Marine Transgressions and Regressions over the Last 3,000 Years in Akkeshi Moor, Hokkaido, Northern Japan.

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  • Sawai Yuki
    Division of Earth and Planetary Sciences, Graduate School of Science, Kyushu University
  • Mishio Wakako
    Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University

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  • 北海道東部厚岸湿原における過去３，０００年間の海進・海退
  • ホッカイドウ トウブ アッケシ シツゲン ニ オケル カコ 3 000ネンカン

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Abstract

In order to clarify paleoenvironmental changes during the late Holocene, we obtained core samples from eight sites in the Akkeshi Moor. Diatom assemblages from the cores were divided into three environmental flora groups based on their environmental characteristics as follows. Assemblage M was characterized by marine species, such as Biddulphia aurita, Cocconeis scutellum, Synedra fasciculata. Assemblage T was characterized by marine and brackish species Melosira sp. 1, Diploneis smitbii, Cocconeis scutellum and Paralia sulcata. Assemblage F was characterized by freshwater species Tabellaria flocculosa, Aulacoseira granulata and Eunotia spp.<br>Three paleoenvironmental stages of the moor were identified based on paleoenvironmental reconstruction of the diatom assemblage and by chronological correlation using radiocarbon dating and the tephra. The process at the marine transgressions and regression were compiled besed on the detailed lithological characteristics of the peat and mud deposits in Akkeshi Moor.<br>(1) Stage 1 (-ca. 2, 600yrs BP)<br>A long intertidal flat was formed from the mid to upstream area of the Chiraikaribetsu River, and a shallow bay was at its downstream area around 3, 000yrs BP. Subsequently, the upper half of the intertidal flat was infilled by marshy deposits due to the marine regression beginning at ca. 2, 600yrs BP.<br>(2) Stage 2 (ca. 2, 600-900yrs BP)<br>A small transgression occurred, and the relative sea-level reached its the highest stand (ca. +1m) over the last 3, 000yrs at ca. 2, 000yrs BP. The sea extended midstream beyond the preceding marshy area. After this transgression, relative sea-level again dropped below the present level and moor conditions prevailed extensively in the lowland again.<br>(3) Stage 3 (ca. 900yrs BP-)<br>There were fewer remarkable changes in sedimentary environments than were observed in the preceding stages. A transgression at ca. 600yrs BP, when the relative sea-level was at ca. +0.4m above the present stand, spread and inundated only the downstream area. After that, the existing Akkeshi Moor was formed.

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