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1.
In order to reconstruct the past variations of the Southeast Asian monsoon intensity and estimate the sedimentary system reactivity to climatic changes in Southeast Asia over the last 450 kyr, mineralogical and sedimentological analyses have been performed on the terrigenous fraction of the South China Sea sediment. End-member modelling coupled with grain size data discriminates three end-members that determine the nature and intensity of the main sediment transport vectors. Low sea-level stands are characterized by sediment reworking that allows transportation of a coarse end-member (20–40 μm) to the deep-basin. By contrast, the other end-members (4–6 μm; 9–13 μm) are controlled by the shoreline position (sea level) and/or by changes of the rivers capacity transport (monsoon). Finally, aeolian input to the northern margin of the South China Sea can be considered negligible compared to the massive fluvial input and the reworking of the sediments.  相似文献   

2.
文章以风尘沉积的粗粒和细粒组分与季风和西风环流的联系为基础,利用黄土高原中部的洛川剖面、西峰剖面和灵台剖面的粒度和沉积速率记录,讨论了晚新生代中国北方季风环流和西风环流的演变历史,总结了这一时期大气环流演变的基本规律和大气环流演变的动力机制.研究表明,自8～7Ma风尘沉积发育至5Ma左右,西风环流和季风环流都有减弱的趋势;自5Ma以来,西风环流和季风环流的强度都在逐步加强;与此同步,季风环流对风尘沉积的贡献增加,而西风环流对风尘的贡献逐渐减小,这一逐渐发展的大气环流趋势与北半球高纬冰盖的逐步发展有关;大气环流的这种趋势变化在8～7Ma,3.4Ma和1.2～0.9Ma这几个时期存在着突变,可能反映了青藏高原的阶段性隆升对中国北方季风环流演化的决定性作用以及对西风环流结构和强度的重要影响.中国北方大气环流在轨道尺度变化的基本特征是,低空季风环流在冰期加强,在间冰期减弱.西风环流和季风环流在冰期和间冰期的强度和格局可能主要与全球冰量的基本状况和青藏高原原面的性质有关.黄土高原的风尘记录在万年尺度和千年尺度的气候事件上都表现出相当明显的区域差异,可能主要与局部地形的大气环流效应有关.  相似文献   

3.
A 650-m-thick sequence of fluvio-lacustrine sediments from the Yuanmou Basin in southwest China was analyzed at 20-cm intervals for grain-size distribution to provide a high-resolution terrestrial record of Indian summer monsoon variations during the Pliocene. The concentrations of the clay and clay-plus-fine-silt fractions are inferred to reflect the water-level status of the lake basin related to the intensity of the Indian summer monsoon and high concentrations reflect high lake levels resulting from the intensified summer monsoon. The frequency of individual lacustrine mud beds is considered to reveal the frequency of the lakes developed in the basin associated with the variability of the Indian summer monsoon and an increased frequency of the lakes reveals an increased variability of the summer monsoon. The proxy data indicate that the Indian summer monsoon experienced two major shifts at 3.57 and 2.78 Ma and two secondary shifts at 3.09 and 2.39 Ma during the Pliocene. The summer monsoon displayed a general trend of gradual intensification during the period of 3.57–2.78 Ma, coeval with an accelerated uplift of the Tibetan Plateau, implying a close link between the monsoon intensification and the plateau uplift. At 2.78 Ma, the summer monsoon was markedly weakened, synchronous with the formation of extensive Northern Hemisphere ice sheets, denoting a quick response of the monsoon regime to the Northern Hemisphere glaciation. The variability of the summer monsoon decreased at 3.09 Ma and increased at 2.39 Ma, presumably suggesting that variations of the Indian monsoon would be modulated by the initiation and periodic fluctuations of ice-sheet covers in Northern Hemisphere high latitudes.  相似文献   

