Origin of the Red Earth sequence on the northeastern Tibetan Plateau and its implications for regional aridity since the middle Miocene

Desiccation in the interior of Asia is an important aspect of paleoclimate change during the Cenozoic era[1,2]. Research[3,4] shows that the widely distributed loess deposits in China were mainly transported by northwesterly and northerly winds from deserts and the Gobi region; in addition, an indispensable re- quirement for the generation of aeolian sediment is the presence of dry lands in central Asia[5,6]. The aeolian deposits in China provide an especially useful record of desiccation pr…     

The provenance of Gansu Group in Longxi region and implications for tectonics and paleoclimate

The Neogene sediments of Gansu Group from northwestern China contain eolian and fluvial deposits.The origins of these sediments are very important for exploring the onset of Asian inland aridification,the pattern of paleo-atmospheric circulation,and the regional tectonics and geomorphic evolution during the Miocene.Here we present detrital-zircon age spectra of typical eolian and fluvial deposits from highlands and subsidence basin,and compare them with those of surrounding eroded mountain(such as the West Qinling Mountains and Liupan Mountains)materials and Quaternary loess derived from the Asian inlands.The results reveal that(1)the detrital-zircon age spectrum of the Miocene eolian sample is remarkably different from the eroded materials of the West Qinling Mountains and the Miocene fluvial deposits from Tianshui region,but very similar to the Quaternary loess deposits.This indicates that the provenance of Miocene eolian sediments is similar with the Quaternary loess,and thus further confirms the previous conclusions that the distribution of Asian arid lands and the pattern of atmospheric circulation during the Miocene are broadly similar with the Quaternary.(2)The detrital-zircon age spectrum of the fluvial deposits(with age about 11.5 Ma)from Tianshui region is different from the eroded materials of West Qinling Mountains,but similar with that of the Liupan Mountains to the east,which may suggest that the Liupan Mountains have already been exhumed by11.5 Ma.     

基于地磁场相对古强度变化的下蜀黄土年代序列及其古气候意义

分布于宁镇地区的下蜀黄土年代标尺研究薄弱.我们以镇江大港钻孔岩芯的下蜀黄土为例,探讨了下蜀黄土记录地磁场相对古强度的可靠性.岩石磁学的实验显示,下蜀黄土大体上符合估计相对古强度对沉积物的要求.以低频磁化率()作为天然剩磁(NRM300)的归一化因子,我们获得了下蜀黄土记录的地磁场相对古强度变化.通过与邻区和全球的单个及合成曲线的对比,我们发现大港钻孔岩芯的相对古强度记录展现出主要的全球偶极场的变化特征,因而可用于建立下蜀黄土的年代标尺.新的年代标尺表明,大港钻孔的磁化率年代序列与北方黄土无法直接对比,证实了该地点的下蜀黄土磁化率变化机制与北方的不同.大港钻孔下蜀黄土的沉积速率与磁化率相关,低风尘沉积速率对应低磁化率,是降水增加所导致的结果.沉积速率与磁化率年代序列显示,下蜀黄土记录的本区季风变化过程可分为4个阶段.阶段Ⅳ(819～700ka)对应中更新世转型阶段,东亚季风降水较少.阶段Ⅲ(700～412ka)对应中更新世的大间冰期,东亚季风降水最多.阶段Ⅱ(412～197ka)时东亚季风降水减少,较阶段Ⅳ略少.阶段Ⅰ(197～34ka)东亚季风降水最少.因此,中更新世气候转型后,长时间尺度东亚季风降水持续减少,可能受全球温度阶段性降低驱动.     

Mineral magnetic variation of the Jingbian loess/paleosol sequence in the northern Loess Plateau of China: Implications for Quaternary development of Asian aridification and cooling

