A detailed textural, mineralogical, and geochemical investigation of beach sands and seabed sediments from Thorikos and Oxygono bays of the eastern coast of Lavrion is performed, with the objective ...the provenance of the ore types exploited, the processing and beneficiation types employed, and the respective exploitation periods. The Oxygono Bay beach and seabed sands are highly heterogeneous, predominated by lithic clasts originating from surrounding lithologies. Examination of the fine-grained fraction from the seabed core revealed that only the upper 50 cm was affected by recent and ancient mining activity. Combining the mineralogy and geochemistry of Oxygono Bay sands with the radiochronological model of Pappa et al. (2018), four periods of recent exploitation (mid-19th—late 20th century) are distinguished: (1) The “1860–1875 A.D.”, involving exploitation of the ancient smelter slags, (2) the “1875–1900 A.D.”, with ongoing ancient smelter slag processing and the commencement of underground sulfide ore exploitation, (3) the “1900–1930 A.D.”, where heavy mining of the carbonate-hosted Pb–Zn–Ag ore occurs, (4) the “1930–1980”), where the implementation of flotation-type processing assisted in the exploitation of the poor skarn and porphyry-type ores. The latest “1980 A.D.—to date” period depicts the cessation of all mining and processing activities. The southern Thorikos Bay beach sands are homogeneous and fine-grained, are mainly composed of gangue and pyrite, and show elevated Fe, As, Pb, Zn, and Mn content. The southern Thorikos Bay beach sands clearly point to exploitation and processing by flotation of the carbonate-hosted Pb–Zn–Ag sulfide ore, and the tailings were disposed of from the nearby facilities to southern Thorikos Bay without any environmental concern during the “1930–1980 A.D.” period.
Abstract
Sederhana Beach is located in the waters of Muara Gembong, Bekasi Regency, West Java Province, northeast part of Jakarta Bay. The observation data in 2018 showed the morphology of the ...damaged coast and the massive fallen mangrove vegetation. Loss of land and mangrove populations has a major impact on the economic and environmental aspect. How to mitigate the impact of abrasion? This study aims to map the type of sediment, sediment distribution and its depositional environment, to obtain a correlation with the type and direction of currents causing abrasion. The results would become the base for determining the appropriate actions to overcome further abrasion, in an integrated coastal area planning program. Seabed sediments were taken using a grab sampler, megascopic descriptions and analysis of the depositional environment off sediments were held on site. The results are plotted onto a map, to determine the distribution of sediments. Sediment types found are: coarse sand, clay, sandy clay, medium-fine sand, and silt. 75% of the study area is: scattered sandy clay, mostly covering the coastline to the north. This is a common phenomenon because the mainland of research location is dominated by mangrove vegetation. The Coarse sand took 20% of the research location. An interesting phenomenon is: the coarse sand reaches the coast directly adjacent to the mangroves, where 5% of silt is found outside this coarse sand zone. The presence of coarse sand on the shoreline with mangrove vegetation indicates that the main current that triggers abrasion is longshore current, moving parallel to the coastline, namely northeast - southwest. So, the recommended steps that can be taken in mitigating further abrasion at Sederhana Beach is the construction of a current breaking structure that is perpendicular to the coast line
The anthropogenic impact in the German Exclusive Economic Zone (EEZ) is high due to the presence of manifold industries (e.g., wind farms, shipping, and fishery). Therefore, it is of great importance ...to evaluate the different impacts of such industries, in order to enable reasonable and sustainable decisions on environmental issues (e.g., nature conservation). Bottom trawling has a significant impact on benthic habitats worldwide. Fishing gear penetrates the seabed and the resulting furrows temporarily remain in the sediment known as trawl marks (TM), which can be recognized in the acoustic signal of side-scan sonars (SSS) and multibeam echo sounders (MBES). However, extensive mapping and precise descriptions of TM from commercial fisheries at far offshore fishing grounds in the German EEZ are not available. To get an insight into the spatial patterns and characteristics of TM, approximately 4800 km2 of high-resolution (1 m) SSS data from three different study sites in the German EEZ were analyzed for changes in TM density as well as for the geometry of individual TM. TM were manually digitalized and their density per square kilometer was calculated. In general, TM density was highest in August and October. Moreover, different gear types could be identified from investigating individual TM in SSS data. Beam trawl marks were observed to have widths of up to 22 m whereas otter board marks showed widths up to 6 m. The persistence of TM was estimated to 2–7 days minimum for all three sites based on the SSS data from 2015–2019. A maximum persistence could be defined at one site (Dogger Bank) and it was five months for the investigation period 2016–2017. Besides the main factors driving TM degradation (wave-base impact, sediment-type), different methods for TM detection (SSS, MBES, under-water video) are discussed. The study provides valuable information on the physical impact of bottom trawling on the seabed and can support existing monitoring strategies.
