三门峡会兴沟剖面黄土—古土壤序列的岩石磁学研究

岩石磁学是古地磁学和环境磁学研究的基础,是鉴定岩石和沉积物中磁性矿物种类、粒度和含量的有效途径。对黄土高原东南部三门峡盆地的水沟—会兴沟旧石器遗址会兴沟剖面黄土—古土壤序列(S_0~S_8)进行系统的岩石磁学研究表明:本剖面沉积物的主要载磁矿物为磁铁矿、磁赤铁矿和赤铁矿,显示准单畴(PSD)磁性颗粒特征。所有磁学参数曲线均表现出基本一致的变化特征,与深海氧同位素曲线能够很好的对应,反映了第四纪以来的冰期—间冰期旋回中,东亚季风影响下的风尘黄土堆积中磁性矿物种类、粒度和含量的周期性变化特征。黄土中高矫顽力磁性矿物的相对含量要高于古土壤中的,而随着成土作用的加强,在古土壤中细粒的低矫顽力磁性矿物显著增加的同时,其中高矫顽力磁性矿物的绝对含量也相应增加。质量磁化率(χ)与非磁滞剩磁磁化率(χ_(ARM))和饱和等温剩磁(SIRM)及磁粒度参数χ_(ARM)/SIRM和χ_(ARM)/χ均呈明显的正相关关系,表明由成土作用产生的单畴(SD)颗粒和较小PSD颗粒对磁化率增强有显著的贡献。     

末次间冰期以来沙漠－黄土边界带的环境演变

沙漠－黄土边界带地处东亚季风边缘，这一地区的风成沉积即古风成砂－黄土－古土壤序列很好地反映了东亚季风环流的演变历史。末次间冰期的沉积(Ｓ１)由三层古土壤和夹于其间的两层黄土组成，可与深海氧同位素阶段５的五个亚阶段(ａ－ｅ)则很好对比。末次冰期的沉积(Ｌ１)由三层古风成砂和夹于其间的两层黄土构成，由此反映了与上述三层古风成砂对应的阶段２、阶段４及阶段３的中期是沙漠－黄土边界带冬季风活动加剧、沙漠扩大的时期。全新世的沉积是由一层草原上(Ｓ０)和其上的流沙组成，但这层流沙是在最近两千年来不合理的人类活动和当地特定风场条件的共同作用下形成，已不具备明显的气候指示意义。     

末次冰消期以来内蒙古东部气候变化——基于风成砂-古土壤序列的地球化学记录

通过对内蒙古东部现代东亚季风界线两侧的浑善达克沙地和呼伦贝尔沙地风成砂-古土壤剖面的光释光、~(14)C年代和沉积物主、微量元素地球化学的分析,重建内蒙古东部末次冰消期以来气候变迁过程。浑善达克沙地的浩来呼热剖面(hsh1-09)显示了末次冰消期以来3次暖湿气候期,分别对应末次冰消期、全新世大暖期、中世纪暖期。呼伦贝尔沙地的嵯岗剖面(CG-09)表现了全新世大暖期和中世纪暖期两次的暖湿气候期。这一过程是对东亚夏季风的响应,大致反映了内蒙古东部末次冰消期以来的东亚夏季风两次不同程度增强及大暖期结束的南退过程。通过对比前人研究结果,全新世中期东亚夏季风较强,在内蒙古东部大约距今4 ka开始减弱、南退;中世纪暖期夏季风强度较弱,持续时间短。     

中国黄土环境磁学

中国陆相风成沉积序列蕴含了晚新生代以来丰富的古地磁场和古气候信息.第四纪黄土-古土壤序列的岩石磁学性质可以揭示第四纪亚洲内陆地区干旱化过程和东亚古季风演化历史以及末次冰期-间冰期旋回的气候不稳定性特征.文章主要评述近年来中国黄土环境磁学研究所取得的一系列重要进展,同时提出目前需要深入研究的几个重要问题,如成土作用对黄土沉积物沉积剩磁的影响、微生物对磁性矿物的改造、亚洲内陆地区气候过程和环境演变的磁气候学记录等.     

