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1.
Radiocarbon dates obtained on organic materials overridden by a gelifluction lobe allow some estimate of past gelifluction rates for a site near sea level in northern Labrador. The calculated mean gelifluction rate for the last 400 years is in the order of 8 mm yr?1, somewhat higher than the average gelifluction rate described from other locations in the Canadian Arctic. The lobe contains two lithostratigraphic units: an inner diamicton, probably representing a buried gelifluction lobe, overlain by a silt/clay unit which may have been emplaced abruptly as a solifluction sheet. Mean creep rates for these units were in the order of 5 mm yr?1 and 15 mm yr?1 respectively. The area is presently subsiding, and transgressive beach material overlies terrestrial organics which are approximately 300 years old.  相似文献   

2.
The magnetic susceptibility (MS) of Chinese loess showing a general proportional relationship to pedogenic grade has been widely recognized and used for reconstruction of paleoclimate by Quaternary scientists. The in-situ pedogenic enhancement of ferrimagnetic content is normally believed to be the main reason for the increase of susceptibility in soil units. However, this pattern of high magnetic susceptibility in palaeosols, and low values in loess, are not replicated in some loess deposits. Siberian loess deposits display a completely opposite susceptibility behavior: high values in loess and low values in palaeosols. This inverse relationship has been explained by the idea that magnetic susceptibility is reflecting the magnitude of an aeolian ferrimagnetic component of consistent mineralogy, the grain size of which is related to average wind velocity. Our magnetic study of Siberian samples in this paper suggests that there are notable differences in magnetic properties between Siberian loess and developed palaeosols, not only in magnetic grain-size and concentration but also in magnetic mineralogy. This evidence is difficult to explain fully through variation in wind strength alone, but implies that the low magnetic susceptibility values in the Siberian paleosol units are a reflection, at least in part, of the alteration of the ferrimagnetic content by post-depositional processes. The Loess Plateau is a very arid area where potential evaporation is always higher than precipitation; pedogenesis occurs under dry oxidising conditions. The Siberian Kurtak region is located on the edge of the tundra where it is always wet and saturation during interglacials will lead to a reducing pedogenic environment. Ferrimagnetic minerals under this condition will be destroyed, resulting in lower magnetic susceptibility. Therefore, great care should be taken when using susceptibility values for paleoclimatic reconstruction.  相似文献   

3.
The Younger Dryas (YD) maximum highstand shoreline in SW Norway has traditionally been considered as being slightly concave, gradually steepening in the direction of uplift. This phenomenon is attributed to geoidal and isostatic effects near the former ice-sheet margin. On the basis of isolation basin data from the region, we have reconstructed this shoreline, and a Bølling-Allerød (B-A) lowstand shoreline, along three profiles in SW Norway. Along all profiles there are shore levels which, within the error limits estimated, cannot be captured by a single straight (or curved) shoreline. The anomalous shore levels occur near major fault zones and are interpreted to reflect differential uplift rates on opposite sides of faults, superimposed on the general glacio-isostatic tilting of the region. The inferred faulting is consistent with observations previously reported as neotectonic ‘claims’ in the region and shed new light on the deformational structures observed in seismic profiles of the fjord sediments. Excluding the anomalous shore levels, a straight shoreline with gradient ca. 1.1 m/km provides the best and most consistent representation of the YD shore levels along the three profiles. The B-A lowstand shoreline is constrained by fewer data points, but is approximately parallel-dipping the highstand shoreline. Our reconstructions imply a less steep YD maximum highstand shoreline compared to previous reconstructions, where gradients up to 1.4 m/km have been inferred. This may imply that the ice load effect on the lithosphere in SW Norway during the YD is less than previously assumed.  相似文献   

