2004 Volume 58 Issue 58 Pages 93-103
To investigate the Recent lacustrine sedimentary environments, 16 bottom samples and 2 cores from Lake Amida and 7 bottom samples and a single core from Lake Kamegaike, both in Ehime Prefecture, Japan, were analyzed in terms of sedimentation rates and grain size distribution.
In Lake Amida, the average sedimentation rate calculated from a core taken at a shallower water depth site was 1.35cm/y, which is faster than the average rate of 0.92cm/y calculated from another core taken at a deeper site in the same lake. In the core from Lake Kamegaike, an increase of the sedimentation rate from 0.54cm/y to 1.36cm/y was detected at the core depth of 60cm. The surface of this increase is dated to be ca. 1954, which roughly corresponds the time when a water gate was constructed. This increase would be explained as an effect of the gate that should have prevented the suspended matter in the lake to flow out into the sea.
The sedimentation rates obtained in this study were generally larger than those in other Japanese lakes. This may be due to a probable higher rate of soil erosion and transportation, because the area around the lakes of this study has been exploited largely for agriculture such as orange fields. In addition, the variation of depth profiles of Pb-214 and K of the three cores is well correlated to artifact events such as shore protection and road works, which suggests that such events should have affected even on the distribution of nuclides.
These results have given some important clues to study sedimentation models and environmental changes of lakes.