末次间冰期以来沙漠-黄土边界带移动与气候变化

位于现代季风区边缘的沙漠－黄土边界带，具有高度不稳定性。末次间冰期以来历经多次北进南退移动和暖湿、冷干变化。依据古风成砂－黄土－古土壤叠覆更替的沉积序列和磁化率等气候代用指标分析，对末次间冰期以来，尤其是特征时期边界带的位置进行了讨论。其中，盛冰期时移动幅度最大，南界可能达３０°Ｎ左右；末次间冰期和全新世气候鼎盛期最靠西北，南界在古长城以北。全球冰期－间冰期波动导致的气候变化以及东亚冬夏季风强弱变化是控制边界带移动和气候变化的根本因素。     

末次间冰期以来沙漠－黄土边界带的环境演变

沙漠－黄土边界带地处东亚季风边缘，这一地区的风成沉积即古风成砂－黄土－古土壤序列很好地反映了东亚季风环流的演变历史。末次间冰期的沉积（Ｓ１）由三层古土壤和夹于其间的两层黄土组成，可与深海氧同位素阶段５的五个亚阶段（ａ－ｅ）则很好对比。末次冰期的沉积（Ｌ１）由三层古风成砂和夹于其间的两层黄土构成，由此反映了与上述三层古风成砂对应的阶段２、阶段４及阶段３的中期是沙漠－黄土边界带冬季风活动加剧、沙漠扩大的时期。全新世的沉积是由一层草原上（Ｓ０）和其上的流沙组成，但这层流沙是在最近两千年来不合理的人类活动和当地特定风场条件的共同作用下形成，已不具备明显的气候指示意义。     

四川金川黄土地层

马厂剖面中黄土-古土壤序列共厚15．5m,根据岩性、磁性地层特征、磁化率、CaCO3含量和光释光测年结果,将金川黄土划分为冰后期S0古土壤,末次冰期1-1黄土,末次间冰期S1古土壤、倒数第二冰期L2黄土和倒数第二间冰期S2古土壤等5个地层单位;其中,S1复合型古土壤又可细分为S1LL1、S1LL2黄土和S1SS1、S1SS2和S1SS3古土壤等5个次级地层单位。金川黄土沉积始于中更新世晚期,其底界的年龄人致为200ka BP,其磁化率变化反映了最近200ka来的高原季风演化和气候环境变迁,5个磁化率高值段指示了5个夏季风环流增强的时段,4个磁化率低值段则代表了4次夏季风减弱的时期。     

末次间冰期以来黄土中伊利石结晶度的变化与古环境

对环县、长武和渭南3个剖面末次间冰期以来黄土-古土壤样品进行了粘粒(2μm)提取,采用X射线衍射分析技术对粘粒样品的伊利石结晶度,即KI值(1.0nm衍射峰的半高宽)进行时空变化分析。结果表明,环县剖面的KI值在黄土-古土壤中差异不大,而中南部的长武与渭南剖面能很好地反映冰期与间冰期旋回特征;在空间上,古土壤S_0和S_1的KI值自西北向东南呈现逐渐增加趋势,而黄土L_1层仅在最南部的渭南剖面呈现较高的值,环县和长武剖面差异不大,均呈现较低的值。综合分析影响黄土KI值变化的各个因素后,认为风化成壤作用可能是影响黄土-古土壤伊利石结晶度指标变化的主要因素,可以作为东亚夏季风环流强度演化的指标。     

河南邙山晚更新世黄土的环境记录

数十年来,黄土高原的古环境研究受到了高度重视并取得重大进展,但对其东南部的研究却相对较薄弱。位于黄土高原东南边缘的邙山塬因受气候、构造等营力的交互作用而发育巨厚的黄土地层,记录了东亚古环境演变的详细信息。笔者近年来对邙山黄土地层进行了深入研究,以黄土高原经典的洛川剖面为参照,对位于河南省荥阳市北邙乡刘沟村的赵下峪剖面进行了详细划分。邙山黄土S10以上黄土-古土壤序列,总厚度达172.1m,其中末次间冰期S1古土壤厚15.7m,末次冰期L1黄土厚77.3m,是由早冰阶L1LL1黄土、间冰阶L1SS1古土壤和晚冰阶L1LL2黄土组成的复合黄土     

