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

摘要

海面の水位変動に対する海跡湖の環境変化を検討するために，青森県下北半島にある汽水湖の尾駮沼よりコア試料を採取した．~(210)Pbと~(137)Csの放射能測定による堆積速度は0.075g/cm~2/yrであった．有孔虫群集はAmmonia “beccarii”のみで占有されており，1954年ころ，および1960年代～1970年前期にかけて多産した．しかし，1950年以前と1980年以降は産出が少なかった．このような有孔虫の遺骸群集の変化は，気象庁の八戸験潮所で記録されている経年的水位変化ときわめて調和的であった．A.“beccarii”の多産は，湖水と沿岸海水の交換が活発で，また湖内の基礎生産が高まったことを示唆する．しかし，1980年以降には海面水位の上昇とは逆に，有孔虫数は少なく，炭素／窒素比も減少する．その結果，湖内の環境は1980年～1990年にかけて大きく変化した．%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 ~(210)Pb and ~(137)Cs radioactivity measurement and the calculated sedimentation rate was 0.075 g/cm~2/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.