Pollen evidence from Baode of the northern Loess Plateau of China and strong East Asian summer monsoons during the Early Pliocene

The red clay eolian sequence on the Chinese Loess Plateau (CLP) is an important archive for paleoclimate change from the late Miocene to Pliocene,and can provide significant information for the controversial problems of East Asian monsoon evolution and its forcing mechanism.In this study,we present a pollen record from Baode,northern CLP.The record shows four stages of paleoecological evolution.From 5.6-4.4 Ma,a forest steppe ecosystem developed under an extremely warm period with high seasonal precipitation.Since 4.4 Ma,a drier episode occurred,which prompted parkland landscapes to develop.During 3.5-3.05 Ma,the environment changed to a rather open steppe ecosystem with a much cooler and drier climate.After 3.05 Ma,the vegetation evolved to forest steppe.Using the percentages of arboreal plants to mirror precipitation,and comparing with other published pollen data from the CLP,we find the existence of S-N directionality of the precipitation change and high percentage of arboreal plants in the entire CLP during the Early Pliocene,which suggest the CLP was characterized by a strong East Asian summer monsoon.The strong summer monsoon corresponds well to the low global ice volume,which may illuminate global climate mechanism for the summer monsoon evolution in the early Pliocene.     

Stepwise advance of the Mu Us Desert since late Pliocene: Evidence from a red clay-loess record

Analyses of paleomagnetic polarity reversals, magnetic susceptibility and grain size on the red clay-loess sequence, about 280 m thick, at Jingbian show that this sequence contains most of the stratigraphic units recorded in the standard red clay-loess sections of the Loess Plateau over the past 3.5 Ma. Using sand particle percentage ( > 63 μm%) of the eolian sequence as a proxy indicator of desert extent and aridity in the dust source regions, it is found that during the past 3.5 Ma, there are three important climatic events indicating the advance of the Mu Us Desert. The events occurred at about 2.6, 1.1 and 0.6 MaBP, respectively. The stepwise advance of the Mu Us Desert may be linked closely to increase in global ice volume and uplift of the Tibetan Plateau in the late Cenozoic.     

Magnetostratigraphy of Late Tertiary sediments from the Chinese Loess Plateau and its paleoclimatic significance

The Late Tertiary red clay/bed sediments underlying the Quaternary loess-paleosol in the Chinese Loess Plateau possesses high-resolution paleoclimatic changes related to the uplift of the Tibetan Plateau. Magnetostratigra-phy and susceptibility measurements are discussed in this paper. The paleomagnetic results show that the red clay/bed began to accumulate at about 8.1 Ma, which represents the oldest Late Tertiary deposit continuously in the central Loess Plateau. The magnetic susceptibility curves show stepwise increases since the initiation of red clay, superimposed on several peaks. Moreover, the magnetic susceptibility increased abruptly since 3.8 Ma, which probably indicates the inception of the modern East Asia monsoon system. Between 3.8 and 2.6 Ma, the stepwise increases of susceptibility may imply progressively intensified East Asia summer monsoon activity related with the stepwise uplift process of the Tibetan Plateau.     

Pollen record from red clay sequence in the central Loess Plateau between 8.10 and 2.60 Ma

The origin and development of Asian monsoon circula-tion, the desertification and aridification of Asian inland in relation to vegetation evolution and the soil erosion in northwest China have been more and more drawing inten-sive scientific and society attention. Recently the studiesof monsoon evolution from some Tertiary Red Clay sec-tions located at the central Loess Plateau in the eastern Liupan Mountains, based on chronology, dust flux, grains size, magnetic susceptibility, and rates o…     