六盘山东西两侧黄土古土壤容重的特征及其古气候指示意义

为了进一步了解黄土古土壤容重的特征及其与东亚季风演化的关系,建立了洛川剖面250 ka以来、白草塬剖面140 ka以来黄土古土壤容重序列,并与磁化率、粒度及深海氧同位素对比分析,发现六盘山东西两侧不同气候区黄土古土壤容重具不同的特征和受控因素, 指示了不同的古气候意义。六盘山以东半湿润半干旱气候区容重值受成壤作用影响,容重指标主要由夏季风控制,可作为夏季风演化的替代性指标。而六盘山以西半干旱气候区,容重值受沉积物粒度影响,主要由冬季风控制,可作为冬季风演化的替代性指标。与灵台和三门峡地区黄土古土壤容重值对比,发现空间上由西北向东南容重值呈变大趋势,进一步表明了不同气候区的冬夏季风对容重指标的不同影响。     

Heinrich events recorded in a loess-paleosol sequence from Hexigten,Inner Mongolia

We describe the last glaciation climatic history Marine Isotope Stage(MIS, 2-4) from 66.7 ka to 14.5 ka in Hexigten, northeast Inner Mongolia, North China. The climate of the region experienced frequent and significant fluctuations between dry-cold and less dry-cold during the late MIS4. The climate was generally warm and humid during early MIS3(MIS3 c) and late MIS3(MIS3 a), whereas it was cold and dry in middle MIS3(MIS3 b) and during MIS2. In this study, the cold and dry conditions were correlated with a stronger East Asian winter monsoon and strong dune activity; whereas, warm and humid conditions were related to a stronger East Asian summer monsoon(EASM) and weak dune activity. This study establishes six distinct dry and cold intervals during the last glacial period(66.7-14.5 ka) based on optically stimulated luminescence data, multi-proxies record(magnetic susceptibility, grain size analysis, Rb/Sr, SiO2/TiO2) and chemical index of alteration(CIA). The last glacial period may be correlated with Heinrich events 1 to 6 which were further confirmed by comparison with the Hulu cave stalagmites and Greenland ice core records. It is concluded that the study area was substantially affected by the EASM, as compared with the loess-desert transition zone of the Chinese Loess Plateau, especially in MIS3 c and suggested that the East Asian monsoon played a pivotal role in the last glacial period climate and dune activity.     

Rock magnetic response to climatic changes in west Philippine Sea for the last 780 ka: Discussion based on relative paleointensity assisted chronology

We conducted rock magnetic and paleomagnetic research on two deep-sea sediment cores from the west Philippine Sea, located to the east of Benham Rise with the length of 4 m and water depth of over 5000 m. At the bottom of core 146 occurs a reversal of inclination and deflection of relative declination, which is recognized as Brunhes-Matuyama Polarity Boundary (MBPB). No reversal occurs in core 89, which implies a younger bottom age than that of core 146. Rock magnetic results reveal magnetic uniformities in mineralogy, concentration and grain size along the two cores, thus relative paleointensity variations are acquired. The three normalizers-anhysteresis remanent magnetization (ARM), magnetic susceptibility (k) and saturation isothermal remanent magnetization (SIRM) are used for normalization to obtain relative paleointensities. The three normalization results are averaged to indicate the paleoitensity of the cores and are further stacked together to get a synthetic curve for west Philippine Sea (named asWPS800 in this paper). Based on the magnetic correlation between cores and paleointensity to Sint800, we transfer the changes of rock magnetic parameters from depth to time. Furthermore, the astronomically tuned oxygen isotope from ODP site 1143 in the south China Sea is used for the glacial and interglacial indicator. Three concentration proxies (ARM, k and SIRM) and grain size indicators (k _ARM/SIRM, k _ARM/k) are examined according to the paleointensity-assisted chronology. The grain size changes in the two cores display a consistent pattern with the climatic changes embodied by oxygen isotope. The magnetic sizes are usually coarser in glacial periods and finer in interglacial times, which may reflect the influence of chemical erosion rather than fining from sea level rising on the source sediment. Furthermore, the sub-peaks and sub-troughs in interglaciations almost correspond with that of oxygen isotope records, which means sedimentation can reflect the subtle changes in interglaciations. This kind of revelation of climatic fluctuation by magnetic size is also found in the South China Sea, which shows a common pattern of magnetic signals to climate at least within East Asia. The concentration of ARM (representing more about fine grain) also shows similar response to glacial and interglacial cycles, that is, high in interglacial cycle and low in glacial cycle; but k and SIRM (reflecting more about coarse grain) lack the response to the climatic cycles. At the same time, S-ratio lacks the correlation with aeolian dust record and rhythmic changes, indicating the dominant source of main magnetic carrier (low coercivity magnetite) is the suspended matter instead of dust. The decreasing trend of sedimentation rate from west to east also reveals that the sediments are mainly from west Luzon and adjacent land. Grain sizes first became coarse and then stable around 400 ka B.P., and at the same time all the magnetic contents lowered and amplitude of magnetic mineral changes increased. The magnetic transition around 400 ka B.P. is simultaneous with the decreases of carbonate content, reflecting a global carbonate dissolution event, i.e. mid Brunhes event. The synchronization of magnetic content and grain size with climatic cycles of glacials and interglacials imply the validity of paleointesnityassisted chronology. Also, the response of rock magnetic signals to stable oxygen isotope changes and carbonate variation reveals that rock magnetismmethod can be an effective tool for paleoclimatic and paleoceanographic research. __________ Translated from Quaternary Sciences, 2007, 27(6): 1040–1052 [译自 : 第四纪研究]     