4.
We present a mineralogical and geochemical study of core MD012404, retrieved from the central Okinawa Trough (OT) of the East China Sea. Our studies reveal that the sediment sources of the core have been changed through time during the past 100 ka. Our mineralogical proxies indicate that the sediments source from the Yangtze River correlates well sea-level changes before 24 ka. Our Ti/Al ratios otherwise indicate an increase of sediment supply from eastern Taiwan after 26 ka. The cooler climate of the Last Glacial Maximum (LGM, 23～19 ka) led to a reduction in fluvial sediments from the Yangtze River. However, subsequent climate warming (after ～19 ka) resulted in an abrupt increase in fluvial sediments. After the LGM, the Kuroshio intrusion flow into the OT may have increased. We also infer anomalous changes in eolian sources transported by winter monsoons during the LGM and at 80 ka based on an eolian mineralogical indicator (feldspar). We conclude that the sediment source of core MD012404 is primarily of terrestrial origins, but influenced by sea-level changes and variations in the East Asian monsoon.  相似文献   

5.
The regional patterns of texture and composition of modern continental slope and pelagic sediments off Chile between 25°S and 43°S reflect the latitudinal segmentation of geological, morphological, and climatic features of the continental hinterland. Grain-size characteristics are controlled by the grain-size of source rocks, the weathering regime, and mode of sediment input (eolian off northern Chile vs fluvial further south). Bulk-mineral assemblages reveal a low grade of maturity. Regional variations are governed by the source-rock composition of the different geological terranes and the relative source-rock contribution of the Coastal Range and Andes, as controlled by the continental hydrology. The relative abundance of clay minerals is also predominantly influenced by the source-rock composition and partly by continental smectite neoformation. Latitudinal variations of illite crystallinities along the Chilean continental slope (and west of the Peru–Chile trench) clearly reflect modifications of the weathering regime which correspond to the strong climatic zonation of Chile.  相似文献   

6.
Particle-size analysis is a useful way to determine the source and deposition of sediments. However, there are inconsistencies when this method is used to constrain the origin of the red soils in south-eastern China. To address this problem, we performed a detailed grain-size analysis of two red soil sequences in Xuancheng and Qiliting located along the lower reaches of the Yangtze River. By comparing their particle-size characteristics with those of the loess on the Loess Plateau in northern China, we found that the aeolian samples plot in a particular zone in the C–M (grain size of the cumulative 1% versus median grain size) plot and cluster nearer the lower left corner of the plot as their degree of weathering increases. The grain-size features suggest that the onset of large-scale aeolian deposition along the lower reaches of the Yangtze River occurred at approximately 0.8 Ma. Although both sedimentary sorting and post-depositional weathering control the grain-size variations in the deposits, the extremely strong weathering due to the humid, warm climate along the lower reaches of the Yangtze River primarily modified the grain-size distributions of the primary red soil deposits. Strong weathering increased the very fine silt (2–5 μm) fraction and decreased the coarse (>63 μm) fraction. We also found that certain grain-size parameters of the red soils varied with the weathering intensity, which can be used as indicators of palaeoclimate variations. The grain size variations in both the Qiliting and Xuancheng sequences suggest that the mid-Pleistocene climate transition (MPT) may have affected the lower reaches of the Yangtze River at 0.9 Ma.  相似文献   

7.
2.5Ma以来地球轨道参数变化对黄土粒度变化的线性驱动   总被引：9，自引：7，他引：9  
本文对2.5Ma以来宝鸡黄土剖面粒度曲线与ETP曲线做了互功率谱和凝聚函数分析,以期反映风力强度与地球轨道参数变化的关系。结果表明,2.5Ma以来粒度记录中始终存在着与地轴倾斜度、岁差呈线性响应的41000a、23000a和19000a周期;同时,在0.6—0.0MaB.P.时段存在与偏心率呈线性响应的0.1Ma周期,在2.5—1.6MaB.P.时段存在与偏心率0.4Ma呈线性响应的周期。上述结果佐证了地球轨道变化对内陆古气候变化的线性驱动作用,但该理论不能解释在约0.9—0.6MaB.P.和1.6MaB.P.前后出现的二次较大的主导气候周期转型事件。  相似文献   

