[H-QR04] Quaternary, Diachronic dynamics of human-environment interactions

[HQR04-08] Unveiling the black box: monthly measurements on dissolved inorganic radiocarbon in Fuji Five lakes water to understand the lake specific reservoir ages

*Ota Kosuke^1,2, Yusuke Yokoyama^1,2, Yosuke Miyairi², Shinya Yamamoto³, Toshihiro Miyajima² (1.Earth and Planetary Science Department, Graduate school of Science, University of Tokyo, 2.Atmosphere and Ocean Research Institute, University of Tokyo, 3.Mount Fuji Research Institute, Yamanashi Prefectural Government)

Keywords:Radiocarbon, Fuji Five Lakes, Lake water

Lake sediments are important for paleoenvironmental studies, and it is important to estimate the radiocarbon reservoir effects within lake water with high accuracy. Although radiocarbon dating has a large range of uses, the radiocarbon reservoir effect causes water and sediment ages to appear older. This is due to the exchange of ¹⁴C-depleted substances derived from biological remains and host rocks during the process of fixing, and from transporting modern carbon from the atmosphere. The radiocarbon reservoir effect from lake samples is known to be highly variable, showing a maximum of about 2200 years which can lead to problems for high-precision dating. Yet, there are few studies of radiocarbon reservoir effects on land due to complicated carbon transfer processes and lack of appropriate measurement techniques, and no studies have been performed on seasonal variations.

We study Fuji Five Lakes to unveil the mechanisms behind variability of their reservoir ages. Lakes Motosu, Shoji and Sai only have temporal inflow and outflow rivers, and the lake water is defined by precipitation and groundwater inflow. Therefore, the transportation pathway of water is simple and suitable for estimating the radiocarbon transport process around these lakes. By measuring the radiocarbon concentration (Δ¹⁴C) of precipitation, groundwater, and lake water, it is possible to clarify the effect of groundwater on the reservoir effect of lake water. In this study, for the first time, we measured lake water Δ¹⁴C, δ¹⁸O, and δD monthly at the Fuji Five Lakes, and compared results with groundwater Δ¹⁴C.
The results show that Lake Kawaguchi had the lowest Δ¹⁴C in August and Lake Motosu had the highest Δ¹⁴C in January. In Lakes Motosu and Kawaguchi, Δ¹⁴C was found to increase in winter. No clear seasonal changes were observed in Lake Sai and Lake Yamanaka. Regarding the relationship between the lake water and the sediment, the lake water Δ¹⁴C coincided with surface sediment Δ¹⁴C in the summer at Lakes Motosu and Kawaguchi. Sediment Δ¹⁴C in both lakes may be defined by the fixation of lake water ¹⁴C due to elevated biological production in the summer, and summer lake water Δ¹⁴C can be used to estimate the reservoir effect of lake sediment. Therefore, at least seasonal measurements on lake water are recommended to better understand lake specific reservoir ages.