Palynological evidence for vegetation development and climatic change in the Sub-Himalayan Zone (Neogene, Central Nepal)

Abstract

A palynological study on Miocene–Pleistocene sediments exposed at Surai Khola in central Nepal yields new information on vegetation and climate change over the last ∼11.5 Ma. Our results show that during the late Middle Miocene to early Late Miocene (∼11.5–∼8 Ma, Lower Siwalik) the Himalayan foothills and the Gangetic floodplain of Nepal were mainly forested with subtropical to temperate broad-leafed (b.l.) taxa (e.g. Quercus, Lithocarpus/ Castanopsis, Alnus), and tropical forest taxa respectively. Grasses were present at the time but were not abundant. Between the early to late Late Miocene (∼8–∼6.5 Ma; lower Middle Siwalik) grassland replaced subtropical and temperate b.l. forest. This change may be related to disturbance of the vegetation on the slopes due to uplift, perhaps enhanced by intensification of the monsoon. Between late Late Miocene to Pliocene/Early Pleistocene? (∼6.5–<2 Ma; Middle–Upper Siwalik) the grassland vegetation became well established and the influence of the subtropical climax vegetation is minor, with some occasional revivals. An increase in peak discharge in the fluvial system (also related to monsoonal intensification?) is recognized by an increase of the aquatic taxon Potamogeton and better preservation of all specimens. Enhanced seasonal flooding in the system during the Pliocene (∼5.5–∼3.5 Ma; top Middle Siwalik) may have produced local lacustrine conditions on the overbanks, evidenced by an abundance of algae ( Spirogyra) and pteridophytes in the pollen assemblages. Evidence of climatic cooling between ∼6.5–∼5 Ma ago is indicated by the onset of steppe taxa, decrease of tropical forest taxa and disappearance of certain Dipterocarpaceae. Pollen assemblages from the Surai Khola section suggest complex vegetation changes, of which the shift to C 4 grass dominance was only one. Global climatic cooling, the intensification of the monsoon, and the Late Miocene uplift may all have contributed to the phytogeography in the Sub-Himalayan Zone.