A high-resolution mineral magnetic investigation has been carried out on the Jingbian loess/paleosol sequence at the northern extremity of the Chinese Loess Plateau. Results show that the magnetic assemblage is dominated by large pseudo-single domain and multidomain-like magnetite with associated maghemite and hematite. Variations in the ratios of SIRM_100mT/SIRM, SIRM_100mT/SIRM_30mT and SIRM_100mT/SIRM_60mT (SIRM is the saturation isothermal remanent magnetization; SIRM_nmT represents the residual SIRM after an n mT alternating field demagnetization) have been used to document regional paleoclimate change in the Asian interior by correlating the mineral magnetic record with the composite δ¹⁸O record in deep-sea sediments. The long-term up-section decreasing trend in those ratios in both loess and paleosol units has been attributed to a long-term decrease in the relative contributions of eolian hematite during glacial extrema and of pedogenic hematite during interglacial extrema, respectively, which reveals a long-term decreasing trend in chemical weathering intensity in both glacial-stage source region (the Gobi and deserts in northwestern China) and interglacial-stage depositional area (the Loess Plateau region). We further relate this long-timescale variation to long-term increasing aridification and cooling, during both glacial extrema in the dust source region and interglacial extrema in the depositional area, over the Quaternary period. Changes in those ratios are most likely due to Quaternary aridification and cooling driven by ongoing global cooling, expansion of the Arctic ice-sheet, and progressive uplift of the Himalayan–Tibetan complex during this period.     

Quantitative measurements on the paleo-weathering intensity of the loess-soil sequences and implication on paleomonsoon

The loess-soil sequences in northern China provide a near continuous record of Quaternary paleoclimate. The pedogenetic intensity of the sequences is closely linked with the variations of the East Asian summer monsoon. In this study, 2181 samples from the Changwu and Xifeng loess sections are analyzed and two high-resolution paleo-weathering timeseries of the last 1.2 Ma are generated, using the ratio of CBD extractable free Fe₂O₃ (FeD) versus the total iron (FeT). This new index is compared with micromorphological features, low-frequency magnetic susceptibility, frequency-dependent magnetic susceptibility, and the Rb/Sr ratio^[5,6]. The results suggest that the FeD/FeT ratio is able to better reflect the degree of soil development. Since the chemical weathering of loess in the Loess Plateau region mainly depends upon the summer precipitation and temperature under modern climate condition, which are closely associated with strength of summer monsoon, and the chemical weathering intensity of loess primarily reflects the variations of the summer monsoon circulation.     

Chemical weathering history of the southern Tajikistan loess and paloclimate implications

Major chemical elements and Rb, Sr, Ba abundances were measured for loess samples from Chashmanigar loess-soil sequence in southern Tajikistan. And the degree of element loss and intensity of weathering of the samples were calculated. From these calculations we found that the paleosol horizons of the Chashmanigar section were chemically weathered to some extent and the weathering intensity of the soils is stronger than that of loess horizons, indicating that paleosols in southern Tajikistan were deposited in relatively warm and humid interglacial ages. In addition, the southern Tajikistan loess deposited before 0.9Ma BP was more intensely weathered than that deposited after 0.9Ma BP, which may imply the intensification of aridity in this area since 0.9Ma BP.     

Chemical weathering history of the southern Tajikistan loess and paleoclimate implications

In recent years, the loess deposits in Central Asia have attracted increasing attention of Qua-ternary paleoclimate researchers[1—8]. The centre of the loess deposits in this area is located in southern Tajikistan, where thick and continuous loess sectio…     

临夏盆地新生代沉积物粒度记录与亚洲内陆干旱化

临夏盆地毛沟剖面高分辨率粒度记录研究表明,29-7.4Ma间,临夏盆地的古气候一直保持相对稳定,而其中短暂的沉积相的改变是盆地对该期间青藏高原构造隆升事件的响应;从7.4Ma开始,流域外的风尘物质开始逐步被带人盆地,并经过了6.4Ma和5.3Ma的两次加速过程,揭示了我国西北内陆干旱气候可能从7.4Ma左右开始,且在6.4Ma和5.3Ma左右经过两次加强.通过与青藏高原构造隆升事件记录和全球气候记录对比。揭示高原在9-7Ma开始的逐步隆升和期后的阶段性加速隆升以及同期开始的全球变冷,尤其北极冰盖的形成和扩张可能是亚洲内陆干旱化的重要驱动机制.     

