5:15 PM - 7:15 PM
[BCG06-P08] Crystallinity and geological occurrence of carbonaceous material in marine sedimentary rocks in the Isua supracrustal belt, southern West Greenland.
Estimation of various species of organisms in the early Earth plays an important role in revealing the origin and evolution of life, which are important issues in the history of the Earth. In general, it is considered that the first organism originated in a marine environment, such as hydrothermal environments, because hydrothermal vents supplied electron donors such as H2 and Fe2+, which are necessary for organisms (e.g., Martin et al., 2008). Previous studies have reported putative biogenic carbonaceous material (CM) in various types of sedimentary rocks deposited in shallow or deep marine environments in the early Earth based on low δ¹³C values (-20‰) of CM by whole-rock analyses (e.g., rosing 1999; Ohtomo et al., 2014). However, these values may not necessarily correspond to the fractionation factors of every species of the organism, because whole-rock analysis measures δ13C values of mixtures of CM from various sources: biogenic CM, abiogenic CM, and secondary CM. Thus, it is necessary to perform in-situ carbon isotope measurements of CM in order to estimate their species. We first described the crystallinity and geological occurrence of CM, which was possibly formed biogenetically and syngenetically, in marine sedimentary rocks in the 3.8 Ga Isua Supracrustal Belt (ISB), to estimate the species of the organisms in the marine environment in the early Earth through in-situ analysis of carbon isotopes of CM.
The Isua Supracrustal Belt is located in southern West Greenland and is part of the Itsaq Gneiss Complex. It consists of metasedimentary rocks, such as clastic sedimentary rocks, chert, and BIF, as well as basaltic and ultramafic rocks. The northeastern part is divided into four metamorphic zones: Zone A (Greenschist facies), Zone B (Epidote-amphibolite facies), Zone C (Amphibolite facies), and Zone D (Amphibolite facies) based on their mineral paragenesis and compositions (Komiya et al., 1999; Hayashi et al., 2000).We collected two pelitic rock and three BIF samples in Zone A, two pelitic rock samples in Zone B, one chert sample in Zone C, and three BIF samples in Zone D.
The CM grains in the pelitic rock samples in Zones A and B are included within quartz, stilpnomelane, chlorite, carbonate, and more than two types of minerals. The CM grains in the chert sample in Zone C are included within quartz and amphibole. The CM grains in three BIF samples in Zone A, are included within quartz, amphibole, or both, whereas most CM grains in the three BIF samples in Zone D are included within siderite, cummingtonite, chlorite, magnetite, garnet, and more than two types of minerals.
We estimated the crystallization temperatures of a CM grain in the BIF sample in Zone A, twenty CM grains in the chert sample in Zone C and sixty-two CM grains in the BIF samples in Zone D, using Raman spectroscopy, respectively. According to the calculation formula (Kouketsu et al., 2014), the temperature of the CM grain in the BIF in Zone A was estimated to be up to 340 °C, consistent with the metamorphic grade of Zone A. According to a calculation formula (Beyssac et al., 2002), the CM grains in the samples in Zones C and D were estimated to be between down to 543 ± 50°C. The consistency between the estimated crystallization temperature of the CM grain and the metamorphic grade in Zone A suggests that it was probably formed before metamorphism (ca.3.6 Ga; Komiya et al., 1999). The CM grains in Zones C and D were also probably formed before metamorphism because their crystallization temperatures are also consistent with or higher than the metamorphic temperature of these Zones estimated from mineral paragenesis and compositions (500–600 °C; Arai et al., 2015). In addition, the absence of CM associated with siderite, except for some grains in one BIF in Zone D, suggests that the grains in most samples were not derived from abiogenic processes, such as siderite dissociation. Thus, the CM grains in the chert and BIF samples from Zones A, C, and D were possibly biogenic and syngenetic during their deposition.
This study reports possible traces of life in the early Earth from chert and BIF samples in the ISB. We will measure the δ13C compositions of the CM through in-situ analyses in order to estimate their species in the Eoarchean.
The Isua Supracrustal Belt is located in southern West Greenland and is part of the Itsaq Gneiss Complex. It consists of metasedimentary rocks, such as clastic sedimentary rocks, chert, and BIF, as well as basaltic and ultramafic rocks. The northeastern part is divided into four metamorphic zones: Zone A (Greenschist facies), Zone B (Epidote-amphibolite facies), Zone C (Amphibolite facies), and Zone D (Amphibolite facies) based on their mineral paragenesis and compositions (Komiya et al., 1999; Hayashi et al., 2000).We collected two pelitic rock and three BIF samples in Zone A, two pelitic rock samples in Zone B, one chert sample in Zone C, and three BIF samples in Zone D.
The CM grains in the pelitic rock samples in Zones A and B are included within quartz, stilpnomelane, chlorite, carbonate, and more than two types of minerals. The CM grains in the chert sample in Zone C are included within quartz and amphibole. The CM grains in three BIF samples in Zone A, are included within quartz, amphibole, or both, whereas most CM grains in the three BIF samples in Zone D are included within siderite, cummingtonite, chlorite, magnetite, garnet, and more than two types of minerals.
We estimated the crystallization temperatures of a CM grain in the BIF sample in Zone A, twenty CM grains in the chert sample in Zone C and sixty-two CM grains in the BIF samples in Zone D, using Raman spectroscopy, respectively. According to the calculation formula (Kouketsu et al., 2014), the temperature of the CM grain in the BIF in Zone A was estimated to be up to 340 °C, consistent with the metamorphic grade of Zone A. According to a calculation formula (Beyssac et al., 2002), the CM grains in the samples in Zones C and D were estimated to be between down to 543 ± 50°C. The consistency between the estimated crystallization temperature of the CM grain and the metamorphic grade in Zone A suggests that it was probably formed before metamorphism (ca.3.6 Ga; Komiya et al., 1999). The CM grains in Zones C and D were also probably formed before metamorphism because their crystallization temperatures are also consistent with or higher than the metamorphic temperature of these Zones estimated from mineral paragenesis and compositions (500–600 °C; Arai et al., 2015). In addition, the absence of CM associated with siderite, except for some grains in one BIF in Zone D, suggests that the grains in most samples were not derived from abiogenic processes, such as siderite dissociation. Thus, the CM grains in the chert and BIF samples from Zones A, C, and D were possibly biogenic and syngenetic during their deposition.
This study reports possible traces of life in the early Earth from chert and BIF samples in the ISB. We will measure the δ13C compositions of the CM through in-situ analyses in order to estimate their species in the Eoarchean.