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Resource estimation of Co-rich crusts of seamounts in the Pacific
Abstract
Marine Co-rich crusts are important as potential mineral resources for Co, Ni, Cu, Mn, and other metals, as well as for the paleoenvironment signals stored in their stratigraphic layers. The higher Co, Ni and Pt contents of crusts relative to pelagic polymetallic nodules and hydrothermal deposits have made seamount crusts a potential target for commercial exploitation. In order to obtain the amount of Co-rich crust resources on seamounts in the Pacific, based on the surveying data of Co-rich crust resources on seamounts in the western Pacific by means of dredge hauling, a series of detailed research on the distribution of Co-rich crust resources and parameter index for delineation of Co-rich crust resources on seamounts in the Pacific, each seamount is endowed with the crust thickness according to its height and age of ocean crust and consequent amount of dry crust resources is at first calculated to be (507.06-1014.11) × 108 t, (111.15-222.29) × 108 t manganese, (3.04-6.08) × 108 t cobalt, (2.23-4.46) × 108 t nickel, (0.66-1.32) × 108 t copper, and distributed area of crusts on seamounts in Pacific is 2062862 km2. By means of analyzing relationships between Co-fluxes with amount of Co-deposited in the crust and crust thickness, the endowed crust thickness accounts for 6.10%-12.20% of the theoretical deductive thickness, which is close to Ku's conclusion of "the crusts were actually growing for 4% of their lifetime". It is shows that the endowed number of crust thickness is reasonable and the obtained resource amount is reliable. This paper provides a new method for estimating the amount of Co-rich crust resources on seamounts in a whole deep-sea basin.
... Currently, different techniques are applied to evaluate the crust thickness. These techniques can be mainly divided into the following three categories: (i) The average crust thickness is mostly used for estimation [16]. When there is no grid to determine the stations, the thickness of the adjacent area can be estimated based on the crust thickness of adjacent stations [17]. ...
Seamount cobalt-rich crusts are rich in cobalt resources and are sought after worldwide. Among different affecting parameters, crust thickness is the most important in evaluating cobalt-rich crust resources in seamounts. Generally, there are two challenges to crust thickness evaluation: firstly, due to high operating costs, most geological stations for seamount exploration have sparse sampling distributions so there are insufficient data to estimate the crust thickness distribution; secondly, a single evaluation method has advantages and disadvantages, and it is not feasible to benefit from the advantages only. These methods cannot simultaneously make full use of the sampling data in local areas, providing a more appropriate evaluation of the whole area. As a result, the estimated results cannot fully reflect the thickness distribution. Based on the thickness data of the station survey and topographic data, geostatistical units are divided, and a comprehensive crust thickness assessment scheme is established on the ArcGIS platform. To this end, the adjacent area method is applied to calculate the crust thickness within the influence range of the station. Combined with the station buffer radius and Thiessen polygon method, the crust thickness within 1.5 km of the survey station was estimated. Then the “slope–distance” Kriging interpolation method was used to calculate the crust thickness in the study area, and the crust thickness in the optimal effective radius area was given to compensate for the missing part in the first step. Finally, the geological blocks were divided using the topographic classification method, and the crust thickness of the remaining unassigned regions was estimated using the mathematical expectation method. The proposed method was applied to evaluate the Il’ichev Guyot’s crust thickness and reasonable results were achieved. It was found that the thickness estimation of the area near the station is consistent with the measured values. Since finer topographic data are used in the calculation, the thickness estimation result is more detailed. In this regard, a simple and effective calculation method was established on the ArcMap platform. The mathematical expectation estimation method of the crust thickness, based on the topographic and geomorphological classification from the perspective of the mineralization mechanism, compensates for the drawbacks of the first two methods originating from the lack of data points. The results show that the proposed method is an appropriate scheme to evaluate seamount crust thickness without comprehensive investigation.
... 资源 [16] . 虽然各国都在研制开采深海富钴结壳的新技 术, 但是目前可利用的仍是陆地的钴矿资源(主要分类 见表1, 大陆各类型钴矿的代表性矿床品位和金属量等 见图1 [17~19] ). ...
Under the background of a shortage of conventional resources (such as coal and oil) and overfishing of inshore fishery resources, countries all over the world are paying special attention to energy strategies. China has also been paying special attention to energy strategies, as realized through equal trade. In addition, China also actively promotes the development and utilization of renewable energy, which is undoubtedly an effective method to deal with climate change and ease the energy crisis. Since ancient times, the ocean has been the cradle of human life, and it is also a huge treasure house of resources, including the following: a wide range of biological resources, rich mineral resources, chemical resources, and inexhaustible power resources. Exploitation of marine resources can strongly guarantee the survival of human society and sustainable development during the 21st century. A thorough understanding of the resources involved in the relevant seas will be conducive to the efficient and rational exploitation and utilization of marine resources. In this chapter, the characteristics of marine resources in the South China Sea and the Indian Ocean are analyzed, which will contribute to the construction of the Maritime Silk Road, benefit international marine development and construction, and promote the sustainable development of human society.
