,1,2, 杨萍果,1, 姚娇2, 张鹏2, 张建辉2, 孙鹏飞3, 敖红2Environmental magnetic record of East Asian summer monsoon variability on the Chinese Loess Plateau since 16 ka BP
LIANG Xiao,1,2, YANG Pingguo,1, YAO Jiao2, ZHANG Peng2, ZHANG Jianhui2, SUN Pengfei3, AO Hong2通讯作者:
收稿日期:2020-01-6修回日期:2020-12-6网络出版日期:2021-03-25
| 基金资助: |
Received:2020-01-6Revised:2020-12-6Online:2021-03-25
| Fund supported: |
作者简介 About authors
梁潇(1995-), 女, 山西襄汾人, 硕士生, 环境磁学及古环境方向。E-mail:
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梁潇, 杨萍果, 姚娇, 张鹏, 张建辉, 孙鹏飞, 敖红. 16 ka以来黄土高原东亚夏季风变化的环境磁学记录. 地理学报[J], 2021, 76(3): 539-549 doi:10.11821/dlxb202103004
LIANG Xiao, YANG Pingguo, YAO Jiao, ZHANG Peng, ZHANG Jianhui, SUN Pengfei, AO Hong.
1 引言
第四纪气候变化是古气候研究的重要方向,它对人类生活和社会发展具有重要影响[1]。中国北方的黄土高原（33°N~41°N、100°E~114°E,面积约40万km2）沉积了连续完整的晚渐新世至上新世风成红粘土和第四纪风尘黄土序列,记录了中国北方25000 ka以来详细的环境演化历史,为研究过去气候环境演化及其动力学机制提供了理想载体[2,3,4,5,6,7]。20世纪80年代以来,国内外****对中国黄土高原开展了大量研究,尤其是第四纪黄土已成为与深海沉积物和极地冰芯并列的过去全球气候变化研究的三大支柱之一[8]。与极地冰芯和深海岩芯不同,黄土高原位于人类过去和现在正在居住的陆相地表,其气候环境变化,尤其是全新世的变化对预测人类活动影响下的中国北方气候在全球持续变化背景下的未来变化趋势至关重要[9]。以往对黄土高原开展的研究主要集中在六盘山以东的黄土高原地区,而对六盘山以西的黄土开展高分辨率古气候研究相对薄弱[4, 10]。黄土高原西部的黄土序列具有比黄土高原中部和东部的黄土序列更高的沉积速率[11],是进行高分辨古气候研究的良好材料。环境磁学是一门综合性的交叉学科[12],磁性矿物的形成、搬运、沉积过程都会受到气候和环境过程的控制,因而沉积物的岩石磁学性质可作为环境和气候变化过程的有效代用指标[13,14]。20世纪80年代以来,环境磁学在中国黄土—古土壤序列的古气候研究方面具有长足深远的意义[15],特别是发现第四纪黄土—古土壤序列的磁化率所代表的气候变化与深海沉积物氧同位素结果能很好对比[16],有力地推动了中国第四纪和过去全球变化研究的发展,将中国黄土古气候和环境磁学推向国际全球古气候变化研究的前沿。本文选择黄土高原西部兰州地区的晚更新世至全新世的黄土序列开展高分辨率环境磁学研究,探讨16 ka以来东亚季风的演化特征及其动力学机制。
2 研究区概况及实验材料
2.1 研究区域
黄土高原位于东亚夏季风降雨影响的西北边缘区,因此对季风降雨异常敏感。受夏季风的影响,黄土高原地区年平均降雨量从东南部的~600 mm向西北方向逐渐较少到约~200 mm（图1）。黄土高原的风成堆积厚度从东南到西北逐渐变厚,粒度逐渐变粗,反映出黄土堆积主要受到冬季风控制。古土壤层的发育主要受夏季风降雨影响,强度从东南到西北逐渐减弱[4, 17-18]。图1
新窗口打开|下载原图ZIP|生成PPT图1黄土高原和平镇位置图
Fig. 1Chinese Loess Plateau and the location of Hepingzhen
本文选取黄土高原西部兰州地区的和平镇黄土剖面（36°00′43″N, 103°58′30″E）开展研究（图1）。该区为干旱—半干旱温带大陆性气候,年均温为6.7 °C,年平均降雨量为350 mm,蒸发量1450 mm[19]。剖面厚度为3.5 m,剖面上部为质地紧密、颜色发黑的特征性土壤层（黑垆土S0）,该层土壤受风化影响较大;剖面下部为质地疏松均一的灰黄色粉砂质黄土。本文采集了整个S0古土壤层（2.6 m）和黄土层L1上部的0.9 m开展研究。将剖面明显风化的表土移除后,从耕作层底部开始以1 cm间隔采集散样350件。在实验室内40 oC条件下将样品烘干,装入磁学专用样品盒进行磁化率测试。
