The occurrence phases and enrichment mechanism of rare earth elements in cobalt-rich crusts from Marcus-Wake Seamounts

To explore the occurrence phases and enrichment mechanism of rare earth elements(REEs)in cobalt-rich crusts,this study analyzes the mineral composition and REE contents of the samples from Marcus-Wake Seamounts by XRD,ICP-OES and ICP-MS.The results show that,(1)the cobalt-rich crusts contain the major crystalline mineral(vernadite),the secondary minerals(quartz,plagioclase and carbonate fluorapatite),and a large amount of amorphous ferric oxyhydroxides(FeOOH).(2)The cobalt-rich crusts contains higher Mn(10.83%to 28.76%)and Fe(6.14%to 18.86%)relative to other elements,and are enriched in REEs,with total REE contents of 1563−3238μg/g and Ce contents of 790−1722μg/g.Rare earth element contents of the old crusts are higher than those of the new crusts.Moreover,the non-phosphatized crusts have positive Ce and negative Y anomalies,and yet the phosphatized crusts have positive Ce and positive Y anomalies,indicating that cobalt-rich crusts is hydrogenetic and REEs mainly come from seawater.(3)Analytical data also show that the occurrence phases of elements in cobalt-rich crusts are closely related to their mineral phases.In the non-phosphatized crusts,REEs are adsorbed by colloidal particles into the crusts(about 67%of REEs in the Fe oxide phase,and about 17%of REEs in the Mn oxide phase).In contrast,in the phosphatized crusts(affected by the phosphatization),REEs may combine with phosphate to form rare earth phosphate minerals,and about 64%of REEs are enriched in the residual phase containing carbonate fluorapatite,but correspondingly the influence of Fe and Mn oxide phases on REEs enrichment is greatly reduced.In addition,the oxidizing environment of seawater,high marine productivity,phosphatization,and slow growth rate can promote the REE enrichment.This study provides a reference for the metallogenesis of cobalt-rich crusts in the Pacific.

Distribution Characteristics of Cobalt-rich Ferromanganese Crust Resources on Submarine Seamounts in the Western Pacific

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/m^2 and 50 kg/mz 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 41×104 km^2 and the resource quantity is approximately 27 billion tons.

Biomineralization of organic matter in cobalt-rich crusts from the Marcus–Wake Seamounts of the western Pacific Ocean

Organic matter in cobalt-rich crust （CRC） from the Marcus-Wake Seamounts of the western Pacific Ocean, Sample CM1D03, has been analyzed to understand the source, geochemistry and mineralization of organic matter, and the mineralization environment. Biomarkers, includingn-alkanes, isoprenoids, terpanes and sterols, have been detected in various layers of the CRC sample, using gas chromatography （GC） and gas chromatography-mass spectrometry （GC-MS）. The content of organic carbon （OC） and its stable isotope （δ13C）, and the combined features of the biomarkers show that the mineralized organic matter in CM1D03 CRC was mainly derived from microorganisms and lower plankton （e.g., bacteria and algae, respectively） from marine surface water, with some terrestrial higher plant components. The ratio of chloroform bitu-men ＂A＂： OC was high in the CRC, between 10.51 and 20.66, showing significant migration characteristics of n-alkanes. Four mineralization categories of organic matter were recognized based on GC chromatograms ofn-alkane molecules： （1） primitive type （bacteria and algae）, which is characterized by moderately mature ofn-alkanes preserving the original characteristics of the organic matter from microorganisms and lower plankton; （2） microbial degradation type, which is characterized by low contents ofn-alkanes and rising baseline in the chromatogram, with the ＂bulge＂ being the products of organic matter by biodegradation; （3） organic matter migration type, which is characterized by low carbon number ofn-alkanes withnC18 as the main peak carbon, without odd even predominance, and low concentrations of isoprenoids and hydro-carbons with high carbon number; and （4） organic matter hydrothermal type, which is characterized by relatively low concentration of small molecular weightn-alkanes, pristane, and phytane, accompanied by higher concentration ofn-alkanes with carbon number greater thannC18. This study shows that biomarkers can record controlling factors of mineralization and their variation.