Copper ions can catalyze reduction of manganese dioxide by carbon monoxide in ammonia solution. The paper studied on the reduction ammonia leaching process of ocean Co-Mn polymetallic ores, such as the polymetallic nodules and the cobalt-rich crust. The self-catalysis reduction ammonia leaching was carried out, in which the copper ion leached out of the raw materials, the Co-Mn polymetallic ores, was taken as the catalyst and CO as the reducing agent. The concentration of copper in the leaching solution has great effect on the oxidationreduction reaction rate, and the reaction rate is very fast when the concentration of copper is more than 8g/L. By a suitable mixture ratio of polymetallic nodules to cobalt-rich crusts, the self-catalysis reduction ammonia leaching could be realized. At the condition that polymetallic nodules to cobalt-rich crust 3:1, temperature 45? and the leaching solution with more than Cu 9g/L, Ni 13g/L, and Co 3.5g/L, the average leaching extractions of nickel, cobalt and copper are 94%, 86% and 93% respectively in continuous leaching process. Copyright © 2013 by The International Society of Offshore and Polar Engineers (ISOPE).
Scientific classification of the geological objects is one of the most important basic research topics in geology. In this paper, thorough review of current research situation is made to establish the classification and nomenclature of deep sea sediments which unify the quantification and can be easily operated. Researches on various topics were conducted, eg., the composition and distribution of oceanic sediments, the relationship between water depth, average grain size and clay content, the discrepancy of clay, calcareous and siliceous content determination between smear and chemical analysis. The authors establish the quantitative relationship between calcareous and CaCO 3, between siliceous and biogenic SiO 2. This paper also compares and calibrates sedimentation method and laser method for grain size determination, and discusses generality and comparability of the classification and nomenclature system. Innovative key technique and scheme of classification and nomenclature for deep sea sediments were then put forward. The deep sea sediments are classified as deep sea clay, calcareous ooze, siliceous ooze, and clay-siliceous-calcareous ooze according to the simple classification method. The simple classification satisfies the general requirement of marine geological survey and basic understanding of sediment types. This method considers existing sediments mixture and existing classification methods. The deep sea sediments are further classified into 16 sub-types based on the sophisticated classification method, which therefore gives more detailed and comprehensive descriptions for the deep sea sediments and satisfies the requirement for comprehensive marine investigation. The classification and nomenclature for deep sea sediments are comparable with that for shallow sea sediments on diagram, indices, amount of types, naming methods, representation of mixture sediments and operability. Therefore, the classification and nomenclature for deep sea sediments is designed to be a succession of shallow sea sediments classification.
REE resources from deep-sea sediments is a kind of newly discovered and latent REE resources deposited on the ocean floor. Mineral component, major element and rare earth element analysis of 90 pelagic clay samples by piston core from the central Pacific have been conducted and compared with the geochemical chemistry of pelagic sediments from the central North Pacifc and Minami-Torishima area in the western North Pacific in this study. The results show that the pelagic clay from the central Pacific is rich in phillipsite, P, and REY, and significant fish teeth debris are contained in detrital component of pelagic clay; CaO and P2O5, CaO, P2O5 and ∑REY are positively correlated, and characterized by clear Ce negative anomaly and certain HREE enrichment and Y positive anomaly in REE distribution. The REY enrichment of pelagic clay from the central Pacific is mainly caused by excess phosphate component which may have resulted from the fish teeth debris mixed in the pelagic clay. The REY enrichment mechanism of REY-rich pelagic sediments in the North Pacific with no hydrothermal effect shows uniformity and universality, which can be attributed to the admixture of high-REY phosphate in the pelagic clay.
Combined with field studies, microscopic observations, and EBSD fabric analysis, we defined a possible Early Cretaceous metamorphic core complex (MCC) in the Wulian (五莲) area along the Sulu (苏鲁) orogenic belt in eastern China. The MCC is of typical Cordilleran type with five elements: (1) a master detachment fault and sheared rocks beneath it, a lower plate of crystalline rocks with (2) middle crust metamorphic rocks, (3) syn-kinematic plutons, (4) an upper plate of weakly deformed Proterozoic metamorphic rocks, and (5) Cretaceous volcanic-sedimentary rocks in the supradetachment basin. Some postkinematic incursions cut across the master detachment fault zone and two plates. In the upper plate, Zhucheng (诸城) Basin basement consists of the Proterozoic Fenzishan (粉子山) Group, Jinning period granite (762–834 Ma). The supradetachment basin above the Proterozoic rocks is filled with the Early Cretaceous Laiyang (莱阳) (∼135–125 Ma) and Qingshan (青山) groups (120–105 Ma), as well as the Late Cretaceous Wangshi (王氏) Group (85–65 Ma). The detachment fault zone is developed at the base and margin of the superposed basin. Pseudotachylite and micro breccia layers located at the top of the detachment fault. Stretching lineation and foliation are well developed in the ductile shear belt in the detachment faults. The stretching lineation indicates a transport direction of nearly east to west on the whole, while the foliations trend WNW, WSW, and SE. Protomylonite, mylonite, and ultramylonite are universally developed in the faults, transitioning to mylonitic gneiss, and finally to gneiss downward. Microstructure and quartz preferred orientation show that the mylonites formed at high greenschist facies to low greenschist facies as a whole. The footwall metamorphic rock series of the Wulian MCC are chiefly UHP (ultrahigh pressure) metamorphic rocks. Syntectonic rocks developed simultaneously with the Wulian MCC detachment and extension. Geological research has demonstrated that the MCC is associated with small-scale intrusive rocks developing in the vicinity of the detachment faults, for instance, dike. Geochronology results indicate that the denudation of the Wulian MCC occurred at about 135–122 Ma. Its development and exhumation was irrelevant to the Sulu UHP metamorphism zone rapid exhumation during Triassic Period but resulted from the crustal extension of North China Craton and adjacent area.
