Laguna : 汽水域研究

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Laguna : 汽水域研究 4
1997-03 発行


Paleo-environmental Changes during the Last 1,000 Years from Lake Deposits at Lake Hamana, Central Japan
Honda, Shuichi
Kashima, Kaoru
The bore-hole samples were taken at three sites in Lake Hamana, a brackish lagoon, central Japan, to reconstruct paleo-environmental changes.
The paleo-environmental changes during the last 1,000 years were presumed by lithofaces sequences and diatom assemblages from lake deposits, as follows.
Zone I (AD 1000~1498): A lot of trace fosssils were found from sediments.
Aulacoseira granulata, a freshwater diatom species, was dominated. Sea water hardly came into the lake basin, and salinity of the lake water was nearly fresh water.
Zone II a (AD 1498~1550): The tsunami of the Meiou Earthquake in 1498 and the huge storm in 1499 made a small waterthrough between the lake and the sea, called Imakireguchi at the southern edge of the lake. The salinity of the lake had increased suddenly, and Thalassionema nitzschioides and Thalassiosira spp. , marine diatom species, were dominated.
Zone III b(AD 1550~1700): Salinity of the lake water fell down because of the increase of Cyclotella caspia, a brackish diatom. The distinct white-gray laminated structure formed at lake sediments.
Zone IV c (AD 1700~1850): Salinity of the lake water increased again and Thalassionema nitzschioides and Thalassiosira spp. were dominated.
Zone V d (AD 1850~1970) was dominated again. The decrease of the salinity occurred again.
Zone VI e (AD 1970~): The artificial construction of the water pass at Imakireguchi was done in 1954~1973. The salinity of the lake has been maintained at the higher level since then.