第四纪研究  2017, Vol.37 Issue (5): 963-973   PDF    
中国黄土记录的季风快速变化
孙有斌 , 郭飞①,②     
(① 中国科学院地球环境研究所, 黄土与第四纪地质国家重点实验室, 西安 710061;
② 中国科学院大学, 北京 100049)
摘要:中国黄土高分辨率代用指标研究表明东亚季风气候存有亚轨道尺度的快速波动,与北半球高纬气候和大西洋经向环流变化关系密切。本文回顾前人在黄土高原地区多个典型剖面的研究结果,分末次冰期、最后两个冰期旋回及极端冰期(L9和L15)3个时段,阐述了粒度、元素比值、风化指数等代用指标揭示的冬、夏季风快速变化特征,并与冰芯、石笋和深海记录进行对比,探讨了季风突变事件的区域一致性和遥相关动力过程。结果表明,黄土高原西北部高分辨率黄土剖面代用指标对季风快速变化敏感,而东南缘黄土记录的突变事件不太清晰,这种差异受控于沉积速率和风化成壤的空间变化;在冰期-间冰期不同气候背景下,黄土粒度和元素比值的变化幅度和周期表现出较大差异,冰期幅度大、频率快,间冰期幅度小、频率慢,表明冰量大小对季风快速变化有调制作用;在两个极端寒冷期(氧同位素阶段22~24和38),黄土粒度同样表现出快速变化特征,但与北大西洋浮冰碎屑记录似乎难以对应,可能与海陆记录的分辨率差异和年代标尺不确定性有关。综上所述,季风快速变化特征在沉积速率相对较高、成壤相对较弱的黄土剖面比较清晰,在冰期-间冰期不同下垫面条件下季风快速变化的特征和机理可能不同。未来的季风快速变化研究,一方面在时间尺度上拓展高分辨率敏感代用指标研究,揭示季风快速变化的起源和演化特征;另一方面加强地质记录和数值模拟结果的对比研究,获得季风快速变化动力机理的全面认识。
主题词中国黄土     东亚季风     代用指标     突变事件     季风动力学    
中图分类号     P534.63;P532                     文献标识码    A

1 引言

中国黄土高原的黄土-古土壤序列记录了2.6Ma以来东亚季风气候经历了37次冷暖旋回,对应了冰期-间冰期尺度上冬、夏季风优势期相互交替的演化历史,与北半球高纬度太阳辐射和冰量变化密切相关[1~6]。叠加在轨道尺度气候变化之上的千年-百年时间尺度快速变化,首先从北半球高纬冰芯和海洋沉积记录中发现,表现为从末次冰期到全新世温度变化的不稳定性和突变性特征[7~11]。相对非常成功的深海沉积和冰芯研究,中国黄土的粒度、磁化率、元素比值、化学风化指数等代用指标研究表明,气候快速变化同样存在东亚季风气候系统中,在揭示亚轨道尺度季风事件的特征和机理研究方面也取得很大进展[12~20]。多个黄土序列(如渭南、洛川、环县、会宁、兰州和西宁等)的粒度、磁化率、土壤颜色和化学风化指数变化表明,亚轨道尺度的冬、夏季风波动清晰地存在于最后两个冰期气候旋回中[12~19],类似的气候事件可能存在于更老的极端寒冷期中[20]。随着冰芯[21, 22]、石笋[23~28]和深海沉积[29, 30]研究的深入,揭示出亚轨道尺度气候波动至少存在于整个中-晚更新世[20~30]。但是,基于中国黄土的高分辨率研究则主要集中于最后两个冰期-间冰期旋回的冬季风快速变化[31~33]

