Environmental change implied by the relationship between pollen assemblages and grain-size in N.W. Chinese lake sediments since the Late Glacial

Abstract

This paper contributes to an understanding of the relationship between pollen assemblages and grain-size in north-west China. Based on the relationship, we have reconstructed the environmental and vegetation history from sediments from Zhuye Lake, located in the marginal area of the Asian monsoon. During the Late Glacial (before ~ 13 cal ka BP), the high content of sand is correlated with low pollen concentrations, which implies an arid environment and low vegetation cover in the drainage. The relatively high percentages of Pinus and Quercus pollen, derived from nearby high elevations, may indicate particularly low vegetation cover in the lower part of the watershed. Between 13 and 7.7 cal ka BP, increasing silt content is correlated with high percentages of Picea pollen. Fluvial flow and effective humidity at high elevations, as well as lake-level, all increase. Following this, an abrupt environmental change which is likely caused by some change in lake geomorphology persists for about 300 years. The Holocene Optimum is between 7.4 and 4.7 cal ka BP in this region. Pollen concentrations reach their highest values in the section. The abundant herb pollen reflects the high vegetation cover in the lower part of the watershed. The silt and the fine components in the sediments are high and stable indicating optimal moisture conditions. Between 4.7–1.5 cal ka BP the lake-level fell and silty peat was formed at the site. The pollen concentration decreased in contrast to the high pollen concentration in the Holocene Optimum. In the last 1500 years the lake-level fell while terrestrial sediments were deposited at the site. In this phase Chenopodiaceae, Nitraria and Ephedra reach their highest values in the section reflecting an arid environment. In Zhuye Lake and the surrounding area environmental changes are mainly controlled the combined effects of the East Asian monsoon and the Westerlies since the Late Glacial. In the last ~ 1.5 cal ka BP the intensive arid trend may be correlated with changes in the Westerlies.