Creep behavior of submarine sediments

A series of experiments on drained creep of marine sediment indicates that strength degradation results from the creep process, which implies an associated reduction in slope stability. Furthermore, the highest creep potential of a sediment may be at its preconsolidation stress. Results from the experiments on samples from Georges Bank continental slope were also used in conjunction with a preliminary theoretical model to predict creep displacements. For the case illustrated in this report, steep slopes (>20°) and thick sections (>30 m) give rise to substantial creep and probable creep rupture; as angles or thicknesses decrease, displacements rapidly become negligible.Creep may be a significant geologic process on many marine slopes. Not only can it cause major displacements of surface sediment, but it may also be the precursor to numerous slope failures.     

Geotechnical properties of submarine sediments in the Seto Inland Sea

Abstract

Sampling of submarine sediments by an improved piston corer and a bucket dredger has been carried out since 1973. The length of the core samples ranged from 0.9 m to 5.4 m. The recovery ratio ranged from 39.1 percent to 98.9 percent. The physical and engineering properties of 16 sediment cores and the physical properties of 125 dredged samples were determined, and variation of these properties was analyzed.

Clayey silts and silty clays cover the floor of relatively wide bays and sea areas. Sands and sandy silts blanket the floor of channels that have swift currents, and are also found in the vicinity of estuaries.

The piston core samples showed considerable sample disturbance, which should be estimated quantitatively in the future.     

Phosphorus fractionation in surface sediments of the Amazon continental shelf

The aim of this study is to separate various fractions of phosphorus in surface sediments of the Amazon continental shelf and compare them with other areas of the Brazilian shelf. Surface sediments were collected from the river-mouth transect (RMT) and open-self transect (OST). The extraction techniques available in the literature have been used to separate the following forms: Fe-PO₄, Al-PO₄, Ca-PO₄, and associated organic matter. The results indicate that the concentrations of phosphorus are uniform in the study area and may be compared with many other nonpolluted environments. Among the inorganic forms, Ca-PO₄ is dominant in all the samples, and Fe-PO₄ is present in lower concentrations. Al-PO₄ is more significant on the Amazon continental shelf than other parts of the Brazilian coast and other coastal environments of the world.     

Organic matter and the geotechnical properties of submarine sediments

Continental slope deposits off Peru and Oregon where coastal upwelling is a pronounced oceanographic process possess significant concentrations of organic carbon. Geotechnical properties are altered to varying degrees by the organic matter. Organic matter absorbs water and causes clay-size particles to aggregate forming an open fabric. This causes unusually high water contents and plasticity and exceptionally low wet bulk densities. Some of these deposits show notable increases in shear strength, sensitivity and degree of apparent overconsolidation. Owing to the unique geotechnical properties, sediment stability characteristics are considered to be poor in situations of excess pore pressures. Failure appears to take the form of a fluidized flow somewhat similar to the quick clays of Scandinavia.     

The bottom sediments of the Amazon shelf and tropical Atlantic Ocean

Analytical results of bottom sampling of the equatorial Atlantic Ocean affected by the Amazon River show a concentration of material smaller than 2 μ along the shore northwest of the mouth of the Amazon River for 2000 km. A zone having a high percentage of material coarser than 20 μ extends outward from the Amazon River and along the outer continental shelf northwestward for 2000 km. Areas having high carbonate percentages are encountered only on the outer shelf and in deep water.     

Similarity between trace-element composition of river and seabed sediments in the Amazon system

The aim of this paper was to study the elemental composition of sediments in the Amazon ecosystem by comparing bottom samples from the shelf with those from the floodplain of the Rio Amazonas. Trace elements measured by neutron activation analysis were used. The system is divided into three parts: the basin, including tributaries from the pre-Andean region to Óbidos; the delta—plain region between Óbidos and the mouth; and the continental shelf. Significant correlation was found between the shelf and floodplain sediments, especially those samples close to the mouth. In the shelf surface samples, Ta, Eu, La, and Ce (in this order) have the lowest coefficients of variation in concentration, whereas Hf, Br, and Cs have the greatest.     

