Pollen-recorded Vegetation And Climate Changes During Late Glacial In The Western Part Of Chinese Loess Plateau

In order to further improve our understanding of the climate changes between late glacial and Holocene, we studied an eolian-fluvial section (Haiyuan section) and a loess-lacustrine section (Huining section) in the semiarid western part of the Chinese Loess Plateau. It is expected that the regional reconstruction will be helpful in understanding large-scale climate changes and thus in facilitating our understanding of the controlling mechanisms during the transition from the late glacial to the Holocene.The Haiyuan section is 8.5 m thick and obtained 210 samples. The Huining section is 1300 cm thick and obtained 225 samples. All ages of two sections are AMS C-14 dates, and we obtained the bottom and top ages of Haiyuan section are 12.7 ¹⁴C ka BP, 7. 2 ¹⁴C ka BP respectively, and the bottom age of Huining section is about 8.4 ¹⁴C ka BP.Pollen assemblage zones were identified based on the variations of the pollen concentration and assemblages. Six pollen zones in Haiyuan section were identified from the bottom to the top. Zone I (14.8-14.0 cal kyr BP) are characterized by a moderate pollen concentration and a rather high herb pollen percentage, which includes Artemisia, Chenopodiaceae and Gramineae. Zone 11(14.0-12.6 cal kyr BP) is quite distinguishable by a fluctuating and rather high pollen concentration and the percentage of coniferous tree pollen increases significantly in the expenses of the percentage of herb pollen. Zone III (12.6-11.4 cal kyr BP) characterized by a rather high herb pollen percentage, main includes Artemisia, Chenopodiceae and Gramineae. Zone IV (11.4-10.8 cal kyr BP) is marked by a rising of the pollen concentration, and also characterized by an increase in the percentage of coniferous tree pollen at the expenses of herb pollen and temperate deciduous tree pollen. Zone V(10.8-8.4 cal kyr BP) is characterized by a very low pollen concentration and by the dominance of herb pollen including Artemisia, Gramineae and Chenopodiceae. Zone VI (8.4-7.9 cal kyr BP) is characterized by an abrupt increase in the percentage and concentration of coniferous pollen comprising of Pinus, Picea and Tsuga.Six pollen assemblage zones in Huining section were identified from the bottom to the top. Zone I (9.2-6.6 cal kyr BP) is marked by high pollen concentration and coniferous and broadleaf treepercentage, mainly including Pinus, Picea, Quercus and Ulmus. The pollen concentration and percentage of broadleaf tree pollen in Zone II(6.6-5.7 cal kyr BP) abrupt decreased and the percentage of conifer tree pollen relatively increase compared with Zone I . The pollen assemblage of Zone III(5.7-5.3 cal kyr BP) is marked by high pollen concentration and percentage of tree pollen. Zone IV (5.3-3.7 cal kyr BP) are characterized by low pollen concentration and fluctuated changes of conifer tree pollen and Herb pollen percentage. Zone V(3.7-1.8 cal kyr BP) is marked by increasing pollen concentration and relatively high percentage of conifer tree pollen. The obvious changes are abrupt increasing of herb pollen percentage and low pollen concentration in Zone VI(1.8-0.0 cal kyr BP), mainly including Artemisia, Aster type and Chenopodiaceae.We studied the surface and fluvial pollen assemblage in order to explain the relation between pollen assemblage and vegetation, and the results as follows.â‘ the surface pollen assemblage can represent local vegetation type and different vegetation type has different pollen assemblage.â‘¡the fluvial pollen assemblage also can reflect local vegetation, but include partial upriver and regional vegetation. Compared with the pollen percentage in surface sample, the percentages of Pinus and Picea in fluvial sample are higher than that in surface sample.â‘¢Pinus, Picea, Betula, Artemisia, Chenopodiaceae and Hippophae are belong to super-represented type, but Gramineae is likely to low-represented type.According to the relation between modern pollen assemblages and vegetation, major changes in vegetation and associated climate during late glacial can be reconstructed. A steppe (14.8-14.0 cal kyr BP) was followed by a coniferous forest (14.0-12.6 cal kyr BP). The steppe resumed in during the period of 12.6-11.4 cal kyr BP and further deteriorated into a desert steppe afterward (12.2-11.4 cal kyr BP). The forest-steppe occurred (11.4-10.8 cal kyr BP) and was followed by sparsely-vegetated woodland during early Holocene. Since about 9.0 cal kyr BP, the humid steppe and forest-steppe begin to develop on the northern part of study region, and the forest expanded on south part. The vegetation coverage obvious decreased about 5.3 cal kyr BP and steppe board distribution, until 1.8 cal kyr BP, desolate steppe dominated in the study. The pollen data also reveal many climate events, e.g. 14.0-12.2 cal kyr BP(BOA event), 12.2-11.4 cal kyr BP(YD event), a. 10.9 cal kyr BP, a. 8.1 cal kyr BP and 6.6-5.7 cal kyr BP.In comparison with climatic changes of other sites in Chinese Loess Plateau and other near region, the pollen record of Haiyuan and Huining section evidently confirms that the consistency all these records in large phases. For instance, the climate is humid during the period of BOA; Desert steppe begins to develop during YD and the climate changed arid. The climate is warmer and wetter during middle Holocene, which forest-steppe or forest dominated, than early Holocene and late Holocene, which steppe or sparse-tree steppe and Steppe dominated in the studied region respectively. The different is that climatic change of the late glacial revealed by Haiyuan section is more obvious, and climate change of Holocene revealed by Huining section is more fluctuated and events than others.In compared with the records of east Asian monsoon region and high latitude, the pollen records of studied region evidently confirms that the consistency all these records in large phases between the late glacial and Holocene. The humid climate of the western part of Chinese Loess Plateau corresponded with the strengthening of east Asian monsoon during the Bolling-Allerod period, the climate began to dry in study region and corresponded with the weakening of monsoon when Younger Dryas occurred. The expansion of forests in study region corresponded with the prevailing of east Asian monsoon during the middle Holocene, the shrinking of forest from 5.3 cal kyr BP also corresponding with the weakening of east Asian monsoon. Those suggest that the climate change in study region was droved by east Asian monsoon controlled by solar insolation since the late glacial. The comparison between the pollen record in the western part of Chinese Loess Plateau and the published records of rapid climate change in northern Hemisphere shows that the vegetation can sensitively reflect the rapid climatic changes of the northern Hemisphere although the pollen changes always lags the rapid climate change.