Top Page Outline of varves
Varves in Lake Suigetsu
Varves refer to the sediment with characteristic striped pattern piled up in the lake bed for an extremely long period of years. One new layer made up of light and dark coloured stripes is formed each year and this striped pattern is piled up alternately. The colour difference arises from different types of sediment depending on the season.
One of the means of identifying the age of excavated articles is radiocarbon dating. This is the method for determining the age by using the proportion of radiocarbon (carbon 14) present in the sample and comparing it against the standard air.
The shortcoming of this radiocarbon dating, however, is that inaccuracy occurs in age determination of excavated articles that ranges from several hundred to several thousand years. Radiocarbon contained in animals and plants was originally taken in from the one that exists in the atmosphere. However, the amount of radiocarbon (carbon 14) contained in the atmosphere varies from period to period. Therefore, the amount of radiocarbon of the same kind of animals and plants varies depending on the period.
In order to correct this inaccuracy, it is essential to have the results of radiocarbon dating of samples whose dates are known. It is varves that serve as this accurate measure.
We can identify the exact age of varves and leaves intercalated in the varve in Lake Suigestsu. Measuring the proportion of radiocarbon in the leaves, the relationship between the age and radiocarbon proportion become clear and consequently, the age of excavated artifacts can be accurately determined.
As “a global standard measure” in archaeology and geology, varves of Lake Suigetsu have dramatically improved the precision of age determination compared to conventional measuring methods.
Lake Suigetsu, which is one of the Five Lakes of Mikata known as a scenic spot, is an ideal lake in terms of the environment for varve formation for the following three reasons. ① It has no river to flow in directly. ② Absence of life in the lake bed ③ It has not been filled up by sediment despite the lapse of time. These rare natural conditions are close to a miracle.
Due to the reasons mentioned in ① and ② above, the lake bed has not been stirred, which has contributed to the formation of beautiful striped pattern of varves. Moreover, fault activity around Lake Suigetsu does not allow the lake to be filled up by sediment and, therefore, as mentioned in ③, varves have been continuously formed in the lake bed for as long as 70,000 years. No other lake in the world has longer succession of varves accumulated for such a long period of years than Lake Suigetsu does.
Varves give us clues not only of identifying the specific age of excavated articles, but of finding out what the ancient natural environment was like such as general climate, the appearance of forests in ancient time, etc. Examining the species and amount of pollen from the plants around the lake that are confined in the varves makes it possible to simulate what the appearance of the forest and general view around the lake were like in ancient time.