Late Cenozoic sporopollen records in the Yangtze River Delta, East China and implications for East Asian summer monsoon evolution

The late Cenozoic East Asian summer monsoon (EASM) is generally linked with the Tibetan Plateau uplift and/or global cooling. In this study, four cores (ZK004, ZK005, SZ03 and SZ04) consisting of fluvial-lacustrine deposits spanning the period of ~ 8.0-0 Ma were obtained from the Yangtze River Delta, East China for sporopollen analysis to investigate trends in the EASM evolution and its driving forces. The results show that high percentages of arboreal taxa (such as Fagaceae, Ulmaceae, Juglandaceae, Taxaceae-Taxodiaceae, Liquidambar, Anacardiaceae, Rutaceae, etc.) represent a relatively warm and humid climate between ~ 8.0 and 2.6 Ma. The herb-dominated vegetation (Poceae, Asteraceae, Cyperaceae, etc.) is linked with a relatively cold and dry climate after ~ 2.6 Ma. The compiled thermophilic percentages and AP (arbors)/NAP (non-arbors) ratios of these four cores, which show that the long-term cooling and drying trends are similar to the global cooling since ~ 8.0 Ma, imply that the EASM experienced long-term weakening coincident with the global cooling. However, over this super-long time scale, the thermophilic percentages and AP/NAP ratios have remained at roughly stable levels since ~ 2.6 Ma, possibly due to the effects of the Tibetan Plateau uplifts.