Variation of summer precipitation δ18O on the Chinese Loess Plateau since the last interglacial

Orbital-scale East Asian Summer Monsoon (EASM) variations inferred from loess deposits in northern China and speleothems from southern China display different dominant periods, complicating our understanding of monsoon response to insolation and ice-volume forcings. Here we integrate a new microcodium δ¹⁸O record from a high-resolution last interglacial loess profile with previously published data and provide a composite microcodium δ¹⁸O record on the Chinese Loess Plateau (CLP) since the last interglacial. The composite microcodium δ¹⁸O record displays distinct precessional cycles, consistent with speleothem δ¹⁸O records, but with different amplitude contrast (particularly during the peak interglacials). We propose that both loess and speleothem δ¹⁸O records exhibit covariations at precessional timescale oscillations. The discrepancy between loess and speleothem from southern China can be attributed to the influences of other processes besides summer precipitation on the proxies. A slight difference in amplitude between microcodium and speleothem δ¹⁸O records implies that the EASM is also influenced by inland surface boundary conditions, which has important impacts on the occurrence of EASM precipitation. Therefore, microcodium δ¹⁸O from the Chinese loess–paleosol sequences can be regarded as a representative proxy of EASM precipitation in northern China and then a reliable proxy reflecting the variation of EASM intensity.     

Impacts of grain size sorting and chemical weathering on the geochemistry of Jingyuan loess in the northwestern Chinese Loess Plateau

Major and trace elemental compositions of loess samples collected from the Jingyuan section in the northwestern Chinese Loess Plateau (CLP) were analyzed to investigate the potential impacts of grain size sorting and chemical weathering on the loess geochemistry and to extract appropriate geochemical indices for better evaluating the East Asian monsoon variability. Based on variations of major and trace elements in different grain size fractions, seventeen elements were classified into three types: (1) Si and Na display higher contents with the increased particle sizes; (2) Al, Fe, Mg, K, Mn, Zn, Rb, Cr, V are mainly enriched in fine size fractions; (3) Ti, Ba, Zr, P, Ca and Sr show irregular variations among different size fractions. Comparison of Al-normalized elemental ratios with Zr/Rb and Rb/Sr ratios (two commonly employed indicators for grain size sorting and pedogenic weathering) indicates that Si/Al, Zr/Al, Ti/Al variations match well with Zr/Rb and grain size results, whilst Ca/Al, Sr/Al, P/Al ratios display similar variability as that of Rb/Sr ratio. Comparison of loess based proxies (e.g., elemental ratios, magnetic susceptibility, grain size) of Jingyuan section with speleothem and ice-core records confirms that elemental ratios of high-resolution loess sequences developed in the northwestern CLP can be employed to address fluctuations of the winter monsoon-induced grain size sorting and summer monsoon-related weathering and pedogenesis at glacial–interglacial and millennial timescales.     

