Late Holocene Geoenvironmental Changes around Lake Koyama- Tottori Plain, Western Japan

Abstract

Lake Koyama-ike, located in the northwestern part of the Tottori plain, is a coastal lagoon sheltered by sand dunes/bars along the western San'in region, western Japan. We took depositional facial samples from the lake and alluvial deposits in coring and trench excavations in the Takazumi lowland at the southern coast of Lake Koyama-ike. In addition to facial observations of the deposits, diatom fossil assemblages of the cores were analyzed to examine Late Holocene geoenvironmental changes. Chronologic discussion of the cores was based on seven radiocarbon ages found using the AMS method and volcanic ash. Before the K-Ah tephra (ca. 7.3 ka), correlated with the Jomon transgression, the marine area expanded around Lake Koyama-ike, and a sandy tidal flat developed in the inland area of the Takazumi lowland. This tidal flat became a freshwater marsh no later than the K-Ah tephra fall. Subsequently, the freshwater marsh was buried by muddy marsh deposits and sandy flood deposits from nearby rivers and then was transformed into an inland forest ca. 5,200 calBP. However, in the northern lowland, the inner bay formed before ca. 5,800 calBP. Then, this inner bay changed into a brackish lake with the deposition of sandy sediments from rivers after ca. 5,700 calBP. Finally, the salinity of the lake water decreased at ca. 4,600 calBP and it became a freshwater lake. This environmental change was observed in other coastal lowlands around Lake Koyama-ike, suggesting the possibility that the marine water channel between Lake Koyama-ike and the sea are was sheltered around this period.