Determining the upper boundary of the cobalt-rich crust distribution of a guyot is important for estimating the mineral resources available, however, it has also long been an unsolved question. Correlations between the sub-bottom structure, revealed by sub-bottom profiling, and crust distribution can be revealed for the first time by the synchronous application of sub-bottom profiling and deep-sea video recording. The lower boundary of the sediment corresponds with the upper boundary of the crust. By analysis of these two kinds of data, the lower boundary of the sediment can be determined; therefore, the upper boundary of the crust distribution can be deduced. According to this method of analysis, the water depth of the upper boundary of crust distribution of a seamount in the western Pacific is about 1560 m.
Hydrogenetic ferromanganese crusts were dredged from four seamounts in the western Pacific, OSM7, OSM2, Lomilik, and Lemkein, aligned in a NW-SE direction parallel to Pacific Plate movement. The crusts consist of four well-defined layers with distinct textural and geochemical properties. The topmost layer 1 is relatively enriched in Mn, Co, Ni, and Mo compared to the underlying layer 2, which is relatively enriched in Al, Ti, K, and Rb and Cu, Zn, and excess Ba. Textural and geochemical properties of layer 2 suggest growth conditions under high biogenic and detrital flux. Such conditions are met in the equatorial Pacific (i.e., between the Intertropical Convergence Zone (ITCZ) and equatorial high-productivity zone). Layer 2 likely formed when each seamount was beneath the equatorial Pacific along its back track path. On the other hand, layer 1 probably started to grow after seamounts moved northwest from the ITCZ. This interpretation is consistent with the thickness of layer 1 across the four crusts, which increases to the northwest. Ages of the layer 1-layer 2 boundary in each crust, a potential proxy for northern margin of the ITCZ, also increase to the northwest at 17, 11, 8, and 5 Ma for OSM7, OSM2, Lomilik, and Lemkein, respectively. Assuming Pacific Plate motion of 0.3°/Myr, the seamounts were located at 12°N, 11°N, 9°N, and 8°N at the time of boundary formation. This result suggests that the north edge of the ITCZ has shifted south since the middle Miocene in the western Pacific, which agrees with information from the eastern Pacific.
We apply the wide-beam sampling technique of Jordan et al. (1983) to
approximately 157,000 km of wide-beam profiles to obtain seamount
population statistics for eight regions in the eastern and southern
Pacific Ocean. Population statistics derived from wide-beam echograms
are compared with seamount counts from Sea Beam swaths and with counts
from bathymetric maps. We find that the average number of seamounts with
summit heights h ≥ H is well-approximated by the exponential
frequency-size distribution:
ν(H)=νoe-βH. The exponential model for
seamount sizes, characterized by the single scale parameter
β-1, is found to be superior to a power-law
(self-similar) model, which has no intrinsic scale, in describing the
average distribution of Pacific seamounts, and it appears to be valid
over a size spectrum spanning 5 orders of magnitude in abundance.
Large-scale regional variations in seamount populations are documented.
We observe significant differences in seamount densities across the
Murray fracture zone in the North Pacific and the Eltanin fracture zone
system in the South Pacific. The Eltanin discontinuity is equally
evident on both sides of the Pacific-Antarctic ridge. In the South
Pacific, regions symmetrically disposed about the ridge axis have very
similar seamount densities, despite the large difference between Pacific
plate and Antarctic plate absolute velocities; evidently, any
differences in the shear flows at the base of the Pacific and Antarctic
plates do not affect seamount emplacement. Systematic variations in
νo and β are observed as a function of lithospheric
age, with the number of large seamounts increasing more rapidly than
small seamounts. These observations have been used to develop a simple
model for seamount production under the assumptions that (1) an
exponential size-frequency distribution is maintained, (2) production is
steady state, and (3) most small seamounts are formed on or near the
ridge axis. The limited data available from this study appear to be
consistent with the model, but they are insufficient to provide a
rigorous test of the assumptions or determine accurately the model
parameters. However, the data from the South Pacific indicate that the
off-axis production of large seamounts probably accounts for the
majority of seamounts with summit heights greater than 1000 m.
