[H-QR04] Quaternary, Diachronic dynamics of human-environment interactions

[HQR04-P07] Stratigraphy of the wetland deposit on marine terraces in Nemuro Peninsula, Hokkaido, Japan

*Reisuke Kondo¹, Arata Momohara², Norio Kito³, Takashi Inoue⁴, Akihiro Yoshida⁵, Minoru Yokochi⁶, Kazuhiro Kaneko⁷, Tsumoru Sagayama⁸, Mari Sumita⁹, Kiyoyuki Shigeno¹⁰, Kyota Uemura¹², Yosuke Miyairi¹, Tatsuhiko Sakamoto¹¹, Takato Takemura¹², Yusuke Yokoyama¹, Hiroko Fujita¹³ (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Graduate School of Horticulture, Chiba University, 3.Hokkaido University of Education, Hakodate, 4.Research Faculty of Agriculture, Hokkaido University, 5.Faculty of Law, Economics and Humanities, Kagoshima University, 6.Graduate School of Global Food Resources, Hokkaido University, 7.Graduate School of Agriculture, Hokkaido University, 8.Hokkaido Research Centre of Geology, 9.GEOMAR Helmholtz Centre for Ocean Research Kiel, 10.Meiji Consultant Co. Ltd., 11.Faculty of Bioresources, Mie University, 12.College of Humanities and Sciences, Nihon University, 13.Field Science Center for Northern Biosphere, Hokkaido University)

Keywords:mire, peat, marine terrace, 14C dating, tephra, Numuro peninsula

I. Introduction
Large scale mires are distributed on the marine terraces of the Nemuro Peninsula in eastern Hokkaido. These mires are different geomorphological setting from wetlands at the coastal lowland. The mires on marine terraces in this area are distributed on different marine terraces and some are also distributed on hill slope. The inflow rivers were not found in these mires and erosional valleys are distributed at the margins of the mires. The valley heads of these erosional valleys are buried by the mires.
A part of Habomai mire is clarified as the blanket mire, which slope is covered by peat. And historical botany has been reported (Igarashi et al., 2001; Momohara et al., 2009; Kondo et al., 2021). However, it is unclear that the relationship between the stratigraphy of sediments on the terraces and the landform evolution.
We try to clarify the stratigraphy of wetland deposit on marine terraces around the coast of the Nemuro Peninsula.

II. Methods
Several core samples were collected by drilling in the Habomai Mires, Ochiishi Mires, Hamanaka-Chanai Mires and Baran Peatland and those mires are not directly affected by the Jomon transgression. And the field surveys were conducted at the outcrops in the study area.
We applied tephrochronology, ¹⁴C dating of mainly plant macro fossils and pIRIR dating in order to estimate the age of sediments.

III. Results
The peat deposition mainly started in ca.7.5 ka(Ochiishi Mires, HU-OS-1core: N43°11′40.98″ E145°31′21.99″; Habomai Mires, HU-HB-4 core: N43°21′37.00″ E145°45′19.61″; Barasan Peatland, HU-BRS-2 core: N43°26′12.57″ E145°15′44.17″etc.) according to the ¹⁴C dating and tephrostratigraphy of borehole cores and outcrop samples. On the other hand, it was also found that peat sedimentation started locally before ca.14 ka(Habomai Mires, HU-HB-1core; N43°21′32.56″ E145°45′16.20″etc.).
Furthermore, in the Hamanaka - Chanai Mires, located in the clear fluvial landforms on the marine terraces, the ¹⁴C age of plant macro fossil obtained from the bottom of peat showed ca. 3.5 ka(CG-1-P core, N43°8′23.28″ E145°01′23.52″).

IV. Discussions
In this area, the peat deposition has been started since the Late Glacial (around the Ma-l tephra layer) due to local hydrological conditions at the base of terrace cliffs on the surface of marine terraces and at the head of periglacial dendritic valleys. The peatland has been spread over a wide area on the marine terrace since 7.5 ka (around the Ma-f-j tephra layer) after the relatively early peat deposition areas, and the landscape of these mires became similar to the present wetland environment at the same period. It is possible that a similar meteorological environment has been produced around 7.5 ka because the recharge of these mires is greatly affected by precipitation and the low temperature contributed by the summer period sea fog. It is considered that the peat deposition started soon after the fluvial event disappeared in the mid-Holocene at the valley head in the part of the Hamanaka - Chanai mires.