2.2 实验方法
采用捷克AGICO公司生产的AGICO MFK1 FA旋转型卡帕桥多频磁化率仪对所有样品进行低频（976 Hz）磁化率（χlf）和高频（15616 Hz）磁化率（χhf）测试,并计算频率磁化率（χfd = χlf - χhf）。根据磁化率变化特征选取代表性样品进行详细的岩石磁学实验。磁化强度（J）随温度变化曲线（J-T）采用美国Quantun Design公司生产的MPMS3超导磁性测量系统测量,样品用量约0.15 g,测试温度范围为300~1000 K,为了避免在加热过程中样品被氧化,整个实验在氦气环境下进行。低温磁性测量也是在MPMS3超导磁性测量系统上完成。本文测量了极低温（1.8 K左右）饱和等温剩磁在零场中的升温曲线。将样品在零场中从室温降到1.8 K,施加一个强度为5 T的外场,使得样品获得饱和剩磁（Saturated Isothermal Remanence, SIRM）,然后撤销外场,测量在零场中从1.8 K升温到400 K过程中饱和剩磁的变化特征。此外,利用MicroMagTM 3900 VSM型振动磁力仪测试了样品的磁滞回线和FORC（First Order Reversal Curve）图,测量所加最大磁场为1 T,测量间隔为5 mT,在去除顺磁性影响后,得出饱和磁化强度（Ms）、饱和剩磁（Mrs）和矫顽力值（Hc）。上述所有实验均在中国科学院地球环境研究所环境磁学实验室完成。3 结果
3.1 岩石磁学
磁化强度与磁性矿物的种类和含量有关,与磁性矿物颗粒的大小无关。J-T曲线通过磁性矿物在加热和冷却过程中磁化强度随温度变化的特征来识别磁性矿物的种类。和平镇黄土剖面代表性样品的J-T曲线测试结果如图2所示。所有样品的冷却曲线均高于加热曲线,表明样品在加热过程有新的亚铁磁性矿物的产生[13, 20]。~400 °C前磁化强度的逐渐降低与黄土沉积物中磁赤铁矿的分解有关[21]。当加热到~580 °C,磁化强度均显著降低并出现明显的转折,指示磁铁矿是该剖面黄土样品磁化强度最主要的贡献者。由于赤铁矿的饱和磁化强度比磁铁矿低3个数量级[22],因此赤铁矿在J-T曲线上的特征通常会被磁铁矿的信号所掩盖[23],因此难以显示出来。图2
新窗口打开|下载原图ZIP|生成PPT图2和平镇黄土剖面代表性样品磁化强度随温度变化曲线
Fig. 2J-T curves for selected samples from the Hepingzhen loess section
样品的低温磁学结果表明,从1.8 K升温至400 K过程中,样品的饱和剩磁呈逐渐降低趋势,并在~125 K显示出加速降低的特征,这对应磁铁矿在~125 K的Verwey Transition[24]（图3）。因此,与样品的J-T曲线测试结果一致,样品的低温磁学结果表明和平镇黄土的磁性由磁铁矿主导。
图3
新窗口打开|下载原图ZIP|生成PPT图3和平镇黄土剖面代表性样品饱和剩磁随温度变化曲线
Fig. 3Low-temperature saturated isothermal remanence variability for selected samples from the Hepingzhen loess section
FORC图对样品中磁性矿物的粒度反映灵敏[25]。在FORC中Hu反映磁性颗粒间相互作用,Hu越大颗粒间相互作用越剧烈,Hc反映样品矫顽力,峰值处的矫顽力通常在一定程度上反映是样品中磁性矿物平均矫顽力[25]。和平镇黄土剖面FORC图（图4）内部沿Hc轴分布闭合等值线,其最大矫顽力值约为15 mT,指示样品中存在单畴颗粒（Single Domain, SD）。外部等值线沿Hu轴的垂直分布均大于40 mT,与假单畴（Pseudo-single Domain, PSD）磁铁矿特征一致[25,26]。FORC图左下部等值线紧靠Hu轴并与Hu轴近似平行,指示样品中含有超顺磁颗粒（Super Paramagnetic, SP）[25]。因此FORC图表明和平镇黄土的磁性由SP、SD和PSD磁铁矿颗粒主导。
图4
新窗口打开|下载原图ZIP|生成PPT图4和平镇黄土剖面代表性样品的FORC图
Fig. 4First order reversal curve (FORC) diagrams for selected samples from the Hepingzhen loess section
