陕西渭南晚冰期时的环境与气候──蜗牛化石的证据

LATE GLACIATION PALEOENVIRONMENT IN WEINAN,SHAANXI: EVIDENCE BASED ON SNAIL FOSSILS

通过对关中盆地渭南地区晚冰期时黄土地层中陆生软体动物──蜗牛化石的系统研究，划出了３个蜗牛化石组合带，代表了３种不同的气候环境。通过对２２个黄土样品中的蜗牛化石组合、１４种蜗牛化石含量的对应分析，建立了晚冰期时的气候因子变化曲线，与植物硅酸体研究结果比较，两者有很好的一致性。晚冰期时，渭南地区的气候经历了温凉、偏湿－寒冷、干旱－短暂变暖－冷、干的变化过程，环境经历了半干旱、半湿润草原－干旱草原、荒漠草原－干旱草原的发展过程。在１４０００ａＢ．Ｐ．前后有一气候转暖期，预示着黄土高原末次冰消期气候转暖的开始。

heT terrestrial mollusk fossils preserved in loess sediments are good recorders of paleoclimate and paleoenvironment. They are the only biological remains still existing in large quantities. A new discovery on terrestrial mollusk fossils permits to improve the reconstruction of environments during the last glaciation in Weinan.The studied section is located at Weinan, Shaanxi, of the southern-most part of the Loess Plateau, which is a sensitive area to climatic changes and has an annual average temperature of about 12-13.6℃ and an annual mean precipitation of about 570-620mm. In this study, 22 samples, which were collected with an interval of 10cm from a range of 1.54-3.64 m in depth, were analyzed for fossil mollusk fossils. Each sample weights about 10-12kg. All samples were washed and sieved through a 0.5mm mesh to pick out snail shells. Sixteen species of terrestrial snail fossils were identified: Vallonia costata (Milller), V. tenera (Reinhardt), V. Pulchella (Mller), V. sp., Pupilla aeoli (Hilber), P. sp., Kaliella depressa Moellendorff, Macrochlamys rejecta (Pfeiffer), Gastrocopta armigerella (Reinhardt), Cathaica pulveraticula (Martens), C. pulveratrix (Martens), C. fasciola (Draparnaud),C. sp., C. constaniae (H. Adams), Metodontia yantaiensis (Crosse et Debaux), M.huaiensis (Crosse). A method of correspondence analysis suitable to the mollusk data was used to estimate semiquantitatively the temperature and humidity variations. At the same time this analysis method allows to study both species and assemblages. The results obtained are drawn in a diagram that takes into account the present ecology of mollusk species. Generally, the first major factor is interpreted as 'temperature'(ranging from temperate to cold) and second as humidity (ranging from dry to moist) according to the modern ecology of mollusk species. Consequently, the climatic curve could be established using the mollusk fossils identified.Three mollusk fossil zones have been recognized in the Weinan loess section.which represent three different environments in the last glaciation:Mollusk Zone I(3.64-3.04m and 23 000-19 900 aB.P.) M. rejecta was the dominant species in the whole snail fossil assemblage, which is one of the typical representatives of thermophilus and humidiphilus species and is distributed mainly in the south part of China now. Its luxuriant appearance in the Weinan section implies that the condition during that time was relatively warm and moist. K.depressa and M. huaiensis associated with it were two kinds of thermohumidiphilus snail species. However, there exists a certain amount of P. aeoli and V. costata in the section, implying a short-time condition of dry and cold climate. The study of phytolith and spore-pollen indicates that small shrubs and herbs were predominated in the vegetation of this period. In these environmental conditions, weakly-developed paleosol changed into loess. During 21 900-19 900 a B.P.,the amount of M. rejecta decreased dramatically, while that of P. aeoli increased gradually. In addition, G. armigerella and C. pulveratrix appeared at the first time:the former is an aridiphilus and simi-aridiphilus component; the latter is a coldaridiphilus one. Therefore, Weinan during 23 000-21 900 a B.P. underwent the change of a relatively mild and moist climate to a colder and drier climate. Meanwhile, the semijarid-semihumid loess steppe was developing to the dry steppe and the desert steppe.Mollusk Zone II (3.04-2.04m and 19 900-14 700 a B.P.) P. aeoli, a species distributed extensively in the Paleoarctic region now, was a predominant element in the Weinan mollusk fossil assemblage. There are also V.costata, another drought-enduring and cold resistant species, and C. pulveratrix.These species indicate a rather cold and dry environment. Perhaps, it could be a desert steppe appeared during the peak of the last glaciation.Mollusk Zone III(2.04-1.54m and 14 700-12 300 a B.P.)The zone is distinguished both by a marked decline in P. aeoli and a contemporaneous increase in V.tenera and by considering M. reiecta an