09:00 〜 09:15
[MIS13-01] 初期地球での生物多様性
キーワード：生物多様性、初期地球、生物多様性の地質学的判断指標、生物多様性の地球化学的判断指標
The origin and evolution of organisms are key issues of biological evolution on the Earth. It is widely considered that the organisms already appeared by the Eoarchean based on carbon isotope signatures of graphite (chemofossils) in the Eoarchean geologic terrains such as the Saglek Block, Nuvvuagittuq supracrustal belt, and Isua supracrustal belt, but many questions still remain. When, where, and how did the organisms first appear? How did the organism evolve on the early Earth? How many and what kinds of organisms exist on the early Earth? However, most of them may be unable to be solved by geologic evidence, but the latter questions may be able to be answered based on geological evidence.
Recently, we started to identify what kinds of organisms for the fossils found in the Eoarchean terrains based on geological and geochemical evidence. The geologic evidence is useful to estimate the habitat environment such as shallow/deep marine environments, coastal/pelagic environments, or hydrothermal fields. The estimated environments contribute to identifying the kinds of organisms because the organisms should adapt the habitat environments. Geochemical evidence, of course, plays important roles on estimating the kinds of organisms, and carbon, nitrogen, iron, and sulfur isotopes were often used to identify methanogen, diazotroph, phototrophic iron-oxidizing bacteria/microbial dissimilatory iron-reduction, and microbial sulfate reduction, respectively.
The Isua supracrustal belt consists of cherts, banded iron formations, carbonate rocks, ferruginous black shale, and clastic sedimentary rocks such as mafic sediments and conglomerates as well as mafic and ultramafic rocks. The cherts and banded iron formations are not intercalated with the clastic sedimentary rocks but occur on the mafic volcanic rocks. The occurrence suggests that the cherts and banded iron formations were formed in pelagic environments. Especially, it is widely considered that the banded iron formations were formed around hydrothermal vents whereas the cherts were formed far from the vents. Given that alkaline hydrothermal systems were dominated in the Eoarchean, cherts, however, were formed around the hydrothermal vents whereas the banded iron formations were formed far from the vents or in the surface layer of ocean. The carbonate rocks have two types of geologic occurrence: intercalation with cherts or conglomerate, respectively. The former was possibly formed in the pelagic environment whereas the latter was in continental margins. The Nulliak supracrustal suite in the Saglek block also consists of cherts, banded iron formations, carbonate rocks, and clastic sedimentary rocks such as pelitic rocks and conglomerates as well as mafic and ultramafic rocks. The cherts and banded iron formations were formed in the pelagic environments whereas the pelitic rocks and conglomerates were formed in the continental margins. Because the carbonate rocks are intercalated with cherts and banded iron formations, they were formed in the pelagic environments. The Nuvvuagittuq supracrustal belt consists of cherts and banded iron formations. They were formed in the pelagic environments. In summary, precursor minerals in the carbonate rocks and banded iron formations were possibly precipitated in the surface layers of pelagic areas, whereas the chert were deposited in deep ocean near hydrothermal vents. The geological evidence suggests that the former cradled phototrophs, whereas the latter did chemotrophs. The clastic sediments of pelitic rocks, mafic sediments, and conglomerate were formed near continental margins.
Combination of carbon, iron, and sulfur isotopes allows us to estimate the styles of metabolism such as carbon-fixation, iron-metabolism, and sulfur-metabolism, respectively. The high δ56Fe values of sulfide up to +2.35‰ in the banded iron formations of the Isua supracrustal belt suggest that photosynthetic iron-oxidizing bacteria inhabited in the photic zone. On the other hand, the low δ56Fe values of sulfide down to -2.41‰ in the clastic sediments of the Isua supracrustal belt suggest that dissimilatory iron-reduction inhabited in the continental margins. Although the carbon isotope values of graphite are not low enough to identify methanogens, large variations of carbon isotope values related with lithology of host rocks suggest presence of various organisms with different styles of carbon fixation. The establishment of biodiversity in the Eoarchean may constrain the origin and tempo of the evolution of life.
Recently, we started to identify what kinds of organisms for the fossils found in the Eoarchean terrains based on geological and geochemical evidence. The geologic evidence is useful to estimate the habitat environment such as shallow/deep marine environments, coastal/pelagic environments, or hydrothermal fields. The estimated environments contribute to identifying the kinds of organisms because the organisms should adapt the habitat environments. Geochemical evidence, of course, plays important roles on estimating the kinds of organisms, and carbon, nitrogen, iron, and sulfur isotopes were often used to identify methanogen, diazotroph, phototrophic iron-oxidizing bacteria/microbial dissimilatory iron-reduction, and microbial sulfate reduction, respectively.
The Isua supracrustal belt consists of cherts, banded iron formations, carbonate rocks, ferruginous black shale, and clastic sedimentary rocks such as mafic sediments and conglomerates as well as mafic and ultramafic rocks. The cherts and banded iron formations are not intercalated with the clastic sedimentary rocks but occur on the mafic volcanic rocks. The occurrence suggests that the cherts and banded iron formations were formed in pelagic environments. Especially, it is widely considered that the banded iron formations were formed around hydrothermal vents whereas the cherts were formed far from the vents. Given that alkaline hydrothermal systems were dominated in the Eoarchean, cherts, however, were formed around the hydrothermal vents whereas the banded iron formations were formed far from the vents or in the surface layer of ocean. The carbonate rocks have two types of geologic occurrence: intercalation with cherts or conglomerate, respectively. The former was possibly formed in the pelagic environment whereas the latter was in continental margins. The Nulliak supracrustal suite in the Saglek block also consists of cherts, banded iron formations, carbonate rocks, and clastic sedimentary rocks such as pelitic rocks and conglomerates as well as mafic and ultramafic rocks. The cherts and banded iron formations were formed in the pelagic environments whereas the pelitic rocks and conglomerates were formed in the continental margins. Because the carbonate rocks are intercalated with cherts and banded iron formations, they were formed in the pelagic environments. The Nuvvuagittuq supracrustal belt consists of cherts and banded iron formations. They were formed in the pelagic environments. In summary, precursor minerals in the carbonate rocks and banded iron formations were possibly precipitated in the surface layers of pelagic areas, whereas the chert were deposited in deep ocean near hydrothermal vents. The geological evidence suggests that the former cradled phototrophs, whereas the latter did chemotrophs. The clastic sediments of pelitic rocks, mafic sediments, and conglomerate were formed near continental margins.
Combination of carbon, iron, and sulfur isotopes allows us to estimate the styles of metabolism such as carbon-fixation, iron-metabolism, and sulfur-metabolism, respectively. The high δ56Fe values of sulfide up to +2.35‰ in the banded iron formations of the Isua supracrustal belt suggest that photosynthetic iron-oxidizing bacteria inhabited in the photic zone. On the other hand, the low δ56Fe values of sulfide down to -2.41‰ in the clastic sediments of the Isua supracrustal belt suggest that dissimilatory iron-reduction inhabited in the continental margins. Although the carbon isotope values of graphite are not low enough to identify methanogens, large variations of carbon isotope values related with lithology of host rocks suggest presence of various organisms with different styles of carbon fixation. The establishment of biodiversity in the Eoarchean may constrain the origin and tempo of the evolution of life.