Long term monthly measurements on water dissolved inorganic radiocarbon to understand the seasonal radiocarbon changes in three of Fuji Five lakes, Japan

Mr Kosuke Ota1,2, Prof Yusuke Yokoyama1,2, Dr Yosuke Miyairi1, Dr Shinya Yamamoto3, Dr Toshihiro Miyajima1

1Atmosphere and Ocean Research Institute, The University of Tokyo, , Japan, 2Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, , Japan, 3Mount Fuji Research Institute, Yamanashi Prefectural Government, , Japan

The process of water flowing into lakes through precipitation, groundwater, and river varies depending on the region and season. It is important to clarify the details of hydrological cycles for long term in order to know the state of the lake in the future. In this study, we investigated the water pathway of Fuji Five Lakes, Japan by measuring the radiocarbon concentration (Δ¹⁴C) oxygen, and hydrogen stable isotopes of precipitation, groundwater, and lake water from 2018 to 2021.
Three out of five lakes, namely lakes Motosu, Shoji and Sai only have temporal inflow and outflow rivers, and the lake water property is defined by inflow of precipitation and groundwater. Therefore, the transportation pathway of water can be quantitatively estimated including the carbon transport process in the hydrosphere. The mass balance of water was then calculated from the groundwater recharge altitude, lake observation data, and the box model of groundwater and lake water Δ¹⁴C.
In Lake Motosu, continuous surveys of surface water properties have been conducted, such as vanadium concentration, water temperature and water quality surveys in summer and winter, and vertical surveys of water temperature and water quality throughout the year. They show that precipitation is dominant to govern water property to the lake.
A single stage Accelerator Mass Spectrometer and a wavelength scan cavity ring-down spectrometer at the Institute of Atmosphere and Ocean Research, The University of Tokyo were used for the measurement of ¹⁴C, oxygen, and hydrogen stable isotope ratios (δ¹⁸O, δD). In addition, the results of the groundwater measurement and the water balance study were combined to provide a comprehensive picture of water exchanges.
The results show that Lake Motosu had the highest Δ¹⁴C in January. In this lake, Δ¹⁴C was found to increase in winter. No clear seasonal changes were observed in Lake Sai. Isotope ratio in Lake Motosu became lighter in summer.
We will present the three years long record of Δ¹⁴C, δ¹⁸ O, and δD to discuss hydrological cycles for the Fuji Five Lakes.


University of Tokyo

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Nov 19 2021


8:45 am - 9:30 am