8.
The Lower Tagus Valley in Portugal contains a well-developed valley-fill succession covering the complete Late Pleistocene and Holocene periods. As large-scale stratigraphic and chronologic frameworks of the Lower Tagus Valley are not yet available, this paper describes facies, facies distribution, and sedimentary architecture of the late Quaternary valley fill. Twenty four radiocarbon ages provide a detailed chronological framework. Local factors affected the nature and architecture of the incised valley-fill succession. The valley is confined by pre-Holocene deposits and is connected with a narrow continental shelf. This configuration facilitated deep incision, which prevented large-scale marine flooding and erosion. Consequently a thick lowstand systems tract has been preserved. The unusually thick lowstand systems tract was probably formed in a previously (30,000–20,000 cal BP) incised narrow valley, when relative sea-level fall was maximal. The lowstand deposits were preserved due to subsequent rapid early Holocene relative sea-level rise and transgression, when tidal and marine environments migrated inland (transgressive systems tract). A constant sea level in the middle to late Holocene, and continuous fluvial sediment supply, caused rapid bayhead delta progradation (highstand systems tract). This study shows that the late Quaternary evolution of the Lower Tagus Valley is determined by a narrow continental shelf and deep glacial incision, rapid post-glacial relative sea-level rise, a wave-protected setting, and large fluvial sediment supply.  相似文献   

9.
In this study, we present grain-size distributions of the terrigenous fraction of two deep-sea sediment cores from the SE Atlantic (offshore Namibia) and from the SE Pacific (offshore northern Chile), which we ‘unmix’ into subpopulations and which are interpreted as coarse eolian dust, fine eolian dust, and fluvial mud. The downcore ratios of the proportions of eolian dust and fluvial mud subsequently represent paleocontinental aridity records of southwestern Africa and northern Chile for the last 120,000 yr. The two records show a relatively wet Last Glacial Maximum (LGM) compared to a relatively dry Holocene, but different orbital variability on longer time scales. Generally, the northern Chilean aridity record shows higher-frequency changes, which are closely related to precessional variation in solar insolation, compared to the southwestern African aridity record, which shows a remarkable resemblance to the global ice-volume record. We relate the changes in continental aridity in southwestern Africa and northern Chile to changes in the latitudinal position of the moisture-bearing Southern Westerlies, potentially driven by the sea-ice extent around Antarctica and overprinted by tropical forcing in the equatorial Pacific Ocean.  相似文献   

10.
The competing roles of bedrock uplift and climatic change in the formation of fluvial terraces remain uncertain. Most of recent studies have attributed terrace formation to climatic changes and held that, even in tectonically active settings, climate variations control cycles of terrace planation and abandonment. Based on field investigations of loess-paleosol sequences, magnetostratigraphy and optically stimulated luminescence (OSL) dating, we develop a new chronology for a spectacular flight of terraces along the Yellow River near Lanzhou, China over past 1.24 Ma. All the terraces are strikingly similar in that they have several meters of paleosol developed directly above fluvial deposits on the terrace treads, suggesting that the abandonment of each terrace due to river incision occurs during the transition from glacial to interglacial climates. However, the ages of terraces cluster in two relatively short time periods (1.24–0.86 Ma and 0.13 Ma – present). During the intervening time between 0.86 Ma and 0.13 Ma, terraces either did not form or were not preserved. We suggest that this record indicates that rock uplift rates varied through time and influenced terrace formation/preservation. Thus, our results demonstrate the utility of deep chronologic records from fluvial terraces for deconvolving the effects of tectonics and climate on fluvial incision.  相似文献   

