Silica sinters form diverse plant fossils - Small-scale hot springs and forest coexistence key factor

Associate Professor Yasuhiro Iba and Doctoral Student Tomoyuki Ueda from the Faculty of Science, Hokkaido University, Dr. Aya Kubota from the National Institute of Advanced Industrial Science and Technology (AIST) and JSPS Overseas Research Fellow Ryo Taniguchi from the University of Edinburgh (UK) investigated the Nakabusa Hot Springs area in Nagano Prefecture. Their research revealed that "silica sinters" (siliceous hot spring deposits ("yu-no-hana" in Japanese)) formed at Japanese hot springs possess unique petrological and sedimentological characteristics not found in silica sinters from other countries, incorporating diverse plants from the surrounding area as fossils. Their findings were published in Palaeogeography, Palaeoclimatology, Palaeoecology.

Silica sinters have attracted attention as an excellent medium capable of preserving incorporated biological fossils for billions of years. Traditionally, their formation processes and fossil preservation modes have been studied in overseas hot spring environments such as Yellowstone National Park in the United States. At these silica sinter localities, massive geothermal activity and hydrothermal eruptions result in extremely low biodiversity. Organisms incorporated into silica sinter are limited to a small portion of microorganisms and plants adapted to harsh environments such as high temperatures. It has been thought that only extremely localized and specialized biological communities can be preserved as fossils in silica sinter.

In this study, the Nakabusa Hot Springs area in Azumino City, Nagano Prefecture, was used as the main research site. Nakabusa Hot Springs is located at the foot of Mount Tsubakuro in the Northern Japanese Alps and is covered by forests mainly consisting of Japanese cypress growing on steep slopes. The research team thoroughly surveyed the area to record the distribution, temperature, pH, silica concentration, and petrological and sedimentological characteristics of each hot spring source and silica sinter. They also used optical and electron microscopy to observe the fine petrological features of collected silica sinter samples. Additionally, they conducted experiments by placing ceramic tiles in multiple hydrothermal channels under different conditions such as sunlight exposure and water volume, calculated growth rates from the thickness of deposited silica sinter, and compared them under different environmental conditions.

In the Nakabusa Hot Springs area, a total of 32 hot spring discharge points were identified within an approximately 100-meter-square area. These discharge points, less than 10 centimeters in diameter, quietly discharge hot spring water, and the hydrothermal water flows down steep slopes while forming elongated silica sinter outcrops up to 40 meters long and 5 meters wide. These characteristics are completely different from geothermal areas overseas with violently spouting geysers. Most of the discharged hydrothermal water was very hot, at over 80℃, and the high silica concentration and neutral to alkaline pH were found to be suitable for silica sinter formation. Observations and experiments revealed that silica sinter develops on well-sunlit south-facing slopes and develops well at the edges of hydrothermal channels while not occurring at the bottom of channels. These results indicate that silica sinter formation in this area is controlled by water evaporation promoted by sunlight and silica precipitation.

The silica sinter in the investigated area can be classified into four types based on petrological characteristics, with thick-layered silica sinter containing large amounts of plant remains being predominantly distributed. Plants buried in the silica sinter varied widely, including conifers, broadleaf trees, and mosses, and were fossilized with their exteriors covered by silica or their interiors completely filled with infiltrated silica. Some upright trees were observed being buried in silica sinter while still standing, and moss colonies were found with half fossilized and half still living. Here, unlike the large-scale volcanic hot spring systems overseas that destroy forests and create barren landscapes, the small-scale and sporadic hot spring activity unique to island-arc environments allows hydrothermal water and rich forests to coexist, enabling various plants to be preserved in silica sinter.

This research revealed for the first time the existence of silica sinter incorporating diverse plant communities, along with its depositional environment, processes, and fossil preservation modes. This demonstrated that silica sinter formation systems have greater diversity than previously thought, presenting a new model for reconstructing the depositional environments of geological-age silica sinter containing important fossils and their preserved ecosystems.

Besides Nakabusa Hot Springs, Japan has other known silica sinter localities such as Onikobe Hot Springs in Miyagi Prefecture and Akinomiya Hot Springs in Akita Prefecture. Conducting detailed investigations in these areas would enable discussion of island-arc silica sinter formation modes as more universal phenomena. Additionally, some fossil silica sinters show characteristics different from conventional large-scale modern silica sinters, but this gap has often been ignored. For example, the 400-million-year-old Rhynie chert, which provides excellent preservation of the oldest terrestrial ecosystems, shows no evidence of massive silica sinter platforms or high-energy geysers, and the preserved biological fossils show relatively high diversity. Re-examining the characteristics of these fossil silica sinters based on the current findings is expected to enable more detailed and accurate reconstruction of past environments and ecosystems.

Journal Information
Publication: Palaeogeography, Palaeoclimatology, Palaeoecology
Title: Modern silica sinter deposits from an island-arc setting and their potential for fossilizing plants
DOI: 10.1016/j.palaeo.2025.113176