Abstract
This paper is an attempt to gain insight into the environmental changes of Lake Hamana, a semienclosed brackish lagoon, central Japan, over the last six thousands years. The following environmental changes were revealed by the study of the diatom remains of two undisturbed cores at the central portion of Lake Hamana.
Lake Hamana was a brac kish lake that had large inputs of marine water about 6000 years ago. The salinity of this time was above 20 ‰ that was similar to the present level. The salinity decreased lower than 10 ‰ from about 4500 years ago, and Lake Hamana became a freshwater lake at about 3000 years ago. A series of environmental oscillations with 700 - 1200 years intervals between fresh and brackish episodes followed till about 500 years ago (1498AD), as shown by alternation of freshwater planktonic forms of Aulacoseira granulata and Aulacoseira ambigua with the brackish water planktonic form of Cyclotella caspia. A sudden increase in salinity after the Meiou Earthquake and the following tsunami (1498AD) caused an abrupt change of diatom assemblages of the lake.