Grain-size records at ODP Site 1146 from the northern South China Sea: Implications on the East Asian monsoon evolution since 20 Ma

273 samples from Ocean Drilling Program (ODP) Site 1146 in the northern South China Sea (SCS) were analyzed for grain-size distributions using grain-size class vs. standard deviation method and end-member modeling algorithm (EMMA) in order to investigate the evolution of the East Asian monsoon since about 20 Ma. 10–19 μm/1.3–2.4 μm, the ratio of two grain-size populations with the highest variability through time was used to indicate East Asian winter monsoon intensity relative to summer monsoon. The mass accumulation rate of the coarsest end member EM1 (eolian), resulting from EMMA, can be used as a proxy of winter monsoon strength and Asian inland aridity, and the ratio of EM1/(EM2 EM3) as a proxy of winter monsoon intensity relative to summer monsoon. The combined proxies show that a profound enhancement of East Asian winter monsoon strength and winter monsoon intensity relative to summer monsoon occurred at about 8 Ma, and it is possible that the summer monsoon simultaneously intensified with winter monsoon at 3 Ma. Our results are well consistent with the previous studies in loess, eolian deposion in the Pacifc, radiolarians and planktonic foraminifera in the SCS. The phased uplift of the Himalaya-Tibetan Plateau may have played a significant role in strengthening the Asian monsoon at 8 Ma and 3 Ma.     

SAR TT-OSL dating of the loess deposits in the Horqin dunefield (northeastern China)

Palaeoclimatic change in northeastern China during the late Quaternary remains poorly understood. This is partly because of the scarcity of detailed investigations into archives in the region, but especially because of the lack of an appropriate dating method to obtain age control for the area's loess deposits. In this paper, the single-aliquot regenerative-dose (SAR) OSL and thermally transferred OSL (TT-OSL) method have been used to date two typical loess sections (Kulungou and Pingancun) in northeastern China. The results show that the SAR TT-OSL method is suitable for dating the Horqin aeolian sections and suggest that deposition commenced at least since the middle Pleistocene in the Horqin dunefield. A sand layer deposited at about 262 ± 32 ka implies that the dunefield experienced at least one significant period of aridification during the late Quaternary.     

轨道调谐建立洛川黄土地层的时间标尺

对黄土高原中部洛川厚140m黄土剖面进行了黄土层间隔5cm、古土壤层间隔3cm的采样,在室内测量了全部样品的磁化率．以磁化率变化作为东亚夏季风气候变化的替代性指标,利用轨道调谐方法建立了黄土－古土壤序列的时间标尺．结果表明,B／M界线处的年龄是772KaB．P,午城黄土与离石黄土分界线的年龄为1348KaB．P,黄土堆积底界的年龄为2471KaB．P．．通过和古地磁场倒转点绝对年龄对比以及与太阳辐射量变化时间序列进行交叉谱分析,证明了新时间标尺的可信性．     

Tectonic-climatic events in eastern Qilian Mountains over the past 0.83 Ma

The eastern part of Qilian Mountains experienced strong tectonic uplift during the late Quaternary, and climate record there was influenced by Tibetan Plateau to some extent. Based on studies on the fluvial terrace series and eolian loess deposition, we find that the tectonic uplifts of the Tibetan Plateau had coupled with climatic changes in our studied region and others since the mid-Pleistocene. The uplift that occurred at 0.83 Ma corresponded to significant desert expansion in L6 and periodic variation since MIS16, while the 0.14Ma one to the further drying in northwest China. Those coupled events may indicate that tectonic uplift drove climatic changes, and the Tibetan Plateau has important impacts on East Asian Monsoon system.     

Central Asian aridification during the late Eocene to early Miocene inferred from preliminary study of shallow marine-eolian sedimentary rocks from northeastern Tajik Basin

The tempo-spatial development of the Cenozoic Asian aridification across the Eocene-Oligocene and its controlling factors are important scientific topics in Earth Sciences, which are pertinent to regional and global tectonic and climatic events. However, sedimentary rocks preserving the record of aridification during this time from central Asia (ACA) are rare. Here we present a preliminary analysis of sedimentary facies of the lower Paleogene in the northeastern Tajik Basin, which reveals that: the lower part of the studied section is dominated by shallow marine deposits of the Paratethys, the middle part is typical of alternations of eolian dune and fluvial deposits, the upper part is represented by eolian loess-sandy loess (L&SL) like facies, and the top exhibits alternations of fluvial-lacustrine and loess like facies. Based on a chronological framework derived from preliminary magnetostratigraphy, published U-Pb dating of a volcanic ash, and regional litho-stratigraphic correlations, we determine that eolian and L&SL facies accumulated in the northeastern Tajik Basin during the Late Eocene and through most of the Oligocene. These sedimentary units indicate that semi-arid to arid environments of ACA had developed at least since the late Eocene. This initial aridification is closely linked to the westward retreat of the Paratethys that was likely driven by a combination of tectonic activity and sea level changes.     