内蒙古东南缘晚第四纪以来的气候环境演化

基于内蒙古东南缘西拉木伦河上游刘家店河湖相剖面的粒度、磁化率、微量元素地球化学指标,重建该区35 ka BP以来的气候演化过程。结果表明,在MIS 3晚期(35.23~25.15ka BP)研究区气候条件总体温暖湿润,并伴有区域变干的趋势;MIS 2阶段(25.15~11.13 ka BP)气候整体寒冷干燥,但叠加有短暂回暖气候事件。剖面记录的末次盛冰期(LGM)出现于22.25~18.47 ka BP,此时气候极度干冷;MIS 2阶段叠加了两个短暂气候适宜期,分别出现于18.47~16.24 ka BP和14.72~11.13 ka BP。在11.13 ka BP前后研究区进入全新世,气候变得暖湿。刘家店剖面的气候记录与周边气候记录具有可对比性,揭示了区域上东亚夏季风进退具有一致性,并认为自MIS 3晚期以来东亚夏季风受北半球太阳辐射及冰量的共同驱动。此外,刘家店剖面记录揭示的千年尺度气候变化对典型气候事件具有一定的响应,推测这些千年尺度的季风强度变化可能与北大西洋经向翻转环流(AMOC)相关。     

热带西太平洋沉积物的环境磁学特征对东亚冬季风的响应

西太平洋暖池(Western Pacific Warm Pool,WPWP)指位于热带太平洋中、西部(包括南海南部和苏禄海)年平均水温超过 28 ℃的广大海域。由于巨厚的表层暖水覆盖,西太平洋暖池成为全球热量和水汽交互的重要源区,对驱动温盐环流、调节全球气候变化具有重要作用。东亚冬季风(EAWM)是全球气候系统中最活跃的组成部分之一,它可能会通过寒潮侵入热带地区,引起深层对流,以此加强暖池区的对流活动和降水异常,从而影响赤道地区的潜热释放。但地质历史时期西太平洋暖池与东亚冬季风的相互作用关系尚不明确。由于东亚冬季风携带的风尘中往往包含有大颗粒的高矫顽力磁性矿物,我们可以通过沉积物中磁性矿物的组合、含量、颗粒大小和形态变化,分析不同时间尺度上气候环境变化和风尘物质的输入情况,进而反演东亚冬季风的强度变化。本文对取自热带西太平洋B10钻孔的岩心样品进行了环境磁学测试,以揭示地质历史时期西太平洋暖池沉积物环境磁学特征对东亚冬季风的响应。实验结果表明,沉积物中的主要载磁矿物为低矫顽力的磁铁矿,属于亚铁磁性矿物,并含有少量高矫顽力磁性矿物。沉积物中的磁性颗粒以准单畴(PSD)颗粒为主。高矫顽力的磁性矿物含量和细颗粒磁铁矿相对含量在冰期和间冰期呈现出显著相对变化,对东亚冬季风的变化有敏感响应:冰期沉积物中高矫顽力矿物含量增多,磁性颗粒粒径变大;间冰期沉积物中高矫顽力矿物含量降低,磁性颗粒粒径变细。在干燥、寒冷的冰期,由风尘携带而来的高矫顽力磁性矿物相对含量增加,沉积物中的磁性颗粒粒径变大,反映冰期东亚冬季风强度增大;在气候温暖湿润的间冰期,风尘的输入量较小,由风尘携带的高矫顽力磁性矿物含量相对较低,沉积物中的磁性颗粒粒径变小,反映间冰期东亚冬季风的强度减弱。     

Characterization of diamictons on the basis of their mineral magnetic properties in Murchisonfjorden,Nordaustlandet, Svalbard

