【作者】 李颖；

【导师】 王万忠； 穆兴民；

【作者基本信息】 西北农林科技大学 ， 水土保持与荒漠化防治， 2013， 博士

【摘要】 东亚季风是全球大气环流的重要组成部分，亦是控制东亚气候演变的直接驱动力。黄土高原的黄土及古土壤中蕴藏了东亚季风演变历史的信息。中国黄土的堆积始于约2.5百万年前，之后近乎连续沉积至今。完整的第四纪黄土-古土壤序列地质、生物证据表明，黄土是干冷时期冬季风搬运的粉尘堆积，与其相间的古土壤则与携带水分的夏季风密切相关。因此，黄土高原黄土-古土壤序列完整地保存了近2.5百万年东亚季风气候的演变过程（即冬季风和夏季风优势期的交替历史），被认为是全球古气候旋回在东亚地区的良好信息载体。本文以我国黄土高原西北部靖远(36.35°N，104.6°E，海拔2210m)、古浪(37.49°N，102.88°E，海拔2400m)20m探井的黄土-古土壤剖面为研究对象，通过光释光技术定年，采用距平累积、Mann-Kendall趋势检验、小波分析等方法分析黄土粒度组成、磁化率、化学元素含量以及粒度分布分形维数等指标在剖面中的分布，探讨末次晚冰期以来黄土高原西北部东亚冬、夏季风强度的阶段性、趋势性、周期性以及突变性变化特征，揭示地质时期沉积、成壤环境的时空差异以及古气候突变的驱动机制，取得的主要结果有：1.确定了适用于黄土高原西北部的冬季风、夏季风强度代用指标。中值粒径、较粗组分32-63μm颗粒含量、Si/Al、Zr/Al、粒度分布分形维数是靖远、古浪可靠的冬季风强度代用指标。磁化率、细组分<2μm颗粒含量、Sr/Al以及Ca/Al值是研究区敏感的夏季风强度代用指标。但由于末次冰期夏季风对研究区气候的影响非常有限，各夏季风强度代用指标在末次晚冰期L1黄土层中变化趋势不明显。2.分析了靖远末次晚冰期以来冬季风强度的阶段性及趋势性变化特征。根据光释光测年结果，靖远20m剖面沉积年龄为约48kyr，0-1.7m（14.8kyr BP至今）为全新世S0古土壤层，下至20m（47.3-14.8kyr BP）为末次晚冰期L1黄土层，层间未见弱风化古土壤层。各冬季风、夏季风强度代用指标均表现出明显的冰期-间冰期尺度差异。以近47.3kyr时段各冬季风强度代用指标均值为标准，分析了末次晚冰期靖远古季风强度的变化阶段及趋势：（1）47.3-30.1kyr BP时段，冬季风强度较弱，但却呈现出显著的增强趋势；（2）30.1-24.2kyr BP时段，冬季风强度处于均值水平，且继续呈现增强趋势，但增强程度与上时段相比有所减弱；（3）24.2-14.8kyr BP时段，冬季风处于强盛阶段，但强度却呈现显著减弱趋势；（4）14.8kyr BP至今，夏季风强盛，呈现显著增强趋势。冬季风衰退，呈现出显著减弱趋势。3.分析了古浪末次晚冰期以来冬季风强度的阶段性及趋势性变化特征。根据光释光测年结果，古浪20m剖面沉积年龄约为60kyr，0-1.6m（13.7kyr BP至今）为全新世S0古土壤层，下至20m（60-13.7kyr BP）为末次晚冰期L1黄土层，层间未见弱风化古土壤层。古浪剖面冬、夏季风强度代用指标也表现出明显的冰期-间冰期尺度差异。以60kyrBP至今各冬季风强度代用指标均值为标准，将古浪末次晚冰期以来古季风演变过程分为5个阶段：（1）60-50.1kyr BP时段，冬季风强度处于较弱阶段，没有呈现显著变强或变弱趋势；（2）50.1-37.1kyr BP时段，冬季风强度处于平均水平且有减弱趋势，但趋势并不显著；（3）37.1-26.8kyr BP时段，冬季风强度处于较弱阶段，但呈现出显著增强趋势；（4）26.8-13.7kyr BP时段，冬季风处于强盛阶段，且仍旧呈现显著增强趋势，直至全新世开始；（5）13.7kyr BP至今，夏季风强盛，呈现显著增强趋势。冬季风强度处于较弱阶段，呈现显著减弱趋势。4.分析了靖远、古浪末次晚冰期以来冬季风强度的周期性变化特征。末次晚冰期以来，靖远研究区冬季风强度没有呈现出全域性的周期性变化特征，在8-12kyr时间尺度上，冬季风强度周期变化主要分布在48-30kyr BP时段；在15-20kyr时间尺度上，周期变化主要分布在48-25kyr BP时段；在30-35kyr时间尺度上，周期变化主要分布在48-20kyr BP时段。古浪研究区冬季风强度存在明显的约10kyr、21kyr以及43kyr时间尺度上的周期性变化特征，其中第一主周期为约43kyr。古浪冬季风强度在约43kyr主周期的大尺度以及约21kyr中尺度上表现出全时域的周期性波动特征，而在约10kyr的小尺度上，古浪冬季风强度周期变化主要分布在末次晚冰期。目前靖远、古浪冬季风强度小波变换系数已达到零值，冬季风强度将进入高值阶段，气候将由温湿转为干冷。5.比较了靖远、古浪沉积环境以及成壤环境的空间差异。靖远和古浪物源和气候环境一致，沉积、成壤条件相当，没有明显的气候空间差异。黄河泥沙在黄土沉积过程中向靖远输送了较粗颗粒，导致距离粉尘源区较远的靖远中值粒径值和较粗组分颗粒含量均高于古浪，且沉积速率（10-130cm kyr-1）也略高于古浪（9-63cm kyr-1）。全新世以来，靖远研究区的年均温和年均降水量均略高于古浪，具有较高磁化率和更为强烈的Ca、Sr淋溶，成壤环境略优。6.探讨了古浪黄土-古土壤剖面对突变气候事件的响应。古浪剖面清楚地记录了末次冰期向全新世过渡期间的Younger Dryas事件，以及末次晚冰期的多次Heinrich事件和Dansgaard-Oeschger旋回。古浪剖面对气候突变事件的记录与格陵兰冰芯、中国东部葫芦洞石笋基本一致，尤其是在60-27kyr BP时段。利用海-气耦合模型CCSM3，向北大西洋地区加入持续淡水通量减弱大西洋经向反转环流，发现黄土高原黄土沉积源区冬季风环流加强，东亚夏季降水量减少。初步推断大西洋经向反转环流是末次冰期黄土高原西北部突变气候事件的主要驱动原因。更多还原

【Abstract】 The East Asian monsoon plays a key role in the global atmospheric circulation, anddirectly controls and drives the climate variations of East Asia as well. The completeloess-paleosol sequence on the Chinese Loess Plateau is a good recorder of East Asianmonsoon oscillations. The Chinese loess started its deposition since2.5Ma ago, and theprocess continues since then. The geological and biological evidences from Quaternaryloess-paleosol sequences show that when the winter monsoon prevails with a cold and dryclimate, relatively unweathered loess is deposited whereas the interbedded paleosol is formedrepresenting strong summer monsoon with a warm and humid climate. Since the completeloess-paleosol sequence on the Chinese Loess Plateau uninterruptedly preserves the EastAsian monsoon evolution, which is the alternative history between winter and summermonsoon dominant periods, it can be seen as the reflection of global climatic cycles in theEast Asia region. In this thesis, two20m sampling