The Effects of Seamounts on Sound Propagation in Deep Water

A propagation experiment was conducted in the South China Sea in 2014 with a flat bottom and seamounts respectively by using explosive sources. The effects of seamounts on sound propagation are analyzed by using the broadband signals. It is observed that the transmission loss （TL） decreases up to 7 dB for the signals in the first shadow zone due to the seamount reflection. Moreover, the TL might increase more than 30 dB in the converge zone due to the shadowing by seamounts. Abnormal TLs and pulse arrival structures at different ranges are explained by using the ray and wave theory. The experimental TLs and arrival pulses are compared with the numerical results and found to be in good agreement.

Accration and Subduction of Seamounts in West Junggar

Recently,through implementation of the National Nature Science Foundation of China （No.41303027） and Special Fund for Basic Scientific Research of Central Colleges （310827153506,310827153407,2014G1271058）,we focused on basalts in Darbut and Karamay ophiolitic melanges in West Junggar which is a part of the Central Asian Orogenic Belt,and made some advances as follows.

Geochemistry of the lava and its implications in Musicians Seamounts

Chemical and isotopic data of the lava samples dredged in the southern Bach Ridge and the northern Italian Ridge of the Musicians Seamounts province, northeast of Hawaii. Although most of the samples analyzed are generally altered, a few are fresh. The latter exhibits similar geochemical and isotopic characteristics to normal MORB （Mid-Ocean Ridge Basalts）. There are systematic geochemical trends from hotspot to mid-ocean ridge in the province. Incompatible element and isotopic variations suggest that the flow field had at least two distinct parental magmas, one with higher and one with lower MgO concentrations. The two parental magmas could be related by a magma mixing model. The major and trace element modeling shows that the two parental magmas could not have been produced by different degrees of melting of a homogeneous mantle source, but they are consistent with melting of a generally depleted mantle containing variable volumes of embedded enriched heterogeneity enriched interbeds.

The western Durkan Complex(Makran Accretionary Prism,SE Iran):A Late Cretaceous tectonically disrupted seamounts chain and its role in controlling deformation style

The Durkan Complex is a key tectonic element of the Makran accretionary prism(SE Iran)and it has been interpreted as representing a continental margin succession.We present here a multidisciplinary study of the western Durkan Complex,which is based on new geological,stratigraphic,biostratigraphic data,as well as geochemical data of the volcanic and meta-volcanic rocks forming this complex.Our data show that this complex consists of distinct tectonic slices showing both non-metamorphic and very low-grade metamorphic deformed successions.Stratigraphic and biostratigraphic data allow us to recognize three types of successions.Type-Ⅰis composed by a Coniacian-early Campanian pelagic succession with intercalation of pillow lavas and minor volcaniclastic rocks.Type-Ⅱsuccession includes a volcanic sequence passing to a volcano-sedimentary sequence with Cenomanian pelagic limestones,followed by a hemipelagic sequence.This succession is characterized by abundant mass-transport deposits.Type-Ⅲsuccession includes volcanic and volcano-sedimentary sequences,which are stratigraphically covered by a Cenomanian platform succession.The latter is locally followed by a hemipelagic sequence.The volcanic rocks in the different successions show alkaline geochemical affinity,suggesting an origin from an oceanic within-plate setting.Our new results indicate that the western Durkan Complex represents fragments of seamounts tectonically incorporated in the Makran accretionary wedge during the latest Late Cretaceous-Paleocene.We propose that incorporation of seamounts in the frontal prism caused a shortening of the whole convergent margin and possibly contributed to controlling the deformation style in the Makran Accretionary Wedge during Late Cretaceous-Paleocene times.