神木邱家鄢最高阶地的年代及对中游黄河演化的指示

通过详细的野外调查,发现陕西神木邱家鄢地区存在一古河流阶地,根据该阶地上覆晚第三系红粘土推测该阶地为晚第三纪形成的。对该阶地上覆地层进行调查表明,该区晚第三纪红粘土之下存在约53米的风成砂层和河流相砂层。对红粘土和粉砂层进行古地磁测年样品的采集和测试。古地磁研究结果表明,该剖面底部的年代约在6．8 Ma左右,而红粘土与粉砂层之间的界限在6．27 Ma。根据深度－年代之间的关系,推算得出底部粉砂层的沉积速率比上部红粘土的沉积速率要快10倍以上,结合区域气候变化和构造运动历史的研究,邱家鄢剖面在（6．8－6．27）Ma之间快速沉积粉砂层的来源可能是中游黄河的河漫滩,指示了山陕峡谷段南北流向的古黄河可能在中新世晚期的6．8 Ma以前已经在该区出现。     

Carbon isotopic records in paleosols over the Pliocene in Northern China: Implication on vegetation development and Tibetan uplift

Carbon isotopic composition of pedogenic carbonate can be used to estimate the proportion of C₄ and C₃ plants. Here we present carbon isotopic data of carbonate in a red earth section at Xifeng, central Loess Plateau. Results show that C₄ vegetation increased in ∼4.4 Ma B.P., stabilized between 4.0 and 3.0 Ma B.P. The character and timing of C₄ expansion on the Loess Plateau are similar, but different with other localities, e.g. Pakistan and Africa, implying that regional climate changes were main factors driving the expansion of C₄ plants. This event is comparable in timing with increased aridity evidenced by Xifeng grain size and North Pacific eolian dust records. Therefore we argue that the Pliocene expansion of C₄ plants in northern China might have been caused by the increased aridity, which in turn might be related to rapid uplift of the Tibetan Plateau.     

黄土高原构造侵蚀期研究

根据野外调查和实际测量资料，研究了黄土高原构造侵蚀期和期内造成的侵蚀。资料表明，黄土高原约160万年以来发生了6次构造侵蚀期，各次侵蚀期下切深度存在差别，深度变化一般在5－130m之间。黄土高原构造侵蚀期与海、湖盆地构造侵蚀期不同，它所造成的侵蚀主要限于沟谷、丘陵区和塬边。构造侵蚀期的多次发生是黄土高原分割和沟谷发育的主要原因。     

黄土高原极端降水变化及其对植被覆盖度影响

全球变暖背景下,黄土高原极端降水事件频发,对社会经济及植被生态产生了重要影响,研究黄土高原极端降水变化特征及其对植被覆盖度(fractional vegetation cover, FVC)影响,可为生态环境保护和区域可持续发展提供科学支撑。基于2000—2020年黄土高原64个气象站点逐日降水数据和MODIS NDVI数据,采用趋势分析、相关分析等方法,分析了黄土高原地区极端降水和FVC时空变化特征及极端降水对FVC的影响。结果表明：2000—2020年黄土高原地区总降水量P_RCPTOT、中雨日数R_10mm、大雨日数R_20mm、强降水日数R_25mm和降水强度S_DII均呈显著上升趋势(P<0.05),区域西部和北部强降水总体增加,但区域南部呈干旱化。2000—2020年黄土高原FVC总体呈显著上升趋势(P<0.05),集中分布在区域中东部,占区域总面积的32.00%;显著下降趋势主要分布在南部少部分地区,仅占区域总面积的4.91%。2000—2020年黄土高原地区F...     

Magnetic characteristics and their paleoclimatic significance of Fengzhou loess in the Qinling Mountains of China

The Fengzhou loess section is very typical in the Qinling Mountains. The section, about 82 m thick and underlain by the Neogene red clay, consists of 33 layers of loess and 33 layers of paleosol. The section covers Brunnes normal polarity zone and Matuyama reversed polarity zone, and the B/M boundary is located in the middle of L₈. Matuyama reversed polarity zone records Jaramillo, Olduvai and Reunion normal polarity subchrons. The boundary between Matuyama reversed polarity zone and Gauss normal polarity zone (M/Ga) appears in the lithological boundary between loess and the Neogene red clay. Loess accumulation in Fengzhou started before 2.48 Ma. The magnetic susceptibility curve indicates 33 cold-dry and humid-warm cycles of paleoclimatic variation in the Qinling Mountains during the last 2.48 Ma. The above climatic fluctuation may be well compared with that of the oxygen isotopic records from deep-sea deposits.     

