MODERN POLLEN-BASED INTERPRETATIONS OF THE HOLOCENE PALEOCLIMATE IN THE SOUTHERN MARGIN OF THE TENGGER DESERT, NW CHINA

The focus of is on the reconstruction of the middle Holocene climatic changes based on the pollen records contained in a 6.2 m thick section in the southern part of the Tengger Desert, northwestern China. To understand the modern analogues of the middle Holocene changes, modern pollen spectra across different vegetation zones from the Qilian Mountains to the low land Tengger Desert were studied first. Identified were four pollen combinations corresponding to four different modern ecological environments: (1) a Picea - Artemisia combination in a spruce forest and its vicinity; (2) an Artemisia - Chenopodiaceae- Gramineae- Salix combination in a mixture belt of montane grassland and poplar woodland; (3) a Chenopodiaceae - Artemisia -Nitraria combination in a desert steppe belt; and (4) an Elaeagnus - Artemisia - Chenopodiaceae - Nitraria combination in a desert riparian forest. Our study also shows that a relatively low percentage of Picea pollen (30 -10%) seems to be associated with water transportation (alluvial deposits) and a very low percentage of Picea pollen (10-5%) appears to be related to long-distance air transportation. With references to the modern relationships between pollen assemblages and associated bioclimates, the pollen sequence covering the period between 8500 and 3000 yr cal. BP was used to infer the climate change history. The results show that a warmer and drier climate dominated between 8500 and 7950 yr cal. BP. Subsequent decreases in Artemisia and warm desert forbs/shrubs taxa and contemporaneous fluctuating increases in montane coniferous and aquatics elements from 7950 to 7400 yr cal. BP reflect a fluctuating increase in humidity or/and a fluctuating decrease in temperature. The following period (7400 - 5670 yr cal. BP) was generally warm with the Holocene Climate Optimum occurred between 7400 and 6490 yr cal. BP. Followed was a remarkably cool spell (6490 - 6290 yr cal. BP) that was then succeeded by a warmer and more humid period (6290 - 5670 yr cal. BP). The period (5670- 4470 yr cal. BP) was characterized by drastic oscillations in climatic conditions with three humid and probably cold spells intervened by three relatively dry and warm spells. The period (4470 -3510 yr cal. BP) was cold and humid, followed by a wet spell as indicated by a very high percentage of Typha pollen.