中〜後期更新世における古植物相 東北地方を中心として

Abstract

Middle-Upper Pleistocene strata yielding abundant macroscopic plant and pollen fossils are distributed here and there in the coastal plain and inland basin areas in Northeast Honshu, Japan. Among them, those in the southern part of Tohoku district are investigated in full detail. On the basis of stratigraphical distribution of the taxa of woody plants obtained from these strata, the local assemblage zones are recognized as shown in Table 1, namely, TODERA 1-2, TODERA 3, TODERA 4, TODERA 5-6, TODERA 7, TODERA 8, LIG, LG III, LG II and LG I fossil assemblage zones, in ascending order. LIG and LG I fossil assemblage zones can be subdivided into two subzones: the former is subdivided into LIGa and LIGb fossil assemblage subzones, and the latter into LG Ia and LG Ib subzones.The LG I fossil assemblage zone is assigned to the later cold time (about 35, 000-10, 000y.B.P. based on radiocarbon dating); the LG II fossil assemblage zone may be from the middle temperate time (about 50, 000-35, 000y.B.P. based partly on radiocarbon dating), and the LG III fossil assemblage zone may be the early cold time of the period corresponding to the Last Glacial Age in the glaciated areas. The LIG fossil assemblage zone is assigned to the Last Interglacial Age. Moreover, the fossil assemblage zones of TODERA 8, TODERA 7, TODERA 5-6, TODERA 4, TODERA 3 and TODERA 1-2 may date from the middle to early ages of the Middle Pleistocene.The fossil assemblages belonging to the LG II, TODERA 7 and TODERA 1-2 zones and LIGb subzone chiefly consist of the taxa commonly found in the temperate deciduous broadleaf forest in the Japanese Islands. On the other hand, the fossil assemblages belonging to the LG Ia subzone and TODERA 8 zone are chiefly composed of the taxa distributed in the subalpine to montane or the subarctic to boreal mixed conifer and deciduous broadleaf forests in the Japanese Islands, Kurile Islands and Sakhalin. The fossil assemblages belonging to the TODERA 5-6, TODERA 4, TODERA 3 and LG III zones and to the LIGa and LG Ib subzones consist of a mixture of the taxa in the temperate deciduous broadleaf forest and the taxa of needleleaf and broadleaf trees which are distributed in the subalpine to montane or the subarctic to boreal mixed conifer and deciduous broadleaf forests in Japan. The fossil assemblages belonging to the TODERA 4 zone and LIGa subzone include more taxa distributed in the temperate deciduous broadleaf forest than those in the subalpine to montane or in the subarctic to boreal mixed conifer and deciduous broadleaf forest. The fossil assemblages belonging to the TODERA 3, TODERA 5-6 and LG III zones and LG Ib subzone are composed of taxa consisting in opposite ratitos to those belonging to the TODERA 4 zone and the LIGa subzone.Although stratigraphcal gaps are often found in the Middle to Upper Pleistocene strata in Northeast Honshu, it is clearly recognized that the plant fossil assemblage composition periodically changed through the Middle to Late Pleistocene. In Northeast Honshu, the periodic oscillation of vegetation, which shifted between two forests represented by the present ones in the Japanese Islands, the temperate deciduous broadleaf forest and the boreal mixed conifer and deciduous broadleaf forest, became more remarkable with time during the Middle to Late Pleistocene.