中太平洋海山富钴结壳成矿的空间分布规律研究

海山是富钴结壳主要的成矿载体,结壳成矿分布不仅受到海洋最低含氧带、碳酸盐补偿深度、生物生产力、物质来源、海水氧含量等大尺度宏观因素的影响,同时也受到海山地形、沉积作用和底层海流冲刷等小尺度微观因素的控制。通过对以中太平洋R海山为主的水下结壳成矿与分布的深入研究,发现结壳富集区以海山浅水区域为主;地形地貌上的尖顶高地区、顶坡过渡带、山体鞍部、山脊、斜坡上部等区域结壳质量较好,覆盖率高;下斜坡、山谷、山顶平坦区和斜坡区平缓台地是结壳贫乏区,结壳厚度小、覆盖率也低;15°以下的小坡度地形结壳成矿较好,3°~7°最佳,15°以上的大坡度区质量有所下降,陡崖区最差。地形对结壳分布和成矿起基础性影响作用,沉积作用和底流冲刷分别起到阻滞和促进作用,海流是保持低坡度区结壳长期稳定生长的关键因素,地形控矿本质上是和底层流联合对抗沉积堆积作用的过程。     

中西太平洋海山形态类型与钴结壳资源分布关系

通过对中西太平洋海山形态剖面的山体高度、山顶直径、基底直径、山顶直径与基底直径之比、山体坡度、山体高度与基底直径比值六个参数多元统计分析,发现可以根据山体高度与基底直径的比值对海山形态类型进行分类:比值小于0.10的为平顶海山(Ⅰ类),大于0.10的为尖顶海山(Ⅱ类),对于等于0.10的海山需参考平坦度和山体坡度,平坦度大和山体坡度缓的为Ⅰ类,反之为Ⅱ类。西太平洋的麦哲伦海山区、马绍尔群岛基本以平顶海山为主,介于中西太平洋之间的威克—马尔库斯海山区和中太平洋海山区、莱恩群岛平顶海山与尖顶海山共同发育。对各种类型海山上钴结壳分布研究发现,无论是在尖顶海山还是在平顶海山,板状结壳均比较发育,但砾状结壳在平顶海山比在尖顶海山的发育。中太平洋尖顶海山的结壳比平顶海山的发育,但由于山顶面积小,钴结壳资源量不大。仅从平顶海山看,在麦哲伦海山区、威克—马尔库斯海山区板状结壳比中太平洋海山区、马绍尔群岛、莱恩群岛的板状结壳发育,前者的板状结壳平均厚度大于3 cm,后者的板状结壳平均厚度小于3 cm,总体上是西太平洋平顶海山钴结壳比中太平洋平顶海山的发育。两种类型海山各方向上的资源分布明显不同,在平顶海山的西部山坡的资源比东部山坡的丰富,尖顶海山的则刚好相反。     

Dynamics at an elongated,intermediate depth seamount in the North Atlantic (Sedlo Seamount, 40°20′N, 26°40′W)

Observations from a five-mooring array deployed in the vicinity of Sedlo Seamount over a 4-month period, together with supporting hydrographic and underway ADCP measurements, are described. Sedlo Seamount is an elongated, intermediate depth seamount with three separate peaks, rising from 2200 m water depth to summit peaks between 950 and 780 m depth, located at 40°20′N, 26°40W. Currents measured in depth range 750 and 820 m – the layer close to the summit depth of the shallowest southeast peak – showed a mean anti-cyclonic flow around the seamount, with residual current velocities of 2–5 cm s⁻¹. Significant mesoscale variability was present at this level, and this is attributed to the weak and variable background impinging flow. Stronger, more persistent currents were found at the summit mooring as a result of tidal rectification and some weak amplification. Below 1300 m, currents were extremely weak, even close to the seabed. Time series of relative vorticity for the depth layer 750–820 m showed persistent anti-cyclonic vorticity except for two periods of cyclonic vorticity. A mean relative vorticity of −0.06f (f=the local Coriolis frequency) was calculated from a triangle of current meters located at the flanks of the seamount. Modelling results confirmed that anti-cyclonic flow above the seamount was likely due to Taylor Cone generation driven by a combination of steady impinging and tidally rectified flow. The closed circulation pattern over the seamount was found to extend to ∼150 m above the summit level, consistent with simple idealised theory and the supporting hydrographic observations. At shallower depths (<500 m) model simulations predicted a predominantly cyclonic recirculation most likely controlled by topographic steering along the zonal axis of the seamount. There was some indication of flow reversal at these depths from Acoustic Doppler Current Profiler (ADCP) measurements carried out at one hydrographic survey. The model results were in good agreement with observations at the seamount summit, but were unable to reproduce the mesoscale variability patterns recorded in shallower layers. Kinetic energy patterns derived from the model revealed high variability in the oceanic far field downstream of the seamount summit probably as a result of complex flow interaction along the chain of seamount peaks. Possible impacts of the flow dynamics on the biological functioning at Sedlo Seamount and its surroundings are discussed.     

Automated multi-scale classification of the terrain units of the Jiaxie Guyots and their mineral resource characteristics

Given the advances in satellite altimetry and multibeam bathymetry, benthic terrain classification based on digital bathymetric models (DBMs) has been widely used for the mapping of benthic topographies. For instance, cobalt-rich crusts (CRCs) are important mineral resources found on seamounts and guyots in the western Pacific Ocean. Thick, plate-like CRCs are known to form on the summit and slopes of seamounts at the 1 000–3 000 m depth, while the relationship between seamount topography and spatial distribution of CRCs remains unclear. The benthic terrain classification of seamounts can solve this problem, thereby, facilitating the rapid exploration of seamount CRCs. Our study used an EM122 multibeam echosounder to retrieve high-resolution bathymetry data in the CRCs contract license area of China, i.e., the Jiaxie Guyots in 2015 and 2016. Based on the DBM construted by bathymetirc data, broad- and fine-scale bathymetric position indices were utilized for quantitative classification of the terrain units of the Jiaxie Guyots on multiple scales. The classification revealed four first-order terrain units (e.g., flat, crest, slope, and depression) and eleven second-order terrain units (e.g., local crests, depressions on crests, gentle slopes, crests on slopes, and local depressions, etc.). Furthermore, the classification of the terrain and geological analysis indicated that the Weijia Guyot has a large flat summit, with local crests at the southern summit, whereas most of the guyot flanks were covered by gentle slopes. “Radial” mountain ridges have developed on the eastern side, while large-scale gravitational landslides have developed on the western and southern flanks. Additionally, landslide masses can be observed at the bottom of these slopes. The coverage of local crests on the seamount is ~1 000 km2, and the local crests on the peak and flanks of the guyots may be the areas where thick and continuous plate-like CRCs are likely to occur.     

2014年冬季西太平洋雅浦海山与大洋海域大型浮游动物群落比较研究

2014年冬季对西太平洋雅浦Y3海山区和从西太平洋暖池区到黑潮源区之间的大洋海域的浮游动物进行了调查研究，分析了海山区和大洋海域大型浮游动物群落的物种组成、丰度和分布，并比较了两个海域大型浮游动物群落的差异，结合温度、盐度和叶绿素a浓度等环境因子数据，分析了海山对大型浮游动物群落的影响。结果表明，Y3海山区和大洋海域的大型浮游动物群落物种组成存在差异，二者的群落相似度系数为68.4%，两个海域的优势种/类群也不尽相同，海山区的优势种/类群是磷虾各期幼体、尖额磷虾和蛮𧊕，而大洋海域的优势种/类群分别是磷虾后期幼体、手磷虾、蛮𧊕、尖额磷虾和莹虾。大型浮游动物群落在海山区的平均丰度和最大丰度都要高于大洋海域，但海山区的生物多样性指数、均匀度指数和生物量占比都低于大洋海域，因而Y3海山对大型浮游动物群落的海山效应不明显。通过相关性分析和主成分分析结果可以看出，海山区的首要影响环境因子是盐度，而大洋海域则是温度；海山区和大洋海域的大型浮游动物丰度分别与200m层盐度和平均温度呈负相关关系，并且在两个调查海域均未发现大型浮游动物的丰度和生物量分布与叶绿素a浓度之间存在显著的相关关系。     

Interaction of Mediterranean water eddies with Sedlo and Seine Seamounts,Subtropical Northeast Atlantic

During years 2003 and 2004 both Sedlo and Seine Seamounts, lying in the area of influence of the Mediterranean water outflow in the Northeast Atlantic, were investigated in the framework of the EU-funded project OASIS. Analysis of CTD data shows that the levels of major variability in temperature and salinity were confined to the upper ocean layer, which is subject to seasonal air–sea heat exchange, and to the Mediterranean Water (MW) level. In this paper we concentrate on the latter case and show that the variability observed resulted from Mediterranean water eddies (Meddies) colliding with the seamounts. Combined data from CTD surveys, current meter moorings, ship-mounted ADCP and satellite altimetry suggest that during the period of observations there was only one Meddy interacting with each of the seamounts. On Sedlo Seamount intensive interaction resulted in the loss by the Meddy of about 25% of the salt around the seamount, whereas on Seine the Meddy (or portion of it) escaped the seamount after performing several rotations around the summit without any significant interaction.     

A multivariate analysis of larval fish and paralarval cephalopod assemblages at Great Meteor Seamount

Assemblage structures and distribution patterns of larval fishes and paralarval cephalopods were examined in September 1998 at Great Meteor Seamount, an isolated seamount located in the subtropical eastern North Atlantic. Early life stages of fish (n=18555) and cephalopods (n=1200) were collected at 23 stations with a multiple opening–closing net, in seven discrete depth strata from 290 m depth (close to the seamount plateau) to the surface. Oceanic species dominated in both taxonomic groups. A peak in diversity was observed at an intermediate depth, in the 100–150 m water layer. Direct and indirect gradient analyses showed distinct species assemblages in the upper and lower part of the water column, separated by approximately 150 m. The division was statistically significant, although a considerable overlap between species was also observed. Above the summit, vertical gaps were found in the distributions of the deeper assemblages, likely caused by increased predation pressure by benthopelagic fish. Horizontal distribution patterns of fish and cephalopods were similar and corresponded to the structure of closed circulation cells detected above the flanks and the flat plateau area. Fish assemblages were significantly different between the inner and outer seamount regime, which was approximately separated by the 1500 m depth contour. Differences in the taxonomic composition of cephalopods were less pronounced; for only one cephalopod species could a direct association with the seamount be assumed. The study indicates a significant retention potential at the seamount that facilitates local recruitment of resident stocks and generates self-sustainable populations isolated from the continental shelf and oceanic islands.     

Zooplankton metabolism and carbon demand at two seamounts in the NE Atlantic

Zooplankton metabolic rates, determined from electron transfer system (ETS) activity, were studied at two seamounts (Seine: 34°N, 14°W, summit depth ∼170 m; Sedlo: 40°N, 27°W, summit depth ∼750 m) in the northeast (NE) Atlantic during three cruises in November 2003, April 2004 and July 2004. ETS activity and respiratory carbon demand were measured for samples taken at seamount and open-ocean locations in order to probe the hypothesis of locally enhanced seamount productivity. ETS activity and biomass revealed no consistent diel patterns of feeding activity and vertical migration at Seine and Sedlo Seamounts. Spatial differences of biomass-specific ETS activity were observed at both seamounts and coincided with differences in food abundance and quality. At Seine Seamount in April 2004, biomass-specific ETS activity was on average higher at the seamount locations compared to the open ocean, though the enhancement was of a lower magnitude than spatial and temporal variability and had no apparent influence on zooplankton respiratory carbon demand or biomass. A persistent pattern of reduced zooplankton biomass above the summit location at Seine Seamount in April 2004 and July 2004 resulted in a local reduction of respiratory carbon demand. At Sedlo Seamount in November 2003, large spatial differences in biomass-specific ETS activity observed at the seamount locations resulted in a large range of respiratory carbon demand at the seamount, but were not reflected in zooplankton biomass. The depth-integrated (0–150 m) median respiratory carbon demand of the zooplankton community estimated from day and night hauls was 2.1 mg C m⁻² d⁻¹ at Seine Seamount (range: 0.3–6.3) and 2.9 mg C m⁻² d⁻¹ at Sedlo Seamount (range: 1.6–12.0). The sporadic nature and low magnitude of locally higher zooplankton respiration rates at the seamounts, which did not result in locally higher zooplankton standing stock biomass, lead us to reject the hypothesis that locally enhanced seamount productivity provides an autochthonous food supply to the resident faunas at Seine and Sedlo Seamounts. Instead, we conclude that the faunas at both seamounts are more likely supported by advection of food from the surrounding ocean.     

New, high resolution swath bathymetry of Gettysburg and Ormonde Seamounts (Gorringe Bank, eastern Atlantic) and first geological results

High resolution swath bathymetry of shallow water (< 200 m) oceanic seamounts is a relatively rare issue. During the recent Gorringe_2003 cruise over the Gorringe Bank (Eastern Atlantic) we collected multibeam bathymetry on the bank’s two shallow summits, Gettysburg and Ormonde in the –25/–400m depth range at a resolution rarely achieved over an oceanic seamount. We also carried out bottom samplings and ROV dives in the same bathymetric interval. The acquisition parameters and the characteristics of the echosounder employed allowed to generate a Digital Terrain Model (DTM) with metric spatial resolution upto 75–100 m depths. To ensure proper tidal corrections a tide-gauge was deployed at sea-bottom during the survey. DTM reveals for the Gettysburg Seamount an almost perfectly circular summit resulting from the blanket of bioclastic sediments over an igneous ‘core’ consisting of sheared and foliated serpentinites. The core is dissecated by N 10° W trending ridges elevating some tens of metres and filled in between by bioclastic sands. Both foliation and ridge patterns seem related to primary igneous fabric rather than later structural deformation. The overall circular shape confirms the origin of the seamount as a mantle serpentinite diapir in analogy with similar, but subduction-related, circular seamounts observed in the Bonin Trench (western Pacific). In contrast the Ormonde elongated summit follows the regional tectonic trend with a N 60° E active (seismogenic?) fault on its southeastern flank. Its basement morphology corresponds to the outcrops of igneous rocks chiefly consisting of gabbros, volcanic rocks and dyke intrusions. On both seamounts topographic profiles show that the ‘shelf’ area is somewhat convex rather than flat like that of ‘Pacific type’ guyots and is bordered by a depositional, locally erosional shelf break, located between –170 and –130 m. Various terraced surfaces and some geological evidence confirm previous observations and indicate relative sea-level oscillations with partial emersion of the two summits that seem occurred during the last glacial cycle (past 120 ka).     

Bathymetry,substrate and fishing areas of Southeast Atlantic high-seas seamounts

Most of the Southeast Atlantic Ocean is abyssal, and global bathymetries suggest that only ～3.2% of the areas beyond national jurisdiction (ABNJ; also known as the high seas, as defined in the United Nations Convention on the Law of the Sea [UNCLOS]) are shallower than 2 500 m. This study mapped bathymetry and characterised substrates in selected seamount summit areas, including several that have been or may become fishing areas. The southernmost location, the Schmitt-Ott Seamount, has exposed volcanic bedrock with surrounding flats covered by thin biogenic sediments and/or coral rubble that appears ancient. At Wüst, Vema, Valdivia and Ewing seamounts the basaltic base appears to be overlain by coral caps and other coral substrates (sheets, rubble). Adjacent summit plains have biogenic sediments of varying thickness. Vema has a flat, roughly circular summit, <100 m deep, with the shallowest point being a 22-m-deep summit knoll; the upper slopes have ancient coral framework, but the summit has a mixture of coralline and volcanic rock and coarse sediments, including extensive areas with coralline algae and kelp forests. Valdivia Bank is a 230-m-deep, flat, rocky area (～11 × 5 km), protruding steeply from the extensive multi-summit Valdivia subarea of the Walvis Ridge. The distribution of past fisheries in the Convention Area of the South East Atlantic Fisheries Organisation (SEAFO) was considered in relation to the new information on bathymetry and substrate.     

Relation of sea beam echo peak statistics to the character of bottom topography

Histograms of echo peaks from three different topographic regions in the outer beams of a Sea Beam sonar show distinct variation as the topography changes from the North Cleft ridge to the Cascadia abyssal plain in the Pacific Ocean. Parameter estimation of probability density functions (pdfs) shows significant departure from Rayleigh pdf for the nonabyssal plain sites. The spectral character of the sea-floor roughness from an acoustic backscatter model suggests that the ridge area is dominated by large-scale roughness and the abyssal plain by small scale. It appears that change in roughness magnitude effects the observed histogram characteristics.     

基于潜标观测的牛郎海山的深海海流的低频变化特征

本文利用在西太平洋牛郎海山布放的两套锚系潜标获取的长时间海流观测数据,分析了深海的海洋动力环境特征,着重阐释了该海域海流的全水深垂向结构及其低频变化特征。结果表明:（1）年平均海流及其变化幅度均在上层最大、中层和深层次之、中深层最小;（2）年平均上,150 m以浅的海流为东向的副热带逆流,150 m以深和中层为西向流;山顶处的近底层海流为较稳定的弱南向流,山底处的近底层海流为西南向流;（3）在山顶和山底,各深度层次的海流在全年均表现出100 d左右的振荡周期;在2 000 m以浅,各深度层次的海流振荡的位相基本一致,振荡幅度在表层最强、随深度的增加而减小;在2 000 m以深,海流变化的位相与2 000 m以浅相反,振荡幅度在4 000 m最强。     