磁滞特征可以提供亚铁磁性矿物矫顽力和磁畴状态的信息,能够较好的指示磁性矿物的种类[27]。和平镇黄土剖面代表性样品的磁滞回线测试结果如图5。所有样品磁滞回线在400 mT以前均已闭合,饱和磁化强度不超过100×10-3 Am2/kg,与样品磁性特征由磁铁矿主导是一致的[20]。以上多参数岩石磁学结果表明,和平镇剖面黄土的磁性特征由SP、SD和PSD磁铁矿颗粒主导。
图5
新窗口打开|下载原图ZIP|生成PPT图5和平镇黄土剖面代表性样品的磁滞回线
Fig. 5Hysteresis loops for selected samples from the Hepingzhen loess section
3.2 磁化率变化特征及年代标尺的建立
Heller等首次发现中国黄土—古土壤序列的磁化率变化与深海氧同位素变化存在相关性[15],并提出磁化率可作为气候变化的代用指标,开辟了海陆气候耦合研究的新途径。中国黄土的磁学研究证明,黄土—古土壤的磁化率值与成土作用呈现良好的正相关关系,磁化率的高值和低值分别对应于成土作用强的古土壤层和成土作用相对较弱的黄土层[28]。磁化率作为一个反映夏季风强度变化的气候代用指标,能够指示受东亚季风控制的成土作用强度和降水量的变化,极大地推动了中国黄土古土壤全球变化的研究[29]。和平镇黄土剖面的低频磁化率（χlf）和频率磁化率（χfd）随深度呈相同变化特征（图6）,二者之间的线性相关系数R2达0.99（图7）,这表明该剖面的磁化率和频率磁化率主要反映了成土作用形成的细颗粒（SP、SD和较细的PSD）磁铁矿的含量变化,因此与黄土高原中部和东部黄土的磁化率和频率磁化率类似,都可以用来反映季风降雨的变化特征。和平镇剖面黄土—古土壤序列的高分辨率磁化率和频率磁化率记录反映了与中国南方石笋氧同位素（δ18O）[30]相同的季风变化特征,尤其是发生在末次冰消期的BA暖事件（B?lling-Aller?d Warming, BA）和YD冷事件（Younger Dryas, YD）在磁化率和频率磁化率记录中都具有明显的响应。基于S0和L1的边界年代（~11 ka）[31] 以及磁化率变化与石笋氧同位素（δ18O）变化曲线具有的良好对比性（图6）,本文将磁化率与具有精确年代的石笋氧同位素进行对比,结合剖面沉积速率的变化特征,建立了和平镇剖面黄土—古土壤序列的年代框架（图6）。这一年代框架仅有一个年代控制点（即S0和L1的边界年代~11 ka）,主要基于与具有准确年代的石笋记录进行对比建立年代,因此,这一年代框架在短时间尺度（千年）上存在不确定性,但这并不影响在长时间尺度和变化趋势方面的古气候解释。
图6
新窗口打开|下载原图ZIP|生成PPT图6和平镇黄土剖面沉积速率、低频磁化率、频率磁化率与华南三宝洞石笋δ18O的年代学对比
注：YD表示Younger Dryas冷事件,BA表示B?lling-Aller?d暖事件;δ18O数据来自文献[30]。
Fig. 6Stratigraphic variations of sedimentation rate, low frequency susceptibility, and frequency-dependent susceptibility for the Hepingzhen loess section, and their comparison with the Sanbao cave δ18O record from South China
图7
新窗口打开|下载原图ZIP|生成PPT图7和平镇黄土剖面χfd和χlf线性关系
Fig. 7The linear relationship between χfd and χlf in the Hepingzhen loess section
和平镇黄土剖面磁化率和频率磁化率记录一致,两者的值都非常低,表明在末次冰期（16~11.7 ka）,黄土高原西部的夏季风降雨较少,成土作用较弱。随着末次冰期北半球冰盖的逐渐融化,夏季风降雨、磁化率和频率磁化率的值都逐渐增加,并在BA暖期形成显著的磁化率和频率磁化率峰值。然而,随着YD的到来,磁化率和频率磁化率明显降低,反映了夏季风降雨的减少。经历YD冷期后,磁化率和频率磁化率值都进一步升高,并在早全新世达到峰值,反映出夏季风降雨达到最高值。从早全新世至现今,磁化率和频率磁化率都呈现显著的减弱趋势,表明夏季风降雨从早全新世开始呈逐渐减弱趋势。
4 讨论
