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

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.