Environmental analysis by apparent radiocarbon age difference : molluscan shells and bone samples from Lake Biwa

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  • 見かけ上の炭素年代差を用いた環境解析 : 淡水湖産貝，骨試料を例として

Abstract

We measured the radiocarbon ages of the samples such as molluscan shells, fish, Phragmites and pine leaves collected after 1966 year at Lake Biwa to examine the possibility of freshwater reservoir effects at Lake Biwa, Japan. The molluscan shells collected after 1990 year, were hardly affected from the nuclear bomb test, showed 330 -450 14Cyrs older radiocarbon ages than that of the coeval atmosphere. Then, the apparent difference in radiocarbon ages between the shell fossils and wood samples & soft-shelled turtles and terrestrial animals (-300 14C yrs) excavated from the Awazu submarine shell midden at Lake Biwa suggests that the freshwater reservoir effect existed in middle Holocene (the Middle Jomon period) at Lake Biwa. Because the present-day average residence time of Lake Biwa water is less than a decade, its direct influence on the reservoir effect is little, which suggests that old carbon has been supplied into Lake Biwa. The both age offsets of radiocarbon dating strongly indicate that there were likely to be freshwater reservoir effects at Lake Biwa from middle to late Holocene, which was corresponded to about 5000 years. A great deal of organic matter has flowed into Lake Biwa. Some very old radiocarbon ages can result from retention of old carbon in peat layers formed by plants such as the ditch reeds that grow along the shore of Lake Biwa. This may have been the result of the inflow of aged subsurface waters that may have dissolved old carbon from the carbonate rocks of Mt. Ibuki and Mt. Ryouzen. Therefore the degree of freshwater reservoir effects at Lake Biwa had a possibility to fluctuate, depending on change of the lake environment such as inflow into the lake and circulation of the lake water. In this paper, we discuss the relationships between the apparent radiocarbon age offsets and its lake environment to understand carbon cycle from middle to late Holocene at Lake Biwa, Japan.

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