T03_07

Eruption History of Mt. Fuji Further Constrained Through Radiocarbon Dating of Fossil Pollen Automatically Extracted from Lake Motosu Sediments

Ota K^1,2, Yokoyama Y^1,2,3,4,5, Miyairi Y², Obrochta S⁶, Yamamoto S⁷, Aurelia H⁸, V.M.A. H^9,10, M. De B¹⁰, Fujiwara O¹¹

¹University of Tokyo, Kashiwa, Japan, ²Atmosphere and Ocean Research Institute, University of Tokyo, Kashiwa, Japan, ³Graduate Program on Environmental Sciences, Graduate School of Arts and Sciences, The University of Tokyo, Bunkyoku, Japan, ⁴Biogeochemistry Program, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan, ⁵Research School of Physics, The Australian National University, Canberra, Australia, ⁶Graduate School of International Resource Science, Akita University, Akita, Japan, ⁷Yamanashi Prefectural Government Mount Fuji Research Institute, Yamanashi, Japan, ⁸niversity of Liege Department of Geography, Belgium, Belgium, ⁹Geological Survey of Belgium, Royal Belgian Institute of Natural Sciences, Belgium, Belgium, ¹⁰Ghent University Department of Geology, Belgium, Belgium, ¹¹Geological Survey of Japan, AIST, Tsukuba, Japan

Lake sediments continuously record paleoenvironmental changes and/or the history of volcanic eruptions, and the construction of highly accurate age models is important for paleoclimatic studies. Organic fossils such as leaves, which have been widely used for dating, are useful because they directly record atmospheric radiocarbon(¹⁴C) and are deposited quickly, but they are only rarely found in sediments, limiting high time-resolution studies. Pollen fossils, on the other hand, not only directly reflect atmospheric ¹⁴C, but also are not easily degraded and are commonly contained within sediments, making them valuable for high temporal resolution and high precision dating of any desired layer. However, manually collecting the requisite number of grains is prohibitively time consuming.

Therefore, in this study, we applied flow cytometry to separate pollen grains for radiocarbon measurement of the sediments of Lake Motosu, at the northern part of Mt. Fuji, Japan. The ¹⁴C measurements were made using single-stage accelerator mass spectrometer (AMS) at the Atmosphere and Ocean Research Institute, University of Tokyo.

As a result, multiple age values were obtained from the sediments of Motosu Lake. The pollen ¹⁴C age agrees leaf fossils within the analytical uncertainty. Compared to the ¹⁴C age of bulk sediments, ¹⁴C age of pollen fossils are 300 to 350 years younger. Since several thin tephra layers in the sediments of Lake Motosu have been reported possibly to record small-scale eruptions of Mt. Fuji, pollen ¹⁴C dating will be applied above and below each tephra layer to further constrain the timing of Mt. Fuji eruptions.