Seamount Off Research Articles

Tectonic and volcanic implications of a cratered seamount off Nicobar Island, Andaman Sea

The region of the Nicobar earthquake swarm of January 2005 has been explored during a recent cruise using multibeam swath bathymetry, seafloor imaging and TV-guided sampling to decipher the seafloor morphology, nature and tectonic frame work. A seamount with well-developed crater at the summit was discovered near to the center of the Nicobar swarm. Rock samples collected by TV-guided grab from the seamount crater are dacite, rhyolite and andesite type with a veneer of ferromanganese oxide coating. The aggregates recovered from the slope consisted of manganese globules. The geochemistry of the globules suggests 66–97% Mn-oxide indicative of hydrothermal origin. The morphology of the seamount, seafloor video footage and geochemistry of the seabed samples suggest that the cratered seamount has erupted in the recent geological past and is dormant at present. This is the first documented report of submarine arc-volcanism in the Andaman Sea. This finding substantiates the prediction of a submarine volcano east of Nicobar Island. Study of the seismicity and the stress pattern in the region suggest that the earthquake swarm in the region occurred due to 2004 Sumatra–Andaman megathrust earthquake. The derived stress pattern suggests that the 2004 earthquake greatly reduced the normal stress in the region of West Andaman fault from the planes or unclamped them. Thus unclamping of the region by the 2004 Sumatra Andaman earthquake led to the initiation of the swarm through predominantly strike slip faulting. The newly discovered dormant submarine volcano indicates the volcanic nature of the region. We suggest that this submarine volcano is a link between sub-aerial volcanoes of Barren–Narcondam Islands of the Andaman Sea and the volcanoes of Sumatra.

Impacts of bottom trawling on deep-coral ecosystems of seamounts are long-lasting

Complex biogenic habitats formed by corals are important components of the megaben- thos of seamounts, but their fragility makes them susceptible to damage by bottom trawling. Here we examine changes to stony corals and associated megabenthic assemblages on seamounts off Tasma- nia (Australia) with different histories of bottom-contact trawling by analysing 64 504 video frames (25 seamounts) and 704 high-resolution images (7 seamounts). Trawling had a dramatic impact on the seamount benthos: (1) bottom cover of the matrix-forming stony coral Solenosmilia variabilis was reduced by 2 orders of magnitude; (2) loss of coral habitat translated into 3-fold declines in richness, diversity and density of other megabenthos; and (3) megabenthos assemblage structures diverged widely between trawled and untrawled seamounts. On seamounts where trawling had been reduced to <5% a decade ago and ceased completely 5 yr ago, there was no clear signal of recovery of the megabenthos; communities remained impoverished comprising fewer species at reduced densities. Differences in community structure in the trawled (as compared to the untrawled) seamounts were attributed to resistant species that survived initial impacts, others protected in natural refugia and early colonisers. Long-term persistence of trawling impacts on deep-water corals is consistent with their biological traits (e.g. slow growth rates, fragility) that make them particularly vulnerable. Because recovery on seamounts will be slow, the benefits from fishery closures may not be immedi- ately recognisable or measureable. Spatial closures are crucial conservation instruments, but will require long-term commitments and expectations of performance whose time frames match the bio- logical tempo in the deep sea.

Ichthyofauna on three seamounts off southern and central California, USA

In many regions of the world, commercial fisheries for seamount-aggregating species have collapsed. Most are trawl fisheries, which have been implicated in the destruction and loss of fragile coral and sponge habitat and potentially endemic seamount species. Even with the intense fishing effort over seamounts, our understanding of processes regulating the distribution and abun- dance of seamount fish assemblages, which is vital to conservation policy, is weak. Here, we describe the abundance and distribution of demersal fishes found on 3 seamounts off central and southern California. Video observations were taken during 27 dives of a remotely operated vehicle (ROV), and were annotated in detail using the Monterey Bay Aquarium Research Institute's (MBARI) Video Annotation Reference System (VARS). Video analysis yielded 2151 observations of 36 identified taxa. No aggregations of fishes were observed during the surveys. Video transects were analyzed to quan- tify organism density. Similarity among seamounts was compared using Bray-Curtis similarity esti- mates. Our results indicate high similarity among seamount faunas at similar depths, a shift from provincial to abyssal/cosmopolitan species with increased depth, and no evidence of seamount endemism.

