10:45 AM - 12:15 PM
[BCG07-P02] Evidence for early ecosystem preserved in banded iron formation of the Isua Supracrustal Belt
Origin and evolution of life are one of most important issues for history of the Earth. Previous studies have reported traces of early life in the 3.7-3.8 Ga Isua Supracrustal Belt (ISB), southern West Greenland: very low δ13C values (e.g. Rosing, 1999) and peculiar nano-structures (Ohtomo et al., 2014) of carbonaceous materials in metasedimentary rocks. Furthermore, high δ56Fe values of sulfides up to +2.35‰ in banded iron formations (BIFs) and low δ56Fe values down to -2.41‰ in mafic clastic sedimentary rocks suggest iron-oxidizing bacteria and microbial dissimilatory iron reduction in the Eoarchean, respectively (Yoshiya et al., 2015). The lines of evidence suggest not only presence of life but also establishment of ecosystems even in the Eoarchean. Generally speaking, formation of ecosystem plays an important role on not only evolution of life but also atmospheric evolution. For example, a model calculation suggests that presence of only methanogens would not result in methane haze, but combination of phototrophic iron-oxidizing bacteria and methanogens would lead to methane haze (Watanabe et al., 2021). Therefore, it is quite important to investigate whether the early ecosystem was already established in the Eoarchean, and what kinds of organisms inhabited in the ecosystems based on geological, geochemical and geobiological evidence. However, it is still ambiguous because it is difficult to estimate the kinds of organisms for the Eoarchean fossils.
The Isua Supracrustal Belt is located in approximately 150 km northeast of Nuuk, Greenland, and is a part of the Itsaq Gneiss Complex. It consists of metasedimentary rocks of clastic sedimentary rocks, chert, carbonate rocks and BIF, as well as basaltic and ultramafic rocks. The northeastern part is divided into three Units: Northern, Middle, and Southern Units, based on the metamorphic grades (Komiya et al., 1999). We collected twenty-seven BIFs in the Northern (Greenschist facies metamorphic condition) and thirteen BIFs in the Southern (Amphibolite facies metamorphic condition) units.
Our microscopic observation and SEM-EDS analysis show that the mineral assemblages of BIFs in the Northern Unit mainly comprise quartz, magnetite, hornblende, actinolite, stilpnomelane, and apatite, whereas those in the Southern Units mainly contain magnetite, chlorite, grunerite, siderite, calcite, almandine, ilmenite, and apatite.
We found two grains of carbonaceous material in a BIF sample in the Northern Unit and more than ten grains of carbonaceous material in eight BIFs samples in the Southern Unit. The carbonaceous material comprises mainly disordered graphite because two bands at 1355 (D1-band) and 1582 (D2-band) cm-1 are present on their Raman spectra. They are classified into two types based on their occurrence: Inclusions within various types of minerals such as quartz, magnetite, chlorite, grunerite and calcite, and disseminated grains along grain boundary between magnetite and grunerite.
Forty pyrite grains with obvious bands at 342, 377, and 428 cm-1 on their Raman spectra were present in five BIF samples. They are also classified into two types: Subhedral–euhedral (square, hexagon or rectangular) pyrite grains within goethite grains in quartz-rich bands in three BIF samples from the Northern Unit, and small (< 20 μm) rounded pyrite/pyrrhotite inclusions within magnetite grains in two BIF samples from the Northern and Southern units, respectively.
We estimated crystallization temperature of one carbonaceous material using the Raman spectrum. According to a calculation formula (Kouketsu et al., 2014), the temperature is estimated at ca. 150-280 ℃, lower than the metamorphic grade in this area. The discrepancy is possibly because the carbonaceous material was deformed during making the thin section.
It is considered that the subhedral to euhedral pyrite grains were recrystallized or formed in late diagenesis. Furthermore, the goethite associated with pyrite was probably formed from pyrite under oxic condition during later metamorphism. The rounded sulfide grains may be of early diagenetic or detrital origin.
We will measure sulfur isotopic compositions (δ34S) of the sulfides and carbon isotopic (δ13C) compositions of the carbonaceous materials in order to estimate their origins and reconstruct the ecosystem in the Eoarchean.
The Isua Supracrustal Belt is located in approximately 150 km northeast of Nuuk, Greenland, and is a part of the Itsaq Gneiss Complex. It consists of metasedimentary rocks of clastic sedimentary rocks, chert, carbonate rocks and BIF, as well as basaltic and ultramafic rocks. The northeastern part is divided into three Units: Northern, Middle, and Southern Units, based on the metamorphic grades (Komiya et al., 1999). We collected twenty-seven BIFs in the Northern (Greenschist facies metamorphic condition) and thirteen BIFs in the Southern (Amphibolite facies metamorphic condition) units.
Our microscopic observation and SEM-EDS analysis show that the mineral assemblages of BIFs in the Northern Unit mainly comprise quartz, magnetite, hornblende, actinolite, stilpnomelane, and apatite, whereas those in the Southern Units mainly contain magnetite, chlorite, grunerite, siderite, calcite, almandine, ilmenite, and apatite.
We found two grains of carbonaceous material in a BIF sample in the Northern Unit and more than ten grains of carbonaceous material in eight BIFs samples in the Southern Unit. The carbonaceous material comprises mainly disordered graphite because two bands at 1355 (D1-band) and 1582 (D2-band) cm-1 are present on their Raman spectra. They are classified into two types based on their occurrence: Inclusions within various types of minerals such as quartz, magnetite, chlorite, grunerite and calcite, and disseminated grains along grain boundary between magnetite and grunerite.
Forty pyrite grains with obvious bands at 342, 377, and 428 cm-1 on their Raman spectra were present in five BIF samples. They are also classified into two types: Subhedral–euhedral (square, hexagon or rectangular) pyrite grains within goethite grains in quartz-rich bands in three BIF samples from the Northern Unit, and small (< 20 μm) rounded pyrite/pyrrhotite inclusions within magnetite grains in two BIF samples from the Northern and Southern units, respectively.
We estimated crystallization temperature of one carbonaceous material using the Raman spectrum. According to a calculation formula (Kouketsu et al., 2014), the temperature is estimated at ca. 150-280 ℃, lower than the metamorphic grade in this area. The discrepancy is possibly because the carbonaceous material was deformed during making the thin section.
It is considered that the subhedral to euhedral pyrite grains were recrystallized or formed in late diagenesis. Furthermore, the goethite associated with pyrite was probably formed from pyrite under oxic condition during later metamorphism. The rounded sulfide grains may be of early diagenetic or detrital origin.
We will measure sulfur isotopic compositions (δ34S) of the sulfides and carbon isotopic (δ13C) compositions of the carbonaceous materials in order to estimate their origins and reconstruct the ecosystem in the Eoarchean.