中文摘要：

为探讨更新世我国北方植被演化特征，对北京凹陷新5孔河湖相沉积进行孢粉植物群初步研究。结果显示，过去2．3Ma尤其1．7Ma以来北京地区总体上为疏林草原一草原植被所覆盖，西部和北部的山地上生长以松（Pinus）为主的针叶树。主要孢粉类型的含量变化显示，2．3～2．1Ma期间气候温和半干旱。i．70～1．45Ma期间，北京地区冬季气温下降，导致山地针叶树增加；同时，夏季风也得到了增强，导致落叶阔叶树和喜湿草本得到发展；这表明这一时期北京地区的季节性较早期（2．3～2．1Ma）明显增强。1．45～1．05Ma期间研究区气候延续了前期气候特征，并维持相对稳定。1．05～0．94Ma期间，针叶树松属花粉含量明显降低，随后自0．94Ma开始含量明显增加。这可能是对中更新世过渡期及随后全球冰量增加的响应。极地冰量增加，北半球径向温度梯度变陡，导致东亚冬季风加强。另一方面，1．05～0．94Ma期间，无论是落叶阔叶树花粉含量，还是喜湿草本和蕨类孢子的含量均有不同程度的增加，而且显示向上减少的变化趋势，暗示东亚夏季风在1．05～0．94Ma期间总体增强随后逐步减弱的变化趋势。据此我们推测，自中更新世气候过渡期开始，东亚冬季风和夏季风均明显增强。但伴随极地冰盖发展和全球气温下降，东亚冬季风持续增强而东亚夏季风则显示逐步减弱的变化趋势，使得我国中北部许多地区出现植被与环境短暂恢复后逐步衰退的演化趋势。自约0．5Ma开始，研究区以松为代表的针叶树花粉逐步减少，落叶阔叶树花粉含量也开始降低，反映研究区明显变干，相对喜湿的草本和木本逐步为喜干旱的蒿（Artemisia）和藜科（Chenopodiaceae）所代替。研究区这样的孢粉植物群演化模式与东亚、北太平洋乃至南半球具有较好的一致性。不过，中更新

英文摘要：

To explore evolutional characteristics of vegetation in North China during the Pleistocene period, pollen analysis was conducted on the fluviolacustrine sediments of the X5 core in Beijing. The X5 core （39°57＇04.3＂N, 116°30＇55.3＂E） has stratigraphie units of Quaternary, Neogene,Palaeogene and Cretaceous age, and is 862.7m in total depth. The upper 426. 35m contains multiple conglomerate-sand-sih-elay cycles. Magnetostratigraphic results suggest that the upper 422.12m spans the past 3.58Ma and is generally continuous. Fifty-one pollen samples from the upper 422. 12m were analyzecl and 38 samples contained 100 -351 pollen and spore grains（ mean 202 grains/ sample）. A total of 52 pollen and spore types were identified. Shrub and herb taxa were dominant in most of the samples,ranging from 10.3% to 80.9% in abundance with an average of 46.7%. They were dominated by Corylus, Artemisia,Chenopodiaceae and Cyperaeeae. Also present were Gramineae, Polygonaceae, Compositae, Thalictrum, Labiatae,and Myriophyllum. Arboreal pollen taxa were less dominant, fluctuating between 5.9% and 80.6% and averaging to 31.4%. They were dominated by Pinus, Quereus and Ulmus. Also present were Pieea, Abies, Podocarpus, Tsuga, Cupressaeeae,Alnus, Betula, Juglans, Carya, and Castanea. Fern spores and algae ranged from 1.3% to 11.4% in content with an average of 5.6%. These suggest that the study area has generally been covered by sparse-wooded steppe or steppe with some conifers growing on the northern and western mountains since 2.3Ma, especially since 1.7Ma. Content variations of major pollen taxa indicate a warm and semi-arid climate in the study area during 2.3 - 2.1Ma. From 1.7Ma to 1.45Ma, temperature in winter probably decreased in Beijing, resulting in distinct increase of coniferous trees on the western and northern mountains. In the meanwhile,the summer monsoon also strengthened,leading to evident development of broadleaved trees and hygrophilous herbs. These suggest that, relative to the early period （2.3 - 2.1 M