末次冰期高分辨率黄土沉积与格陵兰冰芯和北大西洋深海沉积物记录对比表明,中国黄土同样保存着北大西洋浮冰碎屑(IRD)事件、Dansgaard-Oeschger(DO)旋回、新仙女木(Younger Dryas)事件等气候突变事件的印记,证明中国黄土可记录千年尺度的东亚冬季风快速变化[12, 13, 17~19, 34],类似的千年尺度季风波动也存在于更老的冰期旋回中[14, 15, 20, 35]。中国石笋记录同样表明,东亚夏季风快速变化至少存在于最近几个冰期旋回中,其变化频次至少在最后两个冰期旋回基本一致[24, 26]。尽管东亚冬、夏季风均有明显的千年尺度波动,由于黄土年代标尺的不确定性和石笋氧同位素解释的复杂性等问题,两者的直接对比仍有困难。本文旨在总结前人在中国黄土揭示季风快速变化研究方面取得的进展,重点从末次冰期、最后两个冰期旋回及极端冰期(L9和L15)3个时段,分析季风快速变化的特征、区域相似性及其同北大西洋高纬气候变化的联系,尝试提出未来利用中国黄土开展高分辨率季风快速变化研究的可能方向。

2 材料和方法

在20世纪末期,利用洛川黄土剖面石英粒度变化,首次揭示出末次冰期冬季风多次加强与北大西洋变冷事件的动力遥相关[12, 17]。随后,中国黄土记录的季风快速变化事件研究取得很大进展,多个黄土剖面(如渭南、洛川、西峰、姬塬、靖远、临夏和古浪,见图 1)的代用指标变化表明,亚轨道-千年尺度的冬、夏季风波动存在于最后两个冰期气候旋回中[12, 14, 15, 17, 36],类似的快速变化事件可能同样出现在更老的极端寒冷期中(如L9和L15)[37~39]。本文选择9个典型黄土剖面探讨季风快速变化的时空特征,这些剖面的位置和相关信息如图 1表 1所示。

图 1 黄土高原典型黄土剖面位置图 Fig. 1 Map showing locations of representative loess profiles on the Chinese Loess Plateau

表 1 典型黄土剖面的位置、时间跨度、代用指标和参考文献 Table 1 The location, time span, proxies and references of nine representative loess profiles

中国北方黄土沉积以黄土高原地区最为典型,厚度最大,地层最全。因此,前人研究季风快速变化的典型黄土剖面多分布在该地区(图 1)。黄土高原的黄土沉积厚度从东南到西北厚度逐渐变厚,古土壤层的发育逐渐减弱,典型黄土塬边出露的剖面最为清晰,比如洛川塬和董志塬记录了第四纪完整的黄土-古土壤序列L33~S0[48]。其中,S13、S5和S1等作为古土壤标志层,清晰地记录了增强的间冰期夏季风环流效应;而L33、L15、L9和L1等黄土标志层,记录了冰期干冷的冬季风环流影响[1, 48]。以末次冰期黄土沉积厚度为例,在黄土高原地区从东南向西北逐渐增加,厚度变化范围为8~40m不等。最后两个冰期旋回黄土剖面在黄土高原北部和西部地区沉积厚度大致为60~80m(如兰州、西宁、靖远、古浪等),而在黄土高原东南缘地区约20余米(如洛川、渭南、蓝田等)。东南部黄土剖面通常由于速率低、成壤作用强,古土壤相对发育,不利于保留季风快速变化印记;西北部剖面沉积速率大、成壤作用弱,季风快速变化信息得到较好的保存。

黄土高原连续的第四纪黄土-古土壤序列的地质、生物证据表明[49~53],黄土是干冷的偏北冬季风搬运的粉尘堆积,黄土的粒度指标(石英中值或平均粒径、全样的中值或平均粒径、粗颗粒组分含量)[17, 40, 49, 54]、地球化学指标(Si/Al和Zr/Rb比值等)[55, 56]等可作为冬季风的替代指标。古土壤的发育则与来自大洋暖湿的偏南夏季风密切相关,磁化率[57~59]、土壤颜色[19]、游离铁/全铁[60]、赤铁矿/针铁矿[61, 62]、元素比值(Fe/Al和Rb/Sr)[15, 55, 56]、稳定碳同位素[46, 63~66]、碳酸盐含量[67]、有机质含量[68]、Be含量和化学风化指数[15, 67, 69, 70]等可作为夏季风气候的替代指标。在这些代用指标中,碳同位素变化、化学风化指数、游离铁/全铁比值同降水影响的植被发育和成壤强弱关系密切,可敏感地揭示夏季风强度变化。粒度与不受风化作用影响的某些元素比值,能够敏感响应冬季风的强度变化。前人对最后两个冰期旋回黄土记录的亚轨道尺度季风快速变化开展过多代用指标的研究,粒度和地球化学分析主要集中于风化成壤作用相对较强的黄土高原中南部地区[12, 15, 17, 18],而粒度和同位素指标分析多针对黄土高原西北部地区的黄土序列[31, 33, 40, 59, 63]。相比而言,针对两个极端干冷期季风快速变化的研究相对薄弱,主要集中在黄土高原中部几个经典的黄土剖面,如洛川、西峰和灵台。迄今为止,不同代用指标对冬、夏季风快速变化的敏感性如何?敏感指标变化是否存有时空差异?仍缺乏系统研究。

