[B-CG07] Geo-Bio Interactions and its Applications

[BCG07-P01] Microbial influence on peloid formation in modern hot spring carbonates, Satono-yu, southwestern Japan

*Hannes Stengel¹, Naotaka Tomioka², Yoshio Takahashi³, Fumito Shiraishi¹ (1.Hiroshima University, 2.JAMSTEC Kochi, 3.The University of Tokyo)

Keywords:cyanobacteria, travertine, hot spring, peloid

Microbialites, organosedimentary deposits formed by a benthic microbial community, constitute the earliest macroscopic evidence of life on Earth and have priorly been acknowledged as proxies for the search for extraterrestrial life on Mars or elsewhere in the universe. Microbial involvement in microbialite formation includes a wide range of physical, chemical, and microbial processes from trapping/binding of allochthonous particles to mineral precipitation through metabolisms and their products (e.g., EPS). The relative importance of these processes is thought to vary depending on the depositional environment and time period, the details of which are, nevertheless, still not fully understood. Peloids are a particularly controversial, yet major fabric component of both ancient microbialites as well as tufa and travertine deposits as their commonly studied modern analogues, among many others. Peloids are observed as structureless microcrystalline particles in thin section, but a few previous studies observed their cross sections and found coccoidal cyanobacteria inside (Adachi et al., 2004; Shiraishi et al., 2017), suggesting the possibility that the peloids represent calcified colonies of them. To evaluate the generality of such a peloid formation process, this study investigated peloids from hot spring travertine at Satono-yu (Oita Prefecture, Japan) by microscopical and geochemical-mineralogical techniques. Based on in-detail observations of their (sub-)microstructure on a to-date unprecedented scale, we propose a new peloid formation mechanism through variable, passive involvement of photosynthetically active microbes at the travertine-water interface.


References

Adachi N., Ezaki Y., Liu J. (2004) The fabrics and origins of peloids immediately after the end-Permian extinction, Guizhou Province, South China. Sedimentary Geology 164, 161–178.
Shiraishi F., Hanzawa Y., Okumura T., Tomioka N., Kodama Y., Suga H., Takahashi Y., Kano A. (2017) Cyanobacterial exopolymer properties differentiate microbial carbonate fabrics. Scientific Reports 7, 11805.