Palynological evidence reveals an arid early Holocene for the northeast Tibetan Plateau

Situated within the triangle of the East Asian monsoon, the Indian monsoon, and the westerlies, the Holocene patterns of climate and vegetation changes on the northeast Tibetan Plateau are still unclear or even contradictory. By investigating the distribution of modern pollen taxa on the east Tibetan Plateau, we infer the past vegetation and climate since 14.2 ka BP (1000 years before present) from a fossil pollen record extracted from Gahai Lake (102.3133 degrees E, 34.2398 degrees N; 3444ma.s.l.) together with multiple proxies (grain size, contents of total organic carbon and total nitrogen) on the northeast Tibetan Plateau. Results indicate that the Gahai Basin was covered by arid alpine steppe or even desert between 14.2 and 7.4 ka BP with dry climatic conditions, and high percentages of arboreal pollen are thought to be long-distance wind- transported grains. Montane forest (dominated by Abies, Picea, and Pinus) migrated into the Gahai Basin between 7.4 and 3.8 ka BP driven by wet and warm climatic conditions (the climate optimum within the Holocene) but reverted to alpine steppe between 3.8 and 2.3 ka BP, indicating a drying climate trend. After 2.3 ka BP, vegetation shifted to alpine meadow represented by increasing abundances of Cyperaceae, which may reflect a cooling climate. The strange pollen spectra with high abundances of Cyperaceae and high total pollen concentrations after ca. 0.24 ka BP (1710 CE) could be an indication of disturbance by human activities to some extent, but needs more direct evidence to be confirmed. Our study confirms the occurrence of a climate optimum in the mid-Holocene on the northeast Tibetan Plateau, which is consistent with climate records from the fringe areas of the East Asian summer monsoon, and provides new insights into the fluctuations in the intensity and extent of the Asian monsoon system.