中国黄土能否记录气候快速变化,受到黄土沉积的连续性和分辨率等问题的制约,一定程度上限制了对中国黄土古气候意义的深入解读。由于荒漠化的作用,沙漠进退可能造成黄土高原北部剖面沉积间断[71, 72],洪水事件可能造成黄土高原南部剖面的侵蚀间断[73],高分辨率释光测年似乎表明黄土也存在沉积间断[74, 75]。此外,通过对比黄土高原西北-东南断面的靖远、西峰、渭南剖面的粒度变化,结果表明风化成壤和表层混合作用,也会影响黄土记录的千年尺度气候事件[40]。但是,在黄土高原平坦塬面上发育的黄土序列,是可以分辨出亚轨道尺度的季风波动[40, 41]。独立的证据来自末次冰期黄土的古地磁和释光测年结果,揭示出古浪、西峰、洛川和渭南等剖面沉积基本连续,清晰地记录了拉尚事件(Laschamp Geomagnetic Excursion),表明黄土能记录持续仅千年的地磁场漂移事件[76, 77]。具有光释光测年控制的高分辨率粒度变化,与石笋和冰芯记录的千年尺度气候波动的良好对应关系,显示黄土高原尤其是西北地区沉积速率较高、风化成壤较弱的黄土剖面[40, 76, 77],适合开展高分辨率多代用指标的研究,有助于更好理解在千年尺度上冬、夏季风变化的特征及机理。

3 黄土记录的季风快速变化 3.1 末次冰期季风快速变化

自末次冰期千年尺度气候快速变化事件在格陵兰冰芯中得到辨识以来,在全球各类沉积中找寻类似IRD事件和DO旋回的气候信号成为国际研究热点[2, 18, 34]。中国黄土的研究也不例外,多个典型剖面的粒度、磁化率等代用指标变化[2, 15, 17, 18, 31, 32, 38, 40~47],表现出与格陵兰冰芯[78]及石笋氧同位素[23, 25]相似的变化特征(图 2)。通过分析洛川黄土剖面中石英颗粒的粒度变化,并与北大西洋浮冰碎屑记录对比,首次揭示出末次冰期东亚冬季风增强与北大西洋变冷的遥相关[2, 17]。随后,渭南、洛川和榆林等地黄土剖面中的风化指数和游离铁/全铁比值变化,表明夏季风减弱也与北大西洋浮冰碎屑记录的冷事件有较好的对比性[5, 79]。对比黄土高原兰州剖面马兰黄土的磁化率、磁学指标、土壤色度与格陵兰冰芯记录,揭示了末次冰期内气候快速变化具有全球性[5, 59];利用黄土-沙漠过渡带4个黄土剖面(榆林蔡家沟、横山石峁、会宁李家塬和定边姬塬)粒度变化与格陵兰冰芯对比,发现两者记录的千年尺度气候事件(包括IRD事件和DO旋回)基本一致[43]。此外,来自祁连山中部的祁连剖面和北缘的沙沟剖面、郑州邙山的高分辨率剖面均显示出末次冰期季风的快速变化特征[80, 44]。需要指出的是,早期研究中黄土发育的时间标尺,多通过与深海氧同位素对比并利用粒度模式建立的[2, 14, 38],因此,在对比气候突变事件的时间上存有一定偏差。