East Asian paleoclimate change in the Weihe Basin(central China)since the middle Eocene revealed by clay mineral analysis

Obtaining a continuous sedimentary record of Cenozoic East Asian paleoclimate change is key to understand the origin, evolution and driving mechanism of the East Asian monsoon and climate change. Based on the continuous Cenozoic fluvial and lacustrine sedimentary sequence in the Weihe Basin, central China, we carried out research on the content and crystal parameters of clay minerals. The paleoclimate change since the middle Eocene was reconstructed accordingly. The results show that smectite and illite are the dominant clay minerals in fluvial-lacustrine sediments of Weihe Basin. The crystallinity of illite,the chemical index of illite, the ratio of smectite to illite and chlorite decrease gradually, which indicates that chemical weathering in the Weihe River watershed stepwise weakened since the middle Eocene, under the background of a semi-arid and semi-humid climate. The formation of palygorskite may be affected by a weak diagenesis. In this case, the content of smectite and the illite crystallinity caused by the weathering in the corresponding period may be actually higher than that of the current,which indicates that the climate in the Weihe Basin region was relatively warm and humid from the middle Eocene to Pliocene.With the decrease of Cenozoic global temperature and the continuous accumulation of sediments, the intensity of chemical weathering in the Weihe Basin gradually weakened, and the East Asian monsoon climate gradually evolved until becoming dry in the Quaternary. Evidence from clay minerals also indicates that the aridification in the Weihe Basin from the late Eocene to the Oligocene may be a response to the global cooling event at the Eocene-Oligocene transition.     

STUDY ON QUATERNARY MAGNETIC STRATIGRAPHY IN WUHAN AREA,CHINA

With the lateral and vertical expansion of cities, urban geology becomes critical for urban construction. Wuhan City, as one of the largest cities in China, was chosen by China Geological Survey as pilot city to study multiple elements of urban geological survey. 90% of Wuhan area is covered by Quaternary strata, which means that most of the city is built on Quaternary sediments. The study of Quaternary stratigraphic structure of Wuhan area is a crucial groundwork for the urban geological survey of Wuhan. Due to the badly lagging of research on Quaternary stratigraphy of Wuhan area, this study selects four boreholes from hundreds of cores in this area in the project of Wuhan Urban Geological Survey for magnetic stratigraphic study and paleomagnetic analysis. This work mainly focuses on the borehole SK2 which possesses well-developed strata and is representative for magnetic strata division. Wuhan is located in the eastern Jianghan Basin where the Quaternary sediment has a fluctuant bottom edge, forming a half-graben shape boundary in large scale, as revealed by a large amount of boreholes. The borehole SK2 is located in the western Dongxi Lake depression. SK2 reveals continuous deep Quaternary sediment except for a short gap of late Pleistocen-early Holcene. Moreover, the grain size of drill core is generally smaller than other cores nearby, which is more suitable for paleomagnetic study. In this study, we collected 117 samples with an interval of 0.25~0.30m from relative fine grain layers in the borehole for paleomagnetic study. Demagnetization and sample measurement were conducted in State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, CAS. By comparing with other well dated cores nearby, Wuhan's Quaternary magnetic stratigraphic chronology framework is initially established. Our data show that:1)the Brunhes-Matsuyama boundary(B/M)of borehole SK2 is clear, locating at 29.6m, and sediment at 1.2~29.6m belongs to Bmnhes normal polarity chron; 2)Two polarity drifts at 9.3~9.6m and 14.1~15.0m are observed in Brunhes. These samples collected from the Xiashu Loess yield two polarity drifts, which can be compared with the Xiashu Loess at Nanjing. The comparison between Xiashu Loess from borehole SK2 and that at Nanjing suggests that negative polarity drift at 14.1~15.0m is the Blake reverse polarity subchron(0.104~0.128Ma BP)and the polarity drifts at 9.3~9.6m(0.07~0.08Ma BP)is Norwegian-Greenland Sea reversal-polarity subchron; 3)Three polarity drifts were observed in Matsuyama, which is comparable with the borehole at Zhoulao and the borehole R25 in the center of the Jianghan Basin. They are at 38.2~39.8m(Jaramillo:0.99~1.07Ma BP), 66.5~71.9m(Olduvai:1.77~1.95Ma BP)and 75.8m(Reunion:2.01Ma BP); 4)Based on the forementioned paleomagnetic data, the depositional age structure of this borehole is:1.2~14.1m(late Pleistocene), 15~29.6m(middle Pleistocene)and 29.6~78.8m(early Pleistocene); 5)Our data show that deposition rate increased during 1.47~1.57Ma BP(54.9~61.3m)and 1.07~1.21Ma BP(39.8~47.3m)because they are fluvial sediments. This study builds the Quaternary stratigraphy and timescale for Wuhan area for the first time. However, because of the complex Quaternary sediment type in Wuhan area and the coarse sediment in the Matuyama, more comprehensive study is needed in the future to test whether our Quaternary stratigraphy and timescale is the best.     