Sedimentological parameters and erosion behaviour of submarine coastal sediments in the south-western Baltic Sea

The aim of this study was to evaluate the erodibility of submarine coastal sediments for the purpose of modelling sediment dynamics in Mecklenburg Bay, south-western Baltic Sea. Erosion thresholds derived from experiments with a device microcosm on cores of fine sand (n=5, mean grain size=132 µm) and mud (n=5, medium silt size, mean=21 µm), collected at different times of the year, were compared to theoretical critical shear stress velocities based on grain-size measurements. For this purpose, a sedimentological map of natural surface sediments was constructed for the study area. Calculated values for critical shear stress velocities (u* _{cr-Hjulström} ) are 1.2 cm s^?1 for fine sand, and 3.75 cm s^?1 for cohesive mud. At the mud station, erosion experiments showed an initial transport of the fluffy surface layer (u* _cr-initial ) at a mean critical shear stress velocity of 0.39 cm s^?1. Initial rolling transport at the fine sand station for single sand grains was recorded at values of 0.5 cm s^?1. At higher shear stress velocities, the two sediment types showed diverging erosion behaviour. Measurable erosion (ε>5.0×10^?6 kg m^?2 s^?1) of fine sand starts at a mean critical shear stress velocity (u* _cr-erosion ) of 1.15 cm s^?1 whereas fluffy surface material on mud cores was eroded at mean u* _cr-erosion of 0.62 cm s^?1. This indicates that measured erosion thresholds at the fine sand site fit well to calculated critical shear stress velocities whereas calculated erosion thresholds for cohesive mud are roughly 6 times higher than measured values. As erosion behaviour at the mud station was dominated by fluffy surface material, the comparability of measured and calculated threshold values may be reduced. The underlying silt-sized sediment itself was stable due to cohesive effects. This behaviour has to be taken into consideration by using sediment types instead of mean grain sizes for mapping and modelling sediment dynamics. A comparison of the near-bottom hydrodynamic conditions in the study area and experimentally derived critical shear stress velocities suggests that particle transport is controlled by storm events whereas under calm conditions shear stress velocities do not exceed the critical values.     

Ambient and dynamic pore pressures in fine-grained submarine sediments: Mississippi Delta

A multisensor piezometer probe has measured pore pressures in fine-grained submarine sediments of the Mississippi Delta over a period of approximately eight months. Data presented here cover the initial 2650 hours of the experiment. Dynamic and ambient pore pressures were recorded. Analogue data collected from the time of probe insertion include decay characteristics, steady-state (ambient) excess pore pressures, and the response of pore pressures to surface wave activity. The probe was installed in 43–44 ft of water near an offshore platform in the East Bay area of the Delta. Sensors were located at 21, 41 and 51 ft below the mudline. Ambient excess pore pressures were determined to be 0.7, 2.8 and 6.6 psi (lb/in²) at the respective depths. The relatively high excess pressures and the measured laboratory wet unit weights of the soil result in a significantly low effective stress. Pressure fluctuations due to tidal and surface wave activity were observed to produce significant pore pressure response in these soils. Preliminary data obtained using high-airentry and corundum stones indicate that considerably more research is necessary in order to fully understand the peculiarities observed in the data and to assess the role of dissolved and free gas on the pore pressures in submarine sediments.     

钦州湾外湾水下地形地貌特征及沉积物来源

人类对海岸带的开发利用活动改造了岸线形态,严重改变海洋区域性流场、水下地形形态及沉积物分布。利用单波束测深、沉积物采样分析等手段,查明研究区海底地形地貌和沉积物分布特征,探讨其沉积物来源。结果显示:①钦州湾水下地形复杂多变,水深为0～20 m,分为中部钦州湾外湾水下地形区、东部三娘湾水下地形区、西部防城港东侧水下地形区及南部湾外水下地形区等4个区域;②地貌分为潮控三角洲、潮流沙脊群、潮流冲刷槽和水下岸坡4个主要海底地貌单元以及潮滩、海蚀平台等潮间带地貌单元;③钦州湾砂质沉积分布广泛,占70%,自茅尾海口门向外海沉积物粒径逐渐变粗,由砂质泥变为泥质砂,再过渡为细砂和中粗砂。沉积物碎屑矿物主要由石英、长石、岩屑、钛铁矿、白钛石和电气石组成,粘土矿物以高岭石为主,其次为伊利石、蒙脱石,沉积物来源为近源河流的陆源输入为主。随着围填海和堤坝建设,钦州湾外湾海岸形态变化巨大,伴随着落潮流速大于涨潮流速的峡道效应,水下地形呈现出槽脊规模和高差加剧,近岸淤积加重的特点。     

