内蒙古东部植被是我国北方重要的生态系统屏障,东亚夏季风降水变化是影响当地水资源分布、植被演替、农牧业经济发展的重要因素。为了更好预测未来季风边缘带植被景观、降水变化趋势和评估干旱事件可能产生的影响,需要在该区域开展更多地质增温期历史相似型研究。本文选择生态系统脆弱、对夏季风变化响应敏感的内蒙古东部布日敦湖作为研究点,通过钻取长386 cm的湖泊沉积物并选取底部厚度176cm(386~210cm),分析其中的花粉指标,结合高精度AMS 14C测年,来重建该地区末次冰消期高分辨率植被演替过程和降水不稳定性特征。结果表明,布日敦湖岩芯386~210cm沉积物年代跨度为14918 ~ 10693 a B. P.。湖泊周围植被在14918~ 14167 a B.P.为以蒿属、禾本科等草本植物为主的草原景观;而在14167~ 12695 a B.P.,以桦属为代表的先锋树种出现在湖泊西部山地丘陵区,并逐渐发展成为以桦属为建群种的温带落叶阔叶林;12695~ 11505 a B.P.,桦属和其他阔叶乔木明显减少,湖泊西部地区植被迅速退化为开阔的疏林草原景观;11505~ 10693 a B.P.,植被逐渐恢复为与现今类似的森林草原交错生长群落。乔木/非乔木花粉含量比值(AP/NAP)和主要陆生植物花粉百分比主成分分析(PCA)结果说明,研究区植被演替主要受降水控制。将该结果与沉积相、沉积速率变化相结合,证明14512a B.P.布日敦湖流域内降水突然增加,在14512~12695 a B.P.期间气候最为湿润,并伴随3次百年尺度降水波动事件。而12695 a B.P.时降水迅速减少,在12695~ 11505 a B.P.发生千年尺度干旱事件。布日敦湖地区降水变化总体趋势与北方季风边缘带其他高分辨率湖泊记录相似,但内部结构存在一定差异。在年代误差范围内,降水突变与北高纬博令-阿勒罗德(BA)间冰阶和新仙女木(YD)冰阶温度异常事件发生时间相一致。北半球夏季太阳辐射岁差周期变化、北大西洋经向反转环流(AMOC)循环速率、西风急流强度和位置等均可能导致西太平洋副热带高压位置异常偏移,进而影响东亚夏季风北部边缘带降水变化。
英文摘要:
Vegetation in eastern Inner Mongolia is an important part for protecting ecological system of semi-arid area in north China. Local precipitation is controlled by the East Asian Summer Monsoon ( EASM) and it affects water resources, vegetation succession, agriculture and husbandry economy. In northern margin of the EASM,it is essential for developing analog of warming period in geological history,for better predicting vegetation response, rainfall change and evaluating environment effect of abrupt arid climate events. Buridun Lake ( 43°02'54"N, 119o03'31"E) region is a vulnerable ecotone and sensitively responses to the EASM. A 386-cm long core was drilled and only 176 cm at the bottom of the core was used. The lithology was described as follows, 386 ~375 cm, grey coarse silt; 375~355 cm,brownish fine sand; 355~341 cm,transition zone from silt to clay; 341 ~286 cm,brown laminated layer with grey brownish interlayer; 286~271 cm, brownish clay; 271 ~247 cm, yellowish brown clay and thin white calcium clay interbedding; 247 ~222 cm, greyish brown clay; 222 ~219 cm,grey coarse silt;219~214 cm,whitish grey silt clay; 214~210 cm,yellowish brown clay. Pollen and AMS 14C dating were used to reconstruct high resolution vegetation succession and precipitation history. The result showed that the date of 386 ~210 cm sediments spanned 14918 ~10693 a B. P. Pollen types in total 40 included Pinus, Picea, Betula,Quercus,Ulmus,Salix, Zygophyllaceae, Rosaceae, Cruciferae, Artemisia, Chenopodiaceae, Compositae, Poaceae, Polygonaceae, Leguminosae, Typha, Sparganium and other genus. Shrub and herb pollens accounted for prominent component 54% ~96% since 14918 a B. P., such as Artemisia(24% ~75% ),Poaceae(0 ~8% ) and Leguminosae (0 ~7% ). Tree pollen percentage distinctly increased at 14167 a B. P. with high Betula and low other tree pollen content. Based on the pollen spectra, vegetation types were divided into four zones. Steppe dominated by Artemisia and Poaceae plants was main vegetation landscape in 14918 ~14167 a B. P. Pioneer tree represented by Betula expanded in the upland and hill located in western Buridun Lake in 14167 ~12695 a B. P., and then formed temperate deciduous broad-leaved forest with Betula as constructive species. In 12695 ~11505 a B. P., Betula and other broad-leaved trees significantly decreased and vegetation cover degraded and became steppe with sparse trees in the western catchment of the lake. Forest and steppe ecotone formed like modern time in 11505 ~10693 a B. P. Results of arboreal/non-arboreal pollen percentage ratio and principal component analysis of main terrestrial pollen percentage indicated that vegetation of Buridun Lake area was mainly controlled by precipitation. According to other evidences from lithology and accumulation rate, precipitation abruptly rose at 14512 a B.P. Wet climate appeared in 14512~12695 a B.P., but precipitation was fluctuated on centurial time scale. In 12695 ~11505 a B.P., arid climate was prevalent in this region. The trend of precipitation revealed above resembled other high resolution records of lakes lied in the north fringe of the EASM,but differentiated in details. Within age uncertainties, the timing of abrupt precipitation change in Buridun Lake area was synchronous with Bolling-Allerod interstadial and Younger Dryas stadial reverse in north high latitude. Precession cycle of solar radiation, Atlantic Meridional Overturning Circulation rate and westerlies jet strength and position might force Western Pacific Subtropical High anomaly shift and further affected precipitation change in north fringe of the EASM.