图 2 末次冰期以来典型黄土剖面粒度记录的冬季风快速变化与冰芯[78]和石笋[23, 25]记录对比 实心方块为靖远、古浪和临夏剖面的释光测年结果;数字为DO旋回,灰色条带指示北大西洋浮冰碎屑事件 Fig. 2 Rapid monsoon changes recorded by grain-size proxies of several representative loess profiles and their correlation with speleothem[23, 25] and ice-core[78] records Diamond denotes the OSL dates at the Gulang, Jingyuan and Linxia loess profiles since the last glacial stage. Numbers and grey bars indicate the DO cycles and IRD events in the North Atlantic, respectively

近十年来,随着释光测年精度的提高,针对靖远、古浪和临夏等地开展了释光测年[31, 40, 47],建立了末次冰期黄土发育的独立年代标尺,高分辨粒度变化记录的冬季风加强与北大西洋地区6次浮冰碎屑事件对应,并与石笋和冰芯记录的DO事件具有良好的对应关系(图 2)。这3个剖面的高分辨率光释光测年结果,表明在黄土高原西北部位于粉尘沉积中心的高沉积速率黄土剖面是基本连续的。从空间上看,磁化率在冰期段变化幅度较小、相对平滑,对夏季风突变事件变化不敏感;粒度变化差异很大,从西北部(如靖远、古浪和临夏等)到中东部(如西峰、渭南),粒度记录冬季风快速变化的幅度和频次都逐渐降低。这种亚轨道尺度的粒度空间变化特征表明,黄土高原西北地区风尘粒度对传输动力变化更敏感。由于黄土高原西北地区靠近沙漠地区,沉积速率大,成壤作用弱,能够较好地记录沙漠进退及风力强度的快速变化[40, 76];相反,黄土高原中南部的沉积速率总体较低,沉积后的成壤和表层混合作用均会影响代用指标的敏感性[47, 77]。因此,这种空间差异主要归因于沉积速率和成壤强度变化,影响了不同的代用指标对季风快速变化响应的敏感程度。

3.2 最后两个冰期旋回记录的季风快速变化

尽管中国黄土记录的末次冰期季风快速变化已被广泛认可[2, 15, 31, 59],但在更老时段的季风快速变化特征,仍存有较大争议。就末次间冰期而言,通过分析洛川和三爻剖面S1古土壤的石英粒度变化[8],揭示出了多次冬季风加强事件,即使在深海氧同位素阶段(MIS)5e,仍有几次波动。然而,黄土高原西部兰州、临夏等地高分辨率S1古土壤的磁学参数和碳酸盐含量变化,却表明末次间冰期存在岁差尺度夏季风变化,在MIS 5e时期夏季风最强但没有明显的突变事件 。通过对黄土高原西北部两个高沉积速率黄土剖面的粒度分析,发现倒数第二次冰期同样存在千年尺度的季风快速变化[4]

最近,Sun等[32]通过高分辨率黄土粒度、元素比值和同位素变化的综合研究,揭示了最近两个冰期旋回季风快速变化事件序列与石笋记录的季风快速变化和北大西洋浮冰碎屑事件具有较好的对应关系。另外,通过对黄土高原西北部辛庄塬、李家塬、姬塬、洪德、子长、华池、环县、临夏等多个高分辨率黄土剖面粒度变化曲线的对比和整合发现,最后两个冰期旋回以粒度变化记录的显著变冷千年尺度的气候波动信号与石笋记录有很好的对应关系,大幅度的快速变化在最后两个冰期旋回多出现在最冷干的时期[81, 82]。确认了黄土高原记录的东亚冬季风快速变化,主要受北半球高纬冰盖相关的浮冰碎屑事件影响,北半球高纬气候变化通过西风环流-蒙古高压变化进而影响到东亚季风变化[2, 14, 31]

