Organic geochemical records of paleo-tsunami in peat sediments of the Akkeshi area, eastern Hokkaido, Japan
Abstract
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 core from the Akkeshi area, eastern Hokkaido, Japan to evaluate the sedimentological and geochemical features of the paleo-tsunami deposit.
We analyzed the peat sediment core collected from the Akkeshi town, eastern Hokkaido, Japan during 2016. The sediment core mainly comprises 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. The carbon preferential index (CPI) of n-alkanes is used as a diagenetic indicator, and the values in the tsunami layers are significantly lower than those of peat layers. This result suggests that the deposits containing diagenetic compounds were possibly supplied. In steroid biomarkers, stanols could be detected in all samples of the Akkeshi core. 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. The C27 stanol (cholestanol) is not biosynthesized compound, but C29 stanol (e.g. stigmastanol) is synthesized by a part of terrestrial plant species as minor component. 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 tsunami layers. The higher C27 stanol/sterol ratios suggest the contribution of marine compounds deposited under more reduced condition in the tsunami layer. In kerogen analysis, the marine dinocyst and inner organic lining of foraminifera could be identified in the tsunami layer, although the dinoflagellate-derived biomarkers (e.g. dinosterol) could not be detected. Also, we suggest that more reddish pollens found in the tsunami layer may be reworked component. From these results, it is suggested that the organic geochemical indicators are applicable to reconstruction and evaluation of paleo-tsunami deposit.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMNH32A..07A
- Keywords:
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- 4315 Monitoring;
- forecasting;
- prediction;
- NATURAL HAZARDSDE: 4333 Disaster risk analysis and assessment;
- NATURAL HAZARDSDE: 4341 Early warning systems;
- NATURAL HAZARDSDE: 4564 Tsunamis and storm surges;
- OCEANOGRAPHY: PHYSICAL