海底热液成因含金属沉积物的研究现状及展望

海底热液成因含金属沉积物广泛分布于全球各大洋与弧后活动扩张中心、大洋玄武岩上覆沉积层的底部以及板内火山的顶部等区域.块状硫化物烟囱体经氧化蚀变发生再沉积作用、热液羽流的扩散和沉降作用或低温弥散流的直接沉淀均可形成含金属沉积物.尽管不同热液区的含金属沉积物在矿物和化学组成上具有一定的差异,但其相对正常远洋沉积物均表现为富...     

Regional geochemistry of sediments from the Hellenic volcanic arc in regard to submarine hydrothermal activity

Some 600 surface sediment samples from the Hellenic Volcanic Arc region have been analyzed by bulk and HCl selective leach geochemical techniques. Geostatisti‐cal analysis, followed by further selective leaches on a number of samples, has been used in order to identify any regional hydrothermal influence on the sediments and/or any local hydrothermal metal enrichments. The two main sediment components affecting the geochemical variability in the region are biogenic carbonate and volcaniclastic/terrigenous detritus. Proximity to islands appears to be the primary factor controlling the variability in these components. Sediments from deeper water, to the south of the Hellenic Volcanic Arc and in the Anydros Basin, exhibit some elevated levels of Mn, Co, and Cu, probably of hydrogenous origin. No widespread hydrothermal influence to the sediments in the region is recognized, although this could in part be due to the high sedimentation rates in the region. Localized hydrothermal inputs to sediments are, however, recognized proximal to Santorini, in particular within the northern part of the central caldera. These are predominantly of Fe and associated adsorbed/coprecipitated elements along with some Mn enrichment. Several samples proximal to Milos, just off the coast in the northern part of the central Milos embayment, also show enrichments of commonly hydrothermally associated elements, including Mn, Cu, Zn, and Pb. Data suggest that these enrichments could, in part, be due to weathering of “on‐land”; mineralization as well as hydrothermal inputs.     

High-resolution analysis of the deposition pattern on the Amazon sub-aquatic delta and outer continental shelf

Continental shelves off major river deltas or estuaries act as an interface between terrestrial environments and marine depo centers like the Amazon Fan. In order to understand sedimentary processes on the Amazon sub-aquatic delta and outer shelf, Late-Quaternary erosional and depositional structures have been investigated with an ultra high-resolution seismic survey system (4 kHz). The wavy morphology of the outer shelf implies the presence of tidal sand ridges or meandering channels. In the latter case, the local presence of channel fill deposits suggests several cut-off loops (ox bows). The inter-channel areas are consequently interpreted as levees, which reach their maximum height of 10 m in the area between the Amazon river mouth and the Amazon canyon. The morphology of terraces in front of the sub-aquatic delta reflect erosional processes which presumably occurred during accelerated sea level rise at the beginning and the end of the Younger Dryas. A carbonate platform is present in the northwestern survey area close to the shelf break. Its water depth of 120–130 m implies an evolution during the last glacial maximum. The asymmetric shape of the Cabo Norte Shoal on the topset of the sub-aquatic delta is typical of bottom current produced shoals. The steeper northwestern flank lies in the lee position of the northwest flowing coastal current. Southeast of the shoal the <10 m thick uppermost sequence of presumably Holocene age consists of lobes that dip with the same apparent angle as the foreset towards the offlap break. Within the area 20 km northwest of the shoal the uppermost sequence forms a smooth depression and may be erosionally truncated at the seafloor. Here, the prograding direction includes a northwest component. A shale diapir, which rises from a transparent underlying sequence, marks the transition to an area where the upper sequence is not resolved in the data.     

