华北平原小冰期以来干湿变化与人类活动特征

气候变化尤其干湿变化对人类活动具有重要影响,高分辨率地重建过去的干湿变化是研究区域干湿变化的基础,同时也为未来气候变化提供依据。目前针对华北地区小冰期高分辨率的气候变化研究还较少,特别是干湿变化研究更少。本研究在3个AMS-14C测年的基础上,通过99个样品的孢粉、炭屑、粒度等指标高分辨率地重建了该地区小冰期以来的气候干湿变化特征及与人类活动强度的关系。结果表明:小冰期之前(约1340 A.D.)乔木花粉含量多在25%以上,指示流域植被以森林草地景观为主,沉积物颗粒较粗,沉积环境不稳定,气候较为湿润。小冰期期间(1340~1920 A.D.)乔木花粉多降低至15%以下,喜干的藜科、荨麻属和葎草属花粉含量明显增加,指示气候整体变干,但不同阶段变干程度又存在明显差异:小冰期早期(1340~1580 A.D.)孢粉组合中喜干的藜科花粉含量逐渐增加,湿生植物花粉含量减少,PCA axis 1得分值多在0~1之间,显示气候偏干。该时期与人类活动有关的炭屑浓度、农作物(禾本科≥35μm)花粉和磷(P)含量均最低,说明小冰期早期研究区人类活动强度相对较弱;小冰期中期(1580~1800 A.D.)喜干的藜科、荨麻属和葎草属花粉含量达到最高值,PCA axis 1得分值较小冰期早期更正偏,指示气候更为干旱。炭屑浓度、伴人植物、磷(P)含量和农作物(禾本科≥35μm)花粉均达最高值,指示人类活动强度明显增强;小冰期晚期(1800~1920 A.D.)湿生的禾本科和水生植物狐尾藻属等花粉含量达到最高值,PCA axis 1得分值负偏,指示气候变湿润。该阶段农作物(禾本科≥35μm)花粉、炭屑浓度和磷(P)含量较上一时期均略有降低,表明人类活动强度较小冰期中期略有降低。小冰期以后(1920年至今)喜湿的禾本科和水生植物狐尾藻属花粉含量减少,与小冰期晚期相比,气候向着干旱化方向发展.

Characteristics of dry-wet changes and human activities in the North China Plain since the Little Ice Age

Climate change,especially dry-wet change,significantly influences human activities.Thus,it lays a foundation for study of dry-wet change in a certain area to reconstruct its past dry-wet change by a high resolution.Up till now,few research has been carried out to study high-resolution climate change,especially dry-wet change during the Little Ice Age in North China.In this study,an area in the Baiyangdian Lake(38°43'~39°02'N,115°38'~116°07'E)nearby Duancun(38°50'15.851"N,115°59'43.018"E)in Anxin County,Heibei Province,was chosen to drill(BYD-D)for its undredged characteristics and less human activities.The altitude and water depth of this area were 7.97 m and 2 m,respectively,and sampling depth was 99 cm.The lithologic character of drill hole showed no significant change,mainly deposit of grey black clay.Based on the results of three AMS-14C dating,through 99 samples of pollen,charcoal and grain size,this study reconstructed the characteristics of climate dry-wet changes and the relationship of human activity intensity in the region since the Little Ice Age.The results show that pollen content of trees before the Little Ice Age(ca.1340 A.D.)was mostly above 25%,indicating that vegetation in the basin was largely composed of forest and grassland.Additionally,it was indicative of coarse grain size of sediments,unstable sedimentary environment and relatively humid climate.In contrast,pollen content of arbor during the Little Ice Age(1340~1920 A.D.)reduced to below 15%.And pollen content of Chenopodiaceae,Urtica and Humulus which prefer dryness increased dramatically,a sign of overall drying climate.Whereas,there were noticeable differences between the degree of drying at different stages.In early stage of the Little Ice Age(1340~1580 A.D.),the pollen content of Chenopodiaceae which has a preference for dryness in sporo-pollen complex increased gradually,and pollen content of hygrophytes showed an opposite trend.Moreover,score of PCA axis 1(PCA 1)primarily ranged from 0 to 1.All these factors suggest a relatively dry climate.This period witnessed the minimum concentration of charcoal,content of crop pollen(Poaceae≥35μm)and phosphorus(P)content which are all associated with human activities,consequently suggestive of relatively weak intensity of human activities in the study area.During medium stage of the Little Ice Age(1580~1800 A.D.),the content of Chenopodiaceae,Urtica and Humulus which prefer dryness reached peak value,and score of PCA axis 1(PCA 1)was more positively biased than that of early stage of the Little Ice Age,which indicated drier climate.Furthermore,the highest concentration of charcoal,synarthropic plant,phosphorus(P)content,and content of crop pollen(Poaceae≥35μm)implied significant increase of human activities.In late stage of the Little Ice Age(1800~1920 A.D.),pollen content of wet plant Poaceae and hygrophytes Myriophyllum reached the highest value,and score of PCA axis 1(PCA1)was negatively biased,indicative of humid climate.During this stage,content of crop pollen(Poaceae≥35μm),concentration of charcoal and phosphorus(P)content slightly reduced as compared to medium period,suggesting slight reduction of human activities as compared to medium period.After the Little Ice Age(1920~present),the pollen content of the wet Poaceae and the hygrophytes Myriophyllum decreased,and the climate developed toward aridification compared with the late Little Ice Age.