|Budget Amount *help
¥1,800,000 (Direct Cost : ¥1,800,000)
Fiscal Year 1992 : ¥400,000 (Direct Cost : ¥400,000)
Fiscal Year 1991 : ¥1,400,000 (Direct Cost : ¥1,400,000)
Piston core KH82-4-14 studied here is sited on a flat-gentle slope, west of the Koshikijima islands, East China Sea. The core's dating is based upon two ash layers: the Ah ash layer(about 6,300yrs. B.P.) and the AT ash layer (about 23,000yrs.B.P.). The core bottom is estimated at approximately 28,000yrs.B.P.
28 species of planktonic foraminifera, which attributed to 11 genera, are contained in the core samples. Meaningful down-core faunal frequency changes are exhibited by five dominant species: Globigerina bulloides, Globigerina quinqueloba, Globigerinoides ruber, Globigerinita glutinata and Pulleniatina obliquiloculata. Based upon the recent distribution of these species in surface sediments obtained from the East China Sea, the down-core frequency changes suggest different characteristics of the surface water in this region during the last 28,000yrs.
Between 28,000yrs. B.P. and 17,000yrs. B.P., G. bulloides were prominant, suggesting that the surface water was strongly influenced by c
oastal water. Between 17,000yrs. B.P. and 8,000yrs. B.P., G. quinqueloba predominantly occurred, indicating that the surface water was under the strong influence of low salinity water. Between 8,000yrs. B.P. and 6,000yrs. B.P., the relative frequency of G. quinqueloba obviously decreased while that of P. obliquiloculata, G. ruber and larger individuals (>150mum) of G.glutinata tended to increase, revealing that the low salinity water's influence was sharply weakened while Kuroshio's influence emerged. Since 6,000yrs. B.P., all five species showed relative frequencies similar to their present ones, indicating that the surface water of this region had been affected by both low salinity water and the Kuroshio warm current.
Piston core YS91-1(6m) studied here is taken from the central part of the Yatsushiro Sea. The core's dating is mainly based on measurements of ^<14>c. The core bottom is estimated at approximately 22,000yrs.-25,000yrs. B.P. on the basis of refractive analysis and EPMA chemical analysis of glass of tuff layer (510-600cm in depth). Erosional surface is recognized at about 480 cm in depth. Ammonia beccari and Elphidium crispum are first found above this horizon. They are distributed at the eastern area of the present Yatsushiro sea which are characterized by influences of fresh water. The level of 362cm in depth of this core is dated at 8911+203 yrs. B.P.based on measurements of ^<14>C. Pseudorotalia gaimardii and Florilus japonicus are commonly found above the level of about 100m in depth which is dated at 4809+140 yrs B.P. This faunal assemblage are similar to the present ones, suggesting that the Yatsusiro Sea has been present marine environments since about 5,000yrs. B.P.,although further study is needed. Less