Geotechnical characteristics of mississippi delta sediments,southeast Louisiana

An investigation of the geotechnical properties of sediments from 57 boreholes drilled in West Delta and South Pass OSC Lease Areas, offshore Louisiana, indicated that sediment texture, shear strength, water content, and liquid limit vary significantly laterally and with depth. Samples are predominantly soft, fine‐grained sediments with high water content and medium to high plasticity. Firm, coarser‐grained sediments were obtained near the shoreline in South Pass OCS Lease Area and in the northern and eastern parts of West Delta OCS Lease Area. Trends in the distribution of geotechnical properties appear to be influenced by sediment texture and rate of sedimentation.     

Excess pore pressure in Mississippi delta front sediments: Initial report

Abstract

In September 1975, a differential piezometer probe was successfully implanted in the soft seafloor sediments of Block 28, South Pass, Mississippi Delta. The probe sensor is located approximately 6.4 m below the mudline in a water depth of 19 m, and has essentially continuously monitored excess pore pressure (the difference between sediment pore pressure and hydrostatic pressure at that depth) since installation. Excess pore pressure will be monitored until March 1976, when the probe will be recovered.

Immediately after deployment, an excess pore pressure of 54 kPa was recorded. An ambient excess pore pressure of approximately 32 kPa remained after dissipation of that developed during probe installation. Because of the possible presence of gas in the sediments in this area, it is not known with certainty whether the measured excess pressure is pore water pressure, pore gas pressure, or some combination of the two. An excess pore pressure of about 32 ±4 kPa was monitored during Hurricane Eloise and subsequent storms. The exact magnitude and time distribution of these pressure fluctuations is presently being evaluated.     

Composition of fluids expelled during compaction of Mississippi delta sediments

Fluids discharged from subaerial springs along faults on a sediment diapir near the mouth of the Mississippi River are derived from buried marine pore waters which have been extensively altered chemically by processes of bacterial respiration, mineral precipitation and, possibly, by fractionation due to the presence of clays of high exchange capacity. Vertical mass transport of dissolved components in many shallow marine sediments is controlled by long-term compaction, diffusion and bioturbation. In areas of rapid sediment deposition, however, these processes can be overwhelmed by catastrophic episodes of sediment failure, vertical mixing and upward discharge of water, gas and dissolved species.     

海南岛周边近海表层沉积中的孢粉及其与周边植被的关系

对海南岛周边近海海底180个表层沉积样品作了孢粉、藻类分析,根据孢粉分布规律及孢粉浓度特点,将研究区划分为3个孢粉分区:1区为松属-凤尾蕨科-水龙骨科-里白科-常绿栎 蕈树属 杨梅属孢粉区;2区为松属-凤尾蕨科-水龙骨科-里白科-铁线蕨属 鳞盖蕨属孢粉区;3区为松属-水龙骨科-凤尾蕨科-里白科孢粉区。研究区沉积中的孢粉组合与该区陆缘植被特征基本一致,两者特征的相关性为研究陆缘地区的古植被、古气候以及海相地层的划分和对比提供了理论依据。     

Geology and geotechnical characteristics of sediments in east bay area,Mississippi delta

Abstract

Deltaic sedimentation has produced an accumulation of clay and silt with a maximum thickness of 90 m in East Bay area. Sediments in the upper 50 m grade from interbedded sand, silt, and clay near South Pass and Southwest Pass to predominantly clay in the central part of the area. The variation in the types of sediments and rates of deposition greatly affected the engineering properties of these deposits. Sediments from boreholes in the central part of the area have shear strengths of less than 10 kPa to a depth of at least 50 m. Near the distributaries, shear strength increases with depth; values as high as 43 kPa were measured in sediments in the upper 50 m. These sediments are generally stronger and coarser, and have a lower water content and liquid limit than do sediments at comparable depths in the central part of the area.     

