Seamount Summit Research Articles

Flatfish at seamount hydrothermal vents show strong genetic divergence between volcanic arcs

AbstractHydrothermal vents near the summits of seamounts in Western Pacific volcanic arcs foster dense populations of flatfishes, a group otherwise unknown at vents. We examined genetic divergence among populations of a symphurine tonguefish described as Symphurus thermophilus Munroe & Hashimoto from sites up to 6000 km apart in the Western Pacific to explore connectivity patterns among seamounts. Average genetic divergence between individuals from the Mariana Arc and the Tonga‐Kermadec Arcs was 14.2% (COI) and 9.0% (16S), whereas within‐arc divergences were <0.3%. We found that the Tonga‐Kermadec individuals represent a cryptic species, Symphurus sp. A, displaying similar phenotypic features and behaviour to the Mariana Arc S. thermophilus. We also sequenced another distinctive symphurine species; Symphurus sp. B. Collections and image records from three expeditions to the Tonga and Kermadec arcs revealed characteristics of the distribution, dispersion, behaviour and morphology of these flatfish species. These two new Symphurus species inhabit vents where native sulphur occurs in excess and depths are <600 m. Substratum range of Symphurus sp. A was wide, including mussel beds, rock surfaces and sediments in densities that may exceed 100 m−2. Reproductive females were present. The complex and diverse nature of volcanic settings of hydrothermalism introduces a wide variety of habitat conditions that likely augments diversity of faunas on seamounts in distinct biogeographic provinces.

Seamounts in a restless ocean: Response of passive tracers to sub‐tidal flow variability

The action of sub‐tidal oceanic flow impinging on an isolated seamount, and its consequences for biological tracer patterns, was assessed using a three‐dimensional ocean circulation model. Passive tracers released above an isolated Gaussian seamount were exposed to variations of flow magnitude, frequency, direction and seamount height. The evolving patterns indicate significant qualitative differences: Steady and amplitude modulated unidirectional inflows generate advective downstream particle loss and patchiness. Non‐uniform inflow reduces advective particle loss, and tracers are systematically redistributed and aggregated around the seamount summit. Tracer retention is mainly controlled by flow magnitude and summit depth. Variations of flow period were of lesser importance. Two important biological implications arise from this study: First, particle dynamics at isolated seamounts is strongly constrained by mesoscale variability of the sub‐tidal far field flow. Secondly, the influence of seamounts on particle distribution is not restricted to local and small‐scale phenomena, but can penetrate deep into the oceanic far field on a scale of tens to hundreds of kilometers.

Environmental predictors and turnover of biota along a seamount chain

AbstractBiological communities on seamounts are widely regarded as being distinct from those in other offshore habitats, but it is unclear what factors drive any differences. We used invertebrate and algal data from the extensive REVIZEE benthic surveys from the continental margin and seamounts in the Southwest Atlantic Ocean off Eastern Brazil to investigate environmental predictors of seamount assemblage composition. We found that seamount summit samples were significantly different from those of the continental shelf for both invertebrates and algae. Invertebrate samples from the shelf showed more spatial variability than those from seamounts, congruent with the increased variability in both sampled sea‐floor habitats and oceanographic conditions recorded along the continental margin. The most important environmental predictors of assemblage composition across all habitats were distance from shore, temperature, dissolved oxygen and particulate organic carbon for invertebrates, and distance from shore, nitrate and phosphate concentrations for algae. Only distance from shore for invertebrates, and distance from shore, nitrate and phosphate concentrations for algae were consistently emphasised in analyses restricted to seamount and island samples. We found no support for the hypothesis that seamount assemblages in this region become less species rich with distance from the continental margin. However, algal samples did form a serial gradient of compositional turnover with distance from shore. In summary, the seamounts shared the same species pool as nearby continental margins but samples contained a different, equally rich, assemblage of species.

Environmental drivers of ophiuroid species richness on seamounts

AbstractBenthic communities on seamounts are frequently characterised as being species rich, yet there is considerable variation in observed species richness. Although large‐scale patterns of species richness have been described from many marine and terrestrial habitats, their environmental drivers often remain poorly understood. We compared species richness of ophiuroids (brittle‐stars) on 60 seamounts throughout the South West Pacific Ocean, and used an information‐theoretic approach and generalized linear models to determine the relative importance of predictor variables. Due to high correlation among many environmental variables, we used a reduced set of predictors in an a priori model framework. Temperature was the only environmental predictor of any importance in these models over the bathymetric range of the study. Post‐hoc analyses of other potential environmental predictor variables showed that depth, calcite saturation state, temperature range, modelled current velocity and latitude all had some predictive value, but were also highly correlated with temperature or other environmental variables included in the a priori model. Longitude, large‐area species richness, habitat suitability for stony corals, and modelled POC flux did not have high predictive value. We hypothesise that temperature affects richness by constraining species distributions; in particular fewer species can tolerate the conditions on relatively warm shallow seamount summits.

