A research group from the Graduate School of Environmental Studies at Nagoya University and the Faculty of Science and Engineering at Kochi University has elucidated the mechanism behind the formation of chert layers on lake bottoms.
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A research group led by Takashi Kuma, a doctoral student in the Graduate School of Environmental Studies at Nagoya University, and Seiji Hasegawa, a lecturer in the Faculty of Science and Engineering at Kochi University, has examined 50-million-year-old lake strata in Utah, USA, and succeeded in elucidating the mechanism by which hard glass layers (chert layers) form on lake bottoms—a process that had previously been a mystery.
Chert layers are commonly found in marine sedimentary strata and are widely distributed throughout Japan. Chert layers in marine sediments are formed by the accumulation of the remains of organisms called radiolarians, which have glassy ( SiO₂ ) shells. However, chert layers are sometimes found in lake strata as well.However, chert layers formed in lakes do not contain the remains of organisms with glassy shells, and the mechanism by which chert layers form in lakes has long been a mystery.
The research group investigated 50-million-year-old lake strata (the Green River Formation) found in Utah, USA, and discovered that the chert layers in these strata contain large amounts of organic matter indicating the proliferation of algae.They revealed that when the organic matter from the algae decomposed on the lake bottom, the glassy (silica) components dissolved in the lake water rapidly precipitated, forming chert. While it had previously been suggested that the pH (acidity or alkalinity) of the lake water was related to the formation of chert layers on the lake bottom, this study is the first in the world to demonstrate that this process was caused by the proliferation of algae and the decomposition of organic matter on the lake bottom.
Furthermore, it was found that the chert layers formed at regular intervals, occurring approximately every 100, 200, and 1,000 years—a frequency that matches the solar cycle. This revealed that periodic changes in solar activity altered the abundance of algae, which in turn triggered the formation of the chert layers.
These findings were published online in the British scientific journal *Scientific Reports* on November 11, 2019 (7:00 p.m. Japan Standard Time).
Please click to view the joint press release issued by Nagoya University and our university on November 8, 2019.