Palynology of a 250-m core from Lake Biwa: a 430,000-year record of glacial–interglacial vegetation change in Japan

Abstract

Lake Biwa is situated in western Honshu, and is the largest and oldest freshwater lake in Japan. During 1982–1983, a long core was drilled to a bottom depth of 1422 m in order to investigate the palaeolimnological record, which contains 911 m of various sedimentary units overlying Palaeozoic–Mesozoic basements. A 249.5-m core of the uppermost bed (T Bed) was palynologically examined at intervals of ca. 2 m. The investigated part of the core covers approximately the last 430,000 years of the mid-Upper Pleistocene and Holocene, and five glacial–interglacial cycles can be recognized. Ten major vegetational zones could be recognized from the bottom to the surface. The zones bearing even numbers, BW-10, 8, 6, 4 and 2, matched glacial periods in which pollen of subarctic taxa (Pinaceae, Betula) and cool–temperate taxa ( Fagus, Lepidobalanus) was dominant. In the glacial periods, high pollen values for temperate conifers such as Cryptomeria, Cupressaceae and Sciadopitys indicate interstadial periods. Zones bearing odd numbers, BW-9, 7, 5, 3 and 1, matched interglacial periods with high pollen values for the warm–temperate taxon Cyclobalanopsis, or showed the characteristic appearance of Lagerstroemia, and temperate coniferous taxa ( Cryptomeria, Cupressaceae). There were two vegetational types in the interglacial periods. One was the type indicated by BW-9 and 1 zones, when warm–temperate evergreen broad-leaved trees such as Cyclobalanopsis and Castanopsis showed high pollen values, and the warm–temperate deciduous broad-leaved tree Lagerstroemia was lacking. The climate of these interglacial periods seems to have been cold and dry in winter, and warm and wet in summer. The other was the vegetational type recorded in BW-7, 5 and 3 zones, where Lagerstroemia showed low pollen values, and the warm–temperate evergreen broad-leaved trees mentioned above were poorly represented. The climate seems to have been mild and wet in winter, and cool and wet in summer. The dominant pollen values for cool–temperate deciduous broad-leaved taxa such as Fagus and Lepidobalanus are very important indicators of the initiation of both glacial and interglacial periods in western Japan.