程海表层沉积物粒度空间分布特征及其影响因素

由于受湖泊动力条件和物质来源的影响,湖泊表层沉积物的各粒度组分表现出不同的空间分布特征。选择滇西北程海作为研究对象,通过测试程海表层沉积物的粒度,分析了其不同粒度组分的空间分布特征,对影响程海表层沉积物粒度空间分布的因素进行探讨。研究结果表明:程海表层沉积物的优势粒级为粉砂,各样品沉积类型为黏土质粉砂。粒度组分及其参数的空间分布特征变化差异明显,黏土组分含量呈现出由南向北逐渐增加的趋势,而粉砂、中值粒级则呈现出相反的变化趋势,表明程海沉积物粒径受盛行南风的控制与影响,表现为湖泊水动力由南向北逐渐减弱。程海表层沉积物的粒度空间分布特征表明,其表层沉积物易受盛行风的影响向北扩散,因此,应加强该地区沿岸,特别是南岸人类活动的管理,有效控制人为物质的输入,减少污染物的扩散。     

Geochemistry of sediments in the back bay and Yellowknife Bay of the Great Slave Lake

Gold mining activities generated wastes with high concentrations of arsenic and zinc in the vicinity of Yellowknife, Northwest Territories, Canada. Some of the waste material has been discharged into Yellowknife Bay of Great Slave Lake. Concentrations of arsenic and zinc were determined in sediment cores collected at the depositional areas of Yellowknife Bay. Sedimentation rates were estimated using two different radiometric approaches: the depth profiles of Cs-137 and Pb-210. Geochemical composition of the sediment cores indicated input of similar material into sampling areas over the past 50 years. Age profiles of the sediment constructed from the radionuclides measurements were used to determine historical trends of arsenic and zinc inputs into Yellowknife Bay. The historical record was in good agreement with implemented remedial actions and usage patterns of both elements.     

Late Quaternary sequence stratigraphy of Lake Malawi (Nyasa), Africa

High resolution seismic data, multichannel seismic data and sediment cores were used to examine the Songwe Sequence, the uppermost of four depositional sequences identifiable on multichannel seismic data from Lake Malawi (Nyasa). The sequence has a maximum thickness of about 115 m in two areas of the lake, but is typically less than 70 m thick over most of the basin. The sequence is distributed along the entire length of the 560 km long lake, and is concentrated in three main depocentres. ¹⁴C age dates from sediment piston cores are extrapolated to provide an age estimate of about 78 000 yr bp for the oldest sediments within the Songwe Sequence. In the North and Central bathymetric basins of the lake, high resolution seismic data indicate a dynamic depositional environment, dominated by turbidity and mass flow deposits. Seismic data from the southern basin show acoustically transparent sediments with relatively low amplitude internal reflections, indicative of pelagic and hemipelagic sedimentation. In many areas the Songwe Sequence is underlain by a pronounced angular unconformity, suggestive of a significant, prolonged, low lake stage prior to deposition of the sequence. Seismic reflectors within the Songwe Sequence can be correlated to younger low lake stages identified from sediment core data. Major late Quaternary low lake level stages in Lake Malawi, interpreted from features identified in the seismic data and sediment core analyses, are tentatively interpreted at 6000 to 10 000 yr bp , 28 000 to >40 000 yr bp , and prior to 78 000 yr bp . Budget calculations indicate mean sediment concentrations from catchment runoff during the period of deposition of the Songwe Sequence to be about 190 mg 1⁻¹, comparable to estimates of modern rainy season discharges from the major river systems. Erosion rates within the drainage basin are estimated to be higher than the African average by a factor of three or more, probably due to the high relief within the Lake Malawi catchment.     