和平镇黄土磁化率（图8a~8b）与中国南方石笋氧同位素（图8c）[30]在过去16 ka以来的变化相同,表明二者在这一时期都较好地反映了东亚夏季风降雨的特征。二者与北半球太阳辐射（图8d）[30]、北极温度（图8e）[32]和南海海表温度（Sea Surface Temperature, SST, 图8f~8g）[33,34]对比,表明东亚季风的驱动机制在11 ka发生了变化。16—11 ka间东亚季风降雨的增加趋势可能受到北半球太阳辐射和北半球区域温度的共同影响,而在大约12 ka的YD冷期,明显受到此时北半球温度降低的影响,降雨量显著减少。和平镇黄土磁化率与中国南方石笋氧同位素一致,揭示了11 ka以来东亚夏季风降雨的减少趋势主要受到太阳辐射降低的影响。通常,太阳辐射增加和北半球温度的增加会导致印度洋和西太平洋形成更多的水汽,这些水汽通过夏季风环流从低纬海洋带入东亚大陆季风区,使得季风降雨可以深入到中国北方地区,包括黄土高原[35]。太阳辐射增加和北半球温度的增加还会导致海陆热力差异增大,进而增强夏季风环流[29],使其可以输送更多的水汽到季风降雨区[36,37]。现代大气研究表明,太阳辐射增加和北半球温度的增加还会促进热带辐合带、哈德利环流圈的北部上升,以及夏季风前缘向西北扩展[37,38,39]。另外,太阳辐射和北半球温度的变化还会影响西风和冬季风的强度及其在黄土高原的作用时间,进而影响夏季风的强度和作用时间[40]。因此,从和平镇黄土记录可以看出,东亚季风在不同时间段和不同时间尺度上的变化机制可能是不同的,在探讨其过去变化时,需要通过详细的区域和全球对比,对具体情况进行具体分析。不宜认为东亚季风系统是对太阳辐射或区域温度的线性响应,尤其是区域温度、季风降雨和太阳辐射在特定阶段可能并不是耦合的。例如,在大约12 ka的YD冷期,东亚季风降雨的减少和区域温度的降低与较高的太阳辐射就是呈现出不耦合关系（图8）。在11 ka以来,南海海表温度的增加趋势和稳定的北极温度与太阳辐射的减少趋势也呈现不耦合关系,而此时东亚季风降雨的减少却与太阳辐射的减少是耦合的（图8）。后续结合气候模拟的综合研究将有助于更加准确的揭示太阳辐射和北半球温度在不同时间段和不同时间尺度上对东亚季风系统的不同影响。图8
新窗口打开|下载原图ZIP|生成PPT图816 ka以来北半球的古气候记录
注：a为和平镇黄土剖面的低频磁化率;b为平镇黄土剖面的频率磁化率;c为华南三宝洞石笋的δ18O[30],d为40°N的太阳辐射[30],e为格林兰冰芯的δ18O[32],f为钻孔GIK17940的海表温度[33],g为钻孔MD972151的海表温度[34]。
Fig. 8The composite paleoclimatic record for the Northern Hemisphere since 16 ka
本次开展研究工作的和平镇黄土剖面位于东亚季风区的边缘,沉积速率较高,甚至达~1 m/ka（图6）,记录了~16 ka至11 ka夏季风降雨的显著增强,清晰识别出BA暖事件和YD冷事件,以及11 ka以来夏季风的持续减弱现象。尤其是和平镇磁化率记录可以与中国南方的石笋记录进行对比,显示了黄土在高分辨率古气候重建方面具有的巨大潜力。以往在黄土高原中部和东部的研究认为,黄土很难记录BA暖事件和YD冷事件,这可能是这些黄土区域的分辨率不够高引起的。在黄土高原西部沉积速率更高和连续性更好的黄土剖面,例如本文涉及到的兰州地区的黄土,是完全可以较好地记录千年尺度的古气候事件的。尤其是以前在黄土高原中部和东部开展的研究,都无法记录类似石笋表现出的20 ka周期特征[41,42,43],这可能也与黄土高原中部和东部黄土的沉积速率较低有关。而最近在黄土高原西部,具有高沉积速率的靖远和古浪黄土就有效地揭示了石笋表现出的2 ka夏季风周期变化特征,虽然其中也含有石笋不具有的40 ka和100 ka周期[44,45]。目前,如何解释中国南方石笋δ18O记录不存在40 ka和100 ka周期,仍然是石笋和古气候研究领域的重要科学问题。其中一种解释是石笋在轨道时间尺度的变化主要受太阳辐射影响,受北半球冰量的影响较少[46]。黄土高原中部和东部的黄土记录由10万年周期主导,而缺乏40 ka和20 ka周期[43],可能是该区域黄土的沉积速率较低引起的。而黄土高原西部沉积速率较高的黄土记录了100 ka、40 ka和20 ka的组合周期则可能反映了太阳辐射、北半球冰量和大气CO2的综合影响[44,45]。
5 结论
