[2poster44-52] T13 ［Topic Session］Latest Studies in Sedimentary Geology

ECSentry

[T13-P-13] Cyanobacterial influence on peloid formation – Insight from recent hot spring travertine, Satono-yu, southwestern Japan★「日本地質学会学生優秀発表賞」受賞★

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

Keywords:

peloid,hot spring,travertine,cyanobacteria,carbonate precipitation

Terrestrial hot spring environments have long been deemed a favourable setting for the dawn of microbial life on Earth and perhaps elsewhere in the universe. As such, they were identified as promising loci for the exploration of potentially extant life on Mars, meanwhile associated carbonate rock (travertine) precipitates are vigorously studied as analogues for ancient microbialites dating back to the Archaean. Although CO2 degassing and abiotic carbonate precipitation has conventionally been deemed a major driver of travertine formation, variable microbial influences are increasingly being acknowledged. Nevertheless, the complex interplay between the two on travertine fabrics (among others) remains a matter of controversy.
A particularly enigmatic travertine micro-component are peloids, (sub)rounded and internally structureless grains ca. 30–100 μm in diameter that are ubiquitously found in the carbonate rock record. These grains are generally considered to have several origins, ranging from biotic to abiotic processes. Recent research suggests potential cyanobacterial mediation based on sundry coccoid-shaped microstructures in peloid interiors (Adachi et al., 2004; Shiraishi et al., 2017), although the spatiotemporal extent of aforementioned hypothesis is yet to be determined. In order to shed new light on said discussion, we herein investigated and reconstructed the formation mechanism of aragonitic peloids from recent travertine deposits at Satono-yu hot spring (Oita Prefecture, Japan). Integration of advanced microscopical and mineralogical-geochemical techniques allowed an in-detail observation of peloid microstructures on a to-date unprecedented scale. This study critically contrasted arguments in favour of both microbial and abiotic mediation, and a novel, four-step peloid-formation mechanism based on (cyano)bacterial influence shall be proposed.

References:
Adachi N., Ezaki Y. and 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. and Kano A. (2017) Cyanobacterial exopolymer properties differentiate microbial carbonate fabrics. Scientific Reports, 7, 11805.