Morphological description and population structure of an ophiuroid species from cobalt-rich crust seamounts in the Northwest Pacific:Implications for marine protection under deep-sea mining

Many seamounts are covered with cobalt-rich ferromanganese crusts,and are increasingly attracting interest for the potential extraction of valuable mineral resources from deep seabed environments.However,the impacts of potential mining activities on the vulnerable seamount ecosystem remain unclear.To enhance the understanding of connectivity in benthic invertebrate populations in the Northwest Pacific,several seamounts were surveyed and biological samples collected.In the present study,the ophiuroid species Ophioplinthaca defensor is reported for the first time from four deep seamounts in the Northwest Pacific,and described,providing more detailed morphological diagnosis characters.To assess the population structure of the species between and within seamounts,two mitochondrial markers(COI and 16 S)were sequenced.In total,20 haplotypes from 32 COI sequences and 8 haplotypes from 3716 S sequences were recovered.The star-shaped TCS networks and nonsignificant pairwise population differences reveal the absence of distinct population structures between and within seamounts.In addition,the O.defensor population seemed to have undergone a demographic expansion in history.This is the first study on the genetic population structure of a benthic invertebrate from seamounts in the Northwest Pacific,and this results suggest a potentially high,long distance dispersal capacity in O.defensor between seamounts,which could inform the development of the Regional Environmental Management Plans for the cobalt-rich crust seamounts in the area.

Comparative Study of Magmatism in East Pacific Rise Versus Nearby Seamounts:Constraints on Magma Supply and Thermal Structure Beneath Mid-ocean Ridge

Major elements of 2202 basalts from the East Pacific Rise （EPR） and 888 basalts from near- EPR seamounts are used to investigate their differences in magma crystallization pressures and mantle melting conditions. Crystallization pressure calculation from basalts with 5.0wt%〈MgO〈8.0wt % shows that magma crystallization pressures beneath near-EPR seamounts are positively and negatively correlated with Nas and Fes, respectively. However, these correlations are indistinct in axial lavas, which can be explained by chemical homogenization induced by extensive mixing processes. In each segment divided by major transforms and over-lapping spreading centers （OSCs）, near-EPR seamount lavas have higher magma crystallization pressures, higher Fes and lower Nas than the EPR lavas, which indicate cooler lithosphere, lower degrees and shallower melting depths beneath near-EPR seamounts than the EPR. The correlations between magma crystallization pressures and melting conditions beneath near-EPR seamounts imply that the source thermal state controls the melting degree and melt flux, and then melting process controls the shallow lithosphere temperature and magma crystallization depth （pressure）. The cooler mantle sources beneath near-EPR seamounts produce a lower degree of melting and a less robust magma supply, which results in a deep thermal equilibrium level and high magma crystallization pressure. The magma crystallization pressure decreases significantly as spreading rate of the EPR increases from ~80 mm/year in the north （16~N） to ~160 mm/year in the south （19~S）, while this trend is unobvious in near-EPR seamounts. This suggests that the magma supply controlled by spreading rate dominates the ridge crust temperature and magma crystallization depth, while the near-EPR seamount magma supply is not dominated by the axial spreading rate. Because most seamounts form and gain most of their volume within a narrow zone of 5-15 km from ridge axis, they provide good constraint on magma supply and thermal structure beneath the EPR. High magma crystallization pressures in seamounts indicate dramatic temperature decrease from the EPR. The crystallization pressures of seamount lavas are well correlated with mantle melting parameters but in a blurry relationship with axial spreading rate. Despite the adjacency of the EPR and nearby seamounts, the thermal structure beneath the near-EPR seamounts are controlled by their own magma supply and conductive cooling, chemically and thermally unaffected by magmatism beneath the ridge axis.