和平镇黄土剖面的磁性矿物以成土作用形成的SD、SP和较细PSD颗粒的磁铁矿为主,磁化率和频率磁化率的变化与黄土高原中部和东部黄土序列的磁化率和频率磁化率类似,可以用来反映季风降雨的变化特征。根据S0和L1的边界年代以及高分辨率磁化率与石笋氧同位素变化特征的精细对比,建立了16 ka以来黄土高原西部东亚夏季风降雨的变化历史,确认了黄土高原西部对末次冰期BA暖事件和YD冷事件具有显著响应,探讨了16 ka以来东亚夏季风降雨的驱动机制,即16—11 ka间东亚季风降雨的变化受到北半球太阳辐射和北半球区域温度的共同影响,而11 ka以来的夏季风降雨减少主要受到太阳辐射降低的影响。致谢：
感谢中国科学院地球环境研究所古地磁实验室权春艳硕士在实验过程中给予的帮助。参考文献 原文顺序
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被引期刊影响因子
DOI:10.1016/j.quascirev.2016.08.004URL [本文引用: 1]
DOI:10.1016/0033-5894(91)90015-WURL [本文引用: 1]
DOI:10.1016/0341-8162(91)90012-MURL [本文引用: 1]
DOI:10.1016/j.epsl.2009.10.038URL [本文引用: 3]
[本文引用: 1]
[本文引用: 1]
DOI:10.1007/s11430-010-4126-5URL [本文引用: 1]
[本文引用: 1]
[本文引用: 1]
[本文引用: 1]
DOI:10.1016/j.quaint.2012.06.015URL [本文引用: 1]
DOI:10.1038/NGEO1326URL [本文引用: 1]
The last glacial period was characterized by abrupt, millennial-scale climate change. These climate fluctuations are particularly pronounced in records of the East Asian monsoon system(1-8), and seem to be linked to changes in North Atlantic circulation. Here we present records of grain size variations from the northwestern Chinese Loess Plateau, dated using optically stimulated luminescence. We reconstruct changes in the strength of the East Asian winter monsoon over the past 60,000 years and find reconstructed millennial-scale variations that are broadly correlated with temperature variations over Greenland, suggesting a common forcing. We investigate the effect of a slow-down of Atlantic meridional overturning circulation on the monsoon system using a coupled climate model simulation with added freshwater flux into the northern North Atlantic, and find a strengthening winter monsoon circulation over the regions that supply dust to the Loess Plateau and a reduction in summer monsoon precipitation over East Asia. We conclude that Atlantic meridional overturning circulation is a driver of abrupt change in the East Asian winter and summer monsoon systems, and that the northern westerlies play a role in transmitting this signal from the North Atlantic to the Asian monsoon regions.
