| The diatoms in early spring and clay minerals in fall and winter gently settle on lake bottoms to form the yearly layers called varves. Japan has four distinct seasons and very distinct changes in water temperature, so these are quite suitable for forming the varves. These conditions are very favorable for the people who conduct research the relationship between civilization and the earth's environment in the past using varves.
We discovered the first annual laminated sediment sequence in Asia and the longest consecutive varve recorded in the world at 100,000 years at Lake Suigetsu in Fukui Prefecture. These sedimentation layers were later discovered throughout Japan, and we were able to discern the relationship between the environment and civilization from the varve in Megata, Akita Prefecture.
The Bohai kingdom that stretched from northeast China to the northern part of the Korean peninsula flourished until it crumbled in 926. The cause is said to have been the large eruption of Mt. Paektusan. Incidentally, an analysis of the varves shows that Mt. Paektusan had a major eruption in 937. Also, an analysis of the type of diatoms in the varves shows a cooling of the climate for three years before Bohai's fall. It is possible that Bohai collapse was due in part to damage from the frigid temperatures when the climate cooled.