11.
Fossils of terrestrial mammals preserved in submarine environment have been recorded in several places around the world. In Brazil such fossils are rather abundant in the southernmost portion of the coast, associated to fossiliferous concentrations at depths up to 10 m. Here is presented a review of such occurrences and the first record of fossils in deeper areas of the continental shelf. The fossils encompass several groups of both extinct and extant mammals, and exhibit several distinct taphonomic features, related to the marine environment. Those from the inner continental shelf are removed and transported from the submarine deposits to the coast during storm events, thus forming large konzentrat-lagerstätte on the beach, called “Concheiros”. The only fossils from deeper zones of the shelf known so far are a portion of a skull, a left humerus and of a femur of Toxodon sp. and a lower right molar of a Stegomastodon waringi, all collected by fishermen at depths around 20 m. The presence of fossils at great depths and distances from the present coastline, without signs of abrasion and far from areas of fluvial discharges does indicate that these remains have not been transported from the continent to the shelf, but have been preserved directly on the area that today correspond to the continental shelf. These remains indicate the existence of large fossiliferous deposits that have developed during periods of sea-level lowstand (glacial maxima) and have been submerged and reworked by the sea-level rise at the end of the last glaciation.  相似文献   

12.
Millennial to submillennial marine oscillations that are linked with the North Atlantic's Heinrich events and Dansgaard–Oeschger cycles have been reported recently from the Alboran Sea, revealing a close ocean-atmosphere coupling in the Mediterranean region. We present a high-resolution record of lithogenic fraction variability along IMAGES Core MD 95-2043 from the Alboran Sea that we use to infer fluctuations of fluvial and eolian inputs to the core site during periods of rapid climate change, between 28,000 and 48,000 cal yr B.P. Comparison with geochemical and pollen records from the same core enables end-member compositions to be determined and to document fluctuations of fluvial and eolian inputs on millennial and faster timescales. Our data document increases in northward Saharan dust transports during periods of strengthened atmospheric circulation in high northern latitudes. From this we derive two atmospheric scenarios which are linked with the intensity of meridional atmospheric pressure gradients in the North Atlantic region.  相似文献   

13.
Fluvial and eolian successions of oxygen isotope stage 3 are compared with global (GCM) and regional climate (RCM) modeling experiments of the stage 3 and last glacial maximum climate in Europe. Differences in precipitation between stage-3 stades and interstades were minor, which is confirmed by the fluvial successions. The fluvial response to climate variation is non-uniform, and in southern Europe more pronounced than in northern Europe. The model simulations indicate a strong western winter circulation over Europe during stage 3, which is supported by the eolian deposits data. Wind speeds in the last glacial maximum simulation appear modest compared with those of stage 3, which contrasts with the abundance of eolian deposits. This suggests that during glacial climates the stabilizing effect of vegetation determines eolian sedimentation rates, rather than wind speed. Stage 3 can be divided into an older part (>45,000 cal yr B.P.) with a relatively stable landscape and moist climate and a younger part with more frequent climate change and decreasing landscape stability.  相似文献   

14.
《Quaternary Science Reviews》2007,26(22-24):2801-2822
Late Cenozoic fluvial successions are widespread in India. They include the deposits of the Siwalik basin which represent the accumulations of the ancient river systems of the Himalayan foreland basin. Palaeomagnetic studies reveal that fluvial architecture and styles of deposition were controlled by Himalayan tectonics as well as by major climatic fluctuations during the long (∼13 Ma) span of formation. The Indo-Gangetic plains form the world's most extensive Quaternary alluvial plains, and display spatially variable controls on sedimentation: Himalayan tectonics in the frontal parts, climate in the middle reaches, and eustasy in the lower reaches close to the Ganga–Brahmaputra delta. Climatic effects were mediated by strong fluctuations in the SW Indian Monsoon, and Himalayan rivers occupy deep valleys in the western Ganga plains where stream power is high, cut in part during early Holocene monsoon intensification; the broad interfluves record the simultaneous aggradation of plains-fed rivers since ∼100 ka. The eastward increase in precipitation across the Ganga Plains results in rivers with low stream power and a very high sediment flux, resulting in an aggradational mode and little incision. The river deposits of semi-arid to arid western India form important archives of Quaternary climate change through their intercalation with the eolian deposits of the Thar Desert. Although the synthesis documents strong variability—both spatial and temporal—in fluvial stratigraphy, climatic events such as the decline in precipitation during the Last Glacial Maximum and monsoon intensification in the early Holocene have influenced fluvial dynamics throughout the region.  相似文献   

