我国季风边缘区湖泊沉积记录的全新世亚洲夏季风衰退事件

亚洲夏季风是全球季风系统的重要组成部分,亚洲夏季风的变化对其控制区域自然生态系统的多样性和生态平衡,以及社会经济发展有重要的影响。本文选择位于现代亚洲夏季风边缘区对季风变化响应敏感的湖泊达连海为研究对象,基于陆生植物残体和全有机质的AMS ¹⁴C定年建立了钻孔顶部24.6 m沉积物的年代框架,利用粒度指标重建了全新世研究区水文变化过程以及亚洲夏季风衰退事件序列。结果显示,沉积物中存在数层砂层,代表了湖泊低水位时期,进而指示了亚洲夏季风衰退事件。这些事件处在11.6~11.3 cal.ka B.P.、10.4~9.5 cal.ka B.P.、6.4~6.0 cal.ka B.P.、4.6~4.4 cal.ka B.P.、3.7~3.4 cal.ka B.P.、3.1~2.9 cal.ka B.P.以及2.0~0.9 cal.ka B.P.,可以发现中晚全新世以来亚洲夏季风衰退事件发生的频率显著增加。进一步与北半球高纬地区与低纬地区的气候突变事件记录对比显示,全新世百年-千年时间尺度上亚洲夏季风强度的变化与低纬ENSO活动存在密切的联系。

     

全新世中期中国气候和东亚季风:PMIP4模式结果

利用国际古气候模拟比较计划(PMIP)最新第四阶段(PMIP4)中14个气候模式的试验数据,集中研究了距今约6 000年的全新世中期中国气候和东亚季风。与早期PMIP第三阶段(PMIP3)多模式结果类似,全新世中期中国年、冬季和春季地表气温较工业革命前期偏冷,而夏季和秋季偏暖,其中年和冬季模拟偏冷与大部分地质记录显示的偏暖不符;所有14个PMIP4模式集合的中国区域平均年和季节温度变化绝对值为0.08~1.69 ℃,较PMIP3多模式平均结果额外偏小0.01~0.45 ℃,这部分源于大气二氧化碳浓度的减少。在用于分析的11个PMIP4模式平均结果中,全新世中期中国年平均降水、蒸发和有效降水(即降水量减蒸发量)相对于工业革命前期分别增加2%、减少1%和增加7%,所有3个物理量在季节上均表现为冬春季减少,夏秋季增加。对比PMIP4模式和PMIP3多模式平均结果,上述3个物理量的中国区域平均值和区域变化差异均在夏、秋季大于年和冬、春季;相比于PMIP3模式,PMIP4模式模拟的年有效降水变化与地质记录更为接近。全新世中期东亚冬、夏季风在14个PMIP4模式中均模拟加强,所有模式平均较工业革命前期分别增强11%和32%;在区域尺度上,与早期PMIP3模式相比,当前PMIP4模式模拟的季风环流增强幅度在东亚北部更强,南部偏弱。     

Late Holocene monsoon climate as evidenced by proxy records from a lacustrine sediment sequence in western Guangdong,South China

The study of a 300-cm-thick exposed lacustrine sediment section in the Hedong village in Zhaoqing area which is located in sub-tropical west Guangdong Province in South China, demonstrates that the lacustrine sedimentary sequence possibly contains evidence for exploring variation of Asian monsoon climate. Multi-proxy records, including the humification intensity, total organic carbon, and grain size fractions, reveal a general trend towards dry and cold conditions in the late Holocene that this is because of a decrease in solar insolation on an orbital scale. Three intensified Asian summer monsoon (ASM) intervals (∼3300–3000 cal yr BP, ∼2600–1600 cal yr BP, and ∼900–600 cal yr BP), and three weakened ASM intervals (∼4000–3300 cal yr BP, ∼3000–2600 cal yr BP, and ∼1600–900 cal yr BP) are identified. Our humification record (HD_cal) shows a good correlation on multi-centennial scale with the tree ring Δ¹⁴C record, a proxy of solar activity. A spectral analysis of HD_cal reveals four significant cycles, i.e., ∼1250 yr, 300 yr, 110 yr, and 70 yr, and most of these cycles are related to the solar activity. Our findings indicate that solar output and oceanic–atmospheric circulation probably have influenced the late Holocene climate variability in the study region.     