Crustal intrusion beneath the Louisville hotspot track

We report here the first detailed 2D tomographic image of the crust and upper mantle structure of a Cretaceous seamount that formed during the interaction of the Pacific plate and the Louisville hotspot. Results show that at ∼ 1.5 km beneath the seamount summit, the core of the volcanic edifice appears to be dominantly intrusive, with velocities faster than 6.5 km/s. The edifice overlies both high lower crustal (> 7.2–7.6 km/s) and upper mantle (> 8.3 km/s) velocities, suggesting that ultramafic rocks have been intruded as sills rather than underplated beneath the crust. The results suggest that the ratio between the volume of intra-crustal magmatic intrusion and extrusive volcanism is as high as ∼ 4.5. In addition, the inversion of Moho reflections shows that the Pacific oceanic crust has been flexed downward by up to ∼ 2.5 km beneath the seamount. The flexure can be explained by an elastic plate model in which the seamount emplaced upon oceanic lithosphere that was ∼ 10 Myr at the time of loading. Intra-crustal magmatic intrusion may be a feature of hotspot volcanism at young, hot, oceanic lithosphere, whereas, magmatic underplating below a pre-existing Moho may be more likely to occur where a hotspot interacts with oceanic lithosphere that is several tens of millions of years old.

Predicting global habitat suitability for stony corals on seamounts

AbstractAim Globally, species distribution patterns in the deep sea are poorly resolved, with spatial coverage being sparse for most taxa and true absence data missing. Increasing human impacts on deep‐sea ecosystems mean that reaching a better understanding of such patterns is becoming more urgent. Cold‐water stony corals (Order Scleractinia) form structurally complex habitats (dense thickets or reefs) that can support a diversity of other associated fauna. Despite their widely accepted ecological importance, records of scleractinian corals on seamounts are patchy and simply not available for most of the global ocean. The objective of this paper is to model the global distribution of suitable habitat for stony corals on seamounts.Location Seamounts worldwide.Methods We compiled a database containing all accessible records of scleractinian corals on seamounts. Two modelling approaches developed for presence‐only data were used to predict global habitat suitability for seamount scleractinians: maximum entropy modelling (Maxent) and environmental niche factor analysis (ENFA). We generated habitat‐suitability maps and used a cross‐validation process with a threshold‐independent metric to evaluate the performance of the models.Results Both models performed well in cross‐validation, although the Maxent method consistently outperformed ENFA. Highly suitable habitat for seamount stony corals was predicted to occur at most modelled depths in the North Atlantic, and in a circumglobal strip in the Southern Hemisphere between 20° and 50° S and shallower than around 1500 m. Seamount summits in most other regions appeared much less likely to provide suitable habitat, except for small near‐surface patches. The patterns of habitat suitability largely reflect current biogeographical knowledge. Environmental variables positively associated with high predicted habitat suitability included the aragonite saturation state, and oxygen saturation and concentration. By contrast, low levels of dissolved inorganic carbon, nitrate, phosphate and silicate were associated with high predicted suitability. High correlation among variables made assessing individual drivers difficult.Main conclusions Our models predict environmental conditions likely to play a role in determining large‐scale scleractinian coral distributions on seamounts, and provide a baseline scenario on a global scale. These results present a first‐order hypothesis that can be tested by further sampling. Given the high vulnerability of cold‐water corals to human impacts, such predictions are crucial tools in developing worldwide conservation and management strategies for seamount ecosystems.

