关键词: 罗布泊, 第四纪, 孢粉, 环境

Abstract: The Lop Nur depression is at the east margin of the Tarim Basin. It is the lowermost place and the conflux center of the basin, covers about 50 000 km 2. The lowermost place is 780 m above sea level. The bio species in the region are limited and their quantity is rare. Especially the species are rapidly reduced after the lake dried up due to human activities in 1970’s. The data from the field survey of 1980 and 1981 show 13 families, 26 geniuses and 36 species. The boring site of K 1 locates at the middle of the dried up Lop lake, at 90°15′E and 40°17′N. Depth of the well is 100.2 m. The core is totally divided into 92 layers according to natural sedimentation, and classified into 26 depositional cycles in terms of rhythmic sedimentation from coarse to fine. In general, Holocene series is about 2.70 m thick, including the layers from No.1 to 7. The surface is salt crust, and then mainly clay underneath, upper Pleistocene series is about 14.65 m thick, including the layers from No.8 to 17, and mainly composed of earthy yellow and earthy gray clay; mid Pleistocene series is about 48.86 m thick, including the layers from No.18 to 72. Its Lower part is mainly composed of earthy gray, dark gray and grayish green mudstone with thin beds of gypsum, and upper part is mainly the exchange of dark gray muddy rocky gypsum, mudstone with gypsum. At the top there is a buried weathering crust. The mid Plesistocene series is unconformable with an angle about 15° on the lower Pleistocene. The lower Pleistocene series is about 34 m of visible thickness, including the layers from No.73 to 92, and mainly composed of dark gray mudstone and earthy yellow mudstone. Many muddy gypsum beds and gypsum mudstone beds are interbedded in the middle part. The strata decline with an angle change from 15° to 5° top down. In Paleogene the bio group was carring very important characteristics of the tethys region, and the climate of the basin was moistly warm, and produced large coruferous forest and broad leaf forest with some bushes, herb and hydrophytes. The climate of the Tarim Basin changed sharply from Miocene to Pliocene. The broad and deciduous leaf forest reduced by means of either species or total quantity. and the ever green species extincted basically. The coniferous forest decreased, but herb and bush plants occupied the region. According to the variety of species and quantity of pollen found, the whole profile is divided into 15 pollen zones (Fig.1).The zones from No.1 to 4 belong to early Pleistocene,from No.5 to 10 belong to mid Pleistocene,from No.11 to 14 belong to later Pleistocene, and No.15 belongs to Holocene including 3 sub zones. In the early Pleistocene the pollen and spore show coniferous broad leaf forest and steppe. The trees composed of Picea, Pinus, Abies, Betula, Ulmus, Salix and other including Tilia, Juglans, Carpinus, Alnus, Carya , etc. Abundant gypsum mudstone and gypsum beds appear in the mid Pleistocene strata of the K 1 core. It implies that the lake water was in high mineral contents and rich of Ca 2+ and SO 2- 4, and that the evaporation capacity was much higher than precipitation, and stand for outstanding arid time. The pollen assemblagres of the mid Pleistocene indicate that the vegetation in the vicinity of the lake had an alternative process from desert steppe type, then steppe, desert, steppe, desert to desert steppe. It was dominated by semiarid or arid climate with clear alternation from arid to moist environment in the surrounding region of the lake. In the later Pleistocene the lake water was not very deep, and surrounded by swamp and peat lands. Hydrophyte was abundant. Mineral contents of the lake water were not very high. The pollen assemblages show 2 circles of alternation of the vegetation evolution from desert to desert steppe. Under the domination of arid climate alternation of the climate was remarkable. In Holocene series, swamps and peats were well developed around the lake. The lake water was in very high mineral contents.

Key words: Lop nur Region, Quaternary, pollen, environment