Gas-bearing sediments are widely distributed in five continents all over the world. Most of the gases exist in the soil skeleton in the form of discrete large bubbles. The existence of gas-phase may ...increase or decrease the strength of the soil skeleton. So far, bubbles’ structural morphology and evolution characteristics in soil skeleton lack research, and the influence of different gas reservoir pressures on bubbles are still unclear. The micro characteristics of bubbles in the same sediment sample were studied using an industrial CT scanning test system to solve these problems. Using the image processing software, the micro variation characteristics of gas-bearing sediments in gas reservoir pressure change are obtained. The results show that the number and volume of bubbles in different equivalent radius ranges will change regularly under different gas reservoir pressure. With the increase of gas reservoir pressure, the number and volume of tiny bubbles decrease. In contrast, the number and volume of large bubbles increase, and the gas content in different positions increases and occupies a dominant position, driving the reduction of pore water and soil skeleton movement.
In situ electrothermal conduction heating technology refers to the setting up of a heat source directly within the sediments, using the heat conductivity of the sediments and the heat radiation of ...the heat source for heat transfer to achieve the in situ heating of the sediments. The in situ electrothermal conduction heating of sediments has the disadvantage of the heating equipment being easily damaged and difficult to operate, and requires the equipment to be able to withstand seawater pressure and marine corrosion. In this paper, based on the combination of numerical simulations (using COMSOL Multiphysics software) and approximate in situ electrothermal conduction heating experiments, the temperature field and other factors of sediments heated by in situ conductive heating (in a specific area) were studied to determine a numerical model for sediment heat flow in a specific area under different pressures and initial temperatures, and the correctness of the numerical model was further verified by comparative experiments. The results of this study have important implications for future studies on the actual thermal properties of sediments and even heat transfer mechanisms during gas hydrate extraction.
The zero load readings of cone resistance (q
c
) and sleeve friction (f
s
) measured by a piezocone (uCPT) shift with the ambient temperature. A method of correcting the effect of temperature on uCPT ...measurements in seabed sediments has been proposed for the case where there is no temperature sensor in the uCPT probe. This method is based on the assumption that the "actual" profile of f
s
of soft shallow seabed sediments linearly increases with depth, and a rate of increase of α = 0.2 kPa/m was obtained using measured ground temperatures and f
s
values in seabed sediments in Isahaya Bay, Japan. An "actual" f
s
profile can then be constructed using the measured f
s
value at the shallow surface and the value of α. Using the differences between the measured and the estimated "actual" f
s
profiles, the ground temperature profile can be obtained, and then the effect of temperature on the uCPT measurements can be corrected. The proposed method was used for temperature corrections on uCPT measurements in Isahaya Bay, Japan. The values of undrained shear strength (s
u
) from the temperature-corrected uCPT measurements agree well with the laboratory measured values of s
u
using the undisturbed soil samples.
Pockmarks are observed worldwide along the continental margins and are inferred to be indicators of fluid expulsion. In the present study, we have analysed multibeam bathymetry and 2D/3D seismic data ...from the south-western Barents Sea, in relation to gas hydrate stability field and sediment type, to examine pockmark genesis. Seismic attributes of the sediments at and beneath the seafloor have been analysed to study the factors related to pockmark formation. The seabed depths in the study area are just outside the methane hydrate stability field, but the presence of higher order hydrocarbon gases such as ethane and/or propane in the expelled fluids may cause localised gas hydrate formation. The selective occurrence of pockmarks in regions of specific seabed sediment types indicates that their formation is more closely related to the type of seabed sediment than the source path of fluid venting such as faults. The presence of high acoustic backscatter amplitudes at the centre of the pockmarks indicates harder/coarser sediments, likely linked to removal of soft material. The pockmarks show high seismic reflection amplitudes along their fringes indicating deposition of carbonates precipitated from upwelling fluids. High seismic amplitude gas anomalies underlying the region away from the pockmarks indicate active fluid flow from hydrocarbon source rocks beneath, which is blocked by overlying less permeable formations. In areas of consolidated sediments, the upward flow is limited to open fault locations, while soft sediment areas allow diffused flow of fluids and hence formation of pockmarks over a wider region, through removal of fine-grained material.
