Constraint on radiocarbon age correction in Lake Biwa environment from the middle to late Holocene

摘要

Using data from previous studies and newly collected data, we compared the measured radiocarbon ages of molluscan shells, common reed (Phragmites australis) and pine needles (Pinus thunbergii) collected in 1966,1970,1990 and 2008 at Lake Biwa in Japan, and of archaeological samples, to examine radiocarbon reservoir effects at Lake Biwa. We also tested for differences in the radiocarbon reservoir effect between species and locations in the lake. The effects of nuclear bomb tests conducted in the 1950s and 1960s are clear, the offset between atmospheric ~(14)C and the Lake Biwa freshwater ~(14)C is larger for this period because the atmospheric ~(14)C is so high. The semiclosed Lake Biwa system is in dynamic equilibrium with the atmosphere, resulting in the ~(14)C content of the water following the changes in atmospheric ~(14)C caused by nuclear testing. The shells collected after 1990 had radiocarbon ages that were 330-450 ~(14)C years older than those of the coeval atmosphere. The apparent differences in radiocarbon age (about 300 ~(14)C years) between shell fossils and wood samples excavated from the same layer of the submerged Awazu shell midden at Lake Biwa suggest that the radiocarbon reservoir effect also existed in the middle Holocene (the Middle Jomon period, about 5000 years ago). Because the present-day average residence time of Lake Biwa water is 3-6 years, its direct influence on the radiocarbon reservoir effect is small, which suggests that old carbon has been supplied into Lake Biwa.