【作者】 孙惠玲；

【导师】 陈发虎； 周爱锋；

【作者基本信息】 兰州大学 ， 自然地理学， 2011， 博士

【摘要】 中国季风边缘区由于同时受到季风-西风以及季风系统内部(东南季风、西南季风)等复杂的气候系统影响,使得位于季风边缘区的黄土高原在全新世以来的气候变化格局扑朔迷离。由于全新世黄土缺乏理想的测年材料,加上黄土-古土壤记录的时问分辨率相对较低以及可能出现的侵蚀现象造成黄土-古土壤序列在全新世记录上的不连续性,使得黄土高原的气候重建难以建立准确的时间标尺。同时,黄土高原也是中华文明的起源地之一,新石器以来文化的兴衰和环境变化密切相相关。因此,高分辨率地重建黄土高原中晚全新世气候变化历史,获得气候快速变化事件的可靠记录,对于探索我国季风边缘区气候演变规律具有重要意义。本研究以黄土高原西部关山天池为研究对象,通过对沉积物岩芯生物标志化合物(Paq; GDGTs)、单体氢同位素(δD)和其它常规代用指标磁化率(MS)、粒度(GS)、有机质含量(TOC)、碳氮比(C/N)、稳定碳同位素(13Corg)等的综合分析,结合28个陆源植物残体的高分辨率AMS测年控制,重建了中晚全新世6000Cal yr BP以来黄土高原西部的温度和季风强度的变化,并在此基础上结合季风区其他区域的记录对黄土高原中晚全新世气候变化的驱动机制进行探讨。主要结论归纳如下：(1)正构烷烃C23/C31、Paq、C/N和δ13Corg指标显示6000年以来水生植物在关山天池中的比率逐渐增加,基于关山天池特殊的湖盆构造,可能指示了关山天池湖泊水位逐渐下降的趋势。而常规指标和植物残体结合孢粉的结果表明,中晚全新世以来,黄土高原六盘山区植被呈逐渐减少趋势,尤其在2500Cal yr BP之后锐减,这或许与人类活动有关。(2)基于Sun et al (2011)的区域校准公式,运用关山天池GDGTs指标定量重建了中晚全新世以来黄土高原西部地区的古温度变化。来源于陆生植物的沉积物脂肪酸δDC28可间接反映大气降水的6D变化,其整体结果与北半球太阳辐射变化趋势一致,指示了黄土高原西部地区主要受季风影响。(3)重建的关山天池高分辨率温度和季风强度的同相变化结果表明,中晚全新世以来黄土高原西部地区属于水热同期的气候特征。即,6000年以来,研究区的气候类型在“暖湿”、“冷干”间转换。六盘山地区6000年以来的气候环境演化特征如下：(a)6000～5200Cal yr BP,还处在全新世大暖期的余韵中,气候类型表现为暖湿。(b)5200～4800Cal yr BP,有快速气候事件发生,表现为突然冷干的气候类型。(c)4800～3800Cal yr BP,快速回暖气候事件发生,温暖异常,表现为典型的暖湿气候。(d)3800～500Cal yr BP,温度和降水都为低值但相对稳定,表现为冷干气候类型。(e)500Cal yr BP～现今,这一阶段不同于其它时期,降水与温度呈反相变化,温度在逐渐升高、降水在逐渐减少,表现为暖干的气候类型。(4)与季风区高分辨率的石笋氧同位素和湖泊记录的区域对比结果显示,黄土高原地区主要受到亚洲季风的影响。(5)信风张驰驱动的ENSO现象以及西太平洋暖池的温度变化等大尺度的太平洋海气循环可能是驱动季风强度变化的主要原因。更多还原

【Abstract】 The Asian monsoon is a key component of the earth’s climate system that directly affects the livelihood of50million people on the loess plateau of central China. At the far edge of monsoonal influence, this region is especially vulnerable to future changes in temperature and evaporation/precipitation. Therefore, paleoclimatic information on the natural sensitivity of the region to changes in monsoon driven aridity is crucial. Despite the need for multiproxy records of Holocene climate from this region, reconstructions are rare, because of the low resolution of loess deposits and the scarcity of other paleoclimate archives (e.g. natural lakes, speleothems). Here we present multiple proxy records from Tianchi Lake, one of the few nature lakes on the western loess plateau. The chronology is well constrained by a high-resolution (28AMS14C dates) radiocarbon age-model, spanning the past6000years. Here we present magnetic susceptibility, grain size, total organic carbon, carbon and nitrogen ratio, plant macrofossils, Glycerol dibiphytanyl glycerol tetraethers (GDGTs), lake macrophytes (paq) and plant-wax hydrogen isotope to reconstruct regional climate change during the middle to late Holocene.Evidence from n-alkane Paq, C/N and δ13Corg data suggest increasing relative abundance of macrophytes over past6000years, which we interpret (based on lake morphology) as decreasing lake-level. Plant macrofossils and other environmental parameters suggest that the vegetation in the catchment was decreased from6000cal yr BP and then more rapidly from2200cal yr BP. This is coeval with documentary records of increasing local population density and infers historical human impact on the catchment.Using Sun et al (2011) regional calibration we derive mean annual GDGT based temperatures (MBT/CBT-MATs) with reasonable ranges. Our temperature reconstruction closely correlates on millennial to centennial timescales with the independent D/H measurements on C28fatty acid (δDC28), whose signal is assumed to derive primarily from terrestrial plant waxes and the δDC28values to reflect local changes in precipitation. Synchronous evolution of reconstructed temperature (MBT/CBT-MAT) and precipitation (δDC28) from Tianchi Lake suggest that the climate on western loess plateau shifted between "warm/wet" and "cold/dry" during the middle to late Holocene. Five evolutionary stages could be divided as follow:(a)6000～5200Cal yr BP, regional climate seems still in the end of Holocene Megathermal, a warm and wet climate can be inferred.(b)5200～4800Cal yr BP, abrupt cold and dry climate event was existed during this period.(c)4800～3800Cal yr BP, abrupt warm and wet climate event was happened. The temperature increased rapidly and was especially high.(d)3800～1200Cal yr BP, the reconstructed temperature and precipitation in this stage was low and relatively constant. A cold and dry climate can be inferred.(e)1200Cal yr BP～present, asunchronous characteristic of increased reconstructed temperature and decreased reconstructed precipitation was showed. The climate seemed to be a warm and dry condition.Comparisons of our independent GDGT temperatures and plant-wax δDC2s records with stalagmite δ18O records from the monsoon region and NH summer insolation suggest strongly that our record reflects regional changes in monsoon strength forced by NH summer insolation. Superimposed on the longer-term insolation driven changes are centennial scale variations, recorded by both the independent reconstructions of precipitation (δDC28) and temperature (MBT/CBT-MAT).Trade-wind driven of the frequency/intensity of ENSO cycle on the longer-term scale and the SST changes of the western tropical pacific might be the key point of the monsoon strength variations in study area.更多还原