黄土高原西北部古浪剖面高分辨率的粒度、元素比值和碳酸盐碳同位素变化,分别记录了最后两个冰期旋回冬、夏季风的快速变化的特征[32, 46](图 3)。高分辨率的Fe/K比值与磁化率曲线在冰期变化幅度存在差异,但在间冰期变化趋势基本一致,能更精细记录风化相关的东亚夏季风快速变化[32, 69]。碳酸盐碳同位素记录的夏季风快速变化幅度要高于元素比值记录,是目前发现的该地区对夏季风变化最为敏感的代用指标[46, 64]。高分辨率Si/K比值和粒度记录的东亚冬季风变化与深海氧同位素变化趋势基本一致,古土壤层S2、S1和S0分别对应MIS 7、MIS 5和MIS 1,指示东亚冬季风强度粒度曲线同指示全球冰量变化的深海氧同位素记录有较好的可比性,但表现出更多高频快速变化信号[32]。如图 3所示,黄土多个代用指标揭示的季风快速变化与冰芯、石笋记录中冷暖-干湿波动高度相似,但明显高于北大西洋浮冰碎屑事件的发生频率。最后两个冰期旋回中北大西洋发生了21次浮冰碎屑事件,但古浪地区粒度曲线记录的末次间冰期2次冬季风增强事件,在北大西洋记录中没有明显的浮冰碎屑事件对应,在倒数第二次间冰期中粒度和碳同位素曲线记录了2次明显东亚冬、夏季风快速变化事件,与之对应的大西洋浮冰碎屑事件很弱。

图 3 最后两个冰期旋回黄土代用指标变化[32, 46]与太阳辐射[83]、南极冰芯CH4含量[21]、石笋氧同位素[23, 25]、黄土粒度叠加曲线[82]、北大西洋浮冰碎屑记录[84]和深海氧同位素曲线[85]的对比 灰色条带代表不同代用指标中北大西洋浮冰碎屑事件的对比 Fig. 3 Comparison of loess proxies[32, 46] with summer insolation at 65°N[83], CH4 content of Dome C[21], speleothem δ18O of Sanbao-Hulu caves[23, 25], stacked loess grain-size record[82], IRD records of ODP 983[84] and stacked benthic δ18O record in the last two glacial-interglacial cycles[85] Grey bars indicate the correlation of IRD signals in different proxy records

千年尺度气候事件的变化幅度在不同指标间有明显差异,比如碳同位素变化幅度最大,其次分别为粒度、Si/K和Fe/K比值,归因于这些指标对不同气候要素响应的差异。在冰期-间冰期背景下变化幅度和频率也不尽相同,表明下垫面变化对千年尺度气候波动有显著调制作用[32, 46]。在每个冰期-间冰期旋回中,高分辨率元素比值和粒度记录分别可以辨别出28个变暖事件(图 3),其中末次冰期气候突变事件在黄土、石笋和冰芯中具有很好的对应关系[31]。最后两个冰期中北大西洋地区记录的十多次浮冰碎屑事件在黄土和石笋中也都有响应,但在倒数第二个冰期旋回这些气候突变事件的变化幅度和持续时间在海洋和陆地的沉积记录中对应关系相对较差(图 3),可能是不同时间序列的年代不确定性造成的。黄土、石笋、冰芯和海洋记录对比表明,尽管突变事件是气候系统普遍存在的现象,但在不同载体和不同指标中的印记是有明显差异的,尤其是突变事件发生时间的不同,表明提高地质记录的测年精度是理解气候突变事件的特征和规律的关键。基于石笋记录的高精度年代框架,详细对比突变事件的发生时间,可整合海陆记录建立统一的气候突变事件序列[86]

3.3 L9和L15记录的季风快速变化

在黄土-古土壤序列中记录了若干极端气候事件,如两个粉砂层(L9和L15)对应的极端干冷冰期时段,粒度变化也表现出类似的快速变化[38, 41](图 4)。黄土高原中部洛川和西峰剖面的粒度和变化,揭示出极端干冷的冰期时段东亚冬季风也存在亚轨道尺度波动[38],在L9对应的MIS 22(距今890~860ka)时段内至少存在过9次大幅度变化,MIS 23间冰期时段冬季风强度比较稳定;在L15对应的MIS 38(距今1275~1250ka)时段内冬季风至少有7次加强事件,这些事件持续时间约千年。与冬季风快速变化相比,磁化率指示的夏季风强度变化在暖湿期和干冷期都比较稳定,可能与该指标对季风快速变化的敏感性较差有关[32, 38, 46]