OSL chronology of loess deposits in East China and its implications for East Asian monsoon history

Controversy exists regarding the chronology of loess in the Nanjing area in Jiangsu Province and the Jiujiang area in Jiangxi Province, East China. The chronology is of special interest, because the age of the initial accumulation for loess in these areas indicates that dust accumulation has been extended from the Loess Plateau in North China to East China at that time. This implies a threshold in the evolution of the East Asian monsoon, with the strengthened winter monsoon transporting aeolian dust further to the south. In this study nine luminescence samples were collected from the Jiujiang area and four samples from the Nanjing area. Quartz grains of 38–63 μm were isolated and the single-aliquot regenerative-dose (SAR) protocol was employed for D_e determination. For all samples from the Nanjing area, during the SAR measurement cycles the test-dose sensitivity was dose dependent and dropped significantly when the regeneration dose reached >300 Gy, while it increased linearly when the regeneration dose value ranged from 25 to 300 Gy. The reason for this is not yet known. The OSL chronology and its implications for paleoenvironmental change are discussed. It is concluded that: (1) loess deposition in the Jiujiang area started in the early Last Glaciation; and (2) the first loess layer in the Nanjing area was deposited during the last Glaciation.     

The record of paleoclimatic change from stalagmites and the determination of termination II in the south of Guizhou Province,China

A high-resolution climate record from 163.00 kaBP to 113.80 kaBP has been obtained through TIMS-U series dating and carbon and oxygen isotope analysis of the three large stalagmites from two caves in the south of Guizhou Province, China. The record of the oxygen isotopes from the stalagmites reveals that the undulation characteristics between the cooling event of the glacial period and the warming event of the interglacial period in the research area can compare well to those of ice cores, lake sediments, loess and deep sea sediments on the scale of ten-thousand years or millennium time scale. The climate undulation provided by the record of the stalagmites has a coherence with the global changes and a tele-connection to the paleoclimate changes in the north polar region. Our results suggest that the direct dynamics of paleo-monsoon circulation changes reflected in the record of the stalagmites might be caused by changes of the global ice volume, and in turn related to various factors, including the solar radiation strength at the mid-latitudes in the Northern Hemisphere, the southern extension of the ice-rafted event in the North Atlantic, and changes of the equatorial Pacific sea surface temperature at the low-latitudes. Using °¹⁸O values, we have calculated the temperatures and the results show that the temperature difference between the penultimate glacial period (with an average temperature of 8.1°C, and a minimum temperature range from 0.65°C to-1.43°C at stage 6) and the last interglacial period (with an average temperature of 18.24°C at sub-stage 5e) was about 10°C. This temperature difference from the record of the stalagmites corresponds in general to the record temperature variation (about 10°C) of measured ice cores. The climate records from the three stalagmites in the two caves have shown that the circulation strength of the Asian summer monsoon and the winter monsoon in the penultimate glacial period and the last inter-glacial period had a clear change. With the TIMS-U series method, termination II of the penultimate glacial period has been precisely dated at an age of (129.28± 1.10) kaBP for the three stalagmites in the south of Guizhou Province, China. This borderline age represents the beginning of the last interglacial period or the boundary between the Middle Pleistocene and the Late Pleistocene, and corresponds to the beginning age of the last interglacial period shown by the ice cores and in the SPECMAP curve of the marine oxygen isotopes. The chronology determination of termination II is not only of stratigraphic and chronological significance, but also lays an important foundation for discussing the short time scales of climate oscillation and rapidly changing events of paleoclimate in the circulation region of the East Asian monsoon.     