Evidence that three seamounts off southern California were ancient islands

Many seamounts world-wide exhibit features that suggest they were once islands. Rodriguez and San Juan Seamounts and Northeast Bank, within 250 km of the continental margin of the western US, were explored with the ROV Tiburon and found to have guyot morphology, beach deposits and erosional features, degassed volcanic lavas and pyroclasts, and cooling and alteration patterns unusual for submarine erupted lavas. These features are evaluated as evidence that the volcanoes stood above sea level. The summit of Rodriguez Seamount is a smooth, gently domed platform. Rough hills stand above the platform and consist of dense, degassed ‘a‘a flows erupted subaerially. Bedded sandstones and rounded cobbles interpreted to be beach deposits occur near the top of a major break-in-slope at about 700 m depth. The summit of Northeast Bank is also a smooth, gently domed platform. Beach deposits were observed just below the pronounced break-in slope at about 500 m. The summit of San Juan Seamount is a northeast-southwest trending series of rough ridges rather than a dome. However, subaerially exposed ‘a‘a-like lava flows were found above 700 m. Oxidized olivine, degassed volcanic glass, and secondary phosphatization of the lavas are found both shallower and deeper than the breaks-in-slope, and their presence may not be robust indicators of subaerial exposure. Rodriguez stood at least 70 m above sea level and formed a small island 6.8 km 2 in area prior to subsiding at least 700 m. Northeast Bank was a large island perhaps 200 m above sea level and 90 km 2 in area. The summit ridge of San Juan emerged as a line of eight small islands with a total area of about 2.8 km 2, and the tallest island rose at least 140 m above sea level. The subsidence of Northeast Bank within the California borderland, Rodriguez Seamount on the continental slope, and San Juan Seamount on the adjacent ocean crust occurred since they formed 7 to 11 million years ago. The minimum subsidence is consistent with cooling and contraction of the underlying lithosphere.

Feeding strategy of the night shark (Carcharhinus signatus) and scalloped hammerhead shark (Sphyrna lewini) near seamounts off northeastern Brazil

A total of 425 stomachs of night shark (Carcharhinus signatus), and 98 stomachs of scalloped hammerhead shark (Sphyrna lewini), from longline and surface gillneters near seamounts off northeastern Brazil, were analysed between 1992 and 1999. Both predators prey upon reef and benthopelagic fishes, migrant cephalopods and deep water crustaceans, showing similar feeding niches (Schoener Index T=0.75). The great prey richness of the diets may reflect the fact that the marine food web for these species is very extensive in this region. Due to the concentration for feeding of both predators in the seamounts, intense fisheries should be monitored to avoid localised depletions.

Two new Hyalonema species (Hyalonematidae: Amphidiscosida) from eastern and south-eastern Brazil, and further Hexactinellida (Porifera) collected from seamounts off south-eastern Brazil by the RV ‘Marion Dufresne’ MD55 expedition

Descriptions of hexactinellid sponges collected by the RV ‘Marion Dufresne’ MD55 expedition on the Vitória–Trindade seamounts chain (off Espírito Santo State, south-eastern Brazil) in 1987 and stored in the MNHN (Muséum National d'Histoire Naturelle in Paris) are presented. Hyalonema (Cyliconema) conqueror sp. nov. (the first finding of this subgenus in the Atlantic Ocean) and H. (Prionema) dufresnei sp. nov. (the second record of this subgenus for the Atlantic Ocean) are described as new species. The holotype of H. (C.) conqueror sp. nov. was collected with a ROV at Campos Basin (off Rio de Janeiro State, south-eastern Brazil), while the paratypes originated from Vitória–Trindade seamounts chain and off Bahía State (eastern Brazil). Other hexactinellids reported here, Farrea sp., Sarostegia aff. oculata, Aphrocallistes aff. beatrix, Dactylocalyx aff. subglobosus and Euplectella suberea were known before to be widely distributed in the Atlantic Ocean. The total number of hexactinellid sponges known from Brazil has risen to 15 and from the south-western Atlantic to 23.

Benthic invertebrate communities on three seamounts off southern and central California, USA

Seamounts are unique and biologically productive deep-sea habitats that have often been described as having high levels of endemism, highly productive fisheries, and benthic commu- nities vulnerable to trawl fishing. We describe the abundance and distribution of benthic megafaunal invertebrates found on 3 seamounts off central and southern California. Video observations were taken during 27 dives of a remotely operated vehicle (ROV) and were annotated in detail using the Monterey Bay Aquarium Research Institute's (MBARI) video annotation reference system (VARS, http://vars.sourceforge.net/). Video analysis yielded 134 477 observations of 202 identified inverte- brate taxa. Video transects were analyzed to quantify organism density. Thirteen new species were observed and collected. Invertebrate communities at Davidson and Pioneer Seamounts were domi- nated by passive suspension-feeding cnidarians (mostly corals), but at Rodriguez Seamount, a guyot, the invertebrate community was dominated by holothurian echinoderms. We found no evidence of endemism among the megafauna at these 3 seamounts, which are all in close proximity to each other and the continental margin.

Widespread strombolian eruptions of mid-ocean ridge basalt

Glassy lava fragments were collected in pushcores or using a small suction-sampler from over 450 sites along the Juan de Fuca Ridge, Blanco Transform Fault, Gorda Ridge, northern East Pacific Rise, southern East Pacific Rise, Fiji back-arc basin, and near-ridge seamounts in the Vance, President Jackson, Taney, and a seamount off southern California. The samples consist of angular glass fragments, limu o Pele, Pele's hair, and other fluidal fragments formed during pyroclastic eruptions. Since many of the sites are deeper than the critical point of seawater, fragmentation cannot be hydrovolcanic and caused by expansion of seawater to steam. The glass fragments have a wide range of MORB compositions, ranging from fractionated to primitive and from depleted to enriched. Enriched magmas, which have higher volatile contents, may form more abundant pyroclasts than depleted magmas. Eruptions with high effusion rates produce sheet flows and abundant pyroclasts whereas those with low effusion rates produce pillow ridges and few pyroclasts. This relation suggests that high effusion and conduit rise rates are coupled to high magmatic gas contents. The eruptions are mainly effusive with a minor strombolian bubble burst component. We propose that the gas phase is an added component of variable amounts of magmatic foam from the top of the magma reservoir. As the mixture of resident magma and foam rises in the conduit, the larger bubbles in the foam rise more quickly and sweep up the smaller bubbles nucleating and growing from the resident magma. On eruption, the process of bubble coalescence is more complete for the slower rising, gas-poor lavas that erupt as pillow lavas whereas the limu o Pele associated with sheet flow eruptions commonly contain several percent vesicles that avoided coalescence during ascent. The spatter erupted at the vent is quench granulated in seawater above the vent, reducing the pyroclast grainsize. The granulated spatter and limu o Pele fragments are then entrained in a rising plume of seawater heated by the eruption, which disperses them to distances as great as 5 km from the vent.

A new species of five-armed feather star, Pentametrocrinus paucispinulus n. sp., from the eastern Pacific (echinodermata: Crinoidea: Comatulida)

ABSTRACT Pentametrocrinus paucispinulus, new species, is described from a specimen collected on Pioneer Seamount off California in 1768 m. Like its closest congeners, it bears the first pinnule on the second brachial, but it differs in several characters. For example, P. varians and P. australis have longer tapered cirri with more segments, and P. spinosus has multiple rows of thorny stumps on the proximal arms. This is the first record of the genus from the eastern Pacific Ocean. A key to the species in the genus Pentametrocrinus is included.