An empirical relationship between ridge elevation and age of the oceanic crust is presented for the Pacific, Atlantic, and Indian oceans. This relationship is accounted for by the thermal contraction of a cooling lithosphere as it moves away from a center of spreading, and thus is compatible with plate theory. Hence, it is possible to use topographic profiles to predict the age of the ocean floor. Detailed examination of profiles in different areas indicates that slow-spreading ridges (half-rate 3 cm/ yr). However, all ridges appear to show a uniform subsidence rate near the ridge crest. This uniform rate makes it possible to use topographic profiles in regions of smooth topography to predict the age of oceanic crust, less than 40 m.y. old, to better than 4-2 m.y. Topographic profiles will be particularly useful for predicting age where magnetic anomaly patterns are absent or difficult to interpret. Magnetic, topographic, seismic reflection, twenty-nine new, and all other heat-flow observations across the east Pacific rise and Mathematician and Clipperton seamount chains in the central eastern Pacific are examined. The magnetic and heat-flow observations aT e too inconclusive to enable a tectonic reconstruction of the area. However, topographic profiles at right angles to the rise between 20øN and the equator show that the Mathematician and Clipperton seamount chains are the old crest of the east Pacific rise. These parts of the rise crest terminated approximately 5 m.y.B.P. by the spreading center jumping 4 ø to the east. Seismic reflection profiles at right angles to the crest support this conclusion. The topography west of these two chains is used as the basis for a proposed evolution of the central eastern Pacific during the past 20 m.y.
The ‘Darwin Point’, where atolls drown, has been redefined as a function of climate, sea-level history, paleolatitude, seawater temperature and light. During the last 34 Ma, in the Hawaiian-Emperor Chain, the Darwin Point has been shown to shift at least between 24 and 30°N latitude. Atoll drowning there is correlated with decreased seawater temperature and light and depends on sea-level history as well as the elevation of atoll summits at the time of sea-level transgressions. If the atoll top falls below 30 m with respect to sea level and the rate of sea-level rise is more than 15 mm/yr, the atoll will drown. Results of the Ocean Drilling Program legs 143 and 144, which investigated the histories of several NW Pacific guyots, indicate that the demise of the shallow-water carbonate platforms there was related to either a temporal (110–100 Ma) eustatic sea-level event, the paleolatitude location (0–10°S) of nutrient-rich water, and/or increased seawater temperature (greenhouse effect) unconducive to the production of calcium carbonate by shallow-water organisms. Hence, the Darwin Point phenomena of atolls and guyots cannot be viewed as a manifestation of any single factor, rather, it results from a combination of factors including: decreased water temperature and light related to higher latitude; the amplitude and rate of sea-level changes; presence of nutrient-rich waters; and/or elevated seawater temperatures. It is indeed dynamic, both in time, space, and cause.
Recent developments in the use of nuclear accelerators as high-energy mass spectrometers1,2 has renewed interest in the study of cosmogenic 10Be (half life 1.5 Myr) as a geophysical time tracer. The accelerator techniques enable us to measure 10Be with a sensitivity of 107 atoms or less. The potential age dating span with 10Be is extended back to at least 15 Myr BP, providing that the 10Be production and deposition rates remain constant or nearly constant over that time period. A knowledge of the long-term constancy in the 10Be deposition rate is thus very important. We report here the first detailed measurements of 10Be and 9Be in two ferromanganese oxide crusts from the sea floor of the equatorial Atlantic (sample no. K-9-21) and the North Pacific (SCHW-1D), each representing >10 Myr of accumulation history. We have found in these crusts that the deposition of 10Be and 10Be/9Be during the past 7–9 Myr has been constant. Averaged over time intervals of ~1 Myr, the variation is within ±6%. Before that time, both crusts show similar significant deviations. The 7–9 Myr demarcation may be related to the reported late Miocene global abyssal circulation change in the ocean3.
Textural and geochemical properties of ferromanganese crust (Fe−Mn crust) samples from four adjacent seamounts near the Marshall
Islands were investigated to delineate the paleoceanographic condition on their growth history. The Fe−Mn crust samples of
this study show four distinct layers (layers 1 to 4 from top to bottom). The uppermost layer 1 is massive and black, and is
enriched in hydrogenetic elements such as Mn, Co, Ni, and Mo. The next layer 2 is porous and filled with sediment. Detrital
(Al, Rb, and Ti) and biogenic (Cu, Zn, and Ba) elements are enriched in layer 2. The layers 3 and 4 are phosphatized layers
which are impregnated with carbonate fluorapatite (CFA), and therefore their primary mineralogy and geochemistry were not
preserved. The property of layer 2 suggested that this layer had grown under the condition of high biogenic and detrital flux.
Such a condition may be met in the Inter-tropical Convergence Zone (ITCZ) where the northeast and southeast trade winds meet.
