Alluvial Pollen on the North China Plain

http://www.paper.edu.cn QUATERNARY RESEARCH 46, 270±280 (1996) ARTICLE NO. 0066 Alluvial Pollen on the North China Plain XU QINGHAI,YANG XIAOLAN,WUCHEN,MENG LINGYAO, AND WANG ZIHUI Institute of Geography, Hebei Academy of Sciences, Shijiazhuang 050011, China Received August 31, 1995 mm (up to 800±900 mm). The mean annual precipitation is Mordern alluvial pollen varies with geomorphic setting and de- 500±600 mm in the plain itself. By contrast the mean annual positional facies in sediments of the Yellow, Hutuo, and Luan evaporation is about 2500 mm. All rivers on the North China rivers and in Baiyangdian and Hengshuihu lakes. Most of the Plain rise abruptly during heavy rains and fall, or even dry arboreal pollen is derived from the mountains, whereas most of up, between storms. the nonarboreal pollen is derived from the plain itself. Alluvium Most vascular plants on the plain have been introduced dominated by Pinus pollen and Selaginella spores was deposited by people. The main crops are Triticum aestivum, Zea mays, during a ¯ood. Hydrodynamic sorting of alluvial pollen exists in the sediments of ¯oodplain, central bar, natural levees, and point Oryza sativa, Sorghum, Gossypium, Arachis, Brassica peki- bar. In reconstructing the ancient vegetation and past climate nensis, Apium graveolens var. dulce, Phaseolus, Spinacia based on pollen in alluvium, it is important to consider sedimen- oleracea, and Allium ®stulosum. Trees of Robinia, Ailanthus, tary facies and geomorphologic setting. q 1996 University of Washington. Ulmus, Populus, Salix, and Paulownia grow in cities, on road sides, and along river banks. Some herbs, such as Digitaria, Imperata cylindrica var. major, Aster, Artemisia, Cheno- INTRODUCTION podium glaucum, Xanthium, and Salsola collina, can be found on farm ®elds, road sides, and river banks. Aquatic Alluvial sediments are frequently used in Quaternary pol- plants, such as Phragmites, Sparganium, Typha, Cypera- len studies in the semiarid and semihumid North China Plain ceae, Nelumbo, Myriophyllum, and Potamogeton can be where lacustrine and peat deposits are rare. However, owing found in depressions and lakes. Tamarix, Suaeda salsa, Sali- to the lack of adequate understanding of alluvial pollen, past cornia europaea, and Artemisia balodendron are dominant studies (Tong et al., 1983; Xu and Wu, 1985) treated alluvial in the coastal plain area. pollen as having been continuously deposited, as if it accu- Forests are present in the Taihang and Yanshan moun- mulated in a lake. However, pollen in alluvium on the North tains. A forest of Quercus, that includes Pinus tabulaeformis, China Plain varies with depositional facies as well as with Ulmus, Carpinus, Beltula, Tilia, Acer, and Juglans, grows changes in vegetation. at elevations below 1200 m. Betula and Populus forests grow between 1200 and 1700 m elevation. Between 1700 and STUDY AREA 2500 m, the forests are dominated by Picea and Larix. Subal- pine meadows dominated by Poaceae, Cyperaceae, and The North China Plain is bounded by the Taihang Moun- Asteraceae families are found above 2500 m. tains on the west, the Yanshan Mountains on the north, and 2 Bohai Sea on the east (Fig. 1). Covering 136,000 km ,it SITES AND METHODS was formed by the Yellow River, the Luan River, and tribu- taries of the Hai River. The plain can be divided into a Pollen samples were collected along the Yellow, Hutuo, piedmont and fan zone, a ¯uvial plain zone, and a coastal and Luan rivers and from Baiyangdian and Hengshuihu zone (Wu et al., 1991). lakes. Samples were collected along three transects each The North China Plain has a continental monsoon climate. across the Yellow and Luan rivers, ®ve sections on the Hutuo Under the in¯uence of Mongolian high pressure in winter, River, and one section each at Baiyangdian and Hengshuihu temperature and precipitation are low. Continental low pres- lakes (Fig. 1). Several samples were collected from several sure in summer allows warm Paci®c air to raise temperature different sedimentary facies at each section. All but the river- and produce abundant precipitation. Precipitation in July, bed samples were collected subaerially. Samples at the sur- August, and September makes up 70% of the total annual face and 2±3 cm under the surface were collected at each rainfall. Mountain front regions and hills receive the greatest site to compare the in¯uence of the airborne pollen rain and rainfall, with mean annual precipitation being more than 600 the local pollen. Both the surface and the underlying samples 270 转载 AID QR 1854 / a605$$$$21 11-12-96 11:22:17 qras AP: QR 中国科技论文在线 http://www.paper.edu.cn CHINESE ALLUVIAL POLLEN 271 FIG. 1. Map of North China Plain showing geomorphic setting of sampled sections. are modern alluvial analogs. As a river ¯ows along a very the surface, it re¯ects the in¯uence of both airborne pollen