[B-BG02] Geo-Bio Interactions and its Applications

[BBG02-P03] Distribution and characteristics of travertine developed in Shirahone hot spring, Nagano Prefecture

*Ai Kiyohara¹, Akihiro Kano², Fumito Shiraishi¹ (1.Hiroshima University, 2.The University of Tokyo)

Keywords:Shirahone hot spring, fossil travertine, travertine, fissure-ridge, stable isotope

Travertine is a calcium carbonate deposit formed by hot spring. At Shirahone hot spring, there is a huge distribution of "fossil travertine" formed by past hot spring activities, and is said to be the largest in Japan. In 1952, this area was designated as a special natural monument by the government due to the rarity of the "Travertine mound" that is thought to be the spring source and the "Oolitic limestone". However, detailed scientific verification has not been conducted since then. In this study, we described the sedimentological, geochemical, mineralogical, and microbiological characteristics of the hot spring water and the modern and fossil travertine of Shirahone hot spring, and examined the environment at the period of fossil travertine formation and the factors that formed it. Fossil travertine is distributed over a wide area in the Jyu-goyahara area and around Ryu-jin Falls in the north, and in the Konashidaira area in the south. The distribution is estimated 8.9 ha in the north and 5.8 ha in the south. On the other hand, modern travertine is local and small. Fossil and modern travertine are composed of calcite and white in color. The spring water is characterized by a very low redox potential (-282 to -273 mV) due to dissolved hydrogen sulfide derived from volcanic gas, and a very low iron concentration (~1 μM). The ratio of the hydrogen carbonate and calcium which are the components of travertine is HCO₃ /Ca = 2:1. This ratio achieved by the dissolution of calcium carbonate, so it is estimated that the Permian limestone in the Shirahone Complex, which is part of the basement rock, has dissolved. In the south, the concentrations are significantly lower except for Mn. Confocal laser scanning microscope observations and microelectrode measurements for the surface of modern travertine suggest the involvement of oxygenic photosynthetic bacteria (cyanobacteria), anoxygenic photosynthetic bacteria (green sulfur bacteria), and sulfate-reducing bacteria. However, the Ca microelectrode may be affected by hydrogen sulfide. The carbon and oxygen isotope ratios show a positive correlation, reflected the progressive isotopic fractionation downstream. Secondary isotopic fluctuations by diagenesis were also observed. Most δ¹³C and δ¹⁸O values of fossil travertine are 0.25 to 5.15‰ and -16.17 to -9.79‰, respectively, except for three samples. Banded travertine of vein facies which have not experienced flow down preserved the low isotope ratios at the period of fossil travertine formation (δ¹³C is from 0.25 to 1.97‰, and δ¹⁸O is from -16.17 to -15.59‰). The isotopic ratios in the south are higher than those in the north, indicating isotopic fractionation was more significant in the south. As a result of the investigation, the fossil travertine in the north was interpreted to be formed by the gushing of a large amount of hot spring water from a fissure ridge at the northeastern rim. The hot spring water that gushed out from the fissure, flowed down the south side and formed a marginal slope (about 10 m high) consisting of crystalline crust with an inclination of 40 degrees to 10 degrees, and the water had a high flow velocity. Far from the fissure ridge, a lateral flat was formed widely, which is consist of marsh facies and crest facies which is composed by reed, paper-thin raft and coated bubble. The lateral flat often contain ostracodes. A steep valley slope composed by crystalline crust and shrub was formed on the valley slope facing the Yukawa River, and some waterfall (about 6-8 m high, about 15 m wide) which is overhanging into the valley also developed. In the south, the facies reflecting high flow velocity are not so prominent. It suggests that there is a low-volume spring source near the travertine mound protected area, and that travertine deposited in a slow-flow environment. Marsh facies consist of reed were widely formed on the gentle slopes, and slope facies consist of shrubs were formed on the valley slope facing the Yukawa River. In outcrop facing the Yukawa River, an inconformity between fossil travertine and its base was observed, and the thickness of the fossil travertine was 13 to 15 m.