15.
Genetically meaningful decomposition (unmixing) of sediment grain-size distributions is accomplished with the end-member modelling algorithm. Unmixing of the loess grain-size distributions of a Late Quaternary loess–palaeosol succession from the north-eastern Tibetan Plateau indicates that the loess is a mixture of three end-members representing very fine sandy, coarse silty and medium silty loess. The unmixing approach potentially enables the unravelling of sediment fluxes from multiple dust sources, opening the way to significant advances in palaeoclimatic reconstructions from loess grain-size distribution data. However, as laser-diffraction size analysis is a volume-based technique, the proportional contributions of the modelled end-members might deviate (significantly) from weight proportions. Hence, calibration of the end-member volume proportions to weight proportions must be established before one can calculate the source-specific dust fluxes. This paper reports the findings of a sediment-mixing experiment which enables calibration of the modelled mixing patterns established for the Tibetan loess–palaeosol succession.  相似文献   

16.
Chinese loess deposits are generally considered to be the product of dust storms and dust falls from the central Asia arid zones that were transported across China by the northwesterly continental monsoon. In contrast, the Zhengzhou Loess found southeast of the Loess Plateau, adjacent to the floodplain of the Yellow River, records a different eolian regime and dust source. The Zhengzhou Loess was investigated by field observations, measurements of magnetic susceptibility, particle-size distribution, loss-on-ignition, CaCO3 and chemical contents. Both field observations and the laboratory results indicate that, during the last glacial, the Zhengzhou Loess was supplied by two different eolian regimes and dust sources, one was from the fresh flood deposits of the Yellow River driven by the northeast winds from the low-lying floodplain, and the other was from the dust storms and dust falls that traveled across the Loess Plateau driven by the northwesterly continental monsoon from the central Asian arid lands. The early Holocene, 11,500–8500 a BP, was a transition during the change in eolian regime and dust source because of the weakened northwesterly monsoon along with the global climatic amelioration. Following the retreat of the northwesterly monsoon from the onset of the mid-Holocene Climatic Optimum at 8500 a BP, dust supply from the drifting sand zone on the Yellow River floodplain became dominant because of the intensified strength of the northeast winds from the Bohai Sea. From 3100 a BP onwards, climatic aridity and extensive human disturbance have resulted in intensive eolian processes causing the incursion of the drifting sand into the Zhengzhou Loess zone. These results show that loess accumulation is more complex than traditionally assumed. The origin of loess deposits elsewhere outside the Loess Plateau may be related to dust sources derived from alluvial sediments of major river systems.  相似文献   

17.
文章以南海北部ODP1146站沉积物岩芯为研究材料,利用浮游有孔虫次表层水种Pulleniatina obliquiloculata壳体的氧、碳稳定同位素,结合该站位浮游及底栖有孔虫氧碳同位素数据,分析中更新世以来南海北部上层水体结构的演化。1.2 Ma以来ODP1146站P.obliquiloculata壳体δ^18O的变化可分3个阶段:1)1.2~0.9 Ma,冰期-间冰期变化幅度较小、主导周期为41 ka斜率周期;2)0.9~0.6 Ma,冰期旋回变化幅度逐渐增强、100 ka偏心率周期开始强化;3)0.6 Ma以来,冰期旋回呈现稳定且幅度较大的100 ka周期变化。0.9 Ma以来南海北部上层海水δ^18O的冰期旋回变幅增强,可能反映东亚冬季风在0.9 Ma之后显著强化。其中,表层水体δ^18O只在冰期变幅增强,P.obliquiloculata所反映的次表层水体δ^18O的变化幅度在冰期和间冰期都显著增强。约0.9 Ma浮游和底栖有孔虫δ^18O的100 ka周期几乎同时显现;但在0.9~0.6 Ma时期P.obliquiloculata的δ^18O偏心率周期更为显著、斜率周期的强度也更高。因此,冰期旋回周期转型及幅度变化两方面的证据共同反映温跃层结构演化在南海北部中更新世转型(MPT)气候转变过程中的特殊性。1.2 Ma以来ODP1146站P.obliquiloculata的δ^13C在0.02 Ma、0.49 Ma和0.99 Ma左右呈明显的碳重值,同时表层种-次表层种之间的δ^13C差值减小到近于0,可以解读为碳重值事件期间南海北部生产力相对减弱。  相似文献   

