安固里淖湖近5000年来环境变化的孢粉及地球化学沉积记录

ENVIRONMENTAL CHANGE SINCE 5000cal.a B.P. IN THE ANGULI-NUUR LAKE AREA BASED ON PALYNOLOGICAL AND GEOCHEMICAL RECORDS

探讨生态敏感区5000年以来，特别是小冰期和中世纪温暖期植被和气候水文环境变化，有助于认识生态系统对气候变化的响应，进一步理解现代气候环境的背景。安固里淖湖位于内蒙古高原的前缘河北省张北县境内，是河北坝上地区最大的内陆封闭湖泊，属半干旱与半湿润过渡带及东亚季风的尾闾区，对气候变化敏感。本研究根据河北坝上地区安固里淖湖泊一个1.2m长钻孔沉积物孢粉、粒度与常量元素分析，结合4个^14 C测年数据，探讨了安固里淖湖近5000cal.a B.P.来的环境变迁历史。研究结果表明，安固里淖湖近5000cal.a B.P.来气候一直向干旱的方向发展，湖泊一直处于退缩过程中，特别是1000cal.a B.P.以来，干旱程度加剧。5030-1070cal.a B.P.是气候相对湿润阶段，花粉浓度高于6000粒/g，干旱指数（（K＋Na＋Ca＋Mg）/（Fe＋Mn））多低于3.42，退碱系数（（CaO＋K2O＋Na2O）/Al2O3）多低于1.44，CaO/MgO多低于0.73，湖水面积和深度较稳定；中世纪温暖期（1070-620cal.a B.P.），平均花粉浓度降至3200粒/g以下，沉积物粒径变粗且变幅增大，干旱指数（平均3.60）、退碱系数（平均1.53）迅速增加，气候干旱化明显，但干旱程度低于小冰期；小冰期（620-45cal.a B.P.）干旱化及寒冷程度进一步加剧，花粉浓度进一步降低至2400粒/g以下，小冰期最盛期出现在490-250cal.a B.P.，该阶段花粉浓度多低于1000粒/g，干旱指数（平均3.63）和退碱系数（平均1.67）最高；45cal.a B.P.以来气候转暖湿，但湖泊内生碳酸盐含量升高，CaO/MgO值〉1，显示水位继续下降，应主要受人类活动影响及上游修建水库所致。安固里淖湖剖面孢粉组合中高含量的松属、云杉属应主要来源于研究区东南部的坝缘山地，由河流携带入湖。

The study of vegetation and climate changes of ecological sensitive area since 5000 years, especially the period of the Little Ice Age and the Medieval Warm Period will contribute to understanding how the ecosystem respond to climate change. A sediment sequence（1.2m in depth）from the Anguli-Nuur Lake（41°18＇-41°24＇N, 114°20＇-114°27＇E; 1313m a.s.l.）, which is one of the largest inland lakes in the transition zone between a semi-humid and semi-arid climate parallel to the present limit of the southeast monsoon along the southeastern Inner Mongolia Plateau in the Bashang Plateau of Hebei Province, was selected to reconstruct the history of vegetation and climate since 5000cal.a B.P., based on pollen analysis, sediment grain size and major chemical elements analysis, combining with^ 14 C dating data. Totally 94 sample were collected, and the pollen analysis results indicated that a total of 49091 pollen and spore grains（more than 435 grains in each sample）and 78 pollen types, including 21 arboreal and shrub pollen types, 47 herbaceous types and 10 fern types were identified. In different period, pollen concentrations, sediment grain size and ratios of chemical elements changes obviously. From 5030cal.a B.P. to 1070cal.a B.P., the pollen concentrations are more than 6000 grains/g, and the drought index （（K＋Na＋Ca＋Mg）/（Fe＋Mn）） is less than 3.42, alkali index （（CaO＋K2O＋Na2O）/Al2O3） are lower than 1.44 and CaO/MgO are values lower than 0.73 respectively, indicating relative humid climate and relative stable water area and depth. In the Medieval Warm Period（1070-620cal.a B.P.）, the average pollen concentrations decreased below 3200grains/g and the average grain diameter and variable amplitude enlarged, in the meantime, the drought index value（average 3.60） and alkali index value（average 1.53）increased rapidly, suggesting that the regional climate not only become very unstable but the drought were enhanced. In the Little Ice Age（620cal.a B.P. to 45cal.a B.P.）, the pollen concentrations averagely decreased below 2400 grains/g, the drought index and alkali index values reach to the maximum in all sequence, indicating the drought were enhanced further more. The maximum of the Little Ice Age is from 490cal.a B.P. to 250cal.a B.P., when the pollen concentrations are less than 1000 grains/g, combined with the highest drought index value（average 3.63）and alkali index value（average 1.67）. Thus it can be seen that the lake has been shrinking since 5000cal.a B.P., and the drought trend has been become more and more serious, especially since 1000cal.a B.P. Since 45cal.a B.P., the climate has become a little warm and wet, but the content of the lake carbonate got much higher than before, represented by higher CaO/MgO values（more than 1）, suggesting that the lake areas and depth continue shrink. That should be mainly affected by serious human activities and the reconstruction of reservoir in the river upstream. The high content of Pinus and Picea in pollen assemblages should mainly come from the mountains located in the south-east of Bashang Plateau, and then transport to the lake by inflow river water.