[M-IS20] Tsunami deposit

[MIS20-P02] Discussion on dating results from sediments beneath tsunami deposits.
-Verification from trench survey at Atsuma town, Hokkaido-

*Takashi Ishizawa¹, Yuichi Nishimura², Yasuhiro Takashimizu³, Daisuke Sugawara¹, Yosuke Miyairi⁴, Yusuke Yokoyama⁴ (1.International Research Institute of Disaster Science, Tohoku University, 2.Institute of Seismology and Volcanology, Hokkaido University, 3.Niigata University, 4.Atomosphere and Ocean Research Institute, the Univeristy of Tokyo)

Keywords:Kuril trench, 17th century tsunami deposit, radiocarbon dating

Dating of tsunami deposits is important to assess paleotsunami history. For example, regional correlation of tsunami deposits is often conducted by the ages of tsunami deposits to estimate the distribution of tsunami deposits that were formed simultaneously. Since a tsunami deposit is reworked sediment, dating results within a tsunami deposit often show older ages than the actual timing of the tsunami. Therefore, dating of tsunami deposits is often conducted using material from above and below the tsunami deposit. In such cases, dating from sediment beneath tsunami deposits often causes problems. Tsunami often erodes surface sediment during the run-up process, and therefore sediments that are currently located beneath the tsunami deposit are not necessarily sediments that were deposited immediately before the tsunami. Although this has been mentioned in many studies, few studies have been quantitatively demonstrated.
This study conducted multiple radiocarbon dating for sediment beneath a tsunami deposit. Samples were taken from a trench wall excavated at Atsuma town, Hokkaido. The trench wall showed ca. 3,500 years of the geological record, and additional excavation with a handy geoslicer confirmed ca. 6,000 years of the geological record in this area. At the trench, we focused on the 17th century tsunami deposit because the tsunami deposit can surely be traceable along the trench walls and the depositional age is known in detail. B-Tm tephra was also continuously observed beneath the 17th century tsunami deposit, but the thickness of peat between the tsunami deposit and the tephra varied due to the erosion or deposition by the tsunami. To confirm the effect of erosion and deposition by the tsunami on dating, we conducted radiocarbon dating at 20 points beneath the 17th-century tsunami deposits. As a result, the ages of sediment beneath the tsunami deposit varied by a few thousand years. For example, sediments several thousand years old were reworked as a block by the tsunami and located directly beneath the tsunami deposits. Moreover, between the tsunami deposit and the underlying peat, a muddy layer, which is several hundred years older than the 17th century, was observed, which is also considered to be a redeposited layer by the tsunami. In this presentation, we summarize these characteristics and discuss some points to be considered when conducting dating beneath tsunami deposits.