Abstract
Four cores, which penetrated into the Holocene, were collected in 1985-1986 from the center to mouth of Lake Hamana, Japan. Sediments were analyzed for the concentrations of organic C, total N, biogenic SiO2, Mg, Al, Fe, Mn, V and P. The age of the sediment near the bottom part of each core, which exhibited a higher C/N mole ratio, changed from 11,000 yr B. P. at the mouth to 7,000 yr B. P. at the center part of the lake. This indicated a higher sedimentation of land plants enriched in carbon, which probably took place during a period of high sealevel. After 4,000 yr B. P. in the inner part of the lake there were some distinctive changes in the distributions of elements, as related to their 14C ages : (1) higher content and large fluctuations in both the sediment, organic carbon and biogenic SiO2, and (2) an excess organic carbon over biogenic SiO2. Yet, in contrast, the C/N mole ratio≈12 remained constant. These observations suggested that an eutrophic and productive lake was formed when the seawater exchange was restricted. The contents of Fe and Mg also increased remarkably after 4,000 yr B. P. These changes are due to increasing of the transport ation of particles, originating from ultra-basic rocks carried by the Miyakoda River in the northern part of the lake. Before this period marine silt was predominantly transported from the Pacific sea side.
