5:15 PM - 7:15 PM
[BBG02-P02] Formation process of peloids seen in hot spring travertine (Nagayu hot spring, Oita)
Peloids are microcrystalline, structureless particles commonly found in carbonate rocks. Peloids have various origins, including many thought to be of microbial origin, but their formation is not fully understood. Peloids have also been reported in travertine, a carbonate precipitate formed in hot springs (Shiraishi et al., 2019), and it is speculated that microbial influence is responsible for the formation of peloids, even though abiotic precipitation is predominant. In this study, we focused on peloids in travertine formed in Nagayu hot spring, Oita Prefecture, and aimed to clarify their origin by utilizing submicron-scale observation techniques.
The results of water chemistry analysis, bulk mineralogical analysis, and microelectrode measurements were similar to those of previous studies, indicating that abiotic factors primarily drive aragonite precipitation. The observation of the precipitate was similar to that of the previous study, except that the distribution of the peloids was further upstream than in the previous study: peloids with diameters of about 20 to 50 µm were accumulated in the depressions between the fan-shaped aragonite, and non-calcified filamentous cyanobacteria were distributed around them. Thin-foil sections of the peloids were prepared by focused ion beam processing and observed using a transmission electron microscope (TEM) and a scanning transmission X-ray microscope (STXM). The results showed that the center of the peloid was composed of granular aragonite with diameters of about 200 to 500 nm, surrounded by acicular aragonite. Filamentous aragonite with widths of about 100 nm and lengths of about 0.5 to 1.5 µm was also observed in the center, and X-ray absorption around it suggests the presence of carboxyl groups. These observations suggest that acidic EPS released by some microorganism acts as a nucleus for the precipitation of granular aragonite, which is then held by filamentous cyanobacteria that possess non-acidic EPS, forming rounded peloid particles. The results of this study indicate that there are various peloid formation processes other than calcification of coccoidal cyanobacteria.
Reference
Shiraishi F., Eno Y., Nakamura Y., Hanzawa Y., Asada J., Bahniuk A.M. (2019) Relative influence of biotic and abiotic processes on travertine fabrics, Satono-yu hot spring, Japan. Sedimentology 66, 459–479.
The results of water chemistry analysis, bulk mineralogical analysis, and microelectrode measurements were similar to those of previous studies, indicating that abiotic factors primarily drive aragonite precipitation. The observation of the precipitate was similar to that of the previous study, except that the distribution of the peloids was further upstream than in the previous study: peloids with diameters of about 20 to 50 µm were accumulated in the depressions between the fan-shaped aragonite, and non-calcified filamentous cyanobacteria were distributed around them. Thin-foil sections of the peloids were prepared by focused ion beam processing and observed using a transmission electron microscope (TEM) and a scanning transmission X-ray microscope (STXM). The results showed that the center of the peloid was composed of granular aragonite with diameters of about 200 to 500 nm, surrounded by acicular aragonite. Filamentous aragonite with widths of about 100 nm and lengths of about 0.5 to 1.5 µm was also observed in the center, and X-ray absorption around it suggests the presence of carboxyl groups. These observations suggest that acidic EPS released by some microorganism acts as a nucleus for the precipitation of granular aragonite, which is then held by filamentous cyanobacteria that possess non-acidic EPS, forming rounded peloid particles. The results of this study indicate that there are various peloid formation processes other than calcification of coccoidal cyanobacteria.
Reference
Shiraishi F., Eno Y., Nakamura Y., Hanzawa Y., Asada J., Bahniuk A.M. (2019) Relative influence of biotic and abiotic processes on travertine fabrics, Satono-yu hot spring, Japan. Sedimentology 66, 459–479.