DOI:10.11928/j.issn.1001-7410.2016.05.09URL [本文引用: 1]
8 loess and MBB.The recording positions of MBB are quite different in various studies.In northwest of the CLP, the recorded position is located near the top of the L8 loess or even the bottom of S7 paleosol, while it is located near the bottom of the L8 or L8/S8 transition zone in the southeastern part of the CLP.Spatial distribution characteristics of representative cross-sections of the MBB showed that the recording position of MBB is closely related to the pedogenic intensity and sedimentary rate of L8 loess.Sequences from northwest of the CLP present a high sedimentary rate, L8 loess is less affected by pedogenesis.Comparatively, sequences from the central and southeastern CLP have relatively low sedimentary rate, and L8 loess is significantly affected by pedogenesis, which showed some diachronism characteristics.MBB recording position is sensitive to pedogenesis.Significant shift of MBB recording position could be induced by small changes in the thickness of L8 loess.We therefore believed that the MBB recording positions in different areas of the CLP were influence by the intensity of pedogenesis which furtherly influence the stratigraphic division of loess-paleosol sequence, and leading to the regional differences of MBB recording position.In addition, overlapping of Lock-in effect and magnetic field information recorded during the polarity transitional period amplified the regional difference of the recording position of MBB.]]>
[本文引用: 1]
[本文引用: 1]
[本文引用: 2]
DOI:10.11821/xb201001005URL [本文引用: 1]
Loess is widely distributed in Chinese Loess Plateau,and the aeolian sediments have been regarded as one of the most vigorous terrestrial records of paleoclimatic changes during the Quaternary.So far,a large number of investigations have been carried out on palaeoenvironmental changes.However,origination,transportation,and deposition of the eolian sediments,as well as the erosion after deposition,have not been well understood.In this study,we examine 84 individual representative loess-paleosol sections collected from our measurements and literatures,and estimate the total dust deposition amounts and average mass accumulation rates on the Loess Plateau of each glacial or interglacial period over the past 250 ka by remote sensing analysis and Kriging interpolation method.The results provide new evidence for the relation between regional dust deposition and orbit-scale climate changes:the dust accumulation rate is higher in the glacial periods than that in the interglacial periods.Furthermore,based on the modern observation,we estimate average surface erosion rates on the Loess Plateau in each glacial or interglacial period over the past 250 ka as well.The primary results show that there exists relatively intense erosion on the Loess Plateau in the recent geological past,and the surface erosion in the glacial periods is mildly stronger than that in the interglacial periods.
[本文引用: 1]
[本文引用: 2]
DOI:10.1016/S0012-8252(01)00044-7URL [本文引用: 1]
[本文引用: 1]
[本文引用: 1]
[本文引用: 1]
[本文引用: 1]
[本文引用: 1]
[本文引用: 1]
DOI:10.1111/gji.2009.177.issue-3URL [本文引用: 2]
[本文引用: 1]
[本文引用: 1]
DOI:10.1016/j.epsl.2016.03.028URL [本文引用: 1]
[本文引用: 1]
[本文引用: 4]
URL [本文引用: 1]
The magnetic properties of rocks or sediments and the stability of their natural remanent magnetism are determined by the composition and grain size of the magnetic minerals. However, because of the complexity and variety of natural magnetic minerals, how to effectively identify the composition and grain size of the magnetic minerals is always the foundation and difficulty of rock magnetic studies. This paper is a detailed review of the approaches which have been developed to identify the composition and grain size of the magnetic minerals. The high and how temperature magnetic properties of common magnetic minerals are described. Finally we will discuss how to use combined technique to characterize the composition and grain size of the magnetic minerals.