Sedimentological proof and dating of the Early Holocene volcanic eruption of Ulmener Maar (Vulkaneifel,Germany)

A minerogeniclayer occurs in early postglacial organic sediments from five maar lakes (West Eifel Volcanic Field, Germany). The mineralogy and stratigraphic position of this tephra suggests that it is related to the youngest German volcano, Ulmener Maar, nearby. Radiocarbon dating of wood from the base of the Ulmener Maar Tephra at two locations provide ages in agreement with an accelerator mass spectrometer ¹⁴C date for the minerogenic layer from sediments of Lake Holzmaar situated 13 km south-west of Ulmener Maar. The mean radiocarbon age is 9 560 years BP. Dating by varve chronology provides an age of 10017 years VT (varve time in years before 1950) or 10 895 years corrected VT. Based on palynology the Ulmener Maar Tephra was deposited at the end of the Preboreal. High values of natural remnant magnetization intensity, typical of pyroclastic material, confirm that this minerogenic layer differs in composition from other clastic deposits of the sedimentary record. Geochemical analyses reveal increased values of total trace elements for the Laacher See Tephra and Ulmener Maar Tephra. An isopach map based on thickness variations of the Ulmener Maar Tephra at five investigated maar lakes indicates that the tephra was mainly transported to the south west.     

Quaternary lacustrine deposits in a high-energy semi-arid mountain environment,Karakoram Mountains,northern Pakistan

Impressive Quaternary lacustrine deposits are present as terrace remnants throughout the Karakoram Mountains, northern Pakistan. They are mainly the result of damming of drainage systems during glacial advances or by catastrophic mass movement deposits. The longevity of most lakes is relatively short, in the order of years to tens of years, but sedimentation rates are extremely high as a consequence of the high sediment loads within the rivers. This results in deposits that frequently exceed 10 m in thickness. The sediments comprise dominantly planar bedded, massive and, less commonly, planar laminated, silts, comprising detrital quartz, feldspar, mica, calcite, chlorite and illite. A facies model for lacustrine sedimentation in a high-energy semi-arid high mountain region is presented, using case studies from a glacially dammed palaeolake (Glacial Lake Gilgit) and a debris-flow dammed palaeolake (Lake Serat). The rapid deposition and absence of organic material restricts the usefulness of these lacustrine sediments as proxies for palaeoenvironmental reconstruction, but they are helpful in reconstructing the former extent of glaciers and illustrating the importance of high-magnitude–low-frequency events, such as landsliding, as formative processes contributing to the evolution of the Karakoram landscape.     

Late Weichselian and Holocene seismostratigraphy and depositional history of the Gulf of Riga,NE Baltic Sea

The Lateglacial and postglacial sequence in the northern Gulf of Riga is sedimentologically subdivided into nine distinctive layers. In the seismo‐acoustic sequence these layers are correlated with seven seismic/acoustic units, which largely reflect different stages in the development of the Baltic Sea. A uniform layer of the Late Weichselian till, a layer of waterlain glacial diamicton (WGD), a varved succession of the Baltic Ice Lake, a brackish‐water/freshwater sandy/silty clay of Yoldia Sea, a FeS‐rich layer of Ancylus Lake and discordantly bedded sand of the Litorina Sea and present‐day gyttja are revealed both in sediment cores and in acoustic recordings. In general, the lateral extent of the distinguished sediment layers is gradually shrinking upwards in the Quaternary sequence towards the deepest, central depression of the gulf. Two distinguished regional discontinuities divide the Lateglacial and postglacial sediment sequence into three allounits: glacial diamicton deposits in the lower part; ice‐proximal WGD, glaciolacustrine and postglacial lake/marine deposits in the middle; and brackish‐water marine deposits in the uppermost part of the sequence. The presented detailed seismostratigraphic subdivision of the Quaternary sediment sequence of the Gulf of Riga permits a correlation/comparison with similar sequences across the Baltic Sea and in other former glaciated basins.     

洞庭湖泥沙淤积数值模拟模式

在确保沙量守恒的非恒定流、非均匀沙的二维水沙数学模型的基础上,通过洞庭湖水沙输运和河床变形计算实践,比较了两种冲泻质与床沙质的转化模式,提出了包括冲泻质在内的泥沙淤积模式和水沙动边界计算模式.从洞庭湖泥沙输运和河床变形计算的结果看,洞庭湖泥沙淤积数值模式具有较好的稳定性和较高的模拟精度,说明所建湖泊泥沙模型的良好性能和具有推广运用的前景.     

