[M-IS02] Astrobiology

[MIS02-P01] Deep genetic link between banded iron formation and non-photoautotrophic microbial community on the early Earth

*Takeshi Kakegawa¹ (1.Graduate School of Science, Tohoku University)

Keywords:early Earth, BIF, microbial community

Archean banded iron formations (BIFs) are often used as a proxy to reveal surface environments of the early Earth. BIFs are often underlain by carbonaceous sedimentary rocks, which indicate high productivities of deep-sea microbial communities before major deposition of BIFs. It has been unknown if such carbonaceous sedimentary rocks, thus microbial activities, influenced the genesis of BIFs. In order to approach the above problem, BIFs from 2.7 Ga Abitibi Greenstone Belt in Canada were examined. Carbonaceous sedimentary rocks associated with BIFs contain abundant sulfides, and often described as sulfidic BIFs. New analytical data of 34S/32S and 33S/32S ratios of pyrite indicate that large amounts of seawater sulfates were accumulated into carbonaceous sediments at the diagenetic stage, allowing the high activities of sulfate reducers. Typical “high-temperature” sulfides also accumulated in the same sediments. Those findings suggest that carbonaceous sediments behaved as cap rocks of benthic hydrothermal systems, and such capping contributed further evolution of Fe2+ dominating hydrothermal fluids. Apatite crystals were often abundant in oxide layers of BIFs, suggesting the temporal enrichment of phosphorous in local ocean waters. Such enrichment was most likely caused by hydrothermal leaching of phosphates from host rocks, utilization and preservation of phosphates by vent communities and release of phosphate by degradation of organic matter at later stage. This step-by-step process lead enrichment of phosphate in local oceans. This further activated photoautotrophic microbial activities, following progressive deposition of Fe hydro-oxides on the sea floor. In other words, the genesis of BIFs was concealed with microbial activities from Fe2+ supply to Fe2+ oxidation stages, and without such microbial activities, giant BIFs could not be formed.