图 4 L9和L15时期洛川粗颗粒含量[38]、灵台和西峰石英平均粒度[41]与深海氧同位素[85]和U1313孔矿物比值[87]对比 洛川剖面年代适当调整到灵台和西峰剖面的天文调谐时间标尺 Fig. 4 Comparison of grain size of L9 and L15 at Luochuan[38], Xifeng and Lingtai[41] profiles with benthic δ18O stack[85] and Quartz/Calcite ratio of ODP site U1313[87] Chronology of Luochuan profile was slightly tuned to the astronomical timescale of the Lingtai and Xifeng profiles

将灵台、洛川和西峰的L9和L15时期的粒度曲线与深海氧同位素[85]、北大西洋U1313钻孔的矿物比值[87]进行对比,L9和L15对应深海氧同位素阶段22~24和38(图 4)。西峰和洛川黄土古土壤序列的粒度变化和深海氧同位素以及U1313孔矿物记录之间存有差异,如极端干冷气候的冬季风加强事件在深海氧同位素记录中没有明显反映。洛川和西峰剖面L9层位高粒度值表明,该时期为相对干冷的气候条件,但与之对应的深海氧同位素记录并不是整个第四纪中最寒冷的冰期,U1313孔石英与方解石的矿物比值表明在该时期北大西洋地区的冰漂碎屑事件记录不明显。U1313孔矿物比值记录了890~880ka时期北大西洋地区冰期的冰漂碎屑事件,但是西峰和洛川等地粒度指示的季风快速变化较弱。尽管西峰、洛川和灵台等地在L15时期的粒度曲线与深海氧同位素具有较好的对应关系,表明MIS 38阶段气候相对干冷,但同样粒度的显著加粗无法仅用与响应北半球冰量的变化控制来解释。

西北地区的沙漠是中国黄土的主要源区,粉尘经由西北冬季风传输运移[88, 89],黄土粒度变化主要取决于冬季风的强度和源区的距离[20, 88]。L9和L15形成于干冷的冰期,但干冷气候和冬季风加强并非是形成这两个粉砂层的唯一因素,因为深海氧同位素揭示的这个两个时段冰量并没有异常增加[85]。寒冷冰期气候条件下沙漠扩展,导致粉尘输入增加对这两个粉砂层的形成起到一定的作用[82];同时,在约1200ka和900ka期间青藏高原及其周边山体发生了快速隆升,导致物理风化作用和河流下蚀作用加强,造成在山前盆地源区碎屑和黄河泥沙的增多,也对两个粉砂层的形成有重要贡献[90, 91]。因此,L9和L15时期黄土粒度的显著变粗,可能是全球变冷和青藏高原区域隆升引起的沙漠扩张和冬季风增强的共同结果[81],记录了中更新世东亚源区干旱化和粉尘排放的阶段性变化[92~95]

4 结论及展望

中国黄土高原的黄土-古土壤沉积序列作为独一无二的陆相古季风气候变化记录,在揭示构造-亚轨道季风气候变化具有不可替代的优势。前人开展了多个黄土剖面高分辨率多代用指标的研究,揭示出末次冰期东亚冬、夏季风的快速变化,同东亚地区石笋记录及高纬地区的海洋和冰芯记录高度相似;最后两个冰期旋回中黄土和石笋记录的季风突变事件数量相当,但幅度和频率在冰期-间冰期不同气候背景下有差异。在中更新世极端寒冷期冬季风存有类似的亚轨道尺度气候波动,但与深海沉积记录的对比似乎比较困难。从动力学角度讲,季风突变事件与高纬地区气候变化关系密切,模拟实验结果表明[31]北大西洋地区淡水注入导致经向环流减弱,可通过北大西洋-东亚大气遥相关过程影响东亚冬、夏季风变化。