pits were excavated at Jingyuan (36.35°N,104.6°E,2210m above sea level) and Gulang (37.49°N,102.88°E,2400m above sea level),respectively, to obtain climatic data such as the particle size distributions, magneticsusceptibility, chemical elements contents, and factional dimensions of particle sizedistributions. Key methods such as the optically stimulated luminescence, cumulativedeparture, Mann-Kendall trend test, wavelet analyses are employed in this thesis to genaeratean independent chronology of Jingyuan and Gulang loess-paleosol sequences. These analysesaim to test the stages, trend, periodical, and abrupt changes of the East Asia Monsoon, and todetermine the driving factors of abrupt climatic changes in the East Asian monsoon system onthe northwestern Loess Plateau since. The key findings presented in this research are outlinedas follows:1. The suitable winter and summer monsoon intensity proxies are determined. The meanparticle size, relatively coarse particle (32-63μm) content, Si/Al, Zr/Al, and fractionaldimensions of particle size distributions are identified as the reliable proxies of the wintermonsoon intensity at the Jingyuan and Gulang sections. The magnetic susceptibility, fineparticle (<2μm) content, Sr/Al, and Ca/Al are sensitive proxies of the summer monsoon intensity during the Holocene at the research sections which are not sensitive in the lastglacial period because of the brief weak summer monsoon intensity.2. The stage and trend characteristics of the winter monsoon intensity at the Jingyuansection are analysed. Optically stimulated luminescence dating shows that the20m pitaccumulated over48kyr, the uppermost1.7m (from14.8kyr BP) unit represents the HoloceneS0soil,1.7-20m (47.3-14.8kyr BP) layer for the late last glacial period L1loess, and noweakly weathered paleosol layers are identified in the L1loess layer. All of the winter andsummer monsoon intensity proxies show clear glacial-interglacial scale differences.Compared to the mean of monsoon intensity proxies in the interval of the recent47.3kyrperiod, the monsoon intensity since the late last glacial period can be divided into4stages:(1)the period47.3-30.1kyr BP at which the winter monsoon was weak but showed a significantincreasing trend;(2)30.1-24.2BP, the winter monsoon intensity steadily was equal to theaverage of the monsoon intensity with a significant increasing trend, which slowed downrelatively compared to the increasing trend in the first stage;(3)during the period24.2-14.8kyr BP, the winter monsoon was quite strong but appeared a significant decreasingtrend;(4) during the period14.8kyr BP till now, the summer monsoon is strong with asignificant increasing trend, but winter monsoon is weak with a significant decreasing trend.3. The stage and trend characteristics of the winter monsoon intensity at the Gulangsection are analysed. Optically stimulated luminescence dating shows that the20m pit inGulang accumulated over60kyr, the uppermost1.6m (from13.7kyr BP) unit represents theHolocene S0soil,1.6-20m (60-13.7kyr BP) layer for the late last glacial period L1loess, andno weakly weathered paleosol layers are identified in the L1loess layer. The winter andsummer monsoon intensity proxies in Gulang also show distinct differences between S0andL1layers. Compared to the mean of monsoon intensity proxies in the interval of the recent60kyr period, the monsoon intensity since the late last glacial period can be separated into5stages:(1) the period60-50.1kyr BP was a time when