陕西渭南北庄村晚更新世晚期古植被的再研究

通过渭南北庄村约31,000—18,000aB.P.沉积物中花粉百分含量、浓度和沉积率分析,对该时期古植被取得一些新认识,即当时云杉、冷杉等亚高山针叶树可能并未形成较大面积的森林,而是呈岛状分布于水体周围,并与区域性蒿草草原呈镶嵌状分布,植被并不稳定,亚高山针叶树曾两度出现又消失。从20,000aB.P.无乔木花粉出现,到18,000aB.P.难以找到任何花粉,本区变为无乔木生长的荒漠草原,气候曾经过数次冷偏湿与冷干的交替后,从18,000aB.P.起变得极度干燥和寒冷。

A RESTUDY OF THE LATEST PLEISTOCENE PALEOVEGETATION AT BEIZHUANGCUN, SHAANXI PROVINCE, NORTHERN CHINA

The outcrop section of late Quaternary sequence at Beizhuangcun, Weinan County, Shaanxi Province （34°20'N, 109°30'E, 490m a. s. 1.） is of particular significance to Quaternary geology in China due to the discovery of a flora with cones and other parts of Picea wilsonii Mast. there in the middle of 1960′s. A Picea-Abies pollen assemblage has been found in the flora-bearing deposits, and radiocarbon dating has given an age of 23,100 ±850 a B. P. In result of recent studies, more complete sections have been found in the same locality, and more detailed palynological analyses have been conducted in a 3m thick muddy segment of the sequence with radiocarbon dates of 30,930±320,27,440±320, 21,030±440 and 18,380±520 a B.P.The extremely low pollen influx, ranging from 9-188 grains/cm²·a, is indicative of a sparse vegetation for the whole time span of deposition, although certain oscillations both in vegetation type and climate are implied by the succession of pollen zones （from Ⅰ to Ⅴ in an ascending order） representing the time interval of some ten thousand years as mentioned above. The pollen zones Ⅰ（ca. 31,000—26,800 a B. P.） and Ⅲ （ca. 24,000—23,000 a B. P.） are similar in pollen composition, being distinguished from others by relatively high pollen influx, arboreal pollen approximating non-arboreal one in proportion, and fairly high percentages of Pinus and other subalpine conifers such as Picea, Abies and Tsuga. Judging from the low values of pollen influx and high proportion of herbaceaous pollen, the regional vegetation must have been grasslands rather than real forests, and the subalpine conifers formed nothing else as open and patchy small woodlands around water bodies. The pollen zones Ⅱ （ca, 26,800—24,000 aB,P.）and Ⅳ （ca, 23,000—20,000 a B. P.） are characterized by an extremely low value of pollen influx and an overwhelming dominance of herbaceous pollen types （primarily Artemisia and Compositae）. The pollen flora is representative of dry grasslands or deserts, probably similar to the present-day western part of Ordos Plateau in Nei Monggol where the average annual precipitation does not exceed 150—300mm. Then, arboreal pollen disappears completely in the pollen zone Ⅴ （ca. 20,000 to 18,000 a B.P.）, suggesting the absence of trees in the entire region. Since 18,000 a B. P. onwards, no pollen has been found from deposits of the studied area. The pollen floras found in the Beizhuangcun section, as the author believes, are well representative of northern China for the time interval about 10,000 years before the Last Glacial Maximum.The paleovegetation was dominantly dry grasslands or grassland-deserts, the climate was cold and dry, with short, some relatively humid stages as climatic oscillations. The subalpine conifers in pollen diagrams of that period of time must have derived from small woodlands scattered in local humid areas, rather than subalpine conifer forests widely distributed in northern China, as suggested by many previous authors.