Late Holocene paleoenvironmental changes in the Yahagi River lowlands, central Japan

Abstract

The Yahagi River lowlands are located along the lower reaches of the Yahagi River in central Japan. The author researched and discussed the geomorphic development and paleoenvironmental changes in the lowlands during the late Holocene, using sediment analysis of boring-core data, AMS 14C dating, diatom analysis, and archaeological data. Some changes in the sedimentary environment were clarified by the facies and diatom assemblages of the sediments. In the northern part of the study area, the uppermost sediments indicate that the delta-platform emerged at ca. 3000 yr BP, and peat and humic soil were formed under stable conditions. After ca. 2000 yr BP, the sedimentary environment became unstable, and the peaty or humic sediments were covered with sandy ones which were deposited by flooding. The fluvial influence was dominant in the periods around 2000 yr BP and ca. 1400-1200 yr BP. In the northwestern part of the present lowlands, the subaerial delta expanded widely during ca. 3000-2500 yr BP. The molluscan species of archaeological shell mounds in the southwestern part indicate that an inlet remained near the region until ca. 1000 yr BP. This suggests that the progradation of the delta was slow before that time. In the eastern part, the sedimentary environment of the brackish lake or marsh changed to a freshwater environment around ca. 1500 yr BP. Around this time, the delta probably emerged and the formation of peaty sediments began. The remarkable expansion of the subaerial delta during ca. 3000-2500 yr BP was possibly influenced by minor sea-level changes in the late Holocene, the so-called Yayoi minor regression. It is also possible that climatic change and/or human activities influenced the deposition of sandy sediments over the peaty or humic ones.