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1.
A 42 m thick loess–paleosol sequence was studied in the dry pre-Andean valley of Tafi-del-Valle, at the La Mesada site (2280 masl), Tucumán Province in northwest Argentina. The sequence contains 28 paleosols interbedded with 26 loess layers. The latter are coarse loamy and have large hexagonal polyhedrons. The soil layers are restricted to fine loamy Bt horizons, with prismatic structure and organo-argillans on the structural surfaces. Surface (A) and eluviation (E) horizons are systematically absent from the whole sequence. Soil truncation is unlikely as no remains of such horizons or erosional uncomformities were identified. The dated part of the section encompasses a time span of 10,080 years, from 17,580 BP at 5.2 depth to 27,660 BP at 42.3 m depth. A total of 20 Bt/C pairs developed during this time interval, corresponding to a climatic change every 500 years on the average between dry–cool conditions, promoting loess influx, and moist–warm conditions favouring soil development. The depth function of the Bt/C clay ratios reflects the climate improvement during the soil formation interstadials, with an optimum in the upper part of the sequence. This might be related with recurrent northward shifts of the polar front, accompanied by the weakening of the mid-latitude South Pacific anticyclone. Similar short-term periodicity has been identified in the ice cap of Greenland, highlighting the global character of such frequent climatic changes during the Pleistocene.  相似文献   

2.
The Guadix–Baza basin is one of a number of intramontane depressions located within the Betic Cordillera (SE Spain), where the geological and geomorphological evolution is controlled by tectonic activity. The basin ceased to be closed after capture by the Atlantic network, when five main land systems developed. Late Pleistocene geological evolution basically consisted of erosional modelling, when badlands were deeply incised in highly erodible materials (marls and silts). Three travertine platforms and several Palaeolithic sites were used to determine that this main incision period fell within a 115,000–48,000 yr range. There are few signs of geomorphological evolution of this basin in the last 48,000 yr. Based on these geomorphological data and soil development, tectonic uplift of the basin probably played a secondary role in its evolution, and climatic conditions in southern Iberia in the 144,000–48,000 yr period were more humid and variable than later.  相似文献   

3.
洞庭盆地两护村孔孢粉组合及其气候与地层意义   总被引:6,自引:0,他引:6       下载免费PDF全文
两护村ZKC1孔位于洞庭盆地安乡凹陷的东南部,孔内第四系(底部跨上新世)厚达294 m,为河流和湖泊沉积,自下而上依次为上新世—早更新世华田组、早更新世汨罗组、中更新世洞庭湖组、晚更新世坡头组以及全新统等。对ZKC1孔第四系进行了详细的孢粉分析,自下而上划分出16个孢粉组合带。ESR年龄和孢粉组合及其反映的气候特征指示华田组下段形成于上新世末。根据孢粉组合特征,结合构造—沉积演化和区域气候背景,重塑洞庭盆地上新世末以来的气候演化过程:上新世末期由孢粉带Ⅰ和Ⅱ指示具暖干气候。早更新世经历了凉干(孢粉带Ⅲ、Ⅳ)→暖湿间凉干(孢粉带Ⅴ~Ⅶ)→冷干间温湿(孢粉带Ⅷ~Ⅹ)→暖较湿(孢粉带Ⅺ,Ⅻ)的气候演变过程。中更新世早期无孢粉样品(洞庭湖组下部砾石层),其沉积环境暗示冷干气候条件;中期由孢粉带ⅩⅢ反映出暖稍湿的气候特征;晚期因构造抬升缺失沉积,同期湿热化事件指示暖湿气候。晚更新世早期缺乏沉积,据区域对比应为寒冷气候;中期由孢粉带ⅩⅣ指示温较湿的气候特征;晚期缺失沉积,系寒冷气候下区域海平面下降所致。全新世经历了暖稍湿(孢粉带ⅩⅤ)→暖稍干(孢粉带ⅩⅥ)的演变。上述气候演变过程与ZKC1孔化学蚀变指数曲线反映的气候演变过程以及中国东部第四纪气候演化基本吻合。以孔深140 m为界,上部孢粉数量显著高于下部,种属也更为丰富。这一变化很可能对应于一次重要的地质事件,其成因有待今后深入研究。  相似文献   

