末次盛冰期以来陕北黄土高原的植被和气候变化

VEGETATION AND CLIMATE CHANGES SINCE THE LAST GLACIAL MAXIMUM IN THE NORTHERN LOESS PLATEAU

本文对黄土高原靖边和富县末次盛冰期以来的黄土沉积做了系统的孢粉分析。结果显示,末次盛冰期以来两地一直发育以蒿为主的草原植被,但植被组成在时间和空间上都有明显的差异。全新世早-中期植物种类比末次盛冰期丰富,沙漠植被减少,落叶阔叶树增加,表明气候变得温暖湿润。末次盛冰期靖边地区是以蒿、藜和其他菊科(非蒿属)为主的荒漠植被,而富县地区藜和其他菊科植物较少,且含少量禾本科植物,植被类型类似于干草原型的冷蒿草原。全新世早-中期,靖边地区仍然有一些藜和其他菊科植物生长,植被类型接近冷蒿-荒漠草原,而富县地区植物种类丰富,榛属植物增加,含少量禾本科植物,藜和其他菊科植物非常少,植被类型接近于蒿类-草甸草原。记录对比进一步显示,无论是末次盛冰期还是全新世早-中期,黄土高原中部和北部之间都存在一定的气候梯度,即越往北气候相对越干冷,和现代的气候格局类似。全新世早-中期植被的空间差异比末次盛冰期显著,表明其气候梯度似乎大于末次盛冰期。

Pollen analyses were carried out on two loess deposits at Jingbian （37°29′N, 108°50′E, 1688m a. s. l ） and Fuxian （ 36°02′, 109°17′E, 1226m a. s. 1. ） of the northern Loess Plateau. At present, the mean annual temperature at Jingbian is 7.8 ℃ and the annual precipitation 395mm, the values for Fuxian are 8.5℃ and 550ram. The Jingbian （4.6m in depth）and Fuxian（4.2m in depth）sections were sampled at intervals of 5 -10cm for the portion deposited since the Last Glacial Maximum. Both sections are loess （L） -soil （S） sequence consisting of the So （Holocene） , L1-1（Last Glacial Maximum, LGM）and the upper part of L1-2 （MIS 3 ）. At Fuxian, a thin loess layer overlying the Holocene soil（ S0） is observed and has been termed L0. Grain size of all the samples was measured for stratigraphic correlation between the two sites and for establishment of an orbital time scale. In the Jingbian and Fuxian sections, soils are characterized by finer particle sizes compared to the overlying or underlying loess horizons, consistent with other loess sections. By correlating the grain size records of Jingbian and Fuxian with Chiloparts（ Chinese loess particle timescale） ,we obtained some age control points for the two sequences, such as the coarsest portion of LI_1 （18kaB. P. ） , the base of SO （llkaB. P. ） and the middle of S0（6kaB, P. ） ,with which age models were constructed for the Jingbian and Fuxian sections by linear interpolation between these assigned dates. Pollen was extracted with ＂the integrative method of heavy liquid＂. Using a Nikon ECLIPSE 50i ,, 150 to 300 pollen grains were identified at^4OOmagnifieations. Results show that the studied area was dominated by steppe vegetation（ mainly Artemisia） during both the LGM and Holoeene. Except for Artemisia, other major vegetation components（ Chenopodiaeeae and Asteraceae）varied both temporally and spatially. During the early to middle Holocene,plants were much more abundant than in the LGM, with less desert taxa and more deciduous trees, indicating a wetter and warmer climate. During the LGM, desert steppe prevailed at Jingbian, mainly composed of Artemisia, Chenopoctiaceae and Asteraceae,while at Fuxian, Chenopodiaceae and Asteraceae decreased and Poaceae increased, indicating the vegetation similar to an arid-steppe-type, Arteraisia frigida steppe. During the early to middle Holocene, the vegetation at Jingbian was close to Arteraisia frigida-desert steppe,with decreased Chenopodiaeeae and Asteraceae. At Fuxian, Corylus increased, and Chenopodiaceae and Asteraceae nearly disappeared in the early to middle Holocene,suggesting the vegetation similar to Arternisia-type meadow steppe. Comparison of the two pollen records shows a north-south climate gradient in both glacial and interglacial periods,i, e. the farther north the colder and drier it becomes, exhibiting a climate pattern similar to the present over the Chinese Loess Plateau. Furthermore, the spatial difference in vegetation composition is more evident in the Holoeene Optimum than in the LGM,indicating a steeper climate gradient in warm period than in cold stage.