A possible middle Pleistocene seasonal deposit in Taipei Basin, Taiwan, inferred from pollen study

Laminated sediments in the Taipei Basin which are tentatively assigned to the antepenultimate glacial period were studied to discern the paleoclimatic significance. Analyses of multi proxies including pollen, mineralogy, grain size, and loss on ignition (LOI) were carried out on three 50 cm thick intervals of a 23.7 m thick laminated section of the Hsinchuan Core, western Taipei. The couplets are commonly composed of 2–7 mm light grey layers and 2–3 mm dark grey layers. Major components in the light grey layer are quartz (>70%) associated with clay minerals (illite). Clay minerals increase in the dark grey layer and can dominate. A pollen study indicates that the light grey layers are characterized by more abundant warm elements such as Cyclobalanopsis and Castanopsis, while the dark grey layers have more abundant cool elements including Pinaceae (Pinus, Picea, Tsuga), Alnus and significant amounts of Poaceae. Study of the present pollen calendar of the Taipei Basin indicates that Pinus appears during winter–spring and Cyclobalanopsis and Castanopsis appear during early summer. The studied couplets are thus very possibly seasonal deposits of the ancient lake. Spectrum analysis for the thickness variation of the light grey laminae, believed to represent most of the annual precipitation changes in summer–autumn, shows prominent El Nino Southern Oscillation (ENSO) peaks of 3–4 and 6–7 year periods. This may indicate that ENSO events occurred during the middle Pleistocene glaciation.