泥河湾盆地上新世末期植被与气候变化的孢粉学记录

摘要

The lacustrine and fluvial sediments in the Nihewan Basin from Late Pliocene to Late Pleistocene contain rich pollen and spore grains,which can be used to reconstruct past climate changes.Sediment samples are collected in the NHA borehole at Haojiatai,eastern part of the Nihewan Basin,Yangyuan County,Hebei Province.The drilling coordinates are 40°13′0.4″N,114°38′32.3″E and 938 m a.s.1.The total sampling depth is 365.82 m.The samples were taken from the top of the Pliocene,19.2 m in thickness and 15600～ 17520 cm in depth.One sample was collected every 20 cm for the core of this section,and a total of 97 samples were obtained.In this study,Haojiatai NHA drilling core in the Nihewan Basin was selected to discuss the characteristics of vegetation and climate changes in Late Pliocene based on the pollen and grain size analysis of 97 samples.According to the existing paleomagnetic and stratigraphic contrast,the age of the sampling strata is from 2.92 Ma to 2.56 Ma.Based on pollen and grain size analysis,4 stages with different climate and vegetation can be divided:Stage 1 (2.92～2.82 Ma),the broad-leaved trees had the highest percentage in the full section with about 13％.Pinus pollen content was more than 30％ and Picea were less than 20％ in most samples and Artemisia and Chenopodiaceae were 10％ ～ 20％.This indicates that broad-leaved forest and pine forest should be the main vegetation type in the sampling point,with some spruce forest existing in the mountainous regions,and that the climate was relatively warm and humid.Stage 2 (2.82～ 2.71 Ma),the pollen percentages of Picea,Artemisia and Chenopodiaceae increased significantly,with the maximum of 90％ for Picea and 80％ for Artemisia and Chenopodiaceae;on the contrary,the pollen percentages of Pinus and broad leaved trees significantly reduced to lower than 10％ and 5％ separately;which indicates that during this period,the surrounding vegetation became Picea forest or steppe and the climate tended to become chilly with severe fluctuation of humidity.Stage 3 (2.71 ～2.66 Ma),Pinus pollen percentages increased significantly,with more than 40％ in most samples.The percentage of fern spore was about 11％,reaching the highest in the whole section;instead,Picea percentage decreased obviously,with less than 30％ in most samples,and broad-leaved trees pollen was rare and the percentage of Artemisia and Chenopodiaceae was less than 10％,indicating that the surrounding vegetation was dominated by pine forests,and the spruce forest retreated to the mountainous regions,and the climate became relatively warm and wet,but the rareness of broad-leaved pollen indicates that the degree of warmth is lower than that in Stage 1.Stage 4 (2.66～ 2.56 Ma),Picea became the dominated pollen type with more than 80％ percentage in most samples,and Pinus was reduced to less than 10％ and broad-leaved forest pollen grains were rarely be recorded suggesting that Picea forest expanded to the nearby of sampling site and the climate was getting much colder and connected to the Pleistocene glacial period.According to the pollen analysis,there are two relative warm periods (2.92～2.82 Ma and 2.71～ 2.66 Ma)and two cold periods in the Late Pliocene (2.82～ 2.71 Ma and 2.66～ 2.56 Ma)respectively.The stages with high Picea pollen percentage,had good correspondence with the deep-sea oxygen isotopes periods of MIS 102,MIS 104,G2,G6 and G10,which represents the cold climate.%泥河湾盆地沉积巨厚的上新世末至晚更新世的河湖相、湖沼相地层,是进行上新世至更新世环境变化研究的理想区域.本研究选择泥河湾盆地郝家台NHA钻孔的上新统稻地组顶部厚19.2m,深度为15600～17520 cm,通过对其岩性段97个孢粉和粒度分析,探讨了该地区上新世末期植被及气候变化特征.根据已有古地磁及地层对比,采样地层的年代为2.92～2.56 Ma.分析结果显示,该时段植被及气候变化可划分为4个阶段:阶段1(2.92～2.82 Ma),松属花粉含量多高于30％,云杉属多低于20％;阔叶乔木约13％,为研究段最高;草本植物以蒿属、藜科为主,含量均在10％ ～ 20％之间,指示采样点周边应以阔叶林和松林为主,山地云杉林存在,气候较温暖湿润.阶段2(2.82～2.71Ma),云杉属花粉含量明显上升,最高达90％,蒿属和藜科花粉比例也增加明显,最高可达80％,为研究段最高值;松属花粉含量明显降低,多低于10％;阔叶树木的花粉减少至5％以下,表明在此期间气候总体趋于变冷变干,且气候变化剧烈,林线下降,周边植被以云杉林或草原为主.阶段3(2.71 ～2.66 Ma),松属花粉含量明显上升,达40％以上,云杉属含量总体下降,多低于30％;阔叶乔木少见;蒿属、藜科含量低于10％;蕨类孢子约11％,达到整段最高值,指示气候转暖湿,周边植被以松林为主,云杉林退至山地,但阔叶树花粉含量较低,显示温暖程度低于阶段1.阶段4(2.66～2.56 Ma),云杉属花粉占绝对优势,多高于80％,松属多低于10％;阔叶乔木近乎消失,暗针叶林扩张,气候进一步变冷,进入更新世的冰期环境.由此可见,上新世末期(2.92 Ma以来)至更新世的气候变化存在明显的2个暖期和2个冷期.其中研究段代表寒冷气候的云杉属花粉高含量阶段与深海氧同位素代表寒冷的MIS 102、MIS 104、G2、G6和G10阶段有较好的对应关系.