Terrestrial mollusk evidence for the origin and sedimentary environment of the Late Tertiary Red Clay Formation in the Loess Plateau, China

The origin and sedimentary environment of theLate Tertiary (Neogene) Red Clay Formation in northernChina had long been controversial. A new mollusk recordfrom the Xifeng red clay (red-earth) sequence in the centralLoess Plateau provides the biological evidence for addressingquestions of its origin and environmental implication. Thestudy of composition and preservation condition of seventymollusk fossil assemblages reveals the initial ecological con-dition of the red clay formation, avoiding the effect ofpost-deposited alteration. The result shows that all of identi-fiable mollusk species are composed of terrestrial taxa, mostof them are the common species found in the overlying Qua-ternary Ioess-paleosol sequence. Most of fossil individualspreserved in the red clay strata are in living conditions basedon the investigation of fieldwork, indicating the originalpopulation. Thus, the mollusk fossil assemblages can be usedas an indicator of primary environment of the red clay for-mation. The mollusk record from the Xifeng red clay se-quence supports the view that the red clay is an aeolian ori-gin, similar to the overlying Quaternary loess deposits. Ourdata also reveal the history of environmental changes atXifeng from 6.2--2.4 Ma, which is coupled in phase with theformation and development of the Arctic ice sheets and theprocess of the Tibetan Plateau uplift. Both may be the majorcause and forcing mechanisms of the late Tertiaryenvironmental changes in the Loess Plateau.     

近20年黄土高原干旱变化特征及其影响因素

为促进黄土高原农业和生态环境可持续发展,基于2001—2020年MODIS蒸散产品和气象站点数据,利用作物缺水指数(crop water stress index, CWSI)、Theil-Sen Median趋势分析、Mann-Kendall检验和偏相关分析等方法,探讨了黄土高原干旱变化特征及其影响因素。结果表明：(1)年际上,2001—2020年黄土高原CWSI呈显著线性下降过程,下降速率为0.005 8 a^-1(P<0.01)。干旱程度呈先减(2001—2018年)后增(2018—2020年)过程;(2)空间上,CWSI以减少趋势为主,显著减少趋势占区域总面积91.24%,其中青海、甘肃、宁夏、陕西南部地区CWSI下降速率较快;(3)黄土高原CWSI与气温以负偏相关关系为主,显著负偏相关占区域面积的7.78%,集中分布于青海东部、山西南部和河南北部;与降水以负偏相关关系为主,显著负偏相关占区域总面积60.88%,集中分布于黄土高原西南部和东部地区。气温升高背景下,降水增加促使黄土高原实际蒸发显著增加、潜在蒸发不显著减少,导致CWSI显著下降。研究结论认...     

Geomagnetic anomalies recorded in L9 of the Songjiadian loess section in southeastern Chinese Loess Plateau

Detailed magnetostratigraphic and rock magnetic investigations on L8-S12 of the Songjiadian loess section in the Sanmenxia area,southernmost margin of the Chinese Loess Plateau were conducted in this study.Matuyama/Brunhes (M/B) boundary occurred at the bottom of the loess unit L8.The top and bottom boundaries of the Jaramillo polarity subchron are found in the middle of L10 and the bottom of L12,respectively.Magnetic fabric of the loess layers maintains the original depositional features and the recorded r...     

History of the temperature-dependence of susceptibility and its implications: Preliminary results along an E-W transect of the Chinese Loess Plateau

An investigation of rock magnetic properties using the temperature-dependence of susceptibility (TDS) and the coercivity of different IRM components was conducted on samples of the Malan loess (L1) and the last interglacial soil (S1) along an E-W transect of the Chinese Loess Plateau. We find that the behavior of the TDS is useful in identifying maghemite (γ-Fe_2O_3) in both loess and paleosol, and that it can be used as a new proxy of the degree of pedogenesis, which is closely linked to climatic conditions. Based on results from the TDS, we conclude that (1) the detrital magnetite grains predominate in the L1 unit to the west of Liupan Mts, where the weathering and pedogenetic processes were weakest; (2) a comparable degree of pedogenesis exists in the L1 at the east of the Liupan Mts and the S1 at the west of the Liupan Mts; (3) a higher concentration of maghemite is found in the S1 unit at the eastern sections, suggesting that the local climate was warmer and more humid in this interval. In addition, we propose that TDS measurements can be used as a rapid means of determining the reliability of the paleomagnetic secular variation (PSV) record in loess/poleoso/sequenees.     