small trough, all sedimentary facies but the river bed are and local pollen. Sampling was done in the September and subaerial at the time of sampling. Pollen in alluvium at the October 1990 during a time of low river ¯ow. At that time, surface may be in¯uenced by airborne pollen and the local the average ¯ow of the Yellow River at Lijin station was pollen. If the surface pollen, especially the pollen concentra- about 200±400 m3/sec (Cheng, 1991); the Luan River at tion, is more than that of samples collected 2±3 cm beneath Luanxian station ¯owed at about 108 m3/sec (Yu et al., AID QR 1854 / a605$$$$22 11-12-96 11:22:17 qras AP: QR 中国科技论文在线 http://www.paper.edu.cn 272 XU ET AL. 1994). The Hutuo River was drying up, with no water on zone and the ¯uvial plain zone mainly comprise pollen from the river bed. drought-tolerant grasses and mesothermal herbs. Chenopodi- Most of the samples collected in the river channels were aceae pollen, probably from salt-tolerant species, is domi- deposited in 1990. The samples collected on the alluvial nant in the coastal plain sediments. Much aquatic pollen plain (outside the river bank) or higher on the ¯oodplain collects in lakes. These pollen assemblages essentially re¯ect were deposited after the exceptional ¯oods in 1963 and 1964. the main characteristics of nearby vegetation, which in turn During the two ¯oods, almost all rivers on the North China re¯ects the local geomorphic setting. Plain over¯owed their banks and deposited a layer of ¯ood sediment on the plain (Department of Hydrology, Hebei ALLUVIAL POLLEN IN DIFFERENT Province, unpublished data of 1991). SEDIMENTARY FACIES Samples of river water were collected from the surface in a plastic pail (2.5 liters). In the laboratory, 80 g of sediment from each sample was We illustrate here the effects of sedimentary facies at the placed in 10% HCl. Tracers of Lycodium spores were added Huangbizhuang section (Figs. 3 and 4), in the piedmont and to permit calculation of pollen concentration (Li, 1985). Sam- fan zone of the Hutuo River, and at the Pinyin section (Figs. ples were then treated with 10% KOH to remove humates. All 5 and 6) in the ¯uvial plain zone of the Yellow River. From water sampled in plastic pails was poured into a 3000-ml bea- these sections we make several quali®cations: ker, and the sediment was then allowed to settle. After concen- (1) River bed: The river bed usually has less pollen, fewer tration, the samples were transfered to glycerol. At least 300 pollen taxa, and lower pollen concentration than the other pollen and spore grains were identi®ed for each sample. facies. (2) Central bar, ¯oodplain, and natural levees: The abundant pollen in these ¯ood-water facies is characterized by GENERAL FEATURES OF ALLUVIAL POLLEN Selaginella spores and Pinus pollen (Figs. 3 and 5). Fall ON THE NORTH CHINA PLAIN (1987) also found high percentages and concentrations of Pinus in such environments. We infer that Selaginella spores Alluvial pollen of the North China Plain is composed of and Pinus pollen grains, being larger and heavier than other plants growing both on the plain and in nearby mountains. pollen, were deposited by ¯oods that carried other pollen Most of the arboreal pollen is derived from such mountain trees away. In that case, high percentages of Selaginella spores as Pinus, Picea, Quercus, Betula, Tilia, Castanea, Juglans, and and Pinus pollen can be used as an indicator of ¯ooding. Alnus. The appearance of their pollen in alluvium, especially (3) Point bar: Point bars have low pollen concentrations, 2±3 cm under the surface, indicates that they were transported typically less than 200 grains/g. Pollen concentrations in- by rivers from the mountains to the plain. By contrast, we treat crease toward the river banks (Fig. 6). Populus, Salix, Ulmus, and Ailanthus as plain trees, because (4) Alluvial plain (outside the river bank): Pollen assem- of their dominance in the plain. Ferns, which grow only in the blages are dominated by pollen from upstream or from the mountains, are represented by spores that were also moved by local area. The greater the distance from the river (over 10's rivers from the mountains to the plain. of kilometers), the more abundant the local pollen is (Xu et Nonarboreal pollen taxa are mainly Artemisia, Cheno- al., 1994a). podiaceae, Poaceae, Xanthium, and Polygonum. They are Pollen assemblages also vary with geomorphic setting produced by plants which are common both in the mountains along the Yellow River. From Figure 7 it is clear that both and on the plain. This distribution makes it dif®cult to deter- surface pollen concentration and pollen concentration 2±3 mine their provenance. cm under the surface are high in the piedmont and fan zone The abundance of arboreal pollen derived from the moun- and in the coastal zone, but low in the ¯uvial plain zone.

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