Effects of oceanographic retention on decapod and gastropod community diversity on seamounts

Using global physical and biological datasets, we tested oceanographic retention (fac- toring out effects of seamount depth and age) as one possible mechanism structuring seamount ben- thic decapod and gastropod communities. We first determined the relative oceanographic retentive potential (such as from Taylor caps or columns) for individual seamounts based on steady-state the- ory. This was then separately compared to decapod and gastropod taxonomic distinctness, our met- ric for the community's response to oceanographic retention. Taxonomic distinctness is a metric based on phylogenetic relatedness between species. Therefore, between-seamount variability in tax- onomic distinctness may reflect the effects of long-term oceanographic retention at the seamount spatial scale. Taxonomic distinctness and retention potential varied between seamounts, but reten- tion did not explain variation in taxonomic distinctness. Among decapod communities, variation in taxonomic distinctness was partly explained by seamount summit depth. With respect to previously suggested causal relationships between seamount retention and biogeographic patterns (divergent seamount communities, high rates of endemism, and inferred speciation at the scale of the seamount), we concluded that seamount-scaled oceanographic retention is weak compared to other ecological drivers of community diversity on seamounts. Alternative processes (such as those related to depth), or alternative spatial scales (within each seamount or between seamount groups and chains) must be considered to explain divergent patterns among seamounts for these taxa.

Description of two new species of Dorsiceratus Drzycimski, 1967 (Copepoda: Harpacticoida: Ancorabolidae) from Sedlo and Seine Seamounts (Northeastern Atlantic) and remarks on the phylogenetic status of the genus

Two new species of Dorsiceratus Drzycimski, 1967 (Copepoda, Harpacticoida, Ancorabolidae), Dorsiceratus wilhelminae sp. nov. and D. dinah sp. nov. are described from Sedlo and Seine Seamounts, respectively (both northeast Atlantic). These are the first records of Dorsiceratus species from seamount summits. Both new species resemble the described species D. octocornis Drzycimski, 1967, D. triarticulatus Coull, 1973, and D. ursulae George, 2006, with respect to most morphological features. On the other hand, D. wilhelminae sp. nov. has long spinules at the inner margin of the A2 enp, while D. dinah sp. nov. bears two, rather than one, tubepores dorsally on third abdominal somite, and a geniculate first outer seta on P1 exp2. These characters are considered as apomorphic relative to the described Dorsiceratus species. As discussed in the present paper, the maintenance of a genus Dorsiceratus appears to be problematic. Although specimens may be assigned without difficulty to a group “Dorsiceratus”, such assignments are based on diagnostic features only; no clear-cut apomorphies have been detected so far to characterize the monophyly of Dorsiceratus. Just two apomorphic characters appear to be synapomorphies for all of the described Dorsiceratus species: 1) P2 enp2 with one rather than two setae and 2) P4 exp sexually dimorphic. Unfortunately, these features are relatively widespread within the Ceratonotus-group sensu Conroy-Dalton (2001) and therefore of rather low value. The authors decided, however, to retain the genus Dorsiceratus until new insights provide more information to support or disprove that hypothesis.

Distribution of zooplankton biomass at three seamounts in the NE Atlantic

Abstract During different seasons of the years 2003–2005 in the NE Atlantic, zooplankton were sampled with a MOCNESS (multiple opening/closing net and environmental sensing system, mesh size 333 μm) above the slopes and summits of Seine, Sedlo and Ampere seamounts and at remote reference sites outside the influence of the seamounts (far field). Wet weights of different zooplankton size classes ( 2 cm) were measured. Night and day hauls were analysed in order to detect diel vertical migrations of the zooplankton, as well as a possible trapping effect due to the shallow topography. Biomass concentrations, independent of daytime, season and summit height, were reduced above the summits at all three seamounts compared to the slope and far-field sites. No trapping effect or retention of biomass was apparent above the seamounts. The vertical distribution patterns of the size class 0.5 cm were nearly absent above the summits of Seine and Ampere seamounts, but considerable numbers were found above the slopes and at the far-field sites. Possible explanations for the observed distribution patterns of zooplankton biomass and size classes are discussed, including retention and lateral advection due to the hydrography at the seamounts, as well as predation by resident seamount fish.

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

AbstractBased 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.

Evidence of a seamount effect on aggregating visitors

It has been suggested that seamounts hold higher abundances of some 'visiting' ani- mals, such as tuna, sharks, billfishes, marine mammals, sea-turtles and even seabirds, but this has been based on sparse records, and warrants further examination. In this paper we use data from a fishery observer program to examine whether the predicted higher abundances of tuna, marine mammals, sea turtles and seabirds actually occur around Azores seamounts and to map the distribu- tion of the various species. Our results indicate that some marine predators (skipjack Katsuwonus pelamis and bigeye tuna Thunnus obesus, common dolphin Delphinus delphis and Cory's shearwater Calonectris diomedea borealis) were significantly more abundant in the vicinity of some shallow- water seamount summits. Our methodology, however, failed to demonstrate a seamount association for bottlenose dolphins Tursiops truncatus, spotted dolphin Stenella frontalis, sperm whale Physeter macrocephalus, terns Sterna hirundo and S. dougalli, yellow-legged gull Larus cachinnans atlantis and loggerhead sea turtles Caretta caretta. Seamounts may act as feeding stations for some of these visitors. Not all seamounts, however, seemed to be equally important for these associations. Only seamounts shallower than 400 m depth showed significant aggregation effects. These seamounts may be considered hotspots of marine life in the Azores, and a special effort should be made in order to ensure a sustainable management of these habitats.

Anthropogenic Pb component in hydrothermal ochres from Marsili Seamount (Tyrrhenian Sea)

Recent hydrothermal Fe-oxyhydroxide precipitates (ochres) were sampled during two submersible expeditions to the NE part of the Marsili Seamount summit (SE Tyrrhenian Sea). The Marsili ochres belong to type 1 primary sulfide-poor Fe-oxyhydroxides according to the classification of Hekinian et al. [Hekinian, R., Hoffert, M., Larqué, P., Cheminée, J.L., Stoffers, P., Bideau, D., 1993. Hydrothermal Fe and Si oxyhydroxide deposits from south Pacific intraplate volcanoes and East Pacific Rise axial and off-axial regions. Econ. Geol. 88, 2099–2121]. They are X-ray amorphous, highly metalliferous [Al/(Al + Mn + Fe) ≪ 0.4], low in Si, transition metals (Co, Ni, Cu, Zn, Pb) and rare earth elements (REE) and enriched in B, V and Mo. Their chemical composition and REE distribution patterns indicate precipitation from low-temperature hydrothermal fluid mixed with seawater. Measured Pb isotope compositions in the ochres indicate that the Pb is dominantly anthropogenic in origin. By contrast, the Pb isotope ratios in the nearby higher temperature hydrothermal deposits from the Panarea arc volcano show that the Pb is derived mainly from local volcanic rocks. The fact that the Pb isotopic signature of the Marsili ochres is controlled by recent anthropogenic input suggests that the Fe-oxyhydroxides precipitated from fluids depleted in transition metals. The low 206Pb/ 207Pb ratios in the ochres reflect the contribution of Pb from Australian ores, indicating precipitation of the ochres somewhere between present-day and the beginning of Australian Pb import to Europe. The young age of the ochres and the presence of other hydrothermal deposits along the Marsili crest suggest that the magmatic chamber beneath the seamount may still be alive. This study represents the first observation of anthropogenic influence on submarine hydrothermal deposits.

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.

Tidally induced turbulent mixing at Irving Seamount—Modeling and measurements

The relationship of tidal currents and turbulence over Irving Seamount is examined by model and measurement. Surface tides are partially scattered into internal tides and, particularly over the seamount summit, current strength and shear greatly increase. Current shear causes the model's turbulent mixing. At Irving, modeled turbulent mixing, as represented by vertical diffusivity, is largest O(10−3 m2 s−1) in the bottom boundary layer and in a turbulent column rising to the sea surface directly above the seamount. Pockets of less intense mixing occur within the turbulent column, but their locations are highly variable. Field measurements confirm a turbulent mixing column atop Irving of comparable intensity. Increased model diffusivity (average of 7 × 10−5 m2 s−1) also occurs to a distance of several hundred kilometers from the seamount flanks in a band vertically centered at ∼800 m depth.

