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.

The eukaryotic clade is composed of stem and crown groups, and the emergence of latter is gathering particular attention because it diversified into all kind of eukaryotes that we see today. Based on eukaryotic fossils reported from worldwide, it is widely believed that they emerged in early Mesoproterozoic. Among them, Mesoproterozoic Chitrakoot Formation, lower Vindhyan Supergroup, India, recently gained attention for its well preserved early eukaryotic fossils including red algae, i.e. crown group (Bengtson, et al. 2017). However, there are still some uncertainties with Chitrakoot Formation: (1) the inhabiting environment of these eukaryotes is unclear, although it is important for understanding O₂ demand of the early eukaryotes, and (2) the depositional age is not well constrained because the upper part (1650 Ma; Bengtson, 2009) is older than the lower part (1406–1483 Ma; Kumar et al., 2001). In this presentation we will describe the lithological characteristics, depositional features, and eukaryotic microfossils from Chitrakoot Formation.The lower part of Chitrakoot Formation is composed of granular dolostone (it was called peloidal dolomite by previous studies), glauconitic sandstone and intraclastic dolostone. Some horizons of granular dolostone have chert matrix, which contains many microfossils including spherical with meshwork, smooth ornamented, and multicellular filamentous forms. The depositional setting of the lower part is tentatively considered as high energy setting and deepening upward.The upper part of Chitrakoot Formation is composed of bedded dolostone with overlying stromatolite-bearing dolostone and phosphorite. The latter is partly deformed by slump folding. The stromatolitic/oncolitic phosphorite contains filamentous structures representing probable mold of filamentous microorganisms. The depositional setting of the upper part is tentatively considered as a relatively low energy environment with flourished microbial mats. In the summary， we believe that detailed investigation is required to elucidate the depositional environment and possible eukaryotic assemblage of Chitrakoot Formation.

References

Bengtson et al. (2009) PNAS 106, 7729–7734; Bengtson et al. (2017) PLoS Biology, 15, e2000735; Kumar et al. (2001) Current Science, 81, 806–809

The Ryukyu Arc is situated between the islands of Kyushu and Taiwan (~1200 km in the full length). This arc is formed by subduction of the Philippine Sea plate beneath the Eurasia Plate along the Ryukyu trench, and is composed of forearc islands, chains of arc volcanoes, and a back-arc rift called Okinawa Trough. Ryukyu Arc is commonly divided into three segments (northern, central and southern) that bounded by the Tokara Strait and the Kerama Gap, respectively (e.g., Konishi, 1965; Kato et al., 1982). The northern Ryukyu Arc (i.e., the Tokara Islands) has more numerous active frontal arc volcanoes occurred at both of subaerial and submarine fields. However, there are sparse active submarine volcanoes related to frontal arc volcanism from the southwest of Iotori-shima in the central-southern Ryukyu Arc (Kimura et al., 1986; Shinjo et al., 1999). In addition, the volcanic activities related to back-arc rifting of the Okinawa Trough well-develop around the central-southern Ryukyu Arc. The existence of volcanic activity associated with volcanic front developed in the central-southern Ryukyu Arc and their petrological characteristics, magmatic features, and contribution of slab-derived materials are still unknown due to few geological surveys and studies.In 2021, detailed geophysical and geological investigation around the Daisan-Miyako Knoll, the Ikema Knoll and the Daiichi-Miyako Knoll was undertaken by KS-21-16 cruise based on the detail morphological data reported from Minami and Ohara (2018). In this presentation, we present the cruise summary of the KS-21-16 and preliminary petrological and geochemical data for these submarine volcanoes.

References:

Kato, S., Katsura, T., Hirano, K., 1982. Submarine geology off Okinawa Island. Report of Hydrographic and Oceanographic Researches, 17, 31–70 (in Japanese with English abstract).

Kimura, M., Kaneoka, I., Kato, Y., Yamamoto, S., Kushiro, I., Tokuyama, H., Kinoshita, H., Isezaki, N., Masaki, H., Oshida, A., Uyeda, S., Hilde, T. W., 1986. Report on DELP 1984 Cruises in the Middle Okinawa Trough. V: Topography and geology of the central grabens and their vicinity. Bulletin of the Earthquake Research Institute, University of Tokyo, 61, 2, 269-310.

Konishi, K., 1965. Geotectonic framework of the Ryukyu Islands (Nansei-Shoto). Journal of the Geological Society of Japan, 71, 437–457 (in Japanese with English abstract).

Minami, H., & Ohara, Y., 2018. Detailed volcanic morphology of Daisan-Miyako Knoll in the southern Ryukyu Arc. Marine Geology, 404, 97-110.

Shinjo, R., Chung, S. L., Kato, Y., Kimura, M., 1999. Geochemical and Sr-Nd isotopic characteristics of volcanic rocks from the Okinawa Trough and Ryukyu Arc: Implications for the evolution of a young, intracontinental back arc basin. Journal of Geophysical Research, 104, B5, 10591-10608.