Rhone River flood deposits in Lake Le Bourget: a proxy for Holocene environmental changes in the NW Alps, France

The Holocene evolution of Rhone River clastic sediment supply in Lake Le Bourget is documented by sub-bottom seismic profiling and multidisciplinary analysis of well-dated sediment cores. Six high-amplitude reflectors within the lacustrine drape can be correlated to periods of enhanced inter- and underflow deposition in sediment cores. Based on the synthesis of major environmental changes in the NW Alps and on the age-depth model covering the past 7500 years in Lake Le Bourget, periods of enhanced Rhone River flood events in the lake can be related to abrupt climate changes and/or to increasing land use since c. 2700 cal. yr BP. For example, significant land use under rather stable climate conditions during the Roman Empire may be responsible for large flood deposits in the northern part of Lake Le Bourget between AD 966 and 1093. However, during the Little Ice Age (LIA), well-documented major environmental changes in the catchment area essentially resulted from climate change and formed basin-wide major flood deposits in Lake Le Bourget. Up to five 'LIA-like' Holocene cold periods developing enhanced Rhone River flooding activity in Lake Le Bourget are documented at c. 7200, 5200, 2800, 1600 and 200 cal. yr BP. These abrupt climate changes were associated in the NW Alps with Mont Blanc glacier advances, enhanced glaciofluvial regimes and high lake levels. Correlations with European lake level fluctuations and winter precipitation regimes inferred from glacier fluctuations in western Norway suggest that these five Holocene cooling events at 45°N were associated with enhanced westerlies, possibly resulting from a persistent negative mode of the North Atlantic Oscillation.     

Use of flood chronology for detailed environmental analysis: a case study of Lake Kizaki in the northern Japanese Alps,central Japan

This paper presents a study of the usefulness of flood layers as a time marker in sediments and a report of a case study of Lake Kizaki in central Japan. A flood layer can be identified as a layer having a higher density, coarser grain size, lower TN content, and higher C/N ratio than those of the upper and lower horizons. It can also be characterized by a hyperpycnal sequence composed of a basal coarsening-upward unit and a top fining-upward unit. When flood layers can be correlated with heavy rains in meteorological records, detailed age markers are well established in the sediment. Five flood layers were identified in the surface sediment of Lake Kizaki, and they could be attributed to the historical heavy rainfalls that took place on July 12, 1995; September 28, 1983; August 25, 1974; September 26, 1959; and September 1, 1949 under the constraint of an age model. A precise age model is essential to clarify the environmental changes such as the pollutant history in detail.     

History of scoria-cone eruptions on the eastern shoulder of the Kenya–Tanzania Rift revealed in the 250-ka sediment record of Lake Chala near Mount Kilimanjaro

Reconstructions of the timing and frequency of past eruptions are important to assess the propensity for future volcanic activity, yet in volcanic areas such as the East African Rift only piecemeal eruption histories exist. Understanding the volcanic history of scoria-cone fields, where eruptions are often infrequent and deposits strongly weathered, is particularly challenging. Here we reconstruct a history of volcanism from scoria cones situated along the eastern shoulders of the Kenya–Tanzania Rift, using a sequence of tephra (volcanic ash) layers preserved in the ~250-ka sediment record of Lake Chala near Mount Kilimanjaro. Seven visible and two non-visible (crypto-) tephra layers in the Lake Chala sequence are attributed to activity from the Mt Kilimanjaro (northern Tanzania) and the Chyulu Hills (southern Kenya) volcanic fields, on the basis of their glass chemistry, textural characteristics and known eruption chronology. The Lake Chala record of eruptions from scoria cones in the Chyulu Hills volcanic field confirms geological and historical evidence of its recent activity, and provides first-order age estimates for seven previously unknown eruptions. Long and well-resolved sedimentary records such as that of Lake Chala have significant potential for resolving regional eruption chronologies spanning hundreds of thousands of years.     