Gas hydrates in sub-seabed sediments is an unexploited source of energy with estimated reserves larger than those of conventional oil. One of the methods for recovering methane from gas hydrates ...involves injection of Carbon Dioxide (CO2), causing the dissociation of methane and storing CO2. The occurrence of gas hydrates offshore Portugal is well known associated to mud volcanoes in the Gulf of Cadiz. This article presents a determination of the areas with conditions for the formation of biogenic gas hydrates in Portugal's mainland geological continental margin and assesses their overlap with CO2 hydrates stability zones defined in previous studies. The gas hydrates stability areas are defined using a transfer function recently published by other authors and takes into account the sedimentation rate, the particulate organic carbon content and the thickness of the gas hydrate stability zone. An equilibrium equation for gas hydrates, function of temperature and pressure, was adjusted using non-linear regression and the maximum stability zone thickness was found to be 798 m. The gas hydrates inventory was conducted in a Geographic Information System (GIS) environment and a full compaction scenario was adopted, with localized vertical flow assumed in the accrecionary wedge where mud volcanoes occur. Four areas where temperature and pressure conditions may exist for formation of gas hydrates were defined at an average of 60 km from Portugal's mainland coastline. Two of those areas coincide with CO2 hydrates stability areas previously defined and should be the subject of further research to evaluate the occurrence of gas hydrate and the possibility of its recovery coupled with CO2 storage in sub-seabed sediments.
Multibeam data have been acquired on eastern Tromsøflaket, a bank in the southwestern Barents Sea comprising a plateau (<
200 m depth) bounded by glacially eroded troughs. These new data reveal the ...morphology of Tromsøflaket in spectacular detail and show that much of the area has been intensively scoured by iceberg ploughmarks. Multibeam bathymetry and backscatter data together with samples and video data have been used to interpret the distribution of surficial seabed sediments. The general pattern of this distribution reflects the morphology. Coarse sediments occur on ridges and shallow bank areas, while finer sediments occur in depressions, on the slopes of the bank and in the deeper areas. One exception to this pattern is observed in the Sørøydjupet trough (ca. 300m depth) where coarse sediments occur. Recent sedimentation is dominated by mud and fine sand.
Detailed examination of the multibeam backscatter data however reveals a high variability of sediments, especially in and around iceberg ploughmarks. We observe coarser sediments on the levees either side of the ploughmarks than inside the furrows. Moreover we frequently note asymmetries in the distribution of fine sediments across these ploughmark features. Drawing inspiration from previous studies of bottom currents which use the distribution of sediments across bedforms (e.g. sand waves) we have interpreted the direction of bottom currents on Tromsøflaket by examining the local distribution of fine sediments across iceberg ploughmarks, which are the most prominent bedform in this area. Further integration of our results with existing grain-size - current-speed models have allowed estimates of the bottom current speeds to be obtained. Such interpretations are particularly valuable in this area where availability of bottom current measurements sparse.
Our approach makes valuable and novel use of multibeam data and results show good agreement with the limited set of available current data and models. We can identify near bottom pathways for the Norwegian Atlantic Current and Norwegian Coastal Current, the two major current systems in this region, and the patterns of circulation help to explain the sedimentation regime and identify recent erosion and deposition areas.
The bentonite-water mixture was selected as the substitute of seabed sediments according to the in-situ measurement data of sediments 15–20 cm deep in China’s ocean poly-metallic mining contract area ...and the soft seabed sediments could be simulated with certain proportion of the bentonite and water; besides, based on the theory on the interaction between the vehicle and ground and referenced to Bekker’s apparatus and related experimental methods, a scenario on the experimental system of the pressure-sinkage characteristics of interaction between the track of tracked vehicle and soft seabed sediments was designed. The pressure-sinkage experiments were performed with different dimensions of penetration plates. The “pressure-sinkage” model based on Bekker’s formula and correlation parameters were obtained to describe the corresponding characteristics of the seabed sediments and a smart calibration model on the pressure-sinkage characteristic of the track was established based on the function chain neural network, which could provide boundary loading conditions for simulation analysis of the tracked vehicle moving on the seabed.
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