4.
The Pleistocene sediments of North European Russia have been studied using different methods to establish their composition and identify the enclosed organic remains. The results obtained elucidate natural climatic environments of the Pleistocene and corresponding sedimentation cycles and development stages. Glaciation centers are identified for different Pleistocene epochs based on lithological criteria. The established secular trends characterizing spatial distribution and composition of different-age glacial horizons facilitate their regional correlation. Climatic conditions and paleogeographic settings of sedimentation during interglacial epochs of the middle-late Pleistocene and Holocene are reconstructed. Variations in development of vegetation, temperature, and humidity are detected for the first time in the Timan-Pechora-Vychegda, Arkhangel’sk and Vologda regions with parallel coordination of obtained data. As is concluded, there was a tendency of climate cooling in North European Russia during the Neopleistocene.  相似文献   

5.
The Cerrado biome is the second largest in Brazil, but the evolution of the Cerrado during the late Quaternary is not yet fully known. This study identifies paleoenvironmental changes during the last 23 000 years, based on a tropical mountain peatland record, in the Serra do Espinhaço Meridional in central-eastern Brazil. A multi-proxy approach was used that involved palynological analysis, stable isotopes (δ13C, δ15N), geochemistry, radiocarbon dating and multivariate statistics derived from a peatland core from Rio Preto (Minas Gerais state). The study reveals a very humid and cold climate during the late Pleistocene, with an increase in temperature and decrease in humidity at the Pleistocene–Holocene transition. During this period there was strong instability in the landscape (episodes of erosion). At the beginning of the Holocene there was a reduction in humidity with greater landscape stability. The current sub-humid climatic conditions seem to have been established in the mid-/late Holocene, with periods of landscape instability. Our findings agree with other Cerrado records that contradict previously established hypotheses, such as the Amazonian Refuge and the Pleistocene Arc.  相似文献   

6.
《Sedimentary Geology》2007,193(1-4):211-220
Central Poland is situated in the centre of the Late Pleistocene and Holocene “European aeolian sand belt”. This area has the highest frequency of round, mat grains of all Europe, due to intense aeolian abrasion. Aeolian processes were very active during several glaciations, but the changes in aeolian activity in the course of an entire glaciation can be studied only for the Vistulian (Weichselian, Wisconsinan). The reason is that a continuous blanket of glacial sediments either covered, or partly eroded older aeolian sediments in central Poland during the Wartanian (Saalian II). Aeolian processes began just after deglaciation and became intense under the periglacial conditions of the Late Wartanian and the whole Vistulian. The evolution of these processes is reconstructed, especially for the interval of the highest and most effective aeolian activity, i.e. the Late Plenivistulian (Oxygen Isotope Stage 2: the coldest stage of the last glaciation). A model of the sequence of periglacial aeolian events during an entire glaciation is presented. This model may be used for the reconstruction of older Pleistocene cycles of high periglacial aeolian activity.  相似文献   

7.
The long-term histories of the neighboring Nakuru–Elmenteita and Naivasha lake basins in the Central Kenya Rift illustrate the relative importance of tectonic versus climatic effects on rift-lake evolution and the formation of disparate sedimentary environments. Although modern climate conditions in the Central Kenya Rift are very similar for these basins, hydrology and hydrochemistry of present-day lakes Nakuru, Elmenteita and Naivasha contrast dramatically due to tectonically controlled differences in basin geometries, catchment size, and fluvial processes. In this study, we use eighteen 14C and 40Ar/39Ar dated fluvio-lacustrine sedimentary sections to unravel the spatiotemporal evolution of the lake basins in response to tectonic and climatic influences. We reconstruct paleoclimatic and ecological trends recorded in these basins based on fossil diatom assemblages and geologic field mapping. Our study shows a tendency towards increasing alkalinity and shrinkage of water bodies in both lake basins during the last million years. Ongoing volcano-tectonic segmentation of the lake basins, as well as reorganization of upstream drainage networks have led to contrasting hydrologic regimes with adjacent alkaline and freshwater conditions. During extreme wet periods in the past, such as during the early Holocene climate optimum, lake levels were high and all basins evolved toward freshwater systems. During drier periods some of these lakes revert back to alkaline conditions, while others maintain freshwater characteristics. Our results have important implications for the use and interpretation of lake sediment as climate archives in tectonically active regions and emphasize the need to deconvolve lacustrine records with respect to tectonics versus climatic forcing mechanisms.  相似文献   

8.
陕西洛川黄土的粒度分维值及其意义   总被引:9,自引:0,他引:9  
对陕西洛川剖面1580个黄土样品的粒度数据分析表明,黄土的基本粒级,即在2-50μm的标度范围内,粒度分布具有分形特征。黄土的粒度分维值在2.16-2.51之间,平均值为2.32;而古土壤的粒度分维值在2.23-2.51之间,平均值为2.41;粒芳分维值对黄土与古土壤的形成环境的演变,有良好的指示意义。粒度分雏值显示,250万年以来,东亚季风不断增强,变化周期和变化幅度逐渐变大。在早更新世,季风较弱,变化周期短.变化幅度小,表现为2~4万年的周期;而在晚更新世,季风较强,冬季风和夏季风相互消长,变化周期长且明显,变化幅度大,表现出10万年的变化周期。  相似文献   

9.
近年来,在东秦岭南洛河流域发现了大批旧石器地点,该区域被认为是中国南北方旧石器工业的交汇和过渡地带。但是,早期人类在这一带的活动时间和环境背景尚不清楚。由于第四纪黄土的定年方法相对成熟,旧石器遗址的黄土堆积可为解决这些问题提供重要证据。对洛南盆地上白川和刘湾两个遗址的黄土剖面进行土壤地层和磁性地层研究以及光释光年龄和磁化率测定结果显示,该地区的黄土堆积可能从110万年前就已经开始,地层有明显的黄土-古土壤旋回,指示了气候和环境在冰期-间冰期时间尺度的变化。黄土地层下部出土的石制品表明,至少在约80万年前后人类就在这一带活动并制作工具。石制品两面加工技术被广泛采用,大量制作精美的手斧和薄刃斧等Acheulian工业类型工具的发现,显示洛南盆地的旧石器工业可能已经突破了"莫氏线(Movius Line)"的框架。  相似文献   

10.
黄土高原西北部黄土沉积的粒度变化同时受控于沙漠进退和冬季风强度波动,因此,此区粒度可视为季风-沙漠系统变迁的替代性指标。本研究以2cm为间隔,对李家塬和新庄源S2以上部分做了粒度分析。结果表明,倒数第二次冰期中的冰阶时段与末次冰期类似,存在着频繁的千年尺度的气候波动;而末次间冰期这类变化并不明显。  相似文献   

11.
McEachern's Deathtrap Cave (G—49/50) is located in the Lower Glenelg region of southeastern Australia and records a Late Pleistocene to Holocene sedimentary record that has been directly influenced by surface processes during its formation. The sedimentary sequence contained within the cave is divided into lower, middle and upper sequences consisting of eight facies. The lower sequence represents the earliest phase of sedimentation, and groundwater fluctuations during the Last Interglacial period resulted in its erosion and redistribution deeper into the cave system. A decrease in the magnitude and frequency of flood events in the cave during the formation of the middle sequence indicates increasingly drier surface conditions prior to the Last Glacial Maximum. The middle sequence has a minimum age of 9840 ± 290 a BP. Moving sand sheets during the Last Glacial blocked the entrance to the cave allowing flowstones to develop on the cave floor. The surface environment surrounding the cave was probably not as dry as contemporaneous inland sites because sedimentation continued to be dominated by flowing water during this period. Holocene sedimentation is represented by the upper sequence and reflects wetter cave conditions between 7680 ± 160 a BP and 5700 ± 110 a BP. A major phase of sediment accretion occurs after 5700 a BP and correlates to a phase of dune instability in the Lower Glenelg region. Flowing water remodelled the sediment cone sometime after 2240 ± 100 a BP, which represents a period of increased surface runoff, although it is not clear whether this is due to climatic or anthropogenic influences.  相似文献   

12.
The study compiles the current knowledge on the Neopleistocene stratigraphy and paleogeography of Central European Russia (including the limits of ice sheets in the first half of the Middle Neopleistocene and Late Neopleistocene) on the basis of new data and provides a draft of the revised regional stratigraphic scale adopted by the regional Interdepartmental Stratigraphic Committee on the Central and Southern Russian platform. The proposed scale incorporates a number of new horizons: the Setun’, Moiseevo, Navlya, and Ikorets horizons of the Lower Neopleistocene; Vologda and Gorki horizons of the Middle Neopleistocene; Chermenino Horizon of the Upper Neopleistocene; and Shuvalovo Horizon of the Holocene. The adopted scale is correlated with the oxygen-isotopic stages and substages identified in the Neopleistocene by the decision of the ISC of Russia, as well as with glacials and interglacials of Lithuania, Poland, Germany, and England. The study discusses the problem of a division of the Pleistocene with respect to duration of Pleistocene stages.  相似文献   

13.
An Early Pleistocene fluvio-lacustrine sequence from the extremely arid southern Negev desert, Israel, indicates that climatic conditions during that period were humid enough to support the formation of a lake (Lake Zihor) there. The lacustrine sediments were deposited in a tectonic valley that developed along the Zihor tectonic line after the deposition of the Pliocene Arava Formation. They intercalate with fluvial sediments that contain several horizons of reddish calcic paleosols. The lacustrine sequence comprises three sedimentary cycles. Each starts with a dark clay layer overlain by a white limestone unit, both of which contain abundant freshwater fauna, and terminates with a green detrital limestone that contains only one type of brackish-water ostracod. The top of each green limestone unit is characterized by pedogenic features and microrelief, indicating desiccation of the lake and soil development. Isotope data support the hypothesis that the white limestone was deposited in an open, freshwater lake. The incision of the present channel of Nahal (wadi) Zihor in the lacustrine sediments is manifested by a series of rock-cut and fluvial terraces (Q1–Q4) capped by gypsic-salic soils, which reflect the onset of the present, extremely arid climate. Over 100 find-spots and larger occurrences of prehistoric artifacts assigned to the Lower Paleolithic were discovered near Lake Zihor. On the basis of techno-typological and stratigraphic considerations, these assemblages are divided into two groups, the first of which may be contemporaneous with the lake, while the second is found mainly on the younger Q1 and Q2 terraces. It is estimated that the lake existed for more than 100,000 years. The climate during this period was probably semiarid, but water budget calculations suggest that, in addition to surface runoff, seepage of groundwater along the Zihor Line contributed a considerable amount of water to the lake.  相似文献   

14.
Interpreting how environmental dynamics respond to global climate change and how this has affected human evolution and dispersal is an on-going topic of debate. During the early Middle Pleistocene (~0.6–0.4 Ma), as compared to earlier, environmental conditions were relatively more stable, with longer climatic cycles alternating between open and forested landscapes. During this interval, humans spread successfully providing an important number of fossil sites where fossils or tools are reported. The Atapuerca-Sima de los Huesos (Burgos, northern Spain) site (Atapuerca-SH) is one of the earliest localities with hominin evidence in the European Middle Pleistocene, with the most important accumulation of Homo heidelbergensis so far. We have analyzed the abundant faunal record from Sima de los Huesos, which is mainly comprised of carnivores, in order to approach an interpretation of the palaeoenvironmental circumstances where these hominids inhabited within the Sierra. Other sites from Sierra de Atapuerca referred to the same Faunal Unit (FU 6), are roughly contemporaneous, and include important ungulates, which are here analyzed with Atapuerca-SH. Additional information provided by isotopic analysis helps elucidate the ancient ecology of taxa present in Sima de los Huesos allowing for an accurate portrayal of the setting in which humans lived. The timing of the spread of Homo heidelbergensis is dominated by a relative climatic and environmental stability and points to a landscape dominated by savannah-like open woodland.  相似文献   

15.
Cucú cave is a small cavity, 1600 m above sea level on the southern slope of Sierra de María (Almería Province, SE Spain), where current mean annual precipitation is < 450 mm. Fossils and palynomorphs contained within a sedimentary sequence, up to 9 m in depth, allow us to consider the prevailing climatic conditions, and the timing of cavern development. The lithological sequence is dominated by clast-supported detrital material with no evidence of alluvial transport. These sediments were formed by freeze-cracking during periglacial conditions, causing further cave enlargement after initial solutional development. The clastic sequence formed during cold climates is covered by a flowstone that was deposited during a period of warmer, wetter conditions. This provides a minimum U–Th isochron age of 40.2 ± 4.5 ka for the timing of periglacial action. Micromammal fossil species indicate a chronology between 140 and 80 ka. Paleoecological data based on the structure of the mammal community indicates that cold conditions prevailed at the time of deposit. In the studied sequence the presence of anthropogenic components has not been documented. The pollen assemblages identified are a common feature of Pleistocene cold stages that are in semi-arid regions.  相似文献   

