摘要:

Pavements of manganese nodules and crusts and outcrops of Miocene limestones were observed on the flanks and flat top of the Tenpo Seamount during threeShinkai 2000dives. The pre‐Miocene volcano supplied nuclei of volcanic rocks and hydrothermal manganese deposits, and subsequent slow or no sedimentation promoted deposition of abundant hydrogenetic nodules and crusts, mainly on the upper flank of the seamount. Nodule pavements generally cover calcareous sand surface sediments, while crusts cover hard outcrops composed probably of volcanic rocks. The fields of crusts and nodules are sparsely distributed with each other on scales of meters to tens of meters. The on‐site observation suggests the deposits have encountered tectonic and/or mass movements that resulted in unusual occurrences of densely stacked nodules and occasionally the nodules resting directly on crusts or hard substrates. Mineralogical and chemical compositions reveal that for nodules and crusts the encrusting manganese layers of around 1 cm thickness are composed of hydrogenetic vernadite, and diagenetic influence is negligible.

ISSN:1064-119X    

DOI:10.1080/10641199309379924

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Manganese nodule distribution is primarily influenced by seafloor topography. Nodule distribution at 479 locations vis‐à‐vis seabed topography is studied by superimposing sampling location on the topographic profile and assigning appropriate domain (hilltop, valley, slope, or plain) for the sampling location. Highest mean abundance is observed at the valleys (6.94 kg /m2), followed by hilltops, slopes, and least on plains. Frequency distributions are regular (Gaussian) on plains, whereas on valleys and hilltops they are irregular (Rayleigh type). Fe and Co content is highest in nodules from hilltops and lowest in those from plains. Conversely, Mn, Cu, and Ni content is highest on plains and least on valleys. Fe: Mn and Co: Mn are negatively correlated in all the domains. Mn and total metal content (Ni + Cu + Co) show direct relationship in all the domains. An inverse relation between nodule abundance and composition is found. Cluster analysis on chemical and abundance data shows two distinct groups in all domains. Abundance and Fe and Co content typically form one group, while all other elements form another group. Genesis of nodules depends on the availability of supply of transition elements to the abyssal environment, maintenance of nodules in the sediment‐water interface, and sedimentation rates.

ISSN:1064-119X    

DOI:10.1080/10641199309379925

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Methods developed for quantitative estimation of seafloor features from seabed photographs and their application for estimation of nodule sizes, coverage, abundance, burial, sediment thickness, extent of rock exposure, density of benthic organisms, and their lebensspuren have been presented. Digitization of the photographs shows variable nodule size (< 1 to 10 cm), coverage (< 1 to 75%) and abundance (< 1 to 20 kg/m2). Nodule population is inversely proportional to the coverage of the sediment (10–100%) and its thickness (0 to > 10 cm), which causes differential burial (0–100%) of nodules. Correlation between nodule parameters (diameter and coverage) in the photographs and grab recovery is used to evolve empirical relationships for estimating nodule abundance in different seabed settings. The rock outcrops (basalts) with a coverage of 6–100% are the sources of nuclei for the nodules, the distribution of which is controlled by the local topography. Higher concentrations of nodules are observed along the slopes, followed by the crests of seamounts, and are lowest in the valleys and plains. A population density of 6–7 benthic organisms per 100 m2belonging to 7 different phyla is observed, with a high frequency of lebensspuren (4–12 traces/m2) in association with nodules. Estimation of these parameters can be used as important inputs in the design of the nodule collector, as it will have to encounter a variety of seafloor conditions, such as patchy nodule distribution, rock outcrops, steep slopes, and frequent microtopographic changes, as well as benthic life. The distribution and relation of various features with one another can also be used to understand the possible impact of nodule mining on the seabed. Estimates show that for a yield of 3 million tonnes of nodules per year, the volume of sediment disturbed will be between 200 × 107and 500 × 107m3over an area of 300–600 km2, depending upon the average abundance of nodules. Hence, the nodule collector will have to be a self‐propelled system, with photographic and acoustic sensors, to enable selective mining and avoid unfavorable areas.