Ocean currents at Axial Volcano, a northeastern Pacific seamount

Axial Volcano is a magmatically active seamount that straddles the Juan de Fuca Ridge (JdFR). With a summit depth of ∼1400 m, the volcano intercepts deep ocean flows at a height well above that of the linear ridge adjoining it to the north and south. Multiyear, yearlong observations of currents at Axial show that mean flow circles the seamount in an anticyclonic sense, consistent with dynamical constraints, with means as large as 6.6 cm s−1. Spectra of currents indicate prominent tidal, inertial, and in some cases weather‐band period (3‐ to 7‐day) peaks, as has been seen elsewhere along the JdFR ridge. For K1, O1, and S2 tidal currents at the seamount, amplitudes are several times larger than the corresponding background amplitudes and rotation around the tidal current ellipses is anticyclonic. M2 tidal currents show none of the same properties. Energy at the inertial frequency decreases by more than an order of magnitude from 100 m above to 400 m below the volcano's summit. In contrast, weather‐band (WB) period energy increases tenfold over the same depth range, with WB energy all but unobservable at summit depth. The presence of WB oscillations makes Axial unlike other seamounts where long observations have been made. The current meter observations also provide a picture of seasonal trends. As expected, during the period Summer 1996 to Spring 1997, largest amounts of WB energy are found during autumn and winter; in contrast, WB motion at Axial Volcano during Winter 2000 was summer‐like in intensity. A linear, baroclinic model of forced damped azimuthal mode‐one oscillatory flow around an idealized axisymmetric seamount shows that amplification of diurnal flow should occur principally at the seamount summit while amplification of the WB should occur, as the observations show, on the seamount flanks. Model and measured phase relationships for temperature and current time series pairs circumscribing Axial support the view that azimuthal mode‐one oscillations are prominent. Over the 0.8‐ to 6‐day range for forcing period (TC) the model indicates that the principal topographic resonance, as indicated by kinetic energy (KE) amplification, occurs for a TC greater than 1 and less than 2 days with only mild dependence in that range on background stratification and seamount shape. Additional but weaker topographic resonances can occur at longer periods in model results, but as TC becomes slightly larger than the frictional timescale, topographic resonances no longer appear. High dissipation and thus short frictional timescales reported by others in regions of rough, sloping topography make topographic resonances at Axial at periods greater than a few days somewhat unlikely. The relatively broad WB spectral peaks at some Axial sites are consequently more likely the result of spectral peaks in forcing than of topographic resonance.

Geotectonic setting of hydrothermal activity on the summit of Lucky Strike Seamount (37°17′N, Mid‐Atlantic Ridge)

We have investigated the relations between volcanic, tectonic, and hydrothermal activity on Lucky Strike Seamount (37°17′N, Mid‐Atlantic Ridge) using a nested survey strategy involving collection of data from different deep‐sea mapping systems. The highly tectonized seamount summit consists of three volcanic cones surrounding a relatively flat depression with a young lava lake in its center. Hydrothermal activity is focused mainly within the summit depression with most of the vents located proximal to the lava lake. Isolated active and inactive chimneys and mounds are widespread throughout the summit depression and occur on both volcanic (pillow lava) and hydrothermal (sulfide rubble and hydrothermally cemented breccias) substrates. The large volume of sulfide rubble, together with the nature of the sulfide structures, indicates that hydrothermal activity has been episodic but ongoing for a long period of time (hundreds to thousands of years). On the basis of the distribution of hydrothermal deposits, we propose a model of alternation between tectonic and volcanic control on hydrothermalism at Lucky Strike Seamount. Midsegment melt focusing produces a spatially and temporally stable heat source that sustains focused high‐temperature hydrothermal activity over long time periods. During periods of amagmatic extension, active faulting within the summit depression provides multiple, near‐surface fluid flow pathways for discharge of high‐temperature fluids and widespread deposition of massive sulfides. During eruptive events, rapid effusion of very hot lava creates a lava lake and hyaloclastite deposits. The new sheet flows form a cap on the hydrothermal system, and fluid upflow is reorganized. Discharge of high‐temperature fluids is restricted to isolated sites with relatively high permeability, for example, the edges of the lava lake. Much of the upwelling hydrothermal fluid pools in the subsurface, conductively cools, and mixes with entrained seawater before discharging as widespread low‐temperature diffuse flow. Hyaloclastites become cemented, further augmenting the sealing of the system. Present‐day activity at Lucky Strike Seamount represents this locally volcanically controlled phase of activity, despite the segment as a whole being dominantly amagmatic.

High abundance of larval rockfish over Cobb Seamount, an isolated seamount in the Northeast Pacific

The larval fish community in the region of Cobb Seamount (500 km west of Oregon) is dominated by myctophid species commonly encountered in the subarctic North Pacific. However, during a survey in June 1992, the ichthyoplankton community within 30 km of the seamount summit was almost completely dominated by larvae of various rockfish (Sebastes) species. Given their very small size (and hence very young age) and the fact that they occurred only rarely in samples collected > 30 km from the seamount summit, we conclude that these Sebastes larvae were produced locally over Cobb Seamount. Previous studies have shown that the Cobb fish fauna is dominated by various Sebastes spp. and that, unlike other fish present on the seamount, the rockfish populations may be self‐recruiting. We suggest that a persistent clockwise (i.e. downwelling) eddy, consistent with a stratified Taylor cone, plays a critical role in retaining larval rockfish over Cobb Seamount and may contribute to the process of self‐recruitment. The key to the success of rockfish on Cobb and other shallow Northeast Pacific seamounts seems to be linked to their viviparous life history.

