Early Miocene terrestrial climate inferred from plant megafossil assemblages of the Joban and Soma areas, Northeast Honshu, Japan

Abstract

Five Early Miocene plant megafossil assemblages from two separate sequences in the Joban and Soma areas on the Pacific side of Northeast Japan, which are interpreted to have accumulated from vegetation in near-sea coastal lowland, are analyzed to discuss the temporal change of floral composition and causal change in terrestrial climate. The Mixed Northern Hardwood forest characterized by various species of deciduous woody dicotyledons and deciduous and evergreen conifers in Stage I (ca. 23.4 Ma) shifted gradually to a Mixed Broad-leaved deciduous forest (Stage II: ca. 21 Ma), a Mixed Broad-leaved Evergreen and Deciduous forest (Stage III: ca. 20 Ma) and a Notophyllous Broad-leaved Evergreen forest (Stages IV and V: ca. 16-17 Ma) with an appearance and subsequent increase in evergreen and deciduous species that favors warmer climatic conditions, and a decrease of taxa which inhabited cooler climates. These changes reflect an increase in mean annual temperature in the first half of the period and a decrease in mean annual range of temperature in the latter half, which were caused by mitigation of winter coldness. A quantitative analysis of mean annual temperature based on the leaf margin analysis also revealed a clear trend of increasing temperature. An abrupt change in temperature observed at the transition from Stages I to III, and warm climate during Stages IV to V can be correlated with the periods influenced by subtropical to tropical shallow-marine climate. A comparison between terrestrial and shallow-marine climates in the Joban and Soma areas probably indicates a synchronous response to climate change occurred during the Early Miocene.