Environmental indicators from comparison of sporopollen in early Pleistocene lacustrine sediments from different climatic zones

Two lacustrine sporopollen records obtained from the Qaidam Basin (in the non-monsoonal region) and the Linxia Basin (in the monsoonal transition zone) indicate that during the early Pleistocene open forest-steppe/steppe vegetation developed in the Qaidam Basin, while in the Linxia Basin an open cypress forest-steppe shifted to a conifer/conifer-steppe and then to open forest-steppe vegetation. Existing sporopollen records from the Chinese Loess Plateau (CLP) (in the monsoonal region) indicate that around the Sanmen Paleolake open forest-steppe shifted to pine/conifer-broadleaved mixed forest. The conifer cover then changed to a subtropical evergreen broadleaved forest distributed around the Nihewan Paleolake. These changes suggest that wetter conditions progressed from western China to the east, and the climate became more humid than that of today indicating a stronger Asian summer monsoon during the early Pleistocene.     

Climate as the dominant control on C<Subscript>3</Subscript> and C<Subscript>4</Subscript> plant abundance in the Loess Plateau: Organic carbon isotope evidence from the last glacial-interglacial loess-soil sequences

Two modes of photosynthesis predominate in terres-REPORTS trial plants: the C3 and C4 modes[1]. The C3 mode is used by all of trees, bushes, and cold season grasses. The C4 mode is used by the most of tropical and warm season grasses. They are disadvantaged relative to C3 plants at high CO2/O2 ratios because of the additional energy ex-pense needed to concentrate CO2 in the bundle-sheath cells. At low CO2/O2, however, C4 plants can achieve a relatively high quantum yield by suppressin…     

Mid-Late Quaternary loess-paleosol sequence in Lantian’s Yushan,China: An environmental magnetism approach and its paleoclimatic significance

The application of rock magnetism methods to investigating the variations of magnetic minerals in the sediments is an important approach to the reestablishment of paleoclimate evolution. Thus we performed fine magnetic measurements on the loess-paleosol sequence (from L15 upwards to S5, in which L is short for Loess, S is short for Paleosol, the same hereinafter) of Yushan stratigraphic section, which is on the southeastern margin of Chinese Loess Plateau, in Lantian County of China’s Shaanxi Province, and the thickness of which is ca. 40 m. Our study shows that the primary magnetic carriers of loess and paleosol in this section are magnetite, maghemite, hematite and goethite. Thermomagnetic analyses on the samples of representative horizons show that the higher pedogenesis degree of the sediments, the smaller variations of magnetization there will be before and after heating, probably related to the pedogenic alteration of loess sediments. Analyses of several magnetic parameters show a significant discrepancy between the paleoclimatic conditions recorded in the strata from the loess unit L15 upward to the paleosol unit S5 in the study area and those recorded in the relative strata of other sections on the Chinese Loess Plateau, and those recorded in marine sediments, indicating the great impact of regional geological background. Similarly, the rapid and intensive change recorded in the segment from L15 to S9-1, and the significant difference between the paleoclimate evolutions of the two periods before and after the change (from L15 to S9-1, and from L9 to the base of S6) indicate the strong alteration of magnetic carriers in the study area as a result of the alternations of summer and winter monsoons in East Asia.     