Planktonic ciliate trait structure variation over Yap,Mariana,and Caroline seamounts in the tropical Western Pacific Ocean

Trait structure is increasingly used in plankton ecology to understand diversity and biogeography.However,our knowledge of micro zooplankton(e.g.planktonic ciliates)trait structure and its variation with hydrography is limited.In this study,we analyzed planktonic ciliate trait structure in waters with different hydrography and deep Chlorophyll a maximum(DCM)layers over three seamounts:Yap,Mariana,and Caroline seamounts.Mariana seamount had a lower surface temperature than the Yap and Caroline seamounts.DCM layers over Mariana and Caroline seamounts were deeper than Yap seamount.There was a weak upwelling in upper 50 m around top of Mariana seamount.The ciliate distribution showed bimodal pattern(high abundance appeared in the surface and DCM layers)over three seamounts.At surface layer,the large size-fraction(>30μm)abundance proportion to aloricate ciliate over Yap seamount(44.4%)was higher than Mariana(32.8%)and Caroline(36.1%)seamounts.For tintinnid abundance proportion to total ciliate,Mariana(12.0%)and Caroline(11.5%)seamounts at about 100-m depth were higher than that of Yap seamount(6.4%).Vertically,tintinnid could be divided into 4 groups over the three seamounts.At30-m depth,group I(species occurring from surface to 100 m only)was dominant component over Yap and Caroline seamounts,while group IV(species occurring at every depth)changed into dominant component over Mariana seamount,the weak upwelling might be the reason.Salpingella faurei was the top dominant species,which corresponded to deeper DCM layers over Mariana and Caroline seamounts.Our results showed that the upwelling and the deeper DCM could influence the planktonic ciliate trait structure.

Establishment of Alloptilella splendida gen.et sp.nov.and resurrection of Scytalium veneris(Thomson&Henderson,1906),two sea pens(Cnidaria:Pennatulacea)from seamounts in the tropical Western Pacific

Sea pens are a highly specialized and morphologically distinct group of octocorallians,but many taxa were poorly described and their phylogenetic relationships are still poorly known.We describe two species of sea pens collected from the tropical Western Pacific:Alloptilella splendida gen.et sp.nov.and Scytalium veneris(Thomson&Henderson,1906).Alloptilella splendida gen.et sp.nov.accords with the definition of the recently resurrected genus Ptilella Gray,1870 in most characters,but differs from the latter in the arrangement of autozooids and the location of mesozooids.The new species mostly resembles Pennatula naresi K(o|")lliker(1880),but differs by the location of me sozooids and the reddish color of sclerite s surrounding siphonozooids.The generic separation is also well supported by their genetic distances and the molecular phylogenetic trees constructed by the concatenated mtMutS-COI-28 S,where Alloptilella clustered with Scytalium Herklots,1858 and formed a clade with the cluster of Ptilella and Distichoptilum.Scytalium veneris,usually considered as a junior synonym of Scytalium sarsii Herklots,1858,is resurrected by recognizing their distinctive differences.This is the first record of Scytalium veneris outside its type locality,and the phylogenetic analysis indicates that Scytalium is a monophyletic group.Both the families Pennatulidae and Virgulariidae are polyphyletic,and more morphological and molecular data are needed to clarify the phylogenetic positions of pennatulacean families and genera.

Morphology and molecular phylogeny of three black corals(Antipatharia,Schizopathidae)from seamounts in the Western Pacific Ocean,with description of a new species

Three deep-sea black corals belonging to the family Schizopathidae,are reported from two seamounts on the Caroline Ridge in the tropical Western Pacific:Umbellapathes parva sp.nov.with depth of 1488-1766 m,Telopathes cf.magna MacIsaac&Best,2013 with depth of 937-1016 m,and Stauropathes cf.punctata(Roule,1905)with depth of 942-1353 m.The latter two species are the first records in the Western Pacific Ocean.U.parva sp.nov.is characterized by monopodial corallum,relatively long unpinnulated stalk with complex pinnulated branche s developing from the lowermost primary pinnules and small spine s.It differs from two known congeners by much smaller spine s and the present of secondary pinnules.T.cf.magna MacIsaac&Best,2013 has sparsely branched corallum,relatively long and simple pinnules arranged in subopposite or alternate pairs.S.cf.punctata(Roule,1905)is characterized by the corallum with almost planar pinnulated branches,and pinnules arranged in subopposite pairs.Our results indicate that the corallum size and shape are variable in conspecific specimens particularly those at different growth stages.By contrast,the size of the polypar spines as well as the abpolypar spines have little variation and can be served as a main differing feature for schizopathid species.The phylogenetic analyses using nuclear internal transcribed spacer region(spanning partial 18 S rDNA,ITS 1,5.8 S,ITS2,and partial 28 S rDNA)and two mitochondrial fragments cox3-IGR-coxl(COI)and trnW-IGR-nad2(NAD2)showed Telopathes,Stau ropathes,and Bathypathes had close relationships,and Umbellapathes formed a sister clade with Alternatipathes.

