Environmental Changes in Brackish Lake Obuchi-numa, Aomori Prefecture, Northeast Honshu, Japan, with Special Reference to Sea-level Variation in the 20th Century

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  • Nomura Ritsuo
    Foraminiferal laboratory, Faculty of Education, Shimane University
  • Nemoto Naoki
    Department of Earth Sciences, Faculty of Science, Hirosaki University
  • Komura Kazuhisa
    Low Level Radioactivity Laboratory, Institute of Natural Environmental Technology, Kanazawa University

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  • 20世紀における海面水位変動と尾駮沼 (青森県) の環境変化

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Abstract

The recent history of brackish Lake Obuchi-numa, Aomori Prefecture, Japan, was investigated, in order to examine the ecological and geochemical response to annual sea-level variations. The core sediment was calibrated by 210Pb and 137Cs radioactivity measurement and the calculated sedimentation rate was 0.075 g/cm2/yr during the 20th century. A foraminiferal assemblage composed exclusively of Ammonia “beccarii ” developed in 1954 and in the 1960s to early 1970s, but it was largely absent before the 1950s and after the 1980s. The occurrences of A. “beccarii ” are mostly in concert with the sea-level rise recorded off Hachinohe, suggesting the active exchange of lagoon and marine water. Geochemical evidence such as chlorine and sulfur supports the influence of marine water when sea-level rose. Total organic carbon to total nitrogen ratios (C/N ratios) decreased in the periods when foraminifera were abundant, and the ratios positively suggest increased supply of N-rich proteins to the sediment. However, there was no clear response of A. “beccarii ” to the sea-level rise that continued after 1990 to the present. The decreased A. “beccarii ” assemblages were probably a result of significant human activities from the 1980s onwards.

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