Considering the present location and paleotracking of seamounts, layer 2 appears to have formed when these seamounts were
beneath the ITCZ. On the other hand, layer 1 may have started to grow after the seamounts moved out of the ITCZ with the northwestward
movement of the Pacific plate. Our study indicates that the Fe−Mn crusts can be used to trace the paleolocation of ITCZ when
precise age determination and information on the plate movement are provided.
A new measurement techique based on a model of a Gaussian seamount loading a thin elastic lithosphere was developed to analyze seamounts that, until then, were not surveyed or seamounts with poor bathymetric coverage. The model predicts that the seamount diameter is equal to the peak-to-trough distance along the vertical deflection profile and that the flexural diameter of a seamount is related to the age of the lithosphere when the seamount formed. This model also suggests that these two measurements are relatively insensitive to the cross-track location from the seamount. These model predictions were confirmed using Seasat altimeter profiles crossing 14 surveyed seamounts in the Pacific. The analysis of the seamount distribution indicated considerable variations in population density and type across the oceans. Most notable among them are the absence of seamounts in the Atlantic, variations in population density across large fracture zones in the Pacific, and the prevalence of small signatures in the Indian Ocean.
The Gilbert Ridge and Tokelau Seamounts are the only seamount trails in the Pacific Ocean with a sharp 60° bend, similar to
the Hawaii-Emperor bend (HEB). These two bends should be coeval with the 47-million-year-old HEB if they were formed by stationary
hot spots, and assuming Pacific plate motion only. New 40Ar/39Ar ages indicate that the bends in the Gilbert Ridge and Tokelau seamount trail were formed much earlier than the HEB at 67
and 57 million years ago, respectively. Such asynchronous bends cannot be reconciled with the stationary hot spot paradigm,
possibly suggesting hot spot motion or magmatism caused by short-term local lithospheric extension.
The mineralizing characters relating to type, shape, composition structure and distribution of cobalt-rich ferromanganese nodule and crust in Pacific ocean seamount have been discussed by investigating information of two surveyings and using the other data. The results indicate that the cobalt-rich development in central Pacific oceanic seamount area, and the mineralizing characters are controlled by the environmental factors, such as landform, water depth and rock types.
Characteristics and distribution of manganese crust on the upper flanks of Horizon and S.P.Lee Guyots in the Central Pacific were investigated by means of seismic and photographic surveys and bottom sampling. Statistical analysis of nearly 1700 near-bottom photographs reveal extensive areas of contiguous crust coverage alternating with similarly extensive areas of soft sediment. Estimated total manganese crust resource between 800 and 2400 m on Horizon Guyot is about 75.5 million metric tons (t) containing 18 million t Mn, 1 million t Ti, 581 thousand t Co, and 340 thousand t Ni. S.P.Lee Guyot resources within the same depth interval are estimated to be 24 million t of crust containing 6.9 million t Mn, 317 thousand t Ti, 286 thousand t Co, and 130 thousand t Ni.-from Author
Marine Co-riched crusts are important as potential mineral resources for Co, Ni, Pt, Mn, and other metals, as well as for the paleoenvironment signals stored in their stratigraphic layers. The higher Co, Ni and Pt content of crusts relative to abyssal polymetallic nodules and hydrothermal deposits have made seamount crusts a potential target for commercial exploitation and has thus led to a surge of interest in their genesis and geochemistry. Twenty-six seamounts and guyots in the Western Pacific (Maggellan Seamounts, Marcus-Wake Mountains, Marshall Islands, Mid-Pacific Mountains, Line Islands) were sampled in detail, environmentally surveyed, and photographed by bottom cameras in order to better understand the distribution, origin, and evolution of marine Co-enriched crusts by Second Institute of Oceanography of SOA and Guangzhou Marine Geological Survey of MLR in the last decade. The major purpose of exploiting Co-riched crusts is to delineate and mine the deposits. Unfortunately, the parameter and index for delineating and evaluating the crust resources are not available so far. Based on the various data collected from the twenty-six seamounts and guyots, together with marine environment information of the Pacific, we have analyzed the changes of crust thickness, abundance, grade, coverage, resource amount, seamount slope and water depth by the quantitative method. We determined the mine area, studied the distribution characteristics of crust resources, delineated and evaluated the crust resources of thirteen typical seamounts and guyots. Accordingly proposed eight important parameters and indexes for delineating and evaluating Co-enriched crust deposits: (1) crust thickness ≥3 cm or ≥4 cm, depending on the different seamount locations; (2) water depth ≤2500 m or ≤3000 m; (3) Co content ≥0.50% or ≥0.60%; (4) abundance ≥60 kg/m2 or ≥70 kg/m2; (5) seamount slope ≤15°; (6) crust coverage ≥30%; (7) application area 17000-20000 km2; (8) reservation area 5000-6000 km2. The parameters and indexes we present here will promote the deposit delineation, resource estimation and evaluation, and will also provide the quantitative parameter index for our country's active participation into the establishment of regulations on exploiting Co-enriched crust resources by the International Sea-bed Authority of United Nations.