Analysis of differences in nutrients chemistry in seamount seawaters in the Kocebu and M4 seamounts in Western Pacific Ocean

Comprehensive surveys were conducted in the Kocebu deep seamount and the M4 shallow seamount in the Western Pacific Ocean in March 2018 and May 2019,respectively.The distributions of nutrients in euphotic zone of the two seamount-areas were revealed,and the causative controlling factors were analyzed.Results show that the vertical distribution of nutrients in the two seamount-areas accorded with the general law of the oligotrophic ocean.The concentrations of NO_(3)-N,PO_(4)-P,and SiO_(3)-Si generally increased gradually with the increase of water depth,and they were extremely low in water layers within100 m.The area of high NO_(2)-N concentration well agreed with the Deep Chlorophyll Maximum Layer.On the other hand,the distribution of water masses and phytoplankton and hydrological conditions in the two seamount-areas were different,resulting in lower concentrations of NO_(3)-N,PO_(4)-P,and SiO_(3)-Si in the water layers below 100 m in the Kocebu seamount area than those in the M4 seamount area.In addition,NO_(2)-N was affected by the distribution of phytoplankton,and distributed mainly in the water layers of 150 and100 m.There was upwelling in the euphotic zone of M4 seamount area,causing accumulations of nutrients and phytoplankton around the seamount,forming a"seamount effect";however,no such an effect was found in Kocebu seamount area.Affected by the composition of biological community and the"seamount effect",the nitrogen limitation in the M4 seamount area was not significant,and the dissolved inorganic nitrogen(DIN):PO_(4)-P and SiO_(3)-Si:DIN were closer to the Redfield ratios.

Ophiuroid fauna of cobalt-rich crust seamounts in the Northwest Pacific Ocean

Seamounts are vulnerable ecosystems in the deep sea and can be heavily impacted by human activities,such as bottom fishing and deep-sea mining.The species composition and distribution patterns of benthic fauna is key information for the designation of marine protected areas and environmental management plans.Three contracts for cobalt-rich crust exploration have been granted to China,Japan and Korea in the Northwest Pacific Ocean by the International Seabed Authority.However,our knowledge of benthic biodiversity in this area is extremely insufficient.During 2013–2020,eight Chinese Ocean Mineral Resources R&D Association(COMRA)cruises were conducted to investigate the benthic assemblages of nine seamounts in this region.In this study,191 ophiuroids collected from seamounts in the Northwest Pacific were identified into 29 species in 11 families.Ophiacanthidae and Euryalidae were the two most dominant families with 12 and 6 species,respectively.Ophiotomidae and Ophiopyrgidae were represented by two species each,while seven families were represented by only one species.Four species were widely distributed among 4–5 seamounts,and 17 species were found only at a single site.An integrated regional taxonomic dataset of Ophiuroidea was generated and analyzed.A total of 23 and 14 species were obtained from the Magellan Seamount Chain(MSC)and the Marcus-Wake seamounts(MWS),respectively,with 8 species shared between the two seamount groups.The individual-based rarefaction curves did not reach an asymptote,suggesting that the sampling effort was inadequate for either the entire region or each single seamount.Most species distributed in a narrow depth range,and the species composition was different between water depths above and below 2000 m.Our results greatly improve the understanding of megafaunal biodiversity from seamounts in the Northwest Pacific Ocean,and highlight the necessity of further surveys to provide more robust information for environmental protection and management in this region.