黄土高原退耕还林（草）以来植被水分利用效率的时空特征及预测

 水分利用效率（WUE）是评估生态系统水碳循环的重要指标。基于PT-JPL模型的区域尺度实际蒸散发（ET）的模拟结果，结合黄土高原的总初级生产力（GPP），分析了退耕还林（草）工程实施后黄土高原水分利用效率WUE的时空变化趋势，结合CMIP6对未来3种情景下黄土高原的ET、GPP和WUE进行了预测。结果表明：2001-2015年，黄土高原植被显著增加， GPP和ET分别以每年3.59 g C·m^-2和4.39 mm的速率增加。WUE在72.68%的地区呈增加趋势，区域增长率为0.003 g C·mm^-1·m^-2·a^-1。在2015-2100年的3种情景中， ET均呈增加趋势，而GPP和WUE在SSP126中变化不大，在SSP245、SSP370情景中显著增加， WUE随着GPP的增加而增加。WUE结合了水资源的“消耗”和“利用”来阐明退耕还林（草）工程的成效，植被恢复虽然增加了区域耗水量，但是显著改善了植被覆盖情况，有效地提高了植被的固碳能力和水分利用效率，整个黄土高原植被的抗旱能力在增强。未来需进一步分析不同树种的WUE，筛选出抗旱性更高的树种进行植被恢复工作。     

Environmental magnetism and paleoclimatic interpretation of the Sanmenxia loess-paleosol sequence in the southeastern extremity of the Chinese Loess Plateau

This is a report of multi-parameter mineral magnetic results of the Sanmenxia loess-pa-leosol couplets in the SE extremity of the Chinese Loess Plateau (CLP) over the interval of L1- L13 spanning the last 1 Ma. These results show that mass-specific magnetic susceptibility (X), XARM and SIRM and their ratios exhibit parallel stratigraphic variations. Since superparamagnetic (SP) particles have significant contribution to X but no contribution to remanence, e.g., ARM and SIRM, the strong linear correlations between X and remanence (e.g., ARM and SIRM) suggest that the pedogenesis-produced magnetic grains are predominantly in small single domain range. Consequently, the significant contribution of SP particles to the bulk X as supposed in previous studies has been obviously overestimated. XARM/X data in Sanmenxia and median grain size records of typical loess sections from the hinterland of the CLP display a strong resemblance, implying that XARM/X may be regarded as a reliable indicator for monitoring the variations of ferrimagnetic grain sizes. The results of S-ratio and HIRM ((SIRM IRM-0.3T)/2) measurements indicate that there is significant enhancement of both soft magnetite/maghemite and hard hematite/goethite in paleosols associated with pedogenesis.     

对黄土高原的新认识

在分析黄土高原地貌与沉积特征的基础上,确定了黄土层下6种不同类型的基底地形,其上黄土层厚度与黄土地貌的空间差异均与基底地形的类型有关.根据黄土高原的地貌结构与成因,结合晋陕峡谷区新生代晚期地壳平均抬升速度的讨论,论证了黄土高原在成因类型上完全不同于剥蚀高原、构造高原、熔岩高原等各种高原类型,它是一种独特类型的叠加高原.正因为其类型的独特性,黄土高原在周缘特征和区域自然环境特征上,均有别于其他任何类型的高原,也使它成为水土流失量和速度最大的高原.     

Biological records of paleoclimate and paleoenvironment changes from Guanzhong area, Shaanxi Province during the last glacial maximum

We have collected a suite ofPaleoloxodon naumanni-Coelodonta antiquitatis fossils from a river terrace profile between Xi’an and Xianyang cities in Guanzhong Area, Shaanxi Province. A detailed examination of the fossils, together with¹⁴C determinations and pollen analyses, have enabled us to reconstruct preliminary features of climate and environment changes and their evolutionary processes, within the last glacial maximum (LGM): about 20 ka ago, the climate was basically cool and wet, and was favourable for the existence ofPaleoloxodon naumanni-Coelodonta antiquitatis fauna. This was followed by a cold, dry phase which was no longer suitable for this type of faunal suite, causing the death of a large number of mammalian assemblages. The available evidence indicates the existence of cold-dry and cool-wet climate and environment fluctuations during the LGM. The cool-wet stage within the LGM reflects a Heinrich event occurring in the high latitude areas, proving the existence of a teleconnection between polar-high latitude areas and the Loess Plateau.