Determination of turbidity current erosional characteristics from reworked coccolith assemblages, Canary Basin, north-east Atlantic

Turbidites contain mixtures of sediments of various ages. These sediments can include both material that was eroded to form the initial turbidity current plus additions derived from erosion of the sea floor during transport. It has been possible to interpret the age range of sediments incorporated into individual turbidites from the Madeira Abyssal Plain by examining the proportions of microfossil (coccolith) species that they contain. The pelagic record of coccoliths is well known for the Quaternary period and shows a succession of dominant species or acmes each lasting a few tens of thousand years. Hence, erosion of sediment representing more than a few tens of thousand years will produce coccolith mixtures not seen in the pelagic record, but dependent upon the age range of the sediments that were eroded. This age range can be estimated by comparison with synthetic ratios of coccolith species created by combining ratios of species from successively older layers in the pelagic record. These can then be compared with the ratios found in individual turbidites until a match is found. The results show age ranges of 54–500 kyr for the sediment mixture in seven turbidites from the Madeira Abyssal Plain. Since the volumes of these turbidites are also known, and accumulation rates in their source areas can be estimated, it is possible to determine both the thickness and the area of the eroded sediment mass that generated the turbidity current. Minimum depths of erosion on the north-west African continental margin vary from 8 to 50 m and minimum areas eroded from 1600 to 5800 km². None of the turbidites examined contains a significant excess of surface sediment, suggesting that, once formed, the turbidity currents that transported them were virtually non-erosional, and that they travelled several hundred kilometres in this state.     

深基坑降水工程试验及降水方案设计

针对长江三角洲第四纪松散沉积层厚度大、地下水位高、地下水量丰富,地下水流动状态复杂等特点,利用深基坑降水试验推求水文地质参数.采用Visual MODFLOW三维有限元差分计算软件,以上海地铁七号线新村路站深基坑降水为例,建立了多层含水层地下水模型,提出了深基坑最优地下水降水方案.     

A numerical model of fine sediment transport on the continental shelf

A numerical model of fine-grained sediment dispersion in the New York Bight of the North American continental shelf is presented. Large amounts of waste material have been dumped in this region and the dispersal patterns of this material are of great interest to environmental managers. The model assumes that fine sediment resuspension is determined by surface wave activity and that transport is determined by tidal currents. Considering surface wave activity to be a random process reduces sediment motion to a random walk which is governed by a diffusion equation. The diffusion equation is solved numerically by an implicit time difference, finite element algorithm for a number of initial conditions. Initial conditions corresponding to ocean dumping sites show patterns of dispersal controlled by the geometry of the study region and the anisotropy of the tides. Material dumped at currently used dump sites reaches sensitive coastal areas before it leaves the continental shelf. Examination of the diffusion coefficients suggests an alternative dump site for fine-grained material. This dump site is relatively near sources of dumped material but produces minimal impact on coastal areas. This site should be considered as a possible alternative to expensive direct disposal at deep ocean sites.     

太湖月亮湾湖泛发生过程模拟及水土物化性质的响应

太湖湖泛具有在非确定区域非特定时间突发的特性,在野外很难进行及时有效的跟踪.利用Y-型沉积物再悬浮发生模拟装置,进行了风浪作用下太湖月亮湾藻源性湖泛发生过程的室内模拟,研究了湖泛发生过程中上覆水体及沉积物基本指标的变化特征.结果表明:湖泛发生过程中,上覆水体经历了显著的好氧-缺氧-厌氧阶段,溶解氧(DO)水平从起始阶段...     

波流作用下太湖水体悬浮物输运实验及模拟

利用室内外实验和数学模型对波浪和湖流共同作用下太湖水体悬浮物输运过程进行了研究.结果表明:当太湖日平均风速在2~8m/s时,水体悬浮物再悬浮通量与平均风速符合正相关关系;太湖悬浮物沉降属于絮凝沉降.悬浮物浓度较低时,其沉降速率与浓度无明显的相关关系;而浓度较高时,沉降速率随浓度升高而增大.悬浮物浓度和沉降速率符合Logistic函数;将太湖湖流模型和波浪模型耦合,有效地反映了太湖的水动力过程;在太湖悬浮物模型中,引入底泥起悬条件,将底泥的起悬量与沉降量分开处理,有效地模拟了太湖悬浮物的输运过程.模拟结果表明太湖悬浮物沿岸区域受湖流的影响较大,湖心区域受波浪影响较大.     