季风快速变化何时出现?如何演化?其与高、低纬气候过程的动力联系如何?仍需要更进行深入的研究。未来的季风快速变化研究可围绕以下几个方面展开:1) 寻找更高分辨率的风尘和湖相记录,比如黄土高原西北部有巨厚的第四纪黄土沉积(厚度超过300m),典型盆地有高分辨率的中-上新世湖相沉积,为揭示季风突变事件的起源和演化提供不可多得的素材;2) 开展高分辨率多代用指标分析,尤其是针对一些高质量岩芯开展多参数的扫描研究,提取对季风快速变化的敏感指标,用于重建季风快速变化的特征(如周期和幅度等);3) 加深季风快速变化规律的认知,结合高精度测年石笋氧同位素记录和其他地质记录敏感代用指标变化,分析气候突变事件的变化特征(如转换和持续时间),为理解和应对未来气候突变发生的可能提供科学认知;4) 全面理解季风突变事件的发生机理,通过加强数值模拟和地质记录的对比,利用高分辨率气候模型开展瞬变试验研究,查明在不同冰盖体积、温室气体浓度和轨道参数配置背景下,季风突变事件发生的规律和机理有何异同,区分气候系统内外部因子及高低纬过程对季风快速变化的不同影响。

致谢: 感谢国家杰出青年科学基金项目(批准号:41525008) 和国家自然科学基金面上项目(批准号:41472163) 共同资助。

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Rapid monsoon changes recorded by Chinese loess deposits
Sun Youbin, Guo Fei①,②     
(① State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710061;
University of Chinese Academy of Sciences, Beijing 100049)

Abstract

Suborbital-scale monsoon variations, revealed by multiple proxies of high-resolution loess sequences, are dynamically linked with changes in high-latitude climate and the Atlantic Meridional Overturning Circulation. In this paper, we synthesized previous invesitgation on representative loess profiles(i.e., Luochuan, Weinan, Xifeng, Lingtai, Jiyuan, Mangshan, Jingyuan, Linxia and Gulang profiles)from the Chinese Loess Plateau(CLP)to elaborate the characteristics of rapid monsoon changes over three time intervals:the Last Glacial stage, the last two glacial-interglacial cycles and the two extreme glacial intervals(corresponding to loess units L9 and L15). Rapid variations of winter and summer monsoons are assessed using mean grain size, elemental ratios and chemical weathering index. By comparing loess proxies with indicators from ice core, marine sediments, and speleothem, we addressed the temporal-spatial characteristics of suborbital monsoon variability and their dynamic links to global climate change. It is suggested that abrupt monsoon events are well recorded by proxy indicators of several representative high-resolution loess profiles, matching well with the Heinrich events and Dansgaard-Oeschger cycles revealed by speleothem and ice-core records for the Last Glaciation. Spatially, loess proxies from high-resolution loess profiles in the northwest Chinese Loess Plateau(CLP)are sensitive to rapid monsoon changes, whereas the loess profiles in the southeast CLP are less sensitive to abrupt monsoon oscillations. Such a spatial difference is likely due to two main reasons:varying sedimentation rates and chemical weathering intensities across the CLP. During the last two climatic cycles, the amplitude and periodicities of loess proxies varied at glacial-interglacial timescales, characterized by large-amplitude and high-frequency fluctuations during the glacial stages. During the interglacials, however, relatively small-amplitude and low-frequency oscillations imply an interglacial modulation on the sub-orbital monsoon variability. Moreover, the winter monsoon also fluctuated on suborbital timescales during two cold climatic extremes(corresponding to marine isotope stages 22~24 and 38). However, it is difficult to match sub-orbital winter monsoon variations with the ice rafting events(IRD)in the North Atlantic during these two glaciations, probably due to the resolution discrepancy and chronological uncertainties between terrestrial and marine records. In summary, previous loess studies reveal that high-resolution and weakly weathered loess sequences can well documented rapid monsoon changes. The features and mechanisms of rapid monsoon changes seem dissimilar under different glacial-interglacial boundary conditions and their amplitude and frequencies are likely modulated by varying glacial-interglacial conditions. So far, the characteristics and dynamics of abrupt monsoon changes during the early to Middle Pleistocene remains poorly constrained. In the future, more high-resolution and sensitive loess proxies should be investigated to better understand the onset and evolution of suborbital-scale monsoon variability during the Quaternary or Neogene. Furthermore, data-model integration should be strengthened to provide a thorough assessment of the origin and dynamics of abrupt monsoon changes.
Key words: Chinese loess     East Asian Monsoon     proxy indicators     abrupt monsoon events     monsoon dynamics