Analysis of multibeam-Hydrosweep echo peaks for seabed characterisation

Echo-peak data of outer and normal incidence beams from five different Southern Ocean bottom topographic regions have been analysed. The Rice and Extremal probability density function (PDF) statistical approaches reveal that the seabed roughness is, in general, Gaussian in nature except in the case of the Kainan Maru seamount summit (area D). The outer beams of the Enderby abyssal plain (area C) echo-peak PDF statistics reveal the highest possible large-scale feature dominance. Interestingly, Extremal PDF fit parameters (α) from the Agulhas Basin (area A) show a less dominant large-scale roughness than in the case of area C. Large-scale feature dominance up to 15° beam angle is observed in the case of mixed roughness seabeds such as area B (southwest Indian ridge) and area E (Meteor Rise). No increase in the 30°-beam Extremal PDF parameters is observed in these areas. Maximum microtopographic roughness is documented in area D on the summit of Kainan Maru seamount. The estimated fit parameters using Extremal statistics of outer-beam peak data provide a better understanding of the scattering process. Previously determined power law and volume roughness parameters using composite roughness theory are given for the five different areas. These support the results obtained using Rice and Extremal PDFs. The existence of higher volume roughness parameters and power law parameters for medium-scale roughness, along with dominant microtopographic features, are evident from the results of this study.

Variability of surface sediments in the Peru basin: dependence on water depth, productivity, bottom water flow, and seafloor topography

Abstract We investigated surficial sediments for physico-chemical composition from numerous sites of seven study areas in the manganese nodule field of the northern Peru Basin as part of a deep-sea environmental study. Major results from this study are strong variability with respect to water depth, productivity in surface waters, locality, bottom water flow, and seafloor topography. Sediment sites are located mostly in 3900 to 4300 m water depth between the lysocline and the carbonate compensation depth (CCD). Large fluctuations in carbonate content (0% to 80%) determine sediment density and compressional-wave velocity, and, by dilution, contents of opal and non-biogenic material. Mass accumulation rates of biogenic components as well as geochemical proxies (barium and phosphorus) distinguish areas of higher productivity in the northwest near equatorial upwelling and in the northeast close to coastal upwelling, from areas of lower productivity in the west and south. Comparisons between the central Peru Basin area (Discol) and western Peru Basin area (Sediperu) reveals, for the Sediperu area, a shallower CCD, more carbonate but less opal, organic carbon, and non-biogenic material in sediments at the same water depth as well as larger down-core fluctuations of organic carbon and MnO2. Bottom water flow in the abyssal hill topography causes winnowing of material from summits of seamounts and ridges, where organic carbon preservation is poor, to basins where organic carbon preservation is better. Down-core measurements in box cores indicate a three-fold division in the upper 50 cm of the sediment column. An uppermost semi-liquid top layer is dark brown, 5–15 cm thick and contains most of the ferro-manganese nodules. A 5–15 cm thick transition zone of light sediment color has increasing shear strength, lowest opal contents and compressional-wave velocities, but highest carbonate contents and sediment densities. The lowermost layer contains stiffer light gray sediments.

The Great Meteor seamount: seismic structure of a submerged intraplate volcano

During R/V METEOR cruise No. 12/2 in summer 1990, an OBH airgun refraction line was shot across the Great Meteor seamount, Northeast Atlantic. Velocity structure was modeled using 1-dimensional velocity analysis and 2-dimensional ray tracing of P-wave travel times. The results represent a seismic image of a submerged intraplate volcano. Upper crustal and apron P-velocities of the seamount are significantly lower than those observed in the oceanic layer 2. We interpreted these low velocities in terms of extrusive lavas and of debris flows with high average porosities. At 3 km beneath the seamount summit, the core of the edifice appears to be dominantly intrusive, with P-velocities >6.0 km/s. A distinct high-velocity body (P-velocities >6.5 km/s) suggests a central conduit or a plutonic suite. Weak evidence was found for the existence of a subcrustal plutonic complex with P-velocities intermediate between those of layer 3 and the upper mantle as it is typical for many volcanic islands.