Airborne pollen-climate relationship based on discriminant analysis in Nam Co, Central Tibet and its palaeoenvironmental significance

Abstract:

Based on the airborne pollen data collected using a Burkard pollen trap, discriminant analysis were conducted to evaluate the relationship between two different atmospheric circulation systems, the Asia summer monsoon (ASM) and the Westerlies, in Nam Co basin, central Tibet. The whole year's samples could be classified into two groups using cluster analysis: one group was from May to September, another group was from October to April of next year, corresponding to monsoon period and non-monsoon period, respectively. The classification represents two different atmospheric circulation systems, ASM in monsoon period and the Westerlies in non-monsoon period. Discriminant analysis was performed. First, the whole year samples were divided into two a priori groups, group A is monsoon period (May-Sep.) and group B is non-monsoon period (Oct.-Apr.). Then percentage data of major pollen taxa were used to establish the discriminant functions, and then the samples were classified into predicted groups. The results of discriminant analysis show that 78.6% of the samples were cross-validated grouped correctly. Thus, airborne pollen assemblages can distinguish two different climate systems: monsoon period and non-monsoon period. According to the discriminant score, the group centroids of group A and group B were negative and positive, respectively. Therefore, we created the discriminant score as a new monsoon index (PDI, Pollen Discriminant Index), small PDI values represented enhanced summer monsoon climate. Using above result, we calculated the PDI of Nam Co NCL core, the PDI values can be coincided with Dryness (moisture indicator) and A/Cy ratio (temperature indicator).