ISSN:1064-119X    

DOI:10.1080/10641199309379926

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

SeaMARC II side‐scan imagery and bathymetry, seismic reflection, and free‐fall core sampling data reveal the morphology, structure, and hydrology controls on the formation and development of a series of crescent‐shaped slab slides along the submarine canyon walls in the Arequipa fore‐arc basin off southern Peru. The crescent‐shaped slab slides occur in a segment of an approximately 25‐km‐long canyon course and generally accord with the meandering of the canyon course. Most of the slab slides commonly consist of four distinctive zones: the fissured zone, scarp zone, voided zone, and frontal zone. The fissured zone is developed on the crown of the sliding walls; the scarp zone is marked by scars with crescent‐shaped slip surfaces and throws ranging from 50 to 120 m. The voided zone is characterized by 1–3 km wide terraces resulting from slab‐type excavating of the seafloor. The frontal zone is normally comprised of debris materials distributed on the seafloor near the canyon thalweg; however, there is lack of debris materials in some frontal zones, probably due to canyon erosion. The formation and development of the slab slides are probably attributed to five major factors: the homoclinal setting, faults and lithology of the strata, extensive weak strata contacts, interstrata‐fluid movement that can be compared to groundwater sapping processes on land, and earthquakes. The homoclinal setting in the uppermost section of the fore‐arc basin prepared a favorable condition for the canyon wall failure by providing a lateral continuity of a potential failure plane. The faults and lithology of the strata control the lateral margins of the slides. The extensive weak strata contacts within the homoclinal section provided a substantial failure surface for the slab slide. The interstrata‐fluid movements may lead to (a) the degradation of the strata strength through undermining of the strata contacts resulting from a lowering of friction, and (b) the collapse of canyon walls by removal of basal support or oversteepening as a result of enhanced erosion by the sapping process of the interstrata fluid. The formation of the crescent‐shaped slab slides was probably further facilitated by the triggering effect of earthquakes associated with the subduction of the Nazca plate under the Peru margin.

ISSN:1064-119X    

DOI:10.1080/10641199309379927

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A continuing series of postdredging surveys of an area in lower Chesapeake Bay from which sand has been mined indicates that the area is recovering through slow filling. Bathymetric data provide significantly more information than side‐scan sonagrams. The side‐scan data are only suggestive of slow filling of the dredged pit.

ISSN:1064-119X    

DOI:10.1080/10641199309379928

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Calibration chamber tests were conducted on open‐ended model piles driven into dried siliceous sands with different soil conditions in order to clarify the effect of soil conditions on load transfer mechanism in the soil plug. The model pile used in the test series was devised so that the bearing capacity of an open‐ended pile could be measured as three components: outside shaft resistance, plug resistance, and tip resistance. Under the assumption that the unit shaft resistance due to pile‐soil plug interaction varies linearly near the pile tip, the plug resistance was estimated. The plug capacity, which was defined as the plug resistance at ultimate condition, is mainly dependent on the ambient lateral pressure and relative density. The length of wedged plug that transfers the load decreases with the decrease of relative density, but it is independent of the ambient pressure and penetration depth. Under several assumptions, the value of earth pressure coefficient in the soil plug can be calculated. It gradually reduces with increase in the longitudinal distance from the pile tip. At the bottom of the soil plug, it tends to decrease with increase in the penetration depth and relative density, and to increase with the increase of ambient pressure. This may be attributed to (1) the decrease of friction angle as a result of increase in the effective vertical stress, (2) the difference in the dilation degree of the soil plug during driving with ambient pressures, and (3) the difference in compaction degree of soil plug during driving with relative densities. Based on the test results, an empirical equation was suggested to compute the earth pressure coefficient to be used in the calculation of plug capacity using one‐dimensional analysis, and it produces proper plug capacities for all soil conditions.

ISSN:1064-119X    

DOI:10.1080/10641199309379929

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

ISSN:1064-119X    

DOI:10.1080/10641199309379923

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor