JpGU-AGU Joint Meeting 2020

Presentation information

[J] Oral

M (Multidisciplinary and Interdisciplinary) » M-TT Technology & Techniques

[M-TT51] Frontiers in Geochemistry

convener:Shogo Tachibana(UTokyo Organization for Planetary and Space Science, University of Tokyo ), Yusuke Yokoyama(Atmosphere and Ocean Research Institute, University of Tokyo), Hiroyuki Kagi(Geochemical Research Center, Graduate School of Science, University of Tokyo)

[MTT51-12] High resolution carbon reservoir effect fluctuations derived from surface water dissolved inorganic radiocarbon of Fuji Five Lakes

*Ota Kosuke1,2, Yusuke Yokoyama1,2, Yosuke Miyairi2, Shinya Yamamoto3 (1.University of Tokyo, 2.Atmosphere and Ocean Research Institute, The University of Tokyo, 3.Mount Fuji Research Institute, Yamanashi Prefectural Government)

Keywords:Radiocarbon dating, Carbon reservoir effect, Fuji Five Lakes

In terrestrial paleoenvironmental research, high time resolution is required for meaningful results; varve deposits in lakes and stalagmites have been used to achieve high time resolution. However, there are limited areas where good geological samples with long growth periods, such as varve deposits and stalagmites, can be found.
Radiocarbon dating is a method applicable to a wide area, but the carbon reservoir effect causes the water and sediments to appear apparently old. This is due to the exchange of 14C-depleted substances derived from biological remains and host rocks during the process of fixing and transporting carbon from the atmosphere. Estimation of reservoir effects in marine samples has been extensively researched, but recent studies have pointed out the possibility of seasonal variation, and detailed studies are still ongoing. There are few studies of reservoir effects on land due to complicated carbon transfer processes and underdeveloped measurement techniques, and no studies on seasonal variations. However, as described above, lake sediment is important for paleoenvironmental analysis, and it is important to estimate the reservoir effects within lake water with high accuracy.
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 carbon transport process in the hydrosphere. 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 Δ14C of lake water once a month at Fuji Five Lakes, compared with Δ14C of groundwater. The water balance was then calculated from the groundwater recharge altitude, lake observation data, and the box model of groundwater and lake water Δ14C. The results of these studies were comprehensively considered, and the mechanism of the reservoir effect in the Fuji Five Lakes was clarified.
The result show that Lake Kawaguchi in August had the lowest Δ14C and Lake Motosu in January had the highest Δ14C. In Lakes Motosu and Kawagushi, Δ14C was found to increase in winter. No clear seasonal changes were observed in Lake Sai and Lake Yamanaka. From the results of the water balance, it was found that the inflow of groundwater reached its maximum in winter, which suggests groundwater inflow is caused by the rise of groundwater pressure due to snowmelt.
Regarding the relationship between the lake water and the reservoir effect of the sediment, the lake water Δ14C in summer and the Δ14C of the surface sediment coincided within Lakes Motosu and Kawaguchi. It is possible that Δ14C of sediment in both lakes are defined by the fixation of lake water 14C due to the expansion of biological production in summer, and that lake water Δ14C in summer can be used to estimate the reservoir effect of lake sediment.