Morphologic investigation of “uncharted” seamount from central Indian basin revisited with multibeam sonar system

Morphology of a seamount at 12°35'E and 76°18.5’ and two abyssal hills in its vicinity is described using the Hydrosweep multibeam‐swath bathymetric system. The height of the seamount is 1350 m, and it occupies an area of 330 km². Its basal width is 22.5 km, and the mount has a gentle and longer western flank and a steep and shorter eastern flank. There is a characteristic terracelike feature on the western flank, about 300 m from the top. A caldera is also observed on top of the seamount. Slope angles in this area are high (over 35"). Results of morphologic studies of the seamount from the multibeam survey are comparable to those from a narrow‐beam echosounding survey. The origin of the seamount may be related to the presence of a fracture zone at 75°45'E.     

热带西太平洋Y3和M2海山区网采浮游植物群落结构特征

研究了热带西太平洋雅浦Y3海山冬季和马里亚纳M2海山春季网采浮游植物群落结构,对调查区浮游植物的物种组成、优势种类、细胞丰度以及多样性指数进行了分析。结果表明,两个海山区共鉴定浮游植物4门50属219种,其中硅藻门30属106种,甲藻门17属112种,蓝藻门1属2种,金藻门2属3种。两个航次研究区浮游植物优势种均以链状硅藻如根管藻(Rhizosolenia)、半管藻(Hemiaulus)和角毛藻(Chaetoceros)等属的种类为主,此外太阳漂流藻(Planktoniella sol)、铁氏束毛藻(Trichodesmium thiebautii)以及部分角藻(Ceratium)物种优势度也比较明显。Y3海山区浮游植物细胞丰度介于1.60～16.61 cells/L,平均值为5.02 cells/L; M2海山区浮游植物细胞丰度介于1.36～10.20 cells/L,平均值为4.12cells/L。两个海山区浮游植物细胞丰度的分布趋势均受硅藻影响较大,甲藻细胞丰度相对较低。在属的水平上,角毛藻、根管藻、角藻和半管藻等属的细胞丰度对两个海山区浮游植物总细胞丰度的贡献较大。多样性指数方面, Y3海山区浮游植物群落香农-威纳指数H′(shannon-wiener index)介于3.95～4.69,平均值为4.30; M2海山区浮游植物群落香农-威纳指数介于3.23～4.46,平均值为3.83。两个海山区浮游植物群落多样性指数均处于较高水平,但站位间的变化不明显。目前,关于热带西太平洋海山区浮游植物群落结构的研究还非常缺乏,亟需后续研究的补充。     

Characterizing Indian Ocean manganese nodule-bearing seafloor using multi-beam angular backscatter

Application of quantitative angular backscatter modelling to manganese nodule-bearing areas of the Central Indian Ocean Basin (CIOB) has been initiated at NIO during the year 1998. Studies were aimed to establish the suitability of seafloor backscattering in delineating seafloor parameters characteristic of nodule-rich sediments. In this paper, processed Hydrosweep multi-beam backscatter data from 45 spot locations in the CIOB (where nodule samples are available) were analysed to estimate seafloor and sediment volume roughness parameters. The application of a composite roughness model to a nodule-bearing region (6,600 km²) of the CIOB, to determine seafloor interface roughness parameters from a multi-beam backscatter dataset, shows only four power law sets. The results attest 80% of the nodule-bearing seafloor to be smooth in terms of interface roughness parameters at micro-topographic level. The sediment volume roughness parameters are dominant only in 29% of the smooth interface roughness sites. This indicates that 51% of the seafloor area possesses negligible (interface and volume) roughness. A critical analysis using pseudo-side-scan records from 12 selected locations in the study area affirms the combined importance of the seafloor interface and sediment volume roughness parameters for precise determination of manganese nodule abundance.     

Hydrothermal manganese oxide deposits from Baby Bare seamount in the Northeast Pacific Ocean

Manganese oxide crusts were recovered from Baby Bare seamount in order to investigate the history of off-axis hydrothermal venting. Baby Bare is a small basement high protruding from a regional sediment cover on the eastern flank of the Juan de Fuca Ridge that acts as a focus for discharging crustal fluids. Stratabound Mn-oxide crusts were collected where warm venting has been observed near the seamount summit. Mn-oxide crusts are composed primarily of 10 Å manganate ± pyrolusite, with minor nontronite, saponite, and/or barite. These assemblage and chemical characteristics such as high Mn/Fe ratios and low trace metal and REE concentrations are indicative of a hydrothermal origin. Minimum ages for these deposits, calculated using growth rates (324 to ∼ 1800 mm/Ma) and estimated thicknesses of manganese outcrops, show that Baby Bare has been hydrothermally active for at least 0.5 Myr, and possibly since its formation (1.7–2.7 Ma). Hydrothermal manganese oxide crusts such as these from Baby Bare record interactions between the hydrothermal fluids and seawater and are important tools for estimating the longevity of off-axis hydrothermal activity.     

