[U-05_30PO1] Interrelation between Life, Water, Mineral, and Atmosphere

[U05-P08] SEM and TEM observations of carbonate, Fe-oxide and silica minerals in Okuoku-hachikuro hot spring, Akita Prefecture

*Shogo TERAJIMA¹, Takeshi KAKEGAWA¹ (1.Graduate School of Science, Tohoku University)

Keywords:aragonite, Fe-oxide, Fe-oxidizing bacteria, SEM, TEM

Recent studies of biomineralization mainly treated biominerals produced by evolved life. It is uncertain if primordial microbes are capable to precipitate biominerals. If biomineralization by early life is well documented, it will help to understand the Precambrian environments more in details.Abundant carbonates are precipitating at Okuoku-hachikuro hot spring, located in Kosaka, Akita Prefecture, Japan. We collected sinters, soft to solidified sediments and microbial mats. Then, those constituents were observed using field emission-scanning electron microscopy (FE-SEM). Minerals around microbial sheath were also observed by transmitted electron microscopy (TEM). Analyses of X-ray diffraction, pH, DO, dissolved amino acids, carbon isotope compositions and chlorophyll compositions were also performed.Hot spring water does not contain appreciable amounts of dissolved oxygen, but Fe-oxides immediately precipitated after discharge. Chlorophyll analyses indicate no presence of anoxic photosynthesizing bacteria. These results suggest that Fe were precipitated by Fe-oxidizing bacteria dominantly, supported by SEM observation of characteristic morphology of the sheath. Cyanobacteria become more dominant in the distance. Most samples contain radial aggregates of needle-shape aragonite. Such morphology was found in bubble in the "first" discharging fluid. Each needle in radial aggregates seems to be bigger depending on a distance from the discharging point. Aggregates of coarser and random orientated needles of aragonite are found in lower stream zone, where evaporation and cooling of hot spring water are more visible. Because of no systematic correlation to biological activities (microbial mat, amino acid, organic carbon, etc.) to those morphological changes, all aragonites are formed inorganically. On the other hand, Fe-oxide covering sheath are found locally. Using dilute hydrochloric acid etching, Fe-oxide is observed clearly, especially in zone 1. It has 3 morphological types: sheath-like, agglomerated and needle in radial aggregates. It is noteworthy that Fe-oxides never grow in large crystals. This can possibly because microbial activities or organic molecules may prohibit the growth of Fe-oxides. Furthermore Si was detected in Fe-oxide. This result suggests that Fe-oxide probably adsorbs amorphous silica selectively. Such unique morphology may help to interpret the origin of hematite in Precambrian banded iron formations.