PALYNOLOGICAL RECORD AND HUMAN IMPACT DURING THE PAST 15,000 YEARS, SONG HONG DELTA AREA, VIETNAM

The obtained palynological sequence from the sediment cores ND-1, VN and GA, which were drilled on the Song Hong delta, revealed climate changes since 15 cal. kyr BP. During 14.5-12.9 cal. kyr BP, the climate was cooler than at present characterized by subtropical species mainly belonging to Castanopsis (Lithocarpus) and Elaeocarpus with abundant temperate species Cryptomeria, Castanea, Alnus, Juglans and so on. Inferred from a widely developed grassland mainly composed by Artemisia; climate during 12.9-11.6 cal. kyr BP was cooler and drier than at present, corresponding to the Younger Dryas cooling period. In a transition period, 11.6-10.9 cal. kyr BP, the climate became warmer than before but still cooler than at present indicated by the appearance of some tropical species in Datiscaceae, Ficus, Piperaceae, Taxotrophis and Vites with the higher percentages of temperate taxa than at present. After 9.4 cal. kyr BP, the five cooling periods during 9.4-9.0 cal. kyr BP, 6.50-5.20 cal. kyr BP, 4.53-3.34 cal. kyr BP, 2.10-1.54 cal. kyr BP, and 0.62-0.13 cal. kyr BP were shown by the increasing of temperate broad-leaved trees and conifers. Each cooling period was followed by warming periods shown by decreasing of temperate-subtropical taxa. The five drying periods during the Holocene of 11.6-9.7 cal. kyr BP, 9.0-7.7 cal. kyr BP, 3.34-2.1 cal. kyr BP, 0.83-0.62 cal. kyr BP and 0.36-0.13 cal. kyr BP were also shown by increasing of drought-tolerant herb and decreasing of wetland/aquatic taxa, which did not always accompany with cooling. Combined the information from pollen, archeology and river sediment discharge data, human had immigrated and settled down again on the delta developed after marine regression at 5.0 cal. kyr BP. The agriculture widely developed inferred from high content of Gramineae after 3.34 cal. kyr BP. The upland cultivated species increased obviously after 2.1 cal. kyr BP, reflecting human activities spreading into montane areas.