JpGU-AGU Joint Meeting 2020

Presentation information

[E] Oral

M (Multidisciplinary and Interdisciplinary) » M-IS Intersection

[M-IS01] Tsunami deposit

convener:Daisuke Ishimura(Department of Geography, Tokyo Metropolitan University), Takashi Chiba(Faculty of Bioresource Sciences, Akita Prefectural University), Masaki Yamada(Department of Geology, Faculty of Science, Shinshu University), Takashi Ishizawa(International Research Institute of Disaster Science, Tohoku University)

[MIS01-01] Evaluation for transport processes of paleo-tsunami by organic geochemical analyses of peat sediments in eastern Hokkaido, Japan

*Hiroyasu Asahi1, Ken Sawada2, Satoshi Furota3 (1.Graduate School of Science, Hokkaido University, 2.Department of Earth and Planetary Science, Facility of Science, Hokkaido University, 3.National Institute of Advanced Industrial Science and Technology)

Keywords:Paleo-tsunami, biomarker, Hokkaido

Tsunami deposit is mainly characterized by mixture of marine sand and sedimentary structures formed by huge tsunami flow. In many cases, however, sedimentological information recorded as the structure of tsunami deposit was erased by erosion and disturbance during the post-depositional processes. In the present study, we performed organic geochemical analyses using biomarker and kerogen of the peat sediment cores from the Akkeshi and Kiritappu areas, eastern Hokkaido, Japan to evaluate the sedimentological and geochemical features of the paleo-tsunami deposits.

We analyzed the peat sediment cores collected from the Akkeshi Town and Kiritappu wetland of Hamanaka Town, eastern Hokkaido, Japan, during August 2016. These sediment cores mainly comprise the peats, intercalated with some sand layers (at most several 10 cm thick), which are presumed to be the layers deposited by the paleo-tsunami.

In kerogen analysis, terrestrial plant fragments are mainly observed in all sand layers, indicating the redeposition of peat-derived matter by tsunami. However, the inner organic lining of foraminifera and the marine dinocyst could be identified in the tsunami layers. These reults can be direct evidences of paleo-tsunami.

In steroid biomarkers, stanols could be detected in all samples of the Akkeshi and Kiritappu cores. The stanol conversion reaction from biosterol is thought to occur by microbial reduction in the sediment–water interface and anoxic water column of marine and lacustrine environments. Therefore, the stanol/sterol ratio can be used as redox indicator. We found that the C27 stanol/sterol (cholestanol/cholesterol) ratios are clearly higher than those of C29 in the sand layers of both Akkeshi and Kiritappu cores. The higher C27 stanol/sterol ratios suggest the contribution of marine compounds deposited under more reduced condition in the tsunami layer. However, some sand layers do not show this trend, and they can be considered event deposits of other origin.

Ring-A degraded triterpenoids (Des-A triterpenoids) are formed by microbial degradation under anoxic environments after deposition, and have been indicated to be abundant in flood sediments (Furota et al., 2014). In the Kiritappu core, the relative abundances of des-A triterpenoids remarkably increased in the sand layers in low C27 stanol/sterol ratios, although these are almost constant throughout the core. From these results, these sand layers may be interpreted as flood deposits, not tsunami deposits. However, the abundances of des-A triterpenoids show different trends for Akkeshi than Kiritappu cores. Thus, it is suggested that organic geochemical methods provide much information to event sediments and this methods are useful for identification of the paleo-tsunami sediments.