[本文引用: 1]
[本文引用: 1]
[本文引用: 1]
DOI:10.1016/j.quascirev.2014.10.026URL [本文引用: 2]
DOI:10.1038/nature18591URLPMID:27357793 [本文引用: 6]
Oxygen isotope records from Chinese caves characterize changes in both the Asian monsoon and global climate. Here, using our new speleothem data, we extend the Chinese record to cover the full uranium/thorium dating range, that is, the past 640,000 years. The record's length and temporal precision allow us to test the idea that insolation changes caused by the Earth's precession drove the terminations of each of the last seven ice ages as well as the millennia-long intervals of reduced monsoon rainfall associated with each of the terminations. On the basis of our record's timing, the terminations are separated by four or five precession cycles, supporting the idea that the '100,000-year' ice age cycle is an average of discrete numbers of precession cycles. Furthermore, the suborbital component of monsoon rainfall variability exhibits power in both the precession and obliquity bands, and is nearly in anti-phase with summer boreal insolation. These observations indicate that insolation, in part, sets the pace of the occurrence of millennial-scale events, including those associated with terminations and 'unfinished terminations'.
DOI:10.1016/S0012-821X(01)00416-2URL [本文引用: 1]
DOI:10.1126/science.aao2473URLPMID:29472480 [本文引用: 2]
Stratification of the deep Southern Ocean during the Last Glacial Maximum is thought to have facilitated carbon storage and subsequent release during the deglaciation as stratification broke down, contributing to atmospheric CO2 rise. Here, we present neodymium isotope evidence from deep to abyssal waters in the South Pacific that confirms stratification of the deepwater column during the Last Glacial Maximum. The results indicate a glacial northward expansion of Ross Sea Bottom Water and a Southern Hemisphere climate trigger for the deglacial breakup of deep stratification. It highlights the important role of abyssal waters in sustaining a deep glacial carbon reservoir and Southern Hemisphere climate change as a prerequisite for the destabilization of the water column and hence the deglacial release of sequestered CO2 through upwelling.
DOI:10.1016/j.quascirev.2019.05.004URL [本文引用: 2]
DOI:10.1016/j.palaeo.2005.11.033URL [本文引用: 2]
DOI:10.1007/BF02657936URL [本文引用: 1]
Primarily based on the 1979 FGGE data an analysis is made of the circulation differences between the East-Asian and Indian summer monsoons together with their oscillation features and also the interplay be-tween various monsoon systems originating from the fact that the Asian monsoon area is divided into the East-Asian and Indian regions, of which the former is demarcated into the Nanhai (the South China Sea) and the Mainland subregions.
DOI:10.1175/JCLI3990.1URL [本文引用: 1]
DOI:10.1073/pnas.1504688112URLPMID:26460029 [本文引用: 2]
40% in the southeast. The spatial pattern of C4 biomass in both the LGM and the mid-Holocene closely resembles that of modern warm-season precipitation, and thus can serve as a robust analog for the contemporary East Asian summer monsoon rain belt. Using the 10-20% isolines for C4 biomass in the cold LGM as a reference, we derived a minimum 300-km northwestward migration of the monsoon rain belt for the warm Holocene. Our results strongly support the prediction that Earth's thermal equator will move northward in a warmer world. The southward displacement of the monsoon rain belt and the drying trend observed during the last few decades in northern China will soon reverse as global warming continues.]]>
DOI:10.1130/G34488.1URL [本文引用: 1]
Paleoclimatic research can provide critical insight on causes of change in the East Asian monsoon, which influences the lives of 1.6 billion people today. In this study, we use paleo-climatic indexes from Chinese loess deposits, which have clear climatic implications and are independently dated, to reconstruct the monsoon precipitation since 21 ka. Our results show that monsoon precipitation persistently decreased from 21 ka to ca. 8 ka, and increased after ca. 8 ka, with a precipitation peak at 8-3 ka. These changes in East Asian summer monsoon precipitation are synchronous with changes in high-northern-latitude ice volume/ice cover and atmospheric CO2. These new data suggest that variation of the monsoon precipitation was probably driven by CO2-forced high-northern-latitude temperature changes, shifting the location of the intertropical convergence zone that dominates monsoon precipitation. Our TraCE-21000 modeling experiment supports this interpretation.