Different grain-size fractions of 12 lithologically defined and 2 non-defined Weichselian glacial stage diamicton units from Nordaustlandet, Svalbard were studied for their mineral magnetic properties. The applied measurements included magnetic susceptibility (χ), anhysteretic remanent magnetization (ARM), saturation isothermal remanent magnetization (SIRM), ‘backfield’ isothermal remanent magnetization (IRM) and several of their parametric ratios. The results indicated a poor correlation between the magnetic fingerprint of dry-sieved and wet-sieved diamicton samples. The dry-sieved samples had higher χ, ARM and SIRM values, suggesting a higher concentration of ferrimagnetic grains. This was probably because washing of the samples resulted in the loss of the most fine-grained ferrimagnetic material. Both sieving methods seem to be capable of discriminating different diamictons, but they cannot be used contemporaneously. The results only partially support previous findings of the chronostratigraphic correlation of diamictons. They indicate that in suitable conditions and within a limited area it is possible to discriminate (and group) glacigenic sequences based on the magnetic signature of their fine-grained fractions in the sample. For example and based on the mineral magnetic fingerprint, a diamicton unit that was described from the marine core (MUR 09/sor1/125–150) obtained from Isvika region most probably represents diamicton material that were derived from provenance areas related to the Middle Weichselian glacial stage. It was also noted that certain mineral magnetic parameters, such as SIRM/χ and SIRM/ARM ratio, had greater potential in discriminating diamicton units than others.     

最后4个冰期旋回中国黄土记录的东亚冬季风变化

开展高分辨率的黄土记录研究, 是揭示过去气候变迁和变率的主要途径.选择黄土高原中部的灵台和赵家川剖面, 对L5以上的黄土-古土壤序列进行了石英颗粒的提取和粒度分析.结果表明, 石英粗颗粒(> 43μm) 体积分数变化揭示了最后4个冰期旋回东亚冬季风存有的快速变化特征, 并同深海氧同位素记录具有良好的对应关系; 然而, 在每一个冰期时段, 石英粗颗粒体积分数的变化幅度和频率存有较大差异, 可能意味着对应的冰期下垫面状况和气候系统内部的非线性响应机制并不相同.值得注意的是由石英粗颗粒体积分数反映的东亚冬季风变化, 在某些时段不仅同磁化率记录的夏季风变化不协调, 而且同深海氧同位素记录存有明显差异.深入研究这种由不同代用指标反映的古气候变化间的差异, 对理解黄土记录的全球性意义显得尤为重要.      

中更新世以来不同时间尺度亚洲中纬度气候演化的区域差异

亚洲中纬度气候主要受两大环流控制,继而划分出受亚洲季风控制季风区和受西风环流控制的西风区。已有研究发现在年代际至千年尺度上,两气候区的湿度演化存在错相位、甚至反相位的现象。文章综述了中更新世以来,多千年至轨道尺度上,两气候区的湿度演化模态的异同,以此深入了解亚洲气候演化的系统性。结果表明:二者的间冰期气候演化的整体趋势存在明显差别,尤其MIS 13以来,二者的演化趋势基本相反。在间冰期的多千年至万年尺度上,二者的气候演化模态呈现非同相位:高精度的释光测年结果显示天山地区的湿度变化滞后于黄土高原地区大概3~5 ka,且该现象在更早的间冰期可重复。在冰期的千年尺度上,两地的湿度演化基本一致,但西风区湿度的波动幅度更大。     

渭河流域典型黄土剖面的环境磁学特征及其古气候意义

对黄土高原南部耀县全新世黄土地层的形成及保存环境进行了研究.结果表明:地层中磁性矿物主要为软磁性矿物,硬磁性矿物对地层的磁学性质贡献小;随着成壤强度的增加,硬磁性矿物的相对含量在减少.本区全新世地层的形成和保存环境为氧化环境,适宜用磁学手段反演全新世古气候.对磁化率和频率磁化率、无磁滞剩磁磁化率的对比分析,可以看出:成壤强度较低时,细粒磁性矿物相对于磁性矿物总量的增加速率相对较快;成壤强度较高时,细粒磁性矿物的相对含量会达到或趋近饱和,相对于磁性矿物总量的增加速率相对较慢.研究表明,在黄土地层中,χfd%和χARM/χ等指示细粒磁性矿物的磁学参数可以显示更细微的气候变化;在古土壤中,χlf的气候指示意义相对更好.