Manganese nodules are common on the deep-ocean floor, but vary considerably in abundance and composition from place to place. They reach their greatest concentrations in deep-water areas at or below the calcium carbonate compensation depth, where detrital sedimentation is at a minimum and overall sedimentation rates are very low. Extensive deposits are also associated with areas of non-deposition or sediment erosion. The localized distribution of nodules within areas of overall high concentrations can be related to a number of factors that include sea–floor topography, sediment distribution, and distribution and abundance of potential nuclei around which the nodules can accrete. The factors that determine environmentally differentiated compositional variations between nodules include seawater composition; biological productivity; the nature of the substrate; and the degree of oxidation of the environment of deposition by its influence, firstly on the oxidation states of metals incorporated into the deposits, and, secondly, on the mineralogy of the nodules themselves that partially determines their receptiveness to different ionic species.
We characterize the seamount distribution on the Pacific Plate using the gridded vertical gravity gradient (VGG, or geoid curvature) derived from Geosat and ERS-1 satellite altimetry. The VGG amplifies short-wavelength information and suppresses longer wavelength components, making it suitable for seamount detection purposes. Furthermore, the VGG over seamounts has a much more pronounced zero crossing than that of the free-air anomaly (FAA); the distance to the zero crossing can be used as a proxy for seamount radius. After removing a regional field obtained by robust median filtering we identify seamount amplitudes and locations from local maxima in the VGG grid. The radius of a seamount is more difficult to estimate since seamounts tend to cluster and overprint each other's signals. Individual seamounts are modeled as Gaussian, axisymmetric objects loading an elastic lithosphere; the VGG over such features can be approximated by a simple analytical expression which we use to determine the zero-crossing distances for overlapping seamounts. By using the VGG the maximum amplitude and distance to the zero crossing become largely independent of the elastic plate thickness and infill density. We do forward modeling of Gaussian seamounts and their gravimetric response and create a look up table that relates seamount FAA (in milligals), VGG (in Eötvös), and zero-crossing distance (in kilometers) to actual height and radius (in kilometers). The frequency-size distribution of these predicted seamount heights follows a power law for heights between 2 and 8 km. The seamount density (number of seamounts per area) is greatest in the central Pacific. We confirm earlier results suggesting that the majority of large seamounts are located in the western region of the Pacific Plate, on older crust. As crustal age increases, so does seamount density, peaking on 100-130 m.y. crust, supporting suggestions of high magmatism in the Cretaceous. We demonstrate that there may be an empirical relationship between the seamount VGG amplitude and the age of the lithosphere at the time of seamount formation and invert this relationship to predict seamount ages from VGG amplitudes. These pseudo ages have large uncertainties but, nevertheless, may be used to investigate temporal fluctuations in Pacific intraplate volcanism. Our results indicate that seamount intraplate volcanism attained a maximum level in the mid-Cretaceous to Late Cretaceous, about 70-120 Ma, apparently contemporaneous with the formation of large oceanic plateaus in the Pacific.
Present-day tectonic concepts of events on the western paleo-Pacific lithosphere must be assessed relative to new data. Data collected by the newer techniques of geophysical surveying reveal leaky fracture zones, trending NNW–SSE and WSW–ENE; non-sequential in-line ages on most seamount chains; and orthogonal intersections of fracture zones. The fracture zones meander, braid, merge, splay, start and stop at any place, and are generally aligned with, or contain, linear chains of seamount. The combination of these in-line features is called megatrends. When the GEOSAT data are compared to the available bathymetry, this seemingly jumbled tectonic structure is verified. As the pole of rotation changes, the stress field changes alignment to agree with the Chandler wobble of Earth. Younger megatrends, propagating ever eastward, cross the older, already imprinted megatrends. During the Cretaceous, the voluminous outpouring of igneous rock created the large Pacific plateaus and rises where the megatrends, active and inactive, orthogonally intersected. The magma floods at the intersections flowed outwardly, and the outward flooding accounts for the fanning magnetic lineations around the Manihiki, Magellan, Shatskiy, and Hess rises. A case study of the Mid-Pacific Mountains (MPM), lying in the north-central Pacific Ocean basin, shows that the MPM formed about 125–110-Ma by overprinting the orthogonal intersections of fracture zones at the Molokai and Easter/Krusenstern–Emperor megatrends and the Murray and Tubai/Mamua megatrends. The MPM have been undergoing distortion into a vortex structure, a feature which has been confirmed by updated bathymetry, GEOSAT altimetry data, and drillsite information.