Reconstruction of climate and vegetation changes of Lake Bayanchagan (Inner Mongolia): Holocene variability of the East Asian monsoon

A high-resolution pollen and Pediastrum record, spanning 12,500 yr, is presented for Lake Bayanchagan (115.21°E, 41.65°N, and 1355 m a.s.l.), southern Inner Mongolia. Individual pollen taxa (PT-MAT) and the PFT affinity scores (PFT-MAT) were used for quantitative climatic reconstruction from pollen and algal data. Both techniques indicate that a cold and dry climate, similar to that of today, prevailed before 10,500 cal yr B.P. The wettest climate occurred between 10,500 and 6500 cal yr B.P., at which time annual precipitation was up to 30–60% higher than today. The early Holocene increases in temperature and precipitation occurred simultaneously, but mid-Holocene cooling started at approximately 8000 cal yr B.P., 1500 yr earlier than the drying. Vegetation reconstruction was based on the objective assignment of pollen taxa to the plant functional type. The results suggest that this region was dominated by steppe vegetation throughout the Holocene, except for the period 9200 to 6700 cal yr B.P., when forest patches were relatively common. Inner Mongolia is situated at the limit of the present East Asian monsoon and patterns of vegetation and climate changes in that region during the Holocene probably reflect fluctuations in the monsoon's response to solar insolation variations. The early to middle Holocene monsoon undoubtedly extended to more northern latitudes than at present.     

全新世东亚夏季风降水穿时性的模拟研究

全新世期间东亚夏季风经历了较为复杂的演变过程,针对其降水的时空变化规律还存在争议。本研究利用TraCE-21 ka气候瞬变模拟的全强迫试验和敏感性试验数据,分析了全新世东亚不同区域降水极大值出现的时间,即东亚季风降水的"穿时性"问题,并就降水量的演变趋势和主要影响因子进行了分析。结果表明,全强迫试验中,全新世期间东亚夏季风总降水和净降水极大值最早在北方出现,然后逐渐南移,直到近代出现在南方及沿海地区,这与全新世期间东亚夏季风强度逐渐减弱相符;利用水汽收支方程对全新世东亚夏季风总降水变化进行分解,北方地区降水变化主要受动力因子的控制,热力因子的贡献占比较小,随着地区的南移,热力因子也起到了一定的贡献,不过动力因子仍是主导因素;敏感性试验进一步揭示,全强迫试验中东亚季风降水的这种"穿时性"主要受到地球轨道变化导致的海陆热力差异变化调控。     

Sensitive grain-size records of Holocene East Asian summer monsoon in sediments of northern South China Sea slope

Changes in paleoenvironments over the last 17,500 yr have been documented by a high-resolution clay mineralogy and grain–size records of Core KNG5 from the northern slope of the South China Sea. Our results indicate that clay minerals are mainly from the Pearl River from 17,500 to12,500 cal yr BP, and the South China Sea modern current system began to form since 12,500 cal yr BP, as a result, Taiwan turns to be the major contributor of clay minerals after 12,500 cal yr BP. Two grain-size populations with high variability through time were identified in the 13–28 μm and 1–2.2 μm grain-size intervals. The 1–2.2 μm grain-size population are mainly controlled by provenance supply and current transport. The 13–28 μm grain-size fraction could be controlled mainly by the sea-level change. The 1–2.2 μm grain-size population record demonstrates that East Asian Summer Monsoon intensity generally follows changes in insolation and that the response is similar for a large area of China and other northern low-latitude records, implying the globality of the monsoon evolution since Holocene. The anomalous environmental conditions in the northern South China Sea may imply intensified ENSO activity during the late Holocene.     

MIS2期南海北部粘土矿物记录的东亚夏季风快速变化

运用伊利石结晶度线性剥离法,定量计算了华南、台湾和吕宋这3个主要物源区对南海北部陆坡MD12-3432和MD12-3434两个岩芯（分别为50.80 m和8.33 m）MIS 2期粘土相对贡献量的时间序列变化。结果显示,台湾对两个岩芯的贡献最大（36%~62%）,吕宋的贡献次之（28%~50%）,二者之和一般大于90%,而来自华南的贡献最小（3%~17%）。台湾和吕宋的贡献量呈现显著反向的千年时间尺度快速波动,二者贡献量随东亚夏季风增强或减弱而显著变化,因此,吕宋/台湾贡献量比值可以用于指示东亚夏季风的快速演化。吕宋/台湾粘土贡献量比值在Bølling-Allerød和Dansgaard-Oeschger 2事件期间的增加,反映了同时期东亚夏季风的快速增强;而在Heinrich 1和Heinrich 2事件期间的减小,则指示了东亚夏季风的减弱。吕宋/台湾贡献量比值对东亚夏季风快速变化的指示,在一定程度上要明显优于蒙脱石/（伊利石+绿泥石）比值。南海北部MIS 2期粘土矿物对东亚夏季风快速变化的响应具有明显的空间差异性,受洋流搬运扰动和物源控制作用影响,陆坡相对浅水区粘土矿物记录的东亚夏季风快速变化信息相对有限,而深水稳定的沉积环境则更有利于保存东亚夏季风快速变化信息。

     

A detailed East Asian monsoon history surrounding the ‘Mystery Interval’ derived from three Chinese speleothem records

The ‘Mystery Interval’ (MI, 17.5−14.5 ka) was the first stage of the last deglaciation, a key interval for understanding mechanisms of glacial–interglacial cycles. To elucidate possible causes of the MI, here we present three high-resolution, precisely dated oxygen-isotope records of stalagmites from Qingtian and Hulu Caves in China, reflecting changes in the East Asian summer monsoon (EASM) then. Based on well-established chronologies using precise ²³⁰Th dates and annual-band counting results, the two-cave δ¹⁸O profiles of ~ 7-yr resolution match well at decadal timescales. Both of the two-cave records document an abrupt weakening (2‰ of δ¹⁸O rise within 20 yr) in the EASM at ~ 16.1 ka, coinciding with the transition of the two-phased MI reconstructed from New Mexico's Lake Estancia. Our results indicate that the maximum southward displacement of the Intertropical Convergence Zone and associated southward shift of polar jet stream may generate this two-phase feature of the MI during that time. We also discover a linear relationship among decreasing EASM intensity, rising atmospheric CO₂ and weakening Atlantic Meridional Overturning Circulation between the MI and Younger Dryas episodes, suggesting a strong coupling of atmospheric/oceanic circulations in response to the millennial-scale forcing, which in turn regulates global climate changes and carbon cycles.     

East Asian monsoon variation and climate changes in Jeju Island, Korea, during the latest Pleistocene to early Holocene

A 4.96-m-long sediment core from the Hanon paleo-maar in Jeju Island, Korea was studied to investigate the paleoclimatic change and East Asian monsoon variations during the latest Pleistocene to early Holocene (23,000-9000 cal yr BP). High-resolution TOC content, magnetic susceptibility, and major element composition data indicate that Jeju Island experienced the coldest climate around 18,000 cal yr BP, which corresponds to the last glacial maximum (LGM). Further, these multi-proxy data show an abrupt shift in climatic regime from cold and arid to warm and humid conditions at around 14,000 cal yr BP, which represents the commencement of the last major deglaciation. After the last major deglaciation, the TOC content decreased from 13,300 to 12,000 cal yr BP and from 11,500 to 9800 cal yr BP, thereby reflecting the weakening of the summer monsoon. The LGM in Jeju Island occurred later in comparison with the Chinese Loess Plateau. Such a disparity in climatic change events between central China and Jeju Island appears to be caused by the asynchrony between the coldest temperature event and the minimum precipitation event in central China and by the buffering effect of the Pacific Ocean.     

Millennial-scale dynamics of the winter cold tongue in the southern South China Sea over the past 26 ka and the East Asian winter monsoon

Millennial-scale variations of the East Asian winter monsoon (EAWM) remain elusive due to sparse and controversial reconstructions. By compiling a variety of alkenone-based sea surface temperature (SST) estimates, we find that the west-east SST gradient in the southern South China Sea (SCS) well documents the temporal dynamics of the winter “cold tongue” off the southern Vietnam and by inference, variations in the EAWM intensity over the past 26 ka. Our results reveal that the winter “cold tongue” SSTs were significantly colder during Heinrich event 1 and the Younger Dryas event, resulting in an increased west-east SST gradient in the southern SCS due to a strengthened EAWM. Within dating uncertainties, an intensified EAWM during cold stadials was coeval with the shutdown or a reduction in strength of the Atlantic meridional overturning circulation (AMOC), exhibiting a strong linkage between the AMOC and the EAWM system. The west-east SST gradient also indicates an enhanced EAWM during the early Holocene, which may be induced by postglacial ice-sheet dynamics and a strong seasonal contrast in solar insolation. Our findings suggest that the EAWM was probably modulated by a complex interplay between the AMOC, solar insolation and ice-sheet dynamics on sub-orbital time scales.     

东亚和南亚夏季风对中国季风区径流深影响

为揭示中国东部季风区径流深对东亚和南亚夏季风变化的响应规律,建立了八大流域VIC(Variable Infiltration Capacity)水文模型以及应用了Mann-Kendall和局部加权回归分析(Locally Weighted Scatter Plot Smoothing, LOWESS)相关性检验方法,并分析了径流深系数空间变化情况。结果表明:东亚夏季风与径流深显著性相关范围要明显大于南亚夏季风,主要位于长江流域以北和松花江流域以南的广大地区以及华南部分地区,并分别成正相关和负相关;而南亚夏季风与径流深显著性正负相关的区域则分别位于华南地区及长江流域上游部分地区。此外,季风区径流深系数空间差异明显,易产流区主要位于长江流域及东南沿海地区,而黄河及海河流域则产流较难。因此,对东亚及南亚夏季风的研究可为预测中国东部不同地区的水文过程及水资源变化提供重要的科学参考。     

近千年东亚夏季风演变*

在分析东亚地区夏季海平面气压场与中国东部6区域干湿指数关系的基础上,重建了公元960—2000年近千年东亚地区夏季海平面气压场的格点资料,并对重建效果进行了检验。同时,依据重建的海平面气压场资料定义了近千年东亚夏季风强度指数,探讨了近千年东亚夏季风的变化特征。结果表明：（1）重建的近千年东亚地区的海平面气压场具有一定的可信度,它为研究更长时间尺度的东亚夏季风变化特征提供了基础;（2）东亚夏季风指数存在60～70 a、30～40 a、10～20 a的显著周期变化;（3）近千年东亚夏季风的强度指数主要经历过9次明显的趋势突变。其中,13世纪30年代东亚夏季风的强度指数经历了最显著的振动。     

全新世太阳活动对东亚夏季风百年尺度变化影响的模拟研究

太阳活动对东亚夏季风(EASM)变化有着重要的影响。本研究利用通用地球系统模式(CESM)开展了全新世气候瞬变模拟试验, 探究了全新世EASM降水百年尺度的变化特征及其对太阳活动外强迫的响应机理。结果表明: 在变化趋势上, 模拟的全新世以来EASM降水呈减弱趋势, 主要受地球轨道参数外强迫的影响。在百年尺度上, 中全新世以来太阳活动外强迫引起EASM降水出现约430年的周期变化特征, 这与高分辨率重建资料记录的周期相似。这一百年尺度变化的空间主模态呈南北偶极型分布, 北方降水与太阳活动存在显著的同期正相关, 当太阳活动增强时东亚海陆热力差异加强, 陆地海平面气压降低, 引起异常的气旋性环流, 有利于北方降水增多。南方降水与太阳活动呈负相关并滞后于太阳活动, 太阳活动通过调制赤道太平洋类拉尼娜态的海温梯度, 引起西北太平洋风-蒸发-海温的正反馈机制, 触发异常的西北太平洋气旋性环流, 从而导致南方降水减少。

     

基于浮游有孔虫重建的南海北部400 ka以来上层水体结构变化及其对东亚季风的响应

现代南海上层水体结构具有鲜明的季节特征,反映了季节性风场通过动力与热力过程施加的强烈影响;这种风场对上层水体结构的调控同样存在于地质历史时期,借此,长时间尺度上的东亚季风强度变化能够在南海沉积中留下记录,对相关高分辨率样品的研究继而可以从海洋的角度发掘出新的季风信息,成为对现有古季风资料的重要补充。本研究利用南海北部岩芯总长为50.8 m的MD12-3432站位（19°16.88'N,116°14.52'E;水深2125 m）高分辨率沉积样品,通过浮游有孔虫属种组合及其转换函数恢复了400 ka以来冬、夏季海水表层温度及温跃层深度,并对其进行频谱分析,同时对比现代观测资料及黄土粒度等古季风记录,讨论了海表温度、温跃层深度变化与东亚季风强度、表层环流格局等因素的关系。结果表明,晚更新世以来南海北部海表温度具有显著的100 ka冰期-间冰期旋回,冬季海表温度降幅可达8℃,是冰期冬季风增强与表层环流变化的共同结果。此外,冬季海表温度与温跃层深度重建结果均具有明显的67 ka周期,可能代表了东亚冬季风风速在南海北部的变化周期。同时,温跃层深度中未出现100 ka周期,可能指示冰期-间冰期旋回中表层环流的变化对其影响较弱,温跃层的加深主要由东亚冬季风的增强造成。

     

Holocene East Asian monsoon variation inferred from species assemblage and shell chemistry of the ostracodes from Hulun Lake, Inner Mongolia

A sediment core from Hulun Lake, Inner Mongolia was analyzed for species assemblages and shell chemistry of ostracodes to investigate changes in the hydrology and climate of the East Asian summer monsoon margin during the Holocene. Darwinula stevensoni was abundant, Ilyocypris spp. scarce, littoral ostracodes absent and Mg/Ca, Sr/Ca and δ¹⁸O were low 11,100 to 8300 yr ago, indicating high lake levels and cool/fresh waters. Darwinula stevensoni declined largely, Ilyocypris spp. throve, littoral ostracodes were rare and chemical indicators remained in low values 8300 to 6200 yr ago, suggesting that the lake continued high stands but water became warm. The lake then contracted and water became cool/brackish 6200 to 4300 yr ago. Littoral ostracodes flourished 4300 to 3350 yr ago, marking the lowest lake levels of the entire Holocene. The lake level recovered and water salinity decreased 3350 to 1900 yr ago. From 1900 to 500 yr ago, the lake maintained the preceding status albeit lowered stands and increased salinities 1100 to 800 yr ago. During the recent 500 yr, the lake expanded and water salinity decreased. The data imply that the East Asian summer monsoon did not intensify until 8300 yr ago and weakened dramatically 4300 to 3350 yr ago.     

东亚夏季风水汽输送带及其对中国大暴雨与洪涝灾害的影响

东亚夏季风每年给中国东部地区带来充沛的降水，是中国水资源的主要来源，同时也常常给中国造成严重的洪涝灾害。东亚夏季风水汽输送的强度、影响范围和持续性在极端暴雨过程中起着关键的作用。这支夏季风气流的水汽输送带可称为东亚季风水汽输送带，与国际上近期提出的"大气河"概念相近，但又不完全相同。东亚夏季风水汽输送带是东亚夏季风最具地区性的特征，也是东亚地区夏季大暴雨和洪涝的制造者。本文根据近百年来的资料，综合评述了东亚夏季风水汽输送带的特征和形成原因，并以海河、黄河、淮河与长江近百年最强的5次持续大暴雨过程为例，分析了季风水汽输送带的重要作用。最后，提出气候变暖可以通过4个方面影响全球水循环，包括气候变暖后大气可容纳更多的水汽、大气环流发生变化、辐射强迫改变以及气溶胶影响的区域性等，这些变化都会对季风水汽输送带产生重要影响。     