Variations in chemical compositions of the eolian dust in Chinese Loess Plateau over the past 2.5 Ma and chemical weathering in the Asian inland

Major and trace elements as well as strontium isotopic composition have been analyzed on the acid-insoluble (AI) phase of the loess-paleosol sequence from Luochuan, Shaanxi Province, China. Results show that the chemical composition of AI phase of loess and paleosols is distinctive to the average composition of upper continental crust (UCC), characterized by depletion of mobile elements Na, Ca and Sr. The distribution pattern of elements in AI phase reveals that initial dust, derived from a vast area of Asian inland, has suffered from Na- and Ca-removed chemical weathering compared to UCC. Some geochemical parameters (such as CIA values, Na/K, Rb/Sr and⁸⁷Sr/⁸⁶Sr ratios) display a regular variation and evolution, reflecting that the chemical weathering in the source region of loess deposits has decreased gradually since 2.5 Ma with the general increase of global ice volume. This coincidence reflects that the aridity of Asian inland since the Quaternary is a possible regional response to the global climate change.     

Magnetic properties of the Tertiary red clay from Gansu

The study on magnetic properties of the red clay indicates that the red clay and loesspaleosol sequence have a common magnetic mineralogy, with magnetite, maghemite, hematite (and possibly goethite) contributing to the magnetic behavior. The red clay magnetic susceptibility is also found to have a positive relation with extrafine superparamagnetic grains. This suggests that, like the Quaternary loess-paleosols, an ultrafine ferrimagnetic component produced during pedogenesis in the red clay under humid conditions also plays an important role in susceptibility enhancement in the soil units. This is supported by the correlation between Rb/Sr ratio and magnetic susceptibility. This signifies that, like the above loess-paleosol sequence, the magnetic susceptibility of the red clay can be used as a general proxy paleoclimatic indicator, although whether its susceptibility in the red clay is comparable to pedogenesis intensity and requires further investigation. Magnetic susceptibility variation in the red clay thus also provides an eolian/pedogenic record of paleoclimatic evolution. Study of the background susceptibility indicates that, on average, the absolute scale of the paleoclimatic shift from red clay development to Quaternary loess deposition is similar to the climatic shift from stage 5 (S1) to stage 2–4 (L1). This may suggest that during the Quaternary there is an evident strengthening of the absolute wind intensity to bring more (about double) coarser and less weathered (non-SP fraction) eolian magnetic input from the source regions to the Loess Plateau than during the Pliocene. The presence of eolian red clay since 7.5 Ma BP in central-northern China implies an important environmental change from the underlying Cretaceous red sandstone. The red clay development was closely related to global drying and climate cooling since the Cretaceous and closely associated with the abrupt uplift of the Qinghai-Xizang Plateau at about that time. This uplift of the plateau intensified the East Asia monsoon system and started red clay deposition.     

Magnetic properties of the Tertiary red clay from Gansu Province, China and its paleoclimatic significance

The study on magnetic properties of the red clay indicates that the red clay and loess- paleosol sequence have a common magnetic mineralogy, with magnetite, maghemite, hematite (and possibly goethite) contributing to the magnetic behavior. The red clay magnetic susceptibility is also found to have a positive relation with extrafine superparamagnetic grains. This suggests that, like the Quaternary loess-paleosols, an ultrafine ferrimagnetic component produced during pe-dogenesis in the red clay under humid conditions also plays an important role in susceptibility enhancement in the soil units. This is supported by the correlation between Rb/Sr ratio and magnetic susceptibility. This signifies that, like the above loess-paleosol sequence, the magnetic susceptibility of the red clay can be used as a general proxy paleoclimatic indicator, although whether its susceptibility in the red clay is comparable to pedogenesis intensity and requires further investigation. Magnetic susceptibility variation in the red clay thus also provides an eo-lian/pedogenic record of paleoclimatic evolution. Study of the background susceptibility indicates that, on average, the absolute scale of the paleoclimatic shift from red clay development to Quaternary loess deposition is similar to the climatic shift from stage 5 (S1) to stage 2-4 (L1). This may suggest that during the Quaternary there is an evident strengthening of the absolute wind intensity to bring more (about double) coarser and less weathered (non-SP fraction) eolian magnetic input from the source regions to the Loess Plateau than during the Pliocene. The presence of eolian red clay since 7.5 Ma BP in central-northern China implies an important envi-ronmental change from the underlying Cretaceous red sandstone. The red clay development was closely related to global drying and climate cooling since the Cretaceous and closely associated with the abrupt uplift of the Qinghai-Xizang Plateau at about that time. This uplift of the plateau intensified the East Asia monsoon system and started red clay deposition.