winter monsoon was weak with noincreasing or decreasing trend;(2) at the period50.1-37.1kyr BP the winter monsoonremained at the average level with an insignificantly decreasing trend;(3) at the stage37.1-26.8kyr BP the winter monsoon was weak but significantly increased;(4) during thestage26.8-13.7kyr BP, the winter monsoon was quite strong, and still exhibited a significantincreasing trend; and (5) at he period13.7kyr BP till now the summer monsoon is strong witha significant increasing trend, but winter monsoon is weak with a significant decreasing trend.4. The periodical characteristics of the winter monsoon intensity at the Jingyuan andGulang sections are analysed. At the Jingyuan section, there is no periodicity over the wholeresearch period since the late last glacial period. During the periods of48-30,48-25, and 48-20kyr BP, the winter monsoon intensity was mainly characterised by the periodicities of,8-12,15-20, and30-35kyr, respectively. The winter monsoon intensity at the Gulang sectionhas obvious periodicities which are about10kyr,21kyr, and43kyr, and the first mainperiodicity is about43kyr. At the time scales of43kyr and21kyr, the winter monsoonintensity shows clear periodic variations since the late last glacial period, but the10kyrtime-scale only characterised the winter monsoon intensity during the late last glacial period.The wavelet transform coefficient of the winter monsoon intensity reaches zero, which means,in the future, that the winter monsoon intensity will get to the strong stage, which will resultin a cold-dry climate in the northwestern part of theChinese Loess Plateau.5. The differences of deposition and pedogenesis environments between Jingyuan andGulang are analysed. There is no distinct spatial climatic difference between these two regions,which are characterised by a similar deposition and pedogenesis environment. Jingyuanlocates farther from the dust source area, however, it has a higher mean particle size, highercoarse particle size content, and even higher accumulation rates (10-130cm kyr-1) than thosein Gulang (9-63cm kyr-1). The reason for these differences is the proximity of Jingyuan to theYellow River which is a likely local dust source. Since the Holocene, both annual meantemperature and precipitation are slightly higher in Jingyuan than Gulang, resulting in ahigher magnetic susceptibility, and stronger leaching of elements Sr and Ca in Jingyuan.6. The driving forces of abrupt monsoonal changes at the northwestern Chinese LoessPlateau are determined. The two20m pits clearly record the abrupt climatic events includingthe Younger Dryas event, the Heinrich events, and the Dansgaard-Oeschger cycles. The rapidchanges in the fractal dimensions of particle size distributions at the Gulang section are wellaligned with those abrupt events recorded in the North Greenland ice core and Chinese HuluCave speleothems, especially over the interval of60-27kyr BP. The evidence is frompublished simulated results from the coupled climate model CCSM3, which shows that anadded fresh water flux into the North Atlantic slowed down the Atlantic meridionaloverturning circulation, consequently an enhancement of the winter monsoon circulation anda decrease of summer monsoon precipitation are found across the dust supply regions of theLoess Plateau. As a result, the Atlantic meridional overturning circulation is deemed as thedriving force of the abrupt climatic events in the northwestern Loess Plateau during the lastglacial period.更多还原