4.
In some of the world's desert and desert‐marginal areas (e.g. Simpson/Strzelecki, Australia) dunefields preserve well‐developed palaeosols, whereas in other regions with broadly similar climatic regimes and topography (e.g. southwest Kalahari), the dunes are characterized by very poorly developed internal stratigraphy. It has been postulated that dunes such as those in the Kalahari may never have had conditions conducive to soil formation, or that soils once formed but any evidence of palaeosols has been lost due to reworking. This study develops and applies a one‐dimensional numerical model to simulate dune development, soil formation and soil preservation. Variables in the model allowed experimentation on the influence of sediment supply, the time taken for soil to form, and the additional resistance to erosion offered by the soil. Reduced sediment supply plays a vital role in landscape development during periods of initial pedogenesis. Although re‐exhumed palaeosols influence sediment supply, the effect is minimal. Although under almost all parameterized conditions more than half (and up to 80‐90%) of those soils initially formed are lost due to reworking, evidence of their past formation remains in the large majority of profiles, and the dominant factor in controlling the preservation of palaeosols is the frequency of their formation. The implication is that where dunes are found without palaeosols, the most likely (albeit not certain) inference is that they have never formed. Counter‐intuitively, the limited sediment supply means their additional resistance to erosion becomes almost inconsequential to their preservation, at least until the unit approaches complete invulnerability. Short chronostratigraphic hiatuses around palaeosols are normal, and although long gaps can occur, they are extremely infrequent. Where such gaps are observed in field studies, external forcing factors (e.g. climatic or environmental changes) are implied, as they are highly unlikely to result from stochastic net preservation. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

5.
Bayesian age-depth models were constructed for two Late Quaternary aged fossil-bearing sedimentary sequences from caves in south eastern South Australia. The deposits in Wet and Blanche Caves contain dense assemblages of vertebrate fossils, largely the result of owl pellet accumulation. While individually calibrated radiocarbon determinations from the fossil sequences have provided a chronology for their accumulation, there was limited capacity available with such data to (a) temporally constrain assemblages associated with different depositional units and layers within the two sites, (b) interpret the chronological relationships among successive units and layers and (c) correlate sedimentary units and layers of similar age between the two deposits. Here, Bayesian age-depth models were constructed in OxCal for the Wet and Blanche Cave sequences, incorporating the available radiocarbon data and stratigraphic information collected during their excavation. Despite the low precision of the age-depth models for Wet and Blanche Caves which results in part from there being only single radiocarbon determinations available for a number of units and layers, the models enabled the relationships within and between the two sites to be established. Of particular utility for future faunal analyses is quantification of the temporal relationship between strata from the two sites, where groups of individual layers from Blanche Cave were found to be temporally equivalent with the longer-duration units in Wet Cave. We suggest that the use of Phase modelling, as performed here, is useful for cave deposits that have complex depositional histories and even in such instances where, as is common for palaeontological sites, few radiocarbon data are collected relative to the time-spans of tens of millennia that are often represented by them.  相似文献   

6.
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7.
Pipelines buried in saturated sand deposits, during earthquake loading could damage from resulting uplift due to excess pore water pressure generation. Several studies have been made to better understand the uplift mechanism and evaluate the effectiveness of mitigating techniques through experiment, but little numerical works have been done to assess the influence of soil properties and field conditions in pipeline floatation. Especially for previously buried pipelines, in order to set the priority for seismic retrofit, evaluating the risk of floatation in each region could be a concern. In this paper, effects of several parameters including dilatancy angle and density ratio of natural soil, diameter and burial depth of pipe, underground water table and thickness of the saturated soil layer on uplift of pipe have been investigated. Results show the prominent role of burial depth in pipe response and that there exits an optimum level for drop of water table to reduce floatation.  相似文献   