Dredge sampling and bottom photography of submarine terraces in the Hawaiian Archipelago find that ferromanganese deposits encrust rock outcrops and form widespread pavements. These deposits are economically interesting because of their cobalt content and location within the 200-mile zone. Ferromanganese crusts and pavements are most prevalent on two deep terrace levels — the Laualualei (370–560 m) and Waho (900–1200 m) — recognized throughout the archipelago, although crust thickness and element content vary considerably on both regional and local scales. The characteristic variability in crust thickness and element content suggests that environmental conditions vary on small spacial and temporal scales. We find no relationship between substrate composition and crust thickness or element content, indicating that ferromanganese oxides are deposited from seawater and accumulate on any exposed, hard substrate surface. We speculate that structural and sedimentary processes regulate the exposure of resistant substrate as a prerequisite for crust layers deposited during intermittent accretion episodes. An enrichment in cobalt is correlated to Mn/Fe ratio and the relative proportions of δ-MnO2 and FeOOH which are the principal crust components. Cobalt content cannot be correlated to water depth, crust thickness, or basement age of the deep terrace localities. Since crust thickness and cobalt content generally increase along the archipelago to the northwest, deposits found on the wide terraces of the leeward archipelago have the most economic potential. This study concludes that hydrogenous deposition of ferromanganese oxides from seawater is affected considerably by local environmental conditions during the evolution of deep terraces on the submarine of slopes of the Hawaiian Archipelago.
Satellite altimetry was used to identify and characterize Pacific intraplate seamounts. The gravimetric amplitudes of seamounts
appear to be related to the age difference between the sea floor and seamounts; by inverting this relation, pseudo ages can
be obtained for undated seamounts. These pseudo ages imply that excursions in seamount volcanism generally correlate with
times of formation of large oceanic plateaus.
One of the most promising applications of the new 10Be detection technique using accelerator mass spectrometry is the determination of growth rates of Mn-nodules and crusts1–5. Because the half life of cosmogenic 10Be (1.5 Myr) is of the same order as the time needed for a few millimetres of nodule material to grow, 10Be is a useful tool for unravelling such evolution during the late Tertiary, when most nodules are assumed to have started their growth6. Mass spectrometric measurement of a 10Be profile across the manganese crust VA 13-2 from the Central Pacific, using a tandem van de Gr£ a ff accelerator, yields growth rates of 2.7 and 4.8 mm Myr−1 for the layers of the crust accumulated between Recent and 6 Myr BP and between 6 and 11 Myr, respectively. Using these measurements as well as 230Th dating, we have been able to distinguish boundaries between zones in the crust with different structures and chemical compositions and to assign ages to them. These ages are 0.12, 2.9–3.4, 5.7–6.7, 7–9, 10–12 and 13–16 Myr BP. All of these boundaries apparently coincide with the times reported for Quaternary and late Tertiary palaeoclimatic events, suggesting that the crust growth has been strongly influenced by palaeoclimate.
Recent geochemical studies on ferromanganese deep-sea nodules have shown that the relationship between Mn and Fe allows us to differentiate between a diagenetic type (Mn/Fe ≥ 2.5) and a hydrogenetic type (Mn/Fe ≤ 2.5)1–7. Diagenetic and the lower parts of mixed type nodules are mainly supplied by an early diagenetic pore water source; hydrogenetic type nodules grow by very slow accumulation of inorganic colloidal particles of hydrated metal oxides from near-bottom seawater. Growth rates of hydrogenetic nodules generally do not exceed 5 mm Myr−1 (refs 8–11), whereas diagenetic nodules were found to grow much faster: an extreme example is the rate of 168 mm M yr−1 that was recently reported for a Mn-rich diagenetic type of nodule from the Peru Basin6. A different inter-element relationship of Co to the main metals Mn and Fe in different type of nodules has also been observed. We show here that the flux of Co into ferromanganese seamount crusts from the central Pacific is similar to the flux into pelagic nodules: the amount of Co supplied per unit of time and area is nearly constant in the oceanic water column. The most important factor governing the Co-enrichment is the growth rate; no local source is needed to explain the observed Co concentrations up to 2% in ore samples from summit areas.
Time markers at about 3 Ma., 6.5 Ma., and 14 Ma. are common to 16
manganese nodules and crusts from different localities in the world
oceans. These time markers correspond to variations in the growth rates,
in the inner structure, and in the chemistry of the samples. The
observed time markers could be related to (1) changes of the bottom
water velocity and (2) variations in the manganese and iron contents in
the deep ocean. The time markers were determined by high-resolution
10Be dating, applying the accelerator mass spectrometry
facility at the Eidgenössische Technische Hochschule Zürich.
Based on the survey data of five submarine seamount provinces (chains) in the Western Pacific, the distribution characteristics of cobalt-rich ferromanganese crust resources have been researched in this paper by using the relative reference data and applying the theories of hotspot and seafloor spreading. The main research results obtained are as follows: The Co-rich crust thickness in the study area is gradually increasing from east to west and from south to north having a negative correlation (r = −0.59) with longitude and a positive correlation (r = 0.48) with latitude. The crust thickness varying along longitude and latitude is influenced by the hotspot and seafloor spreading. The oceanic crusts and seamounts in the northwest part of the study area are older, and the crust resources are superior to those in the southeast part. In the depth of <1500 m, 1500–2000 m, 2000–2500 m in the study area, the cobalt crust thickness is respectively 5.45 cm, 4.34 cm and 3.55 cm, and in the depth of 2500–3000 m and 3000–3500 m, it drops respectively to 2.84 cm and 3.37 cm. The Co-rich crust resources are mainly concentrated in the seamount summit margins and the upper flanks in the depth of <2500 m. There is a strong negative correlation (r = −0.67) between the cobalt crust abundance and the slope of the seamount, 75 kg/m2 and 50 kg/m2 at the slopes of 0°–20° and 20°–34° respectively. Cobalt crusts are mainly distributed in the parts whose slopes are less than 20°. It is consistent with the fractal result that the slope threshold of cobalt crust distribution is 19°, and slopes over 20° are not conducive to the crust growth. The cobalt crusts of high grade are mainly enriched in the region within 150°E-140°W and 30°S-30°N in the Pacific, where there are about 587 seamounts at the depth of 3500–6000 m and over 30 Ma of the oceanic crusts. The perspective area rich in cobalt crust resources is about 41times104 km2 and the resource quantity is approximately 27 billion tons.