16.
The palynological data collected by the author in the course of a detailed study of the loess-paleosol sections in the Russian Plain and Central Asia and the yedoma loess-like sediments of Northern Asia as well as data published by others concerning the pollen-and-spore studies on the loess-paleosol formations of Northern Eurasia revealed discrepancy between the boundaries of the loess-paleosol horizons and the borders of the chronostratigraphic subdivisions and the course of the reconstructed paleoclimatic curves. Loess and soil in a paleogeographic context are not mutually exclusive. On the Russian Plain loess horizons were formed during the glacial climatic rhythms and also during the thermoxerotic maxima and endothermal coolings of the interglacial rhythms, while the formation of fossil soils took place during all stages of the interglacial climatic rhythms and also during interstadial warming and cryohygrotic maxima of the glacial climatic rhythms.  相似文献   

17.
We present a new model for the evolution of volcanic table mountains, based on volcanological and petrological studies of Herdubreid/Herdubreidartögl, an upper Pleistocene volcanic complex within the active Icelandic rift zone. The evolution of these table mountains is highly complex and influenced substantially by different eruptive environments as well as changing magma sources and volcanic and magmatic processes. Whereas current models imply entirely subglacial conditions and continuous compositional (“monogenetic”) evolution for these volcanoes, we subdivide their evolution into four eruptive periods characterized by different environments: (a) Subaerial lava flows erupted during an interglacial period. (b) Lavas and voluminous hydroclastic deposits formed in a lake during a waning period of the last glaciation. (c) Subglacial eruptions during thickening of the ice sheet as a result of a climatic deterioration built pillow lava piles overlain by steep-sided complexes of hydroclastic deposits. These deposits differ from those of the second eruptive period in structure, texture, and degree of alteration. Subaerial lava flows and agglutinates covered these deposits after buildup above the ice sheet. (d) Subaerial lava flows and fallout deposits during a postglacial period. The detailed analysis of volcanic table mountains appears to be a very sensitive indicator of climatic fluctuations. Although most deposits of the studied volcanoes were formed during the waning period of the last glaciation, the subglacial deposits in the upper part of the volcanic complex reflect a temporary, but major, thickening of the ice sheet.  相似文献   

18.
Large-scale fluctuations in global climate and resulting changes in ecology had a profound effect on human evolution and dispersal. Though hominin remains are scarce, studies focussing on the more abundant records of fossil land mammal communities can contribute greatly to our knowledge of the palaeoenvironmental circumstances that influenced and directed the global spread of hominins. To produce a comprehensive and accurate account of the evolution of western Palaearctic habitat diversity between 2.6 and 0.4 Ma BP, information generated from large mammal communities from 221 key sites has been included in this study.The palaeoecological conditions of the western Palaearctic during the Early and early Middle Pleistocene were principally controlled by the following key factors: (1) a widespread trend of temperature decrease, (2) the periodicity of the global temperature record, (3) the intensity of single climatic stages, (4) the temporal pattern of climatic variation, (5) geographical position, and (6) the distribution of continental water resources. A general picture of the evolution of western Palaearctic habitat diversity saw the replacement of extensive forested terrain by an alternating sequence of varied savannah-like and forested habitats during the 2.6–1.8 Ma span, as well as an alternation between different types of predominantly open habitats between 1.8 and 1.2 Ma. Both of these processes were governed by 41 ka temperature periodicity. During the 1.2–0.9 Ma time span, irregular climatic fluctuations were more common and habitat variability increased. The subsequent 0.9–0.4 Ma interval, a period controlled by 100 ka periodicity, was by comparison more stable, with longer climatic cycles alternating between open and forested landscapes. During the entire Early and early Middle Pleistocene, assemblages of large mammal communities reveal a distinct trend of decreasing continentality between Eastern and South-Eastern Europe on the one hand, and South-Western and North-Western Europe on the other. This trend was due to the effect of the Atlantic Ocean, while in Southern Europe the relatively low continentality was balanced by influences from the Mediterranean Sea.When plotted against evidence of hominin occurrence, the data on western Palaearctic habitat diversity inferred from large mammal communities indicate clear environmental stimuli for the earliest human dispersal in Europe. These are: (1) a wide range of habitats, implying a high diversity of resources; (2) mild climates with low seasonality, implying a lack of strong environmental fluctuations. Around 1.8 Ma at the latest, hominins of African origin entered the western Palaearctic for the first time, taking advantage of the diversity of habitats and resources, particularly along large river systems. Their subsequent westward spread between 1.7 and 1.3 Ma was restricted to Mediterranean-influenced areas, which offered a high variability of habitats and relatively low seasonality. The increase in environmental diversity, which occurred from 1.2 Ma onwards, opened up South-Eastern and Eastern Europe for hominin occupation. According to the available records, North-Western and Central Europe were initially colonized during late Early to early Middle Pleistocene interglacials, when these regions experienced periods of low seasonality and considerable habitat diversity.  相似文献   

19.
The Cercadillo Sandstone and Siltstone (CSS) Formation forms the upper part of the Buntsandstein fluvial facies in the western margin of the Iberian Basin. Sedimentation was controlled by two major pre-Triassic tectonic systems: the Iberian Ranges System (NW-SE) and the Central System (NE-SW). Fluvial sedimentation during this period took place in coexisting braided and meandering channels, together with associated ephemeral episodes. This unusual association was due to a combination of structural control by the main normal faults coupled with large discharge fluctuations. The CSS Formation consists of two major fluvial sequences with very similar vertical patterns comprising low sinuosity stream deposits that pass upwards into high sinuosity deposits associated with extensive floodplain sediments. A widespread calcrete horizon is developed at the top of the lower sequence and a surface with pedogenic features is present at the top of the upper sequence. These horizons are interpreted to be the result of subaerial exposure during a period of no subsidence and equilibrium in the erosion-sedimentation processes in the area, probably due to quiescent periods on the master faults that delineate the basin. Minor changes within the sequences are probably due to discharge fluctuations of a climatic origin whereas the general vertical trend of the sequences is suggested to be tectonically controlled. Palaeocurrent data show a general flux of sediments to the south-east, parallel to the axis of the Iberian Basin, but the final part of the upper sequence palaeocurrents to the north-east are interpreted to indicate a local reactivation of secondary, transverse faults in the south-east part of the study area.  相似文献   

20.
Dr. M. Pécsi 《GeoJournal》1987,15(2):151-162
The loess of type localities is subdivided into 3 subseries on the basis of the paleosols allowing even an Eurasian comparison for their lithostratigraphy.
  1. The upper young loess (8–10 m) is most widespread, characterized by 3 sandy loess and 2 intercalated humic loess horizons. The ages of the humic loess horizons H1, H2 are c. 16 ka and 20 ka BP, respectively.
  2. The lower young loess (15–20 m) contains 4 loess packets and 3 chernozem-like paleosols, usually doubled soil complexes (designated MF, BD, BA). The MF paleosol complex, first member of lower young loess is c. 27–28 ka BP (C14 and TL dating). The lower young loess is underlain by a brown forest soil and chernozem-like paleosol-assemblage (MB), probably formed during the last interglacial (c. 105–125 ka BP).
  3. The old loess (c. 20 m) is generally characterized by 6–7 loess, mainly brown forest soil and 2 interbedded fluvial sand layers. The lowermost old loess member and an ochre-red paleosol lie below the Brunhes-Matuyama boundary (0,73 Ma).
  4. The loess is underlain by a subaerial non-loessic sequence (20–40 m) with 5–10 red soils, red clays mostly directly superimposed on and locally interbedded by sand and silty sand. At the base of this sequence of red paleosols usually Uppermost Miocene inland sea deposits are found.
It can be concluded that the reddish paleosol developed under climatic, and in general, ecological conditions (probably warm subhumid with rapid cyclical climatic changes) differing from the cold and semiarid or semidesert environments of typical loess formation. The former conditions were characteristic — from the Lower Pliocene to the Lower Pleistocene — on continental or global scales.  相似文献   

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