8.
Lacustrine fills, including those of oxbow lakes in river floodplains, often hold valuable sedimentary and biological proxy records of palaeo-environmental change. Precise dating of accumulated sediments at levels throughout these records is crucial for interpretation and correlation of (proxy) data existing within the fills. Typically, dates are gathered from multiple sampled levels and their results are combined in age-depth models to estimate the ages of events identified between the datings. In this paper, a method of age-depth modelling is presented that varies the vertical accumulation rate of the lake fill based on continuous sedimentary data. In between Bayesian calibrated radiocarbon dates, this produces a modified non-linear age-depth relation based on sedimentology rather than linear or spline interpolation.The method is showcased on a core of an infilled palaeomeander at the floodplain edge of the river Rhine near Rheinberg (Germany). The sequence spans from ∼4.7 to 2.9 ka cal BP and consists of 5.5 m of laminated lacustrine, organo-clastic mud, covered by ∼1 m of peaty clay. Four radiocarbon dates provide direct dating control, mapping and dating in the wider surroundings provide additional control. The laminated, organo-clastic facies of the oxbow fill contains a record of nearby fluvial-geomorphological activity, including meander reconfiguration events and passage of rare large floods, recognized as fluctuations in coarseness and amount of allochthonous clastic sediment input. Continuous along-core sampling and measurement of loss-on-ignition (LOI) provided a fast way of expressing the variation in clastic sedimentation influx from the nearby river versus autochthonous organic deposition derived from biogenic production in the lake itself. This low-cost sedimentary proxy data feeds into the age-depth modelling. The sedimentology-modelled age-depth relation (re)produces the distinct lithological boundaries in the fill as marked changes in sedimentation rate. Especially the organo-clastic muddy facies subdivides in centennial intervals of relative faster and slower accumulation. For such intervals, sedimentation rates are produced that deviate 10–20% from that in simpler stepped linear age-models. For irregularly laminated muddy intervals of the oxbow fill – from which meaningful sampling for radiocarbon dating is more difficult than from peaty or slowly accumulating organic lake sediments – supplementing spotty radiocarbon sampling with continuous sedimentary proxy data creates more realistic age-depth modelling results.  相似文献   

9.
Mechanical processes operating on the slope surface or at depth control the dynamics of alpine landforms and hold critical information of their geomorphological characteristics, yet they often lack systematic quantification and in-depth interpretation. This study aims to address a long-standing issue concerning geomorphological classification from a kinematic perspective. A group of periglacial landforms consisting of several lobes were discovered in the East Kunlun Mountains of China 30 years ago but were ambiguously classified as rock glaciers and later as gelifluction deposits. Here, we use satellite Interferometric Synthetic Aperture Radar to quantitatively characterize the spatial and temporal changes of the surface movement of these landforms. We observe that: (1) its 17 lobes show a pattern of landform-scale and uniform surface movement, especially during May to October; (2) the lobes move at a spatial mean downslope velocity of 10 to 60 cm/yr and a maximum velocity as high as 100 cm/yr in summer; (3) the landforms are nearly inactive from winter to late spring. Based on these observations, we postulate that the movement of the lobes are driven by deep-seated permafrost creep which typically occurs in rock glaciers. The debris of Lobe No.4 is composed of both boulders and pebbles supported by fine-grained matrix generated from the in situ weathering process. It develops a talus-like oversteepened front around 40° and a convex transverse profile perpendicular to the creep direction, which are also characteristic features of a rock glacier. Piecing these observations together, we identify Lobe No.4 as a debris-mantled-slope-connected rock glacier, with the gelifluction process occurring on the surface as small-scale and discrete events. © 2020 John Wiley & Sons, Ltd.  相似文献   

10.
We investigate the dynamics of suspended sediment transport in a hypertidal estuarine channel which displays a vertically sheared exchange flow. We apply a three-dimensional process-based model coupling hydrodynamics, turbulence and sediment transport to the Dee Estuary, in the north-west region of the UK. The numerical model is used to reproduce observations of suspended sediment and to assess physical processes responsible for the observed suspended sediment concentration patterns. The study period focuses on a calm period during which wave-current interactions can reasonably be neglected. Good agreement between model and observations has been obtained. A series of numerical experiments aim to isolate specific processes and confirm that the suspended sediment dynamics result primarily from advection of a longitudinal gradient in concentration during our study period, combined with resuspension and vertical exchange processes. Horizontal advection of sediment presents a strong semi-diurnal variability, while vertical exchange processes (including time-varying settling as a proxy for flocculation) exhibit a quarter-diurnal variability. Sediment input from the river is found to have very little importance, and spatial gradients in suspended concentration are generated by spatial heterogeneity in bed sediment characteristics and spatial variations in turbulence and bed shear stress.  相似文献   