Interactions between hotspots and the Southwest Indian Ridge during the last 90 Ma:Implications on the formation of oceanic plateaus and intra-plate seamounts

This study investigates the relationship between the hotspot-ridge interaction and the formation of oceanic plateaus and seamounts in the Southwest Indian Ocean.We first calculated the relative distance between the Southwest Indian Ridge (SWIR) and relevant hotspots on the basis of models of plate reconstruction,and then calculated the corresponding excess magmatic anomalies of the hotspots on the basis of residual bathymetry and Airy isostasy.The results reveal that the activities of the Marion hotspot can be divided into three main phases:interaction with the paleo-Rodrigues triple junction (73.6-68.5 Ma),interaction with the SWIR (68.5-42.7 Ma),and intra-plate volcanism (42.7-0 Ma).These three phases correspond to the formation of the eastern,central,and western parts of the Del Cano Rise,respectively.The magnitude and apparent periodicity of the magmatic volume flux of the Marion hotspot appear to be dominated by the hotspot-ridge distance.The periodicity of the Marion hotspot is about 25 Ma,which is much longer than that of the Hawaii and Iceland hotspots (about 15 Ma).

Variations of calcareous nannofossils of cobalt-rich crusts and geological records at the Eocene-OIigocene transition in western Pacific seamounts

Two records of the crust laminae from the Marcus-Wake Seamounts and the Magellan Seamount were biostratigraphically studied. Based on biological imprints of the calcareous nannofossils, the geological ages of the two records were determined, with CM1D03 from the Marcus-Wake Seamounts being of late Paleocene to Pleistocene and CM3D06 from the Magellan Seamount of Late Cretaceous （more than 70.0 Ma）. There are the obvious temporal-spatial differences in the initial formation period and enrichment characteristics of the cobalt-rich crusts of the two seamount chains and differences in the combination and distribution of microfossils in the inner crust layers between the seamounts. These differences are due to the adaptabilities of oceanic species in different environments. Ecological research was carried out in terms of population size of the calcareous nannofossi｜s preserved in the crustal layers to discern the relation of the geological events at the Eocene-Oligocene （E/O） tran- sition. The results show the transitions and recombination of species in the biotic community during the E/O transition obvi- ously corresponded to 25 mm depth in the CM1D03 crust and 58 mm depth in the CM3D06 crust. The changes in biological species and the formation of particular ecological structures indicate the adaptive response of the paleo-biological community in the western Pacific Ocean to the global cold-climate events and the close correlation between the formation of the crust and the global climate change.

Geochemical characteristics and metal element enrichment in crusts from seamounts of the Western Pacific

Elemental geochemistry is an essential part of understanding mineralization mechanisms. In this paper, a data set of 544 cobalt crust samples from seamounts of the Western Pacific are used to study the enrichment characteristics of metal elements. REE normalization is utilized to reveal the origin of the crusts; effects of water depth on Co enrichment and impacts ofphosphatization on mineral quality are discussed to obtain the evolution of these marine mineral deposits, which gives support to further resource assessment. Conclusions are reached as follows： 1） Elemental abundances, inter-element relation- ships, and shale-normalized REE patterns for phosphate- poor crusts from different locations reflect hydrogenetic origin of the crusts. EFs （enrichment coefficients） of REE exhibit exponential increase from surface sediments to phosphorite to polymetallic nodules to crusts, suggesting that the improved degree of hydrogeneous origin induces the enrichment of REE. 2） The crusts in the Western Pacific, formed through hotspot produced guyots trails, have relatively lower REE than those in the Mid-Pacific. The latter could be attributed to the peculiar submarine topography of seamounts formed by intraplate volcanism. 3） The non-phosphatized younger crust layers have 40% higher Co than the phosphatized older layers. This indicates the modification of the elemental composition in these crusts by phosphatization. A general depletion of hydroxide-dominated elements such as Co, Ni, and Mn and enrichment of P, Ca, Ba, and Sr is evident in phosphatized crusts, whereas non-phosphatized younger generation crusts are rich in terrigenous aluminosilicate detrital matter. 4） Co increases above the carbonate compensation depth （CCD） from less than 0.53% to over 0.65% in seamount regions with water depth of less than 2,500 m, suggesting the significance of the dissolution of carbonate in the sea water column to the growth and composition of crusts.