Carbon and biogenic silica export influenced by the Amazon River Plume: Patterns of remineralization in deep-sea sediments

The Amazon River Plume delivers freshwater and nutrients to an otherwise oligotrophic western tropical North Atlantic (WTNA) Ocean. Plume waters create conditions favorable for carbon and nitrogen fixation, and blooms of diatoms and their diazotrophic cyanobacterial symbionts have been credited with significant CO₂ uptake from the atmosphere. The fate of the carbon, however, has been measured previously by just a few moored or drifting sediment traps, allowing only speculation about the full extent of the plume's impact on carbon flux to the deep sea. Here, we used surface (0.5 m) sediment cores collected throughout the Demerara Slope and Abyssal Plain, at depths ranging from 1800 to 5000 m, to document benthic diagenetic processes indicative of carbon flux. Pore waters were extracted from sediments using both mm- and cm-scale extraction techniques. Profiles of nitrate (NO₃) and silicate (Si(OH)₄) were modeled with a diffusion-reaction equation to determine particulate organic carbon (POC) degradation and biogenic silica (bSi) remineralization rates. Model output was used to determine the spatial patterns of POC and bSi arrival at the sea floor. Our estimates of POC and Si remineralization fluxes ranged from 0.16 to 1.92 and 0.14 to 1.35 mmol m⁻² d⁻¹, respectively. A distinct axis of POC and bSi deposition on the deep sea floor aligned with the NW axis of the plume during peak springtime flood. POC flux showed a gradient along this axis with highest fluxes closest to the river mouth. bSi had a more diffuse zone of deposition and remineralization. The impact of the Amazon plume on benthic fluxes can be detected northward to 10°N and eastward to 47°W, indicating a footprint of nearly 1 million km². We estimate that 0.15 Tmol C y⁻¹ is remineralized in abyssal sediments underlying waters influenced by the Amazon River. This constitutes a relatively high fraction (~7%) of the estimated C export from the region.; the plume thus has a demonstrable impact on C_org export in the western Atlantic. Benthic fluxes under the plume were comparable to and in some cases greater than those observed in the eastern equatorial Atlantic, the southeastern Atlantic, and the Southern Ocean.     

基于地球化学参数的海底沉积物端元定量判识方法研究Ⅰ.分离沉积物端元的最优斜交成分因子算法

系统介绍“分离沉积物端元的最优斜交成分因子算法”,此算法包括主成分分析、因子得分成分化和最优斜交变换三部分．此算法可获得成分化的非负的斜交因子得分,弥补了Ｑ型因子分析仅能用于样品间的相互关系及样品的分类研究,而不能胜任海底沉积物端元物质分离工作之不足．方法适合于沉积物物源端元定量分离工作．将此方法应用于冲绳海槽海底表层样,成功地分离出长江陆源物质．此方法同样适合于诸如岩石、水体、土壤污染等混合过程的端元定量分离工作．     

长江远端三角洲表层沉积物中底栖有孔虫分布及其对环境的指示

长江远端三角洲是东海内陆架泥质沉积体重要组成部分,是全新世高海平面时期以来形成的重要地貌单元,其包含了高分辨率沉积环境与气候变化信息,对该地区现代沉积环境及其影响条件的分析将有助于长时间尺度古记录识别与反演。对研究区海域145个站位的表层沉积物样品进行了底栖有孔虫化石分析,鉴定结果表明该地区底栖有孔虫有52种,以玻璃壳体类型为主,平均含量占底栖有孔虫总量的90.6 %,其次为瓷质壳,胶结壳含量最低。进一步因子分析结果表明,研究区底栖有孔虫可划分为4个组合：组合Ⅰ（Ammonia beccarii vars.-Quinqueloculina spp.）分布在研究区西部近岸浅水区,组合Ⅱ（Bolivina robusta-Bulimina marginata-Hanzawaia spp.）分布于研究区东南部水深较大处,组合Ⅲ（Elphidium hispidulum-Fursenkoina schreibersiana）呈带状分布于近岸,组合Ⅳ（Elphidium advenum-Ammonia pauciloculata）分布区域位于组合Ⅰ和组合Ⅱ之间。组合Ⅰ可能指示区域水体环境受闽浙沿岸水团影响;组合Ⅱ可能指示区域水体环境受台湾暖流水团影响;组合Ⅲ的分布可能与夏季闽江水与外海水混合发生水体层化有关;组合Ⅳ可能指示区域水体环境受季节性闽浙沿岸水与台湾暖流水团影响。