This paper presents the fractal distribution of topography of seamounts from the West Pacific and the resource quantity of
cobalt crust therein. The cobalt resource quantity has three to four variable fractal dimensions, corresponding to the distinct
slopes and water depths of the seamount. The multiple fractal property of resource quantity may have resulted from various
factors, such as types and components of cobalt crusts and ages of oceanic crusts hosting the seamounts. Individual seamounts
display complex topography and quantity of cobalt crust, both in the same and different regions.
To investigate the growth rates and absolute time stratigraphy of marine hydrogenetic ferromanganese encrustations, we performed 10Be profiling and ‘Co chronometry’ of crustal layers, as well as and δ18O analysis of phosphatised limestone (francolite) within a ∼ 9.5 cm thick ferromanganese crust from Schumann Seamount in the Hawaiian Archipelago. Together with microfossil stratigraphy, our results indicate that some seamount crusts greatly exceed the commonly accepted Miocene maximum age, in this case probably approaching the Cretaceous age of the seamount. In addition to the unconformity at the crust-substrate boundary, at least eight major disconformities are indicated in the Schumann Seamount crust which probably represent depositional hiatuses or episodes of crust erosion. Three of the six upper disconformities can be placed at the Plio-Pleistocene, Middle Miocene and Paleocene-Eocene based on 10Be, microfossil and Co chronometer evidence. and δ18O values of purified francolite from an inclusion-rich layer between the depths of 44 and 49 mm suggest apparent ages that approach those of Eocene-Late Paleocene microfossils reported in overlying layers, whereas francolite vein infillings in the lower part of the crust and in the basaltic substrate yield values that, if interpreted as ages of phosphatization, suggest a minimum Oligocene age. Paleotracking suggests the phosphogenesis observed here and on other Central Pacific seamounts could not have resulted from upwelling enhanced productivity associated with equatorial divergence if the Oligocene and Middle Miocene isotopic ages reported here and elsewhere are correct; however, a maximum Late Paleocene age for the phosphogenesis, consistent with the stratigraphy, would place these seamounts within 10°N of the equator. Paleotracking also suggests northeast tradewind transport of aluminosilicates in the Cenozoic, in agreement with other evidence for the antiquity of this ferromanganese crust.
Ferromanganese crusts cover most hard substrates on seafloor edifices in the central Pacific basin. Crust samples and their associated substrates from seven volcanic edifices of Cretaceous age along the Ratak chain of the Marshall Islands are discussed. The two most abundant substrate lithologies recovered were limestone, dominantly fore-reef slope deposits, and volcanic breccia composed primarily of differentiated alkalic basalt and hawaiite clasts in a phosphatized carbonate matrix. The degree of mass wasting on the slopes of these seamounts is inversely correlated with the thickness of crusts. Crusts are generally thin on limestone substrate. Away from areas of active mass-wasting processes, and large atolls, crusts may be as thick as 10 cm maximum.The dominant crystalline phase in the Marshall Islands crusts is δ-MnO2 (vernadite). High concentrations of cobalt, platinum and rhodium strongly suggest that the Marshall Islands crusts are a viable source for these important metals. Many metals and the rare earth elements vary significantly on a fine scale through most crusts, thus reflecting the abundances of different host mineral phases in the crusts and changes in seawater composition with time. High concentrations of cobalt, nickel, titanium, zinc, lead, cerium and platinum result from a combination of their substitution in the iron and manganese phases and their oxidation potential.
Ferromanganese oxides in the open oceans are more enriched in cobalt than any other widely distributed sediments or rocks.
Concentrations of cobalt exceed 1 percent in ferromanganese crusts on seamounts, ocean ridges, and other raised areas of the
ocean. The cobaltrich crusts may be the slowest growing of any earth material, accumulating one molecular layer every 1 to
3 months. Attention has been drawn to crusts as potential resources because they contain cobalt, manganese, and platinum,
three of the four priority strategic metals for the United States. Moreover, unlike abyssal nodules, whose recovery is complicated
by their dominant location in international waters, some of the most cobalt-rich crusts occur within the exclusive economic
zone of the United States and other nations. Environmental impact statements for crust exploitation are under current development
by the Department of the Interior.