18.
The Weddell Sea Embayment (WSE) sector of the Antarctic ice sheet has been suggested as a potential source for a period of rapid sea-level rise – Meltwater Pulse 1a, a 20 m rise in ～500 years. Previous modelling attempts have predicted an extensive grounding line advance in the WSE, to the continental shelf break, leading to a large equivalent sea-level contribution for the sector. A range of recent field evidence suggests that the ice sheet elevation change in the WSE at the Last Glacial Maximum (LGM) is less than previously thought. This paper describes and discusses an ice flow modelling derived reconstruction of the LGM ice sheet in the WSE, constrained by the recent field evidence. The ice flow model reconstructions suggest that an ice sheet consistent with the field evidence does not support grounding line advance to the continental shelf break. A range of modelled ice sheet surfaces are instead produced, with different grounding line locations derived from a novel grounding line advance scheme. The ice sheet reconstructions which best fit the field constraints lead to a range of equivalent eustatic sea-level estimates between approximately 1.4 and 3 m for this sector. This paper describes the modelling procedure in detail, considers the assumptions and limitations associated with the modelling approach, and how the uncertainty may impact on the eustatic sea-level equivalent results for the WSE.  相似文献   

19.
对青海湖沉积物进行物质来源进行聚类分析, 并对周边风成黄土与现代湖泊表层沉积物进行粒度分析, 在此基础上, 以青海湖周边典型风成黄土作为风成组分端元, 以青海湖表层沉积物作为水成组分端元, 首次利用已知端元的粒度分布特征对青海湖沉积物中典型粒度分布进行拟合, 进而估算了其中风成和水成组分的比例。结果表明, 青海湖沉积物粒度分布特征可分为三大类:1.水成组分占主导; 2.风成组分占主导(风成黄土主导); 3.两者以不同比例混合。希望此方法能成为未来湖泊沉积物中不同组分的定量判据和古环境的解读提供新途径。   相似文献   

20.
Based on a revised chronostratigraphy, and compilation of borehole data from the Barents Sea continental margin, a coherent glaciation model is proposed for the Barents Sea ice sheet over the past 3.5 million years (Ma). Three phases of ice growth are suggested: (1) The initial build-up phase, covering mountainous regions and reaching the coastline/shelf edge in the northern Barents Sea during short-term glacial intensification, is concomitant with the onset of the Northern Hemisphere Glaciation (3.6–2.4 Ma). (2) A transitional growth phase (2.4–1.0 Ma), during which the ice sheet expanded towards the southern Barents Sea and reached the northwestern Kara Sea. This is inferred from step-wise decrease of Siberian river-supplied smectite-rich sediments, likely caused by ice sheet blockade and possibly reduced sea ice formation in the Kara Sea as well as glacigenic wedge growth along the northwestern Barents Sea margin hampering entrainment and transport of sea ice sediments to the Arctic–Atlantic gateway. (3) Finally, large-scale glaciation in the Barents Sea occurred after 1 Ma with repeated advances to the shelf edge. The timing is inferred from ice grounding on the Yermak Plateau at about 0.95 Ma, and higher frequencies of gravity-driven mass movements along the western Barents Sea margin associated with expansive glacial growth.  相似文献   

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