17:15 〜 18:45
[MIS23-P07] Biodiversity in the early Earth and rapid tempo of early evolution of life: The oldest evidence for sulfur and methene metabolisms
キーワード：初期地球、生物多様性、生命進化、最古の硫黄代謝の証拠
The origin and evolution of organisms are key issues in the evolution of the Earth because life is a fundamental characteristic of the Earth. It is widely accepted that the organisms already appeared by the early Eoarchean based on the carbon isotope signatures of graphite (e.g., Rosing, 1999; Tashiro et al., 2017; Dodd et al., 2017). However, many questions still remain. When and where did the organisms first appear? How did the organism evolve in the early Earth? How many and what kinds of organisms existed on the early Earth? Although it is difficult to obtain answers for most of them, some questions may be solved using geological evidence, and knowledge of the metabolism of early organisms can constrain the tempo of early evolution. Previous studies have suggested the presence of iron metabolism (iron-oxidizing and iron-reducing bacteria) based on the iron isotopes of whole rocks, magnetite, and pyrite (Craddock & Dauphas, 2011; Czaja et al., 2013; Yoshiya et al., 2014), and the morphology of hematite (Dodd et al., 2017). Low carbon isotope values (δ13C) of graphite within apatite, whose nadir to ca. -50‰, in the Akilia banded iron formation (BIF) was considered to be methane metabolism (Mojzsis et al., 1996); however, the evidence is still controversial in terms of the protoliths of the host rock, the origin and formation timing of the apatite, and the occurrence of graphite inclusions. On the other hand, no geobiological evidence for sulfur metabolism was found in the Eoarchean geological terrains.
This paper reviews our recent geomicrobiological studies on three Eoarchean supracrustal belts, including the Isua supracrustal belt, Nulliak supracrustal rocks, and the Nuvvuagittuq supracrustal belt, and provides new evidence for sulfur and methane metabolism, as well as biodiversity in the Eoarchean.
We conducted in-situ analysis of four isotopes of sulfur in sulfide minerals such as pyrite and pyrrhotite from carbonate rocks, chert, BIF, and clastic rocks in ~3.9 Ga Nulliak supracrustal rocks, Labrador, to find evidence for sulfur metabolism. The presence of quite large δ34S values and a negative Δ36S anomaly relative to Δ33S values of the pyrrhotite from the Nulliak supracrustal rocks suggests microbial sulfate reduction in the Eoarchean. Because the oldest geomicrobiological evidence for microbial sulfate reduction was obtained from pyrite in bedded barite in the ~3.47 Ga North Pole area in Pilbara (Shen et al., 2001), this discovery pushes back the previous record by ~450 my. Moreover, the lower Δ36S/Δ33S ratios of -1.35 than the typical Archean value is considered to be the occurrence of methane haze, suggesting that methanogenesis also goes back to the Eoarchean.
The banded iron formations in the Isua supracrustal belt are not intercalated with clastic sedimentary rocks but occur on mafic volcanic rocks, suggesting that they were formed in hydrothermal environments. The northeastern part underwent relatively low-grade metamorphism under greenschist facies condition compared with other areas (Hayashi et al., 2000). We, first, observed a graphite grain from the BIF in a low-grade area under a microscope. The crystallization temperature estimated from the Raman spectrum of the grain is ~300 ℃, corresponding to the greenschist facies.
Although the presence of iron-oxidizing bacteria was suggested based on the morphology of hematite grains in the hematite-type BIFs in the Nuvvuagittuq supracrustal belt, we analyzed the carbon isotopes of graphite in cherts to reveal the biodiversity in the Eoarchean. The δ13C values are correlated with the total organic carbon content; thus, the compositional variation can be explained by three-component mixing. In addition, the δ13C values are also correlated with stratigraphy within the chert unit. These characteristics suggest that biodiversity was established even in the Eoarchean, and the organisms inhabited, depending on environmental conditions.
Our new geomicrobiological discovery suggests that various organisms already inhabited even in the Eoarchean and that the establishment of biodiversity supports a relatively rapid tempo of early evolution of life.
This paper reviews our recent geomicrobiological studies on three Eoarchean supracrustal belts, including the Isua supracrustal belt, Nulliak supracrustal rocks, and the Nuvvuagittuq supracrustal belt, and provides new evidence for sulfur and methane metabolism, as well as biodiversity in the Eoarchean.
We conducted in-situ analysis of four isotopes of sulfur in sulfide minerals such as pyrite and pyrrhotite from carbonate rocks, chert, BIF, and clastic rocks in ~3.9 Ga Nulliak supracrustal rocks, Labrador, to find evidence for sulfur metabolism. The presence of quite large δ34S values and a negative Δ36S anomaly relative to Δ33S values of the pyrrhotite from the Nulliak supracrustal rocks suggests microbial sulfate reduction in the Eoarchean. Because the oldest geomicrobiological evidence for microbial sulfate reduction was obtained from pyrite in bedded barite in the ~3.47 Ga North Pole area in Pilbara (Shen et al., 2001), this discovery pushes back the previous record by ~450 my. Moreover, the lower Δ36S/Δ33S ratios of -1.35 than the typical Archean value is considered to be the occurrence of methane haze, suggesting that methanogenesis also goes back to the Eoarchean.
The banded iron formations in the Isua supracrustal belt are not intercalated with clastic sedimentary rocks but occur on mafic volcanic rocks, suggesting that they were formed in hydrothermal environments. The northeastern part underwent relatively low-grade metamorphism under greenschist facies condition compared with other areas (Hayashi et al., 2000). We, first, observed a graphite grain from the BIF in a low-grade area under a microscope. The crystallization temperature estimated from the Raman spectrum of the grain is ~300 ℃, corresponding to the greenschist facies.
Although the presence of iron-oxidizing bacteria was suggested based on the morphology of hematite grains in the hematite-type BIFs in the Nuvvuagittuq supracrustal belt, we analyzed the carbon isotopes of graphite in cherts to reveal the biodiversity in the Eoarchean. The δ13C values are correlated with the total organic carbon content; thus, the compositional variation can be explained by three-component mixing. In addition, the δ13C values are also correlated with stratigraphy within the chert unit. These characteristics suggest that biodiversity was established even in the Eoarchean, and the organisms inhabited, depending on environmental conditions.
Our new geomicrobiological discovery suggests that various organisms already inhabited even in the Eoarchean and that the establishment of biodiversity supports a relatively rapid tempo of early evolution of life.