Morphology of an uncharted seamount from central Indian Basin

Abstract

The morphology of a seamount at 12°35'S and 76°16'E is studied. This conical seamount has a maximum height of 1275 m (water depth 4135 m) and covers an area of about 189 sq km. The maximum basal extension is about 24 km. The mount is gentler and lengthier on the western flank and steeper and shorter on the eastern flank. A conspicuous flat steplike feature is present on the western flank. Slope angles are high (up to 35°). The trend of the seamount is approximately NNE‐SSW. Dredging yielded basalts containing ferromanganese encrustations indicating its volcanic origin.     

西太平洋雅浦-卡罗琳海区海山多尺度地形特征

海山地形的多尺度特征研究,有助于理解海山形成与演化过程中的地貌过程,并为深入认识俯冲带地球动力过程提供新的线索。本文基于中国科学院海洋研究所“科学”号科考船近年来在雅浦-卡罗琳海区采集的高分辨率海底地形数据,利用统计学及频谱分析等方法,分析了研究区42座海山的形态及地形粗糙度特征,并对海山地形多尺度特征及其成因开展研究。结果表明,雅浦-卡罗琳海区不同构造环境下形成的海山群,在海山的形态参数和多尺度地形特征等方面存在显著差异。相比于索罗尔海槽,帕里西维拉海盆中的海山具有更大的宽高比与更小的平坦度。两区域内海山形态参数具有不同的线性相关关系,表明区域内海山存在不同的形态演化过程。对帕里西维拉海盆中海山地形的频谱分析显示,其大尺度特征信号不显著,意味着该区域内海山地形受小尺度地貌过程的影响更大。索罗尔海槽中海山的粗糙度与海山体积具有线性相关性,可能与不同海山形成过程的差异有关,较早形成的海山受到了更多构造活动及小尺度地貌过程的影响,进而形成了更加粗糙的表面特征。     

Segmentation of the Manila subduction system from migrated multichannel seismics and wedge taper analysis

Based on bathymetric data and multichannel seismic data, the Manila subduction system is divided into three segments, the North Luzon segment, the seamount chain segment and the West Luzon segment starts in Southwest Taiwan and runs as far as Mindoro. The volume variations of the accretionary prism, the forearc slope angle, taper angle variations support the segmentation of the Manila subduction system. The accretionary prism is composed of the outer wedge and the inner wedge separated by the slope break. The backstop structure and a 0.5–1 km thick subduction channel are interpreted in the seismic Line 973 located in the northeastern South China Sea. The clear décollement horizon reveals the oceanic sediment has been subducted beneath the accretionary prism. A number of splay faults occur in the active outer wedge. Taper angles vary from 8.0° ± 1° in the North Luzon segment, 9.9° ± 1° in the seamount segment to 11° ± 1° in the West Luzon segment. Based on variations between the taper angle and orthogonal convergence rates in the world continental margins and comparison between our results and the global compilation, different segments of the Manila subduction system fit well the global pattern. It suggests that subduction accretion dominates the north Luzon and seamount chain segment, but the steep slope indicates in the West Luzon segment and implies that tectonic erosion could dominate the West Luzon segment.     

Broad-band frequency and incident-angle dependence of bottombackscattering on Browns Bank

A two-scale roughness model for bottom backscattering (Novarini and Caruthers) was applied to multibeam sounder data (95 kHz) from Browns Bank (south of Yarmouth, Nova Scotia, Canada). In order to better understand frequency and incident angle dependence of backscattering, acoustic-calibration data (1-6 kHz) were collected from the same area and treated with the same model. The frequency and incident angle dependence of bottom backscattering in the multibeam and acoustic-calibration data were compared. Backscattering due to large-scale roughness was most relevant at near-normal incidence (<7°) and it was more dominant in the low-frequency range, and was strongly dependent on incident angle. Volume scattering was least dependent upon incident angle. It was the dominant factor at the large incident angle. Bragg scattering was the most significant over a very wide frequency range and was more important for high frequency (>5 kHz) and small incidence, but not near-normal incidence