This paper is an attempt to gain insight into the environmental changes of Lake Hamana and Lake Shinji, semi-enclosed brackish lagoons, central Japan, over the last ten thousands years. The following environmental changes were revealed by the study of the diatom remains from two undisturbed cores of the lakes.
Lake Hamana was a brackish lake that had large inputs of marine water about 6,000 years ago. The salinity of this time was above 20%_o similar to the present level.The salinity decreased to less than 10%_o from about 4,500 years ago, and Lake Hamana became a freshwater lake around 3,000 years ago. A series of environmental oscillations with 700 - 1,200 years intervals between fresh and brackish episodes followed till about 500 years ago (1,498AD), as shown by the alternation of the freshwater planktonic form of Aulacoseira granulata with the brackish water planktonic form of Cyclotella caspia. A sudden increase in salinity after the Meiou Earthquake and the following tsunami (1,498AD) caused an abrupt change of diatom assemblages of the lake.
Diatom analysis of a 17m-long core from Lake Shinji showed that the early Holocene was characterized by freshwater taxa, such as Aulacoseira granulata. These freshwater species were replaced by Cyclotella caspia and other euryhaline species indicating a phase of high saline condition until 4,500 yrs BP. Since then, the decrease of water temperature had made the sediment barren of diatom valves. However, the shift of the river course of River Hii by human activities changed hydrological environments and diatoms were found abundantly from the sediments since then.
現在、人類に関わる環境問題に関心が集まり、さまざまな研究がおこなわれている。古環境問題もそのひとつであり、古環境の精度の高い解析が、世界中で試みられ、特に湖の堆積物は細かい環境変動の復元に適しており、多くの研究で用いられている。
珪藻は、淡水性種、汽水性種、海水性種が存在し、地層中に残り易く、地層堆積当時の湖沼環境の復元に適していることが、これまでの研究で明らかになった。本研究では、日本の代表的な汽水湖沼である静岡県の浜名湖と島根県の宍道湖に注目し、気象変動、環境変動による水質変化が激しい汽水域の堆積物中の珪藻遺骸の変化を分析し、古環境復元をおこなった。