DOI:10.1038/nature13636URL [本文引用: 1]
Rainfall on Earth is most intense in the intertropical convergence zone (ITCZ), a narrow belt of clouds centred on average around six degrees north of the Equator. The mean position of the ITCZ north of the Equator arises primarily because the Atlantic Ocean transports energy northward across the Equator, rendering the Northern Hemisphere warmer than the Southern Hemisphere. On seasonal and longer timescales, the ITCZ migrates, typically towards a warming hemisphere but with exceptions, such as during El Nino events. An emerging framework links the ITCZ to the atmospheric energy balance and may account for ITCZ variations on timescales from years to geological epochs.
DOI:10.1038/srep00619URL [本文引用: 1]
DOI:10.1038/nature11493URL [本文引用: 1]
Knowledge of the past variability of climate at high northern latitudes during astronomical analogues of the present interglacial(1) may help to inform our understanding of future climate change. Unfortunately, long-term continuous records of ice-sheet variability in the Northern Hemisphere only are scarce because records of benthic O-18 content represent an integrated signal of changes in ice volume in both polar regions(2). However, variations in Northern Hemisphere ice sheets influence the Siberian High(3) (an atmospheric pressure system), so variations in the East Asian winter monsoon (EAWM)-as recorded in the aeolian dust deposits on the Chinese Loess Plateau-can serve as a useful proxy of Arctic climate variability before the ice-core record begins. Here we present an EAWM proxy record using grain-size variations in two parallel loess sections representative of sequences across the whole of the Chinese Loess Plateau over the past 900,000 years. The results show that during periods of low eccentricity and precessional variability at approximately 400,000-year intervals, the grain-size-inferred intensity of the EAWM remains weak for up to 20,000 years after the end of the interglacial episode of high summer monsoon activity and strong pedogenesis. In contrast, there is a rapid increase in the EAWM after the end of most other interglacials. We conclude that, for both the 400,000-year interglacials, the weak EAWM winds maintain a mild, non-glacial climate at high northern latitudes for much longer than expected from the conventional loess and marine oxygen isotope records. During these times, the less-severe summer insolation minima at 65 degrees N (ref. 4) would have suppressed ice and snow accumulation, leading to a weak Siberian High and, consequently, weak EAWM winds.
DOI:10.1016/j.quascirev.2005.07.005URL [本文引用: 1]
[本文引用: 2]
DOI:10.1038/s41467-019-13786-yURLPMID:31862888 [本文引用: 2]
The HML2 (HERV-K) group constitutes the most recently acquired family of human endogenous retroviruses, with many proviruses less than one million years old. Many maintain intact open reading frames and provirus expression together with HML2 particle formation are observed in early stage human embryo development and are associated with pluripotency as well as inflammatory disease, cancers and HIV-1 infection. Here, we reconstruct the core structural protein (CA) of an HML2 retrovirus, assemble particles in vitro and employ single particle cryogenic electron microscopy (cryo-EM) to determine structures of four classes of CA Fullerene shell assemblies. These icosahedral and capsular assemblies reveal at high-resolution the molecular interactions that allow CA to form both pentamers and hexamers and show how invariant pentamers and structurally plastic hexamers associate to form the unique polyhedral structures found in retroviral cores.
DOI:10.1016/j.quascirev.2015.03.009URL [本文引用: 2]
DOI:10.1126/science.214.4516.59URLPMID:17802573
Values for the precession and obliquity of the earth 9000 years ago indicate that the global average solar radiation for July 9000 years ago was 7 percent greater than at present. When the estimated solar radiation values are used in a low-resulation climate model, the model simulates an intensified continent-scale monsoon circulation. This result agrees with paleoclimatic evidence from Africa, Arabia, and India that monsoon rains were stronger between 10,000 and 5000 years ago than they are today.
