陕西定边地区2400cal.aB.P.以来高分辨率的花粉记录及环境变化

通过对位于黄土/沙漠过渡带定边地区苟池剖面的高分辨率的花粉分析,基于9个 14C年代数据重建了2400cal.aB.P.以来定边的植被与气候变化,结果表明 2400～1740cal.aB.P.地带性植被为以栎为主的森林草原,气候较温暖湿润; 1740～1610cal.aB.P.森林退化,草原发育,气候变干; 1610～1240cal.aB.P.乔木成分增加,植被类型为疏林草原,气候较温和湿润; 1240～1010cal.aB.P.森林退缩,草原发育,气候变干; 1010～690cal.aB.P.草原进一步发育,气候干旱; 690～500cal.aB.P.阔叶树成分增加,气候略温湿; 500～190cal.aB.P.草原发育,气候干旱; 190～0cal.aB.P.乔木成分增加,气候变湿。本区花粉记录的植被和气候变化可与区域的世纪初前后(秦汉)暖期、隋唐暖期、中世纪暖期和小冰期等事件对比。

VEGETATION AND ENVIRONMENT CHANGE BASED ON HIGH RESOLUTION POLLEN RECORDS OF THE LAST 2400 YEARS IN DINGBIAN|SHAANXI PROVINCE|CHINA

Based on the chronology of seven AMS ¹⁴C and two ¹⁴C dates,a detailed history of vegetation and climate change of the last 2400 years was reconstructed by a high­resolution pollen record from Gouchi,Dingbian(37°45′N,107°31′E; 1300m a．s.1.),at the desert­loess transition in Northern China(Fig.1).
Dingbian is near the Maowusu desert in the north of Shaanxi Province,with the mean annual precipitation about 300mm. A section(200cm thick)was excavated by hand in the center of Gouchi Lake(a nearly dry salt lake,about 30km north of Dingbian),in and around which water system is poor except several seasonal rivers. 100 samples for pollen analysis were obtained at ca.2cm intervals. The chronology is based on seven bulk AMS ¹⁴C dates(NSF AMS Facility at University of Arizona)and two ¹⁴C dates(Lanzhou University). The calendar ages are calibrated using the Calib5.0 calibration database.
At least 300 pollen grains were counted for each sample. Pollen types were identified using reference material and published keys and photographs. 41 families and 57 genera of pollen were identified. Three pollen assemblage zones and seven sub­zones were divided in terms of the variations of main pollen percentages,pollen concentration and the result of CONISS analysis from the bottom to the top.
A detailed history of vegetation and climate changes of the past 2400cal.a B．P.was reconstructed by a high­resolution pollen record. Between 2400cal.aB.P.and 1740cal.aB.P.,the pollen assemblage mainly consisted of Quercus (0.45%～21.55%), Pinus (0.34%～42.1%),Chenopodiaceae(0～45.32%),Artemisia (4.16%～42.05%)and Ulmns (0.18%～4.6%)seems to represent a forest steppe dominated by Oak landscape and a warmer and moister climate. During 1740～1610cal.aB.P.,the increase of Artemisia (11.41%～40.22%)at the expense of Quercus (0.31%～7.05%)and Ulmns (0.34%～3.64%)probably might reflect that the steppe to expand,the forest to retreate and the climate to become drier. During 1610～1240cal.aB.P.,the increase of Pinus (15.27%～33.53%)and Quercus(2.39%～13.25%)and the decrease of Chenopodiaceae(9.44%～22.99%)and Artemisia(9.28%～24.83%)suggest an open forest steppe vegetation and a mild and moister climate. During 1240～1010cal.aB.P.,Quercus (0.43%～8.72%)and Pinus(5.42%～20.48%)declined while Artemisia(10.36%～50.00%),Chenopodiaceae(0～25.69%)and Gramineae(2.96%～9.12%)increased,which might reflect that the forest to shrink,the steppe to expand: the climate was drier than before. During 1010～690cal.aB.P.,the drastic rise of Artemisia(15.96%～56.00%)probable show that the steppe to further develop and a dry climate to prevail. During 690～500cal.aB.P.,Quercus (2.58%～10.38%)increased while Artemisia(21.19%～42.09%)declined,which might suggest a temperate climate. During 500～190cal.aB.P.,the pollen concentration was the lowest,whereas Artemisia (30.17%～64.11%)was the highest in the section,which might reflect a steppe vegetation and a dry climate: an ecosystem degradation. During 190～0cal.aB.P.,the rise of Pinus (11.35%～48.54%)and Quercus (2.49%～9.86%)and the decline of Chenopodiaceae(2.93%～13.26%)and Artemisia (13.39%～53.30%)might reflect an open forest steppe vegetation and a moister climate.
The climate of the last 2400 years in Dingbian showed three warm periods. The warmer and moister period of 2400～1740cal.aB.P.should correspond to the Qin­Han Dynasty Warm Period,the mild and moister period of 1610～1240cal.aB.P.to the Sui­Tang Dynasty Warm Period; and the dry period of 1010～690cal.aB.P.to the Medieval Warm Period. In addition,the ecosystem degradation interval of 500～190cal.a B．P.might be compare with the “Little Ice Age”,though the fluctuation of warm/cold climate was not obvious.