Evidence for mid- to late-Holocene palaeotsunami deposits,Kakawis Lake,Vancouver Island,British Columbia

Kakawis Lake situated four metres above sea level on western Vancouver Island, British Columbia, Canada, was the target of a palaeotsunami investigation. Six percussion cores recovered from this lake contain six anomalous deposits interbedded within the unconsolidated lacustrine sediments. Detailed sedimentological, geophysical and macro-fraction analyses were performed. The methods new to palaeoseismic approaches proved to be successful tools to characterize the anomalously coarse layers enriched in terrestrial plant detritus and marine shells. Based on at least eight types of evidence, six tsunami inundations are suggested as mechanisms responsible for the anomalous deposition, spanning from 3,634 to 2,534 cal yrs BP. Each tsunami event consists of a combination of different lithological facies resulting from different stages of tsunami inundation and settling of the material in the lake basin (pulses and inter-pulses). Tsunami deposits in lakes are shown to be less vulnerable to erosional and bioturbation processes than those found in marshes or beaches as well as underwater marine environments. However, few palaeoseismic studies have been carried out in low-elevation lakes along the Cascadia Subduction Zone region. The three last tsunami events known to have inundated areas along the Pacific shores of southern British Columbia, Canada and northern USA are not present at Kakawis Lake, establishing a current <4 m above mean sea level vertical limit as possible maximum tsunami height for areas located away from fjord heads on Vancouver Island. The anomalous deposits found in Kakawis Lake may be the oldest geological evidence of inferred tsunami on Vancouver Island, providing a possible recurrence interval between 200 and 400 years.     

Contamination of two Oregon reservoirs by cinnabar mining and mercury amalgamation

 Two reservoirs in western Oregon contain mercury-contaminated sediment and fish as a result of historic mercury mining in the Cottage Grove Lake watershed and mercury amalgamation used in gold mining in the Dorena Lake watershed. On average, sediment in Cottage Grove Lake contains ten times as much mercury as sediment from Dorena Lake (2.720 versus 0.242 ppm). Mercury content in Cottage Grove Lake sediment shows a sharp initial decrease and leveling off with time that reflects the end of the major cinnabar mining phase; deposition of other heavy metals appears to be linked to the clay content of sediment. Mercury input to Dorena Lake has remained fairly constant with time, but small increases in mercury are associated with the deposits of large floods. Copper, lead, and zinc input to Dorena Lake exhibits a marked decrease and leveling off related to the end of commercial mining for these metals. Received: 12 October 1999 · Accepted: 22 March 2000     

The occurrence of distal Icelandic and Italian tephra in the Lateglacial of Lake Bled,Slovenia

The discovery of sites preserving tephra layers from multiple volcanic centres is key to constructing a single European tephrostratigraphic framework for the Late Quaternary. Until now, the tephrostratigraphy of Europe has been divided into two halves: sites in the North Atlantic and northern Europe regions link the Icelandic, Eifel, and the Massif Central volcanic histories; whilst sites in southern Europe record the sequence of tephra layers produced by circum-Mediterranean volcanic provinces. The missing link, able to tie together these two halves, is found in the tephrostratigraphic record of Lake Bled, Slovenia.Lake Bled, in the Julian Alps, Slovenia, holds a high resolution multi-proxy palaeoenvironmental archive for the Lateglacial of south-central Europe. Cryptotephra investigations have revealed three tephra layers: two closely spaced within Younger Dryas stadial sediments and one shortly after the start of the Bølling-Allerød interstadial warming. Two of the tephra layers (Bld_T120 and Bld_T240) are of Campanian origin and are correlated to deposits of the Pomici Principali (PP) and Neapolitan Yellow Tuff (NYT) eruptions, respectively. The third layer (Bld_T122) correlates to the Icelandic Vedde Ash (VA), extending the known fallout of this widespread marker layer farther to the southeast.The Lake Bled record also allows the stratigraphic relationship and relative ages of the VA and the PP eruption to be discerned for the first time. Whilst existing numerical age estimates for these two deposits are indistinguishable within errors, their close occurrence in the same lacustrine sediment sequence shows that the VA was erupted shortly prior to the PP eruption.The tephrostratigraphy of Lake Bled developed here helps us to tie together regional volcanic stratigraphies into a broader, continental-scale lattice of sites, with the potential to allow the transfer of dates between remote sequences and the construction of relative chronologies, beneficial in particular for environmental and archaeological research.