T03_11

Radiocarbon dating and differentiation of tsunami deposits from the Isumi River lowland, Eastern Boso Peninsula, Japan

Soichiro O¹, Obrochta S¹, Fujiwara O², Yokoyama Y³, Miyairi Y³, Hatakeyama Y¹

¹Akita University, Akita, Japan, ²National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan, ³University of Tokyo, Kashiwa, Japan

We use high-density radiocarbon dating to assess the age of – and duration between – coarse overwash deposits that punctuate lagoonal to lacustrine sediments in a core recovered from the Eastern Boso Peninsula, Japan. The region has experienced seismic uplift, is now located several meters above sea level, and is prone to both tsunami and typhoon inundation. Several of the overwash beds appear stacked with a fining and thinning upward structure, which is characteristic of deposition by a tsunami “wave train” resulting from a single seismic event. However, without evidence of co-seismic uplift or run-up immediately followed by backwash, it is difficult to rule out a storm origin. We use the age modeling routine undatable to account for radiocarbon age uncertainty, sediment sample thickness, and dating material quality. Age model results indicate that the apparent groups of overwash beds lower in the core are separated by up to decades and therefore represent multiple washover events, and are therefore inconsistent with a tsunami “wave train”. The sediment accumulation rate then increased drastically, exceeding 1000 cm/ky at ~2500 cal. BP, after which, many of the grouped overwash beds appear to have accumulated geologically instantaneously, which is consistent with a seismic origin, though further study is needed to identify whether uplift events occured simultaneously to overwash bed deposition. Carbon, nitrogen, and sulfur concentrations further provide information on environmental changes caused by the washover events.