Abstract: The Sanjiaocheng section is located at the end of Shiyang River which is an internal river system in arid area in Northwest China. This study focuses on the lower portion (292-600 cm) of the Sanjiaocheng section. Six carbon dates show the strata are bracketed between 7 000 and 13 000 a BP. The time interval represented by 2 cm of deposition between near pollen samples is 50 years. Fossil pollen and spore of 155 samples were extracted and analyzed using standard techniques. The pollen assemblage shows that the percentage of Picea varies inversely with that of Sabina,the combined percentage of the two being as high as 40%-60%. According to modern analogues of relationships among the ecological habits, transporting characteristics, and overall pollen assemblage, it was proposed that both Picea and Sabina were transported by water from the upper reaches of the Shiyang river, being indicative of effective soil moisture in the region. Specifically, high percentage of Picea implies high moisture, while high percentage of Sabina high aridity. This idea is supported by the pollen data derived from surface samples. Wind has a very limited capacity to carry Picea and Sabina pollen from the mountain to the lowlands. There are several reservoirs in the upper and middle reaches of Shiyang River, without water transporting, the assemblage of pollen is dominated by local xerophyte. Therefore, Picea and Sabina suggest the vegetation of the mountain in the upper reaches of the Shiyang River.

Key words: arid area, lacustrine sediments, pollen assemblage, Picea, Sabina, environmental signals