitrogen release profiles and nitrogen isotope compositions (δ15N). Bulk rocks of graywacke and black shale samples, representing shallow and deep sedimentary environments, were chosen for the analysis, and were combusted at temperature range from 500 oC to 800 oC at 50 oC steps, and 1200oC, to separate components released at different temperatures. As a result, both samples showed similar trend. The amounts of released nitrogen and δ15N values showed bimodal pattern, indicating two nitrogen components with δ15N values of 0‰ (component 1) and +12‰ (component 2). Nitrogen release amounts of component 1 correlated with 40Ar released amounts, indicating that the 0‰ nitrogen was hosted in clay minerals, or organic matter having similar combustion profile with clay mineral. On the other hand, the component 2 had no correlation with 40Ar. This suggests that the component 2 was not related to the clay minerals, but from other carrier, possibly an organic matter. The δ15N of +12‰ is one of the heaviest values reported from the ZF, indicating that biological nitrogen cycle under oxic condition.
The origin of ammonium-nitrogen, hosted by the clay minerals, is unknown. Although maturation of organic matter would release ammonium ion that can substitute for potassium and be trapped in clay minerals, it is unlikely that the δ15N of 0‰ could produce from the +12‰ nitrogen in the organic matter. Thus, regardless the nitrogen source of component 1, it was distinctly different from that of component 2.
Our result suggests that the shallow oxic ecosystem was existed during accumulation of the 2.0 Ga ZF, while it is possible that other type of ecosystem, which may have utilized a different nutrient source, was presented near the sediment/water interface. Both ecosystems co-existed in the same sedimentary basin, which is consistent with those of predicted by the previous study in the same core sample. This study further suggests that not only carbon but also nitrogen cycle was more complicated that thought in previous studies for the Paleoproterozoic era.