11:22 〜 11:44
[BCG07-07] Flourishment of early cyanobacteria and its relationship to juvenile continents
★Invited Papers
キーワード：シアノバクテリア、初期大陸
When and where the early cyanobacteria flourished are still “big” questions. Those problems are often discussed along with the problems of the Great Oxidation Event (GOE). Here I present new geological. And geochemical records of potential activity of cyanobacteria well before GOE.
Sedimentary rock of 3.2 Ga Moodies Group were drilled by a part of ICDP BASE project. Those sedimentary rocks cover fluvial, alluvial and coastal sedimentary rocks, deposited on or along the juvenile continent. Fossilized microbial mats are found in those samples with appreciable amounts of organic carbons. Those microbial mat community most likely included early cyanobacteria, because of habitation difficulties for chemoautotrophic or anoxygenic photosynthesizing bacteria in fluvial to alluvial environments. This is consistent with previous reports of cyanobacteria in 3.2 Ga Moodies Group (1,2). Significant amounts of banded iron formations or jasper are also found in the coastal sedimentary rocks. They are accompanied by highly carbonaceous shales. Organic matter in those shales may represent the bloom of photosynthesizing bacteria. The step-wise combustion analyses of nitrogen isotopes are also performed on other 3.2 Ga sedimentary organic matter. The results indicate microbial nitrogen fixation was widespread in 3.2 Ga Barberton oceans.
Those geological and geochemical records imply that cyanobacteria were already active at 3.2 Ga, although oxic environments were limited. In addition, continental environments were the preferential habitation areas of early cyanobacteria. Based on those findings, here I hypothesize that juvenile continental environments not only provided nutrients (e.g., P and metals) to early cyanobacteria, but also created suitable environments for the Mn-cluster formation in photosystem II. This further speculatively implies that cyanobacteria had roots in juvenile continents.
References: (1) Javaux et al., 2010, Nature, 463, 934-936; (2) Homann, 2019, Earth-Sci. Rev. 196, 10.1016/j.earscirev.2019.102888
Sedimentary rock of 3.2 Ga Moodies Group were drilled by a part of ICDP BASE project. Those sedimentary rocks cover fluvial, alluvial and coastal sedimentary rocks, deposited on or along the juvenile continent. Fossilized microbial mats are found in those samples with appreciable amounts of organic carbons. Those microbial mat community most likely included early cyanobacteria, because of habitation difficulties for chemoautotrophic or anoxygenic photosynthesizing bacteria in fluvial to alluvial environments. This is consistent with previous reports of cyanobacteria in 3.2 Ga Moodies Group (1,2). Significant amounts of banded iron formations or jasper are also found in the coastal sedimentary rocks. They are accompanied by highly carbonaceous shales. Organic matter in those shales may represent the bloom of photosynthesizing bacteria. The step-wise combustion analyses of nitrogen isotopes are also performed on other 3.2 Ga sedimentary organic matter. The results indicate microbial nitrogen fixation was widespread in 3.2 Ga Barberton oceans.
Those geological and geochemical records imply that cyanobacteria were already active at 3.2 Ga, although oxic environments were limited. In addition, continental environments were the preferential habitation areas of early cyanobacteria. Based on those findings, here I hypothesize that juvenile continental environments not only provided nutrients (e.g., P and metals) to early cyanobacteria, but also created suitable environments for the Mn-cluster formation in photosystem II. This further speculatively implies that cyanobacteria had roots in juvenile continents.
References: (1) Javaux et al., 2010, Nature, 463, 934-936; (2) Homann, 2019, Earth-Sci. Rev. 196, 10.1016/j.earscirev.2019.102888