Peatland GDGT records of Holocene climatic and biogeochemical responses to the Asian Monsoon

Branched and isoprenoidal glycerol dialkyl glycerol tetraether (GDGT) membrane lipids have been widely used to reconstruct past climate and environmental change. They are not, however, widely applied to peat deposits and the controls on their distributions in peats remain unclear. Here, we present a high resolution record of branched and isoprenoid GDGT concentrations and distributions from a peat core from the Tibetan Plateau that spans the last 13 kyr, a period characterised by distinct dry and wet periods in the region. The lowest concentrations of total branched glycerol dialkyl glycerol tetraethers (brGDGTs) occurred during a presumably dry interval in the mid-Holocene, suggesting that brGDGTs-producing bacteria are less productive under such conditions, perhaps reflecting their putative anaerobic ecology. The mean annual air temperature (MAT) estimates derived from the methylation index of brGDGTs and cyclisation ratio of brGDGTs (MBT′/CBT) are higher than present mean annual temperature in the region and closer to summer temperatures, perhaps due to seasonal production of brGDGTs. The downcore distributions of isoprenoidal and branched GDGTs are dominated by GDGT-0 and brGDGT II, respectively. The high fractional abundances of GDGT-0 in warm and especially wet intervals suggest that these conditions are favourable for some groups of methanogenic archaea. The mid-Holocene dry interval is associated with an increase in the fractional and absolute abundance of crenarchaeol, which could be indicative of enhanced ammonia-oxidising archaeal-mediated nitrogen cycling under these conditions. Taken together, variations of GDGT concentrations in peats appear to document the response of microbial processes to climate change and variations in the biogeochemical environment.     

Palynological records of Holocene monsoon change from the Gulf of Tonkin (Beibuwan), northwestern South China Sea

Palynological records in cores C4 and B106 from the Gulf of Tonkin reveal signals of paleo-monsoon and paleoenvironmental change during the late Pleistocene and Holocene. Before ∼ 13.4 cal kyr BP, the Gulf of Tonkin was exposed to the atmosphere and covered by grassland. Starting at ∼ 11.7 cal kyr BP, the Gulf of Tonkin was inundated by brackish water, indicated by the appearance of the brackish algae Cleistosphaeridium, Sentusidinium and Spiniferites, a decrease of herb content, and an increase of Pinus. After Hainan Island was completely separated from the Leizhou Peninsula by Qiongzhou Strait at ∼ 8.5 cal kyr BP, a continuous marine sedimentary environment was found. The current patterns were similar to those of the present, with a general trend of current homogenization reflected by gradually decreasing quantities of Quercus pollen and a narrowing gap between the palynological concentrations of the southern and northern parts of the region. The data suggest that three short periods of strengthened winter monsoons and currents were centered at ∼ 6.0 cal kyr BP, ∼ 2.7 cal kyr BP and ∼ 0.2 cal kyr BP and that two short periods of strengthened summer monsoons and currents were centered at ∼ 7.5 cal kyr BP and ∼ 3.4 cal kyr BP.     

西太平洋深海沉积物记录的～80ka以来风尘物质输入与东亚冬季风强度

大洋沉积物中的风尘记录良好地揭示了风尘源区干旱化过程和季风强度变化。以位于西太平洋采薇海山附近的柱状沉积物(MABC19孔)为研究对象,在基于地磁场相对古强度对比获取年代框架的基础上,从沉积物粒度组分和磁学参数中提取研究区~80ka以来风尘物质的记录。沉积物高矫顽力磁性矿物参数结果指示,研究区风尘输入量自~80ka以来总体呈逐渐增加趋势,在MIS3/MIS2转换阶段风尘输入量增加明显,揭示了风尘源区古气候条件的转变。通过碎屑沉积物粒径—标准偏差方法提取代表风尘输入的敏感组分,其平均粒径记录了东亚冬季风强度在MIS3/MIS2转换阶段显著增大,该记录与黄土地区风尘记录指示的东亚冬季风强度变化在MIS3阶段高度一致,在其余时段存在着差异,初步推测这是由于高空风尘输送机制的海、陆差异所致。