11.
Optical microscopy and chemical analyses for major and trace elements have been performed on 27 Proterozoic metavolcanics from southern Sweden. The metavolcanics and associated metasediments are part of a large arced structure around a vast region with granitic batholiths.The compositional data show that the rocks are altered basalts and andesites and the Si, Ti, Zr and Cr relations demonstrate that almost half of the metabasalts are in accordance with present day arc volcanics and that another large fraction represents ocean floor tholeiites. These relations suggest that major plate tectonic processes occurred already during the Proterozoic in southern Sweden and that during these processes arc volcanics and ocean floor basalts were mixed at a subduction zone.  相似文献   

12.
Physical modelling experiments have been carried out in a cold room to test on a small scale, the effects of water supply during the thaw of an experimental slope with permafrost. Permafrost was maintained at depth and a thin active layer was frozen and thawed from the surface. Data from the experiments relate to two different conditions, first with moderate rainfall, and second with heavy rainfall during the thaw period. When moderate rainfall is applied during thaw phases, the experimental slope is slightly degraded. At the scale of the experiment, erosion processes involve frost jacking of the coarse blocks, frost creep and gelifluction that induce slow and gradual down slope displacements of the active layer, but also small landslides leading to large but slow mass movements with short displacements. Changes in experimental slope morphology are marked by the initiation of a small‐scale drainage network and the development of a little crest line which shows a progressive upslope migration. With such boundary conditions, there is not enough water supply to evacuate downslope the whole of the eroded material and a topographic smoothing is observed. When heavy rainfall is applied during thaw periods, rapid mass wasting (small mud‐flows and debris flows) become prominent. Slope failures are largely controlled by the water saturation of the active layer and by the occurrence of steeper slopes. At the scale of the experiment, rates of erosion and maximum incision increase by about 100% leading to significant slope degradation with marked and specific scars comparable to gullying. These morphological changes are dependant on both the size and the frequency of catastrophic events. These experiments provide detailed data that could improve the knowledge of the physical parameters that control the initiation, at a small‐scale, of erosion processes on periglacial slopes with a thin active layer and/or with thin cover of mobilizable slope deposits. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

13.
This paper provides a review on some of the electrostatic coupling effects relevant for generating/modifying plasma irregularities during nighttime in the low latitude ionosphere based on recent observations. Emphasis is given to the role of large polarization electric field associated with an unstable region affecting another region remotely located. Recent radar observations on valley region and E region irregularities from low latitudes show convincing evidence in support of effective electrostatic field coupling along the magnetic field line for their manifestation. Interestingly, the low latitude observations clearly show the ineffectiveness of plasma bubble related fringe fields in generating low latitude valley region irregularities unlike that over the dip equator. Velocity perturbations associated with the unstable low latitude E region relevant for studying the seeding of equatorial spread F are also shown. These new observations have been critically examined in the light of existing experimental knowledge and current understanding of the electrostatic coupling effects for the generation/modification of plasma irregularities in a remote region.  相似文献   

14.
Physical modelling has been developed in order to simulate the effects of periglacial erosion processes on the degradation of slopes and scarps. Data from 41 experimental freeze–thaw cycles are presented. They attest to the efficiency of periglacial processes that control both erosion and changes in scarp morphology: (i) cryoexpulsion leads to an increase of scarp surface roughness and modifies significantly the internal structure of the active layer; (ii) combined effects of frost creep and gelifluction lead to slow and gradual downslope displacements of the active layer (0·3 cm/cycle); (iii) debris flows are associated with the most significant changes in scarp morphology and are responsible for the highest rate of scarp erosion; (iv) quantification of the erosion rate gives values close to 1 cm3 cm?2 for 41 freeze–thaw cycles. These experimental results are consistent with field data acquired along the La Hague fault scarp (Normandy, France) where an erosion rate of 4·6 ± 1 m3 m?2 per glacial stage has been computed from the volume of natural slope deposits stored during the Weichselian glacial stage. These results show that moist periglacial erosion processes could lead to an underestimation of Plio‐Quaternary deformation in the mid‐latitudes. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

