奈良盆地西部における大阪層群Ma2層の前期更新世植物化石群

摘要

The paleovegetation and paleoclimate during the marine oxygen isotope stage 25 were reconstructed based on pollen and plant mac-rofossils from the Pleistocene Osaka Group in the Ma2 layer at Inaba in the western Nara Basin. The paleovegetation was characterized as a deciduous broad-leaved forest dominated by Quercus subgen. Lepidobalanus and Fagus. The paleotemperature of 10-13°C in the western Nara Basin was estimated based on the present distributions of the warmest species Stephania japonica and the coolest species Cha-maecyparis pisifera in the plant macrofossil assemblages.rnIn the Ma2 layer, the cupule fossil of Fagus was identified as F. microcarpa because the fossil was small and the base of the cupule scale protruded. Fossil pollen morphology of Fagus, which would be derived from F. microcarpa, is different from the modern Fagus species in the following two ways : 1) it is small in size, and 2) it has a coarse pattern on the surface, observed under a differential interface microscope. A comparative review of the occurrence of Meta-sequoia shows that there is a significant difference in the amount in the Ma2 layer in the Nara Basin and around Osaka Bay.%奈良盆地西部の生駒市高山町稲葉に分布する大阪層群海成粘土層Ma2層から産出した花粉化rn石と大型植物化石に基づき，Ma2層が堆積した前期更新世MIS25の古植生と古気候を推定しrnた．当時の植生は，ブナ属やコナラ亜属を主体とする落葉広葉樹林が卓越していた．植物化石群rnのうち，最も温暖な地域に生育するハスノハカズラと，最も冷涼な地域に生育するサワラの分布rn域の気候から，当時の奈良盆地西部の気温条件を年平均気温約10～130℃と推定した．Ma2層にrn含まれるブナ属殻斗化石は，小型で基部が隆起する殻斗鱗片から，シキシマブナと同定した．微rn分干渉顕微鏡による観察から，シキシマブナ由来である可能性が高い同層準のブナ属花粉は，小rnさな粒径と粗い表面模様を持つ点で，現生のブナ属と異なることがわかった．また，奈良盆地とrn大阪湾周辺のMa2層のメタセコイア化石産出状況を比較・再検討した結果，大型植物化石の産rn出の有無や花粉の産出率に地域差があることが明らかになった．