Pollen Records And The Quaternary Environment Changes In The Middle Reach Of Heihe River

Arid land in NW China, located at the juncture of westerlies and monsoon, embodys significant amounts of interior rivers and inland lakes. We focus on the middle reach of Heihe River to rebuild the paleaovegetation and paleaoclimate since Quaternary. To achive our aim, we analysis the fossil pollen records from sedimental core combined with pollen modern processes.To verify the influences of the Aeolian process and fluvial process on the pollen transportation, we analyzed 8 riverbed samples and 7 surface soil samples in the middle reach of Heihe River, located in the Hexi Corridor. The results show that Aeolian process could carry the pollen within limited distance, the pollen diagram of surface soi samples mirrors the local vegetation of sample sites, while fluvial process could carry the pollen from the upstream and strongly affect the pollen transportation, reflecting the vegetation of upstream and local vegetation. Confirming the influences of these two processes on the pollen transportation is significant for recovering the palaeoclimate by using the fossil pollen records.We discuss the pollen assemblages of different sediment environments and pollen source by 26 surface sporopollen samples from five different sediment environments(sand dune, alluvial-proluvial fan, washland, riverbed, wet land) along three transverse sections in the middle reach of Heihe River. The results show that the five different geomorphic sediment types have significant different sporopollen assemblages:(1) sand dune: Chenopodiaceae- Artemisia – Nitraria- Poaceae;(2) alluvial- proluvial fan: Nitraria- Chenopodiaceae;(3) flood plain: Picea- Cyperaceae-Chenopodiaceae – Artemisia- Asteraceae;(4) riverbed: Picea- Cyperaceae –Polypodiaceae-Chenopodiaceae – Artemisia;(5) wet land: Poaceae- Typhaceae- Chenopodiaceae – Artemisia. The auxiliary of the principal component analysis also indicatesthe significant difference in sporopollen assemblages among the five different sediment types, verifying the accurace of the representativeness of pollen assemblages from five different sediments, improving the reliability of research result. The results provide the foundation to reconstruct the paleovegetation and paleoclimate based on sporopollen records of different sediment types.Based on the study about pollen modern processes in the middle reach of Heihe River, we reconstructed the evolutionary series of palaeovegetation and palaeoclimate by the pollen records from DWJ core, combining chronology data and sedimental facies. The results show that the percentages of Chenopodiaceae and Artemisia are abundant in the pollen diagram, indicating that the climate kept arid since 1.7Ma BP.ZoneⅠ: 1.7~1.3Ma BP, the percentages of arid herbaceous and shrubs plants, Chenopodiaceae and Artemisia dominated the vegetation, suggesting that the vegetation was desert – steppe vegetation. From 1.7Ma BP, the increasing amounts of Chenopodiaceae and Tamarix indicate that the climate became drier with the progress of retreating lake gradually;Zone Ⅱ: 1.3~1.1Ma BP, the amounts of Chenopodiaceae increased smoothly and the plants of Tamarix rise apparently, indicating that the climate become drier than zone I.Zone Ⅲ: 1.1Ma~now, the vegetation was consisted of large amount of Chenopodiaceae and Tamarix belongs to the typical desert – steppe vegetation, indicating the arid climate during 1.1Ma~now. At the same time, Typhaceae grew in some humid environment by the river.