15.
A number of lakes in Southeast Norway have developed biogenic meromixis and have typically also considerable concentrations of dissolved iron and manganese in the bottom waters. Eight such lakes are described. Products of biological decomposition like carbon dioxide, methane and some inorganic salts gave a difference in density between mixo- and monimolimnion and hence chemical stability to the lakes. The concentration of iron and manganese gave minor contributions to the density gradients, and these substances are regarded as less important for the stability because of their reduced solubility under oxic conditions. Quantitative expressions of wind influence on lake surfaces were obtained by calculation of the work of the wind during the heating season. These values were low compared with a more exposed lake in the same region. Water movements generated by the wind have eroding effects on the chemical stratification only during short periods of homothermy before icelaying. During the rest of the year the monimolimnion is protected by either thermal stratification or ice cover. During the autumnal partial circulation a substantial reduction in chemical stability took place. For some of the lakes it is assumed that full circulation may take place in years with favourable weather conditions.  相似文献   

16.
The success of optically stimulated luminescence (OSL) dating relies to a large extent on suitable characteristics of the analysed mineral, in this case quartz. Previous OSL dating of Quaternary sediments in Scandinavia has shown that quartz characteristics vary widely across the region, resulting in dating studies with varied success. The aim of this study is to provide an overview of quartz luminescence characteristics in Sweden and Norway, evaluate their effect on dating results and discuss the underlying causes of their variability.A qualitative assessment of luminescence signal characteristics of quartz from Late Quaternary sediment deposits, from a range of geological and geographical settings, has been made by re-analysing data from samples previously dated at the Lund Luminescence Laboratory, Sweden. This allowed a general characterisation of signals and a study of the relationship of these properties to dating result ‘quality’. To quantify the results, selected samples were further analysed with single-grain measurements and with small aliquots.The results show that the average luminescence signal from quartz is fairly dim but dominated by a fast signal component and changes little during measurement. Dose determination precision is ∼4% for 8-mm aliquots and ∼6% for 2-mm aliquots. However, the luminescence signal characteristics have a spatial variation across Sweden and Norway, which appears to correlate with large-scale bedrock units. In areas of sedimentary bedrock outside the Scandinavian mountains and within the Blekinge-Bornholm province, the quartz is brighter and has a stronger fast signal component, while in the Caledonian orogenic belt, the signal is very weak and lacks a fast component. These differences lead to a range in precision of doses, from ∼2% to >40% (for doses in the order of 5–400 Gy), and in the number of rejected aliquots (0–100%) depending on location, but also implies that quartz luminescence can be used as a provenance indicator in part of Sweden and Norway.  相似文献   

17.
邢伟  鲍锟山  韩冬雪  王国平 《湖泊科学》2019,31(5):1391-1402
沼泽湿地是陆地生态系统的重要组成部分,在维护区域环境稳定中起着重要作用.随着社会经济的发展,人类活动导致湿地大面积退化和消失,严重影响了区域生态安全;恢复退化湿地已成为各国政府和学者关注的焦点.而了解历史时期沼泽湿地发育过程及影响因素则是建立合理湿地恢复目标的重要前提.东北地区是我国最大的沼泽湿地集中分布区,其中70%的湿地面临不同程度的退化威胁;但由于数据的缺乏,东北地区沼泽湿地发育过程及其与气候变化的关系仍不清楚.基于此,本研究系统分析了全新世以来东北地区沼泽湿地形成发育的动态变化过程,并探讨了东北地区不同区域沼泽湿地的发育规律及其对气候变化的响应机理.研究发现东北地区沼泽湿地约从12 ka（1 ka=1000 cal.）开始发育,在距今8.6 ka以后开始广泛形成,约有35%的沼泽湿地形成于全新世暖湿期（8.0-4.0 ka）;而沼泽湿地发育的高峰期则集中在全新世晚期.这种发育趋势与全球北方主要区域沼泽湿地大规模发育趋势显著不同.古气候重建表明,全新世早期东北地区气候温暖湿润,处在有利于沼泽发育的时期,促进了沼泽湿地的形成;而在全新世晚期,东北地区呈现冷湿的气候组合特征,冷湿的气候条件不利于有机质的分解,进而促进了沼泽湿地的大规模形成和发育.此外,研究结果也表明全新世以来东北地区不同区域沼泽湿地发育的时间和规模呈现显著的空间差异,而温度和降水则是影响不同区域沼泽湿地发育的最重要因素.本研究将为我国东北地区沼泽湿地的保护和恢复提供一定的理论和数据支持.  相似文献   