Deformation history and processes during accretion of seamounts in subduction zones:The example of the Durkan Complex(Makran,SE Iran)

The Durkan Complex is a tectonic element of the Makran Accretionary Prism(SE Iran)that includes fragments of Late Cretaceous seamounts.In this paper,the results of map-to micro-scale structural studies of the western Durkan Complex are presented with the aim to describe its structural and tectonometamorphic evolution.The Durkan Complex consists of several tectonic units bordered by mainly NNW-striking thrusts.Three main deformation phases(D_(1),D_(2),and D_(3))are distinguished and likely occurred from the Late Cretaceous to the Miocene–Pliocene.D_(1) is characterized by sub-isoclinal to close and W-verging folds associated with an axial plane foliation and shear zone along the fold limbs.This phase records the accretion of fragments of the seamount within the Makran at blueschist facies metamorphic conditions(160–300℃ and 0.6–1.2 GPa).D_(2) is characterized by open to close folds with subhorizontal axial plane that likely developed during the exhumation of previously accreted seamount fragments.An upper Paleocene–Eocene siliciclastic succession unconformably sealed the D_(1) and D_(2) structures and is,in turn,deformed by W-verging thrust faults typical of D_(3).The latter likely testifies for a Miocene–Pliocene tectonic reworking of the accreted seamount fragments with the activation of out of sequence thrusts.Our results shed light on the mechanism of accretion of seamount materials in the accretionary prisms,suggesting that seamount slope successions favour the localization and propagation of the basal décollement.This study further confirms that the physiography of the subducting plates plays a significant role in the tectonic evolution of the subduction complexes.

Distribution characteristics of seamount cobalt-rich ferromanganese crusts and the determination of the size of areas for exploration and exploitation

In 2001, the International Seabed Authority （ISBA） initiated the consideration relating to the Regulations for Prospecting and Exploration for Hydrothermal Polymetallic Sulphides and Cobalt-rich Ferromanganese Crusts in the Area at its 7th session. Since then, the consideration of the Regulations has been mainly focused on the size of areas to be allocated for exploration and exploitation of the crusts. This paper, based on the investigation data and the analysis of the distribution characteristics of the crusts, suggests a model for determining the size of areas for exploration and exploitation of the crusts, taking into account various factors such as production scale, crust thickness and grade, mineable area proportion, recovery efficiency, exploration venture, and so on. Through the modeling, the paper suggests that the exploration area （the area covered by each application for approval of a plan of work for exploration of cobalt-rich crusts） shall be 4 856 km2 and the exploitation area （the mine site area） shall be 1 214 km2, for 20 years of 1 million wet tonnes annual production.

The geomorphological features and continuity of the Kyushu-Palau Ridge (KPR)

The Philippine Basin,surrounded by a series of oceanic trenches,is an independent deep ocean basin in the West Pacific Ocean.Its middle part is divided into three marginal sea sub-basins by the Kyushu-Palau and West Mariana Ridges,namely,the West Philippine Basin,the Shikoku and Parece Vela Basins and the Mariana Trough.This paper,through the analysis of the geomorphologic features and gravity and magnetic characteristics of the basin and identification of striped magnetic anomalies,suggests that the entire Philippine Basin developed magnetic lineation of oceanic nature,and therefore,the entire basin is of the nature of oceanic crust.The basin has developed a series of special geomorphic units with different shapes.The KPR runs through the entire Philippine Basin.From the view of geomorphologic features,the KPR is a discontinuous seamount chain （chain-shaped seamounts） and subduction beneath the Japanese Island arc at the Nankai Trough which is the natural boundary between the basin and the Japanese Island arc.At the positions of 25 N,24 N,23 N and 18 N,obvious discontinuity is shown,which belongs to natural topographic discontinuity.Therefore,the KPR is topographically discontinuous.