18.
—?Detailed studies of the low to intermediate seismicity in two coastal regions of Norway have been used in a comparison between earthquake locations from local high-precision networks on the one side and locations using a sparse regional array network on the other side. To this end, a reference set of 32 low-magnitude earthquakes have been located using two local temporary networks in northern and western Norway, with estimated epicenter accuracies better than 5 and 10?km, respectively. Comparisons are made between the local network solutions and the NORSAR Generalized Beamforming (GBF) system, which provides automatic phase association and location estimates using the Fennoscandian regional array network. The median automatic GBF location error is of the order of 20–30?km when four or more arrays detect the event, increasing to about 80–100?km when only two arrays are available, and the automatic GBF bulletin is essentially complete down to magnitude ML = 2.0. Most of the mislocation vectors of the NORSAR GBF solutions are oriented perpendicular to the Norwegian coast, and with a tendency to pull the location in a southeasternly direction. The GBF performance is clearly better, both in terms of accuracy and completeness, than the performance of the automatic bulletin of the Prototype International Data Center (PIDC) which uses data from essentially the same network. The analyst reviewed NORSAR and PIDC bulletins show, not unexpectedly, an improvement in location accuracy compared to the automatic solutions and appear to be of similar quality for the few common events, with an average mislocation of about 20?km. The NORSAR reviewed bulletin is more complete at low magnitudes compared to PIDC, and there appears to be a potential for significant improvements in the PIDC processing of small seismic events in this region.  相似文献   

19.
Age-depth modeling using Bayesian statistics requires well-informed prior information about the behavior of sediment accumulation. Here we present average sediment accumulation rates (represented as deposition times, DT, in yr/cm) for lakes in an Arctic setting, and we examine the variability across space (intra- and inter-lake) and time (late Holocene). The dataset includes over 100 radiocarbon dates, primarily on bulk sediment, from 22 sediment cores obtained from 18 lakes spanning the boreal to tundra ecotone gradients in subarctic Canada. There are four to twenty-five radiocarbon dates per core, depending on the length and character of the sediment records. Deposition times were calculated at 100-year intervals from age-depth models constructed using the ‘classical’ age-depth modeling software Clam. Lakes in boreal settings have the most rapid accumulation (mean DT 20 ± 10 yr/cm), whereas lakes in tundra settings accumulate at moderate (mean DT 70 ± 10 yr/cm) to very slow rates, (>100 yr/cm). Many of the age-depth models demonstrate fluctuations in accumulation that coincide with lake evolution and post-glacial climate change. Ten of our sediment cores yielded sediments as old as c. 9000 cal BP (BP = years before AD 1950). From between c. 9000 cal BP and c. 6000 cal BP, sediment accumulation was relatively rapid (DT of 20–60 yr/cm). Accumulation slowed between c. 5500 and c. 4000 cal BP as vegetation expanded northward in response to warming. A short period of rapid accumulation occurred near 1200 cal BP at three lakes. Our research will help inform priors in Bayesian age modeling.  相似文献   

20.
已有地质和地球物理研究结果表明,北京平原区存在有多条第四纪隐伏活动断裂和隐伏盆地.为了研究该区的地壳浅部结构、断裂的空间展布、断裂活动性以及深浅构造关系,2006年,在北京平原的西北部地区完成了1条60次覆盖的中深层地震反射剖面和跨断裂的浅层地震反射剖面.结果表明,沿剖面结晶基底埋深约为3～6 km在结晶基底以上,地震反射剖面揭示了一套连续性较好的强反射震相,应是新生代、中生代和古生代的沉积岩系;在结晶基底之下,为一系列横向连续性较差、能量较弱的短小反射事件,可能代表了变质程度较高的结晶变质岩系或不成层的其他岩体.中深层和浅层地震剖面揭示的断裂具有上下一致的对应关系和明显的第四纪活动,对本区地堑-地垒状盆岭构造和新生代地层厚度具有重要的控制作用.本项研究不仅可进一步提高对北京地区新构造活动的认识,而且研究中所采用的方法技术对其他地区的深浅构造探测研究也有借鉴意义  相似文献   

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