A new Western Pacific Tonguefish (Pleuronectiformes: Cynoglossidae): The first Pleuronectiform discovered at active Hydrothermal Vents

Symphurus thermophilus n. sp., described from 16 specimens collected by submersibles, ROV, epibenthic sled and dredge, occurs on a variety of substrata at several active hydrothermal sites located at 239–733 m between 21°N and 35°S in the western Pacific Ocean. Symphurus thermophilus, the only pleuronectiform fish known to inhabit hydrothermal vent areas, is characterized by the combination of a 1–2–2–2–2 pattern of interdigitation of dorsal proximal pterygiophores and neural spines, 14 caudal-fin rays, 5 hypurals, 9 abdominal vertebrae, 47–51 total vertebrae, 88–94 dorsal-fin rays, 74–80 anal-fin rays, 100–112 scales in longitudinal series, ocular-side pigmentation pattern featuring 5–8, black, mostly incomplete crossbands, uniformly white blind side, and black peritoneum. Of specimens examined, seven including the holotype, were collected on Kaikata Seamount off southern Japan; one specimen was collected at the Kasuga-2 hydrothermal vent, Marianas Islands; and six were collected at sites on the Kermadec Ridge. In addition to specimens captured, many other S. thermophilus were observed from submersibles and ROVs at hydrothermal sites in the western Pacific including those in the Marianas Islands, at Nikko Seamount near Minami-Iohjima Island, and at Minami-Ensei Knoll, Mid-Okinawa Trough. Many of the specimens examined have skeletal anomalies including fused bones in the caudal skeleton, and missing or partially developed and/or misshapen fin rays.

Partitioning of the Pelagos Sanctuary (north-western Mediterranean Sea) into hotspots and coldspots of cetacean distributions

This study used Monte Carlo methods to generate simulations that considered the effort distribution to determine the locations of significant aggregations of cetacean sightings inside the northern Pelagos Sanctuary (north-western Mediterranean Sea). For three years, monitoring has been conducted from five motor vessels covering about 30,050 km. The most frequently encountered species were the striped dolphinStenella coeruleoalbarepresenting 64.7% of all sightings, the fin whaleBalaenoptera physalus(17.0%), Cuvier's beaked whaleZiphius cavirostris(9.9%), Risso's dolphinGrampus griseus(4.3%) and the sperm whalePhyseter catodon(2.2%). Sightings' positions and effort coverage were distributed over a grid of 5′ longitude and 5′ latitude. Spatial concentrations of sightings were analysed according to the distribution of effort to identify ‘hotspots’ (locations where the species occurred at a significantly greater frequency than expected), and ‘coldspots’ (locations with a significantly lower frequency than expected). Most fin whale hotspots (14) were located on the bathyal plain between 2000 and 2500 m, four hotspots were around the 1000 m isobaths, and one is located close to the seamount off Genoa. Fin whale coldspots were mainly along the coast. Striped dolphin hotspots were widely distributed over two main areas, in waters with depths between 2000 and 2500 m and at the continental slope; coldspots for this species were also mainly along the coast in the northern part of the study area. Many hotspots were found in the Genoa Canyon, and hotspots of striped dolphins, Cuvier's beaked whales, sperm whales and Risso's dolphins overlapped in this region. Some of the hotspots of Cuvier's beaked whales were identified at the seamount in the study area, where no other species was sighted frequently. Risso's dolphin hotspots were mainly near the 1000 m isobath. For sperm whales, several hotspots were identified: three associated with steep slope features (such as canyons or the continental slope), and one was in the centre of the flat area of Pelagos where the depth is 2500 m. This study highlights the ecological importance of particular locations inside the Pelagos Sanctuary—locations that should be protected from anthropogenic degradations for marine mammal conservation.

6+ years of acoustic thermometry in the North Pacific Ocean: A cycle of the Pacific decadal oscillation

Acoust. measurements of basin-averaged temperatures have been made in the North Pacific over the last 8 years. The acoustic observations give integral measurements of ocean temperature with the averaging needed to detect weak, gyre-scale signals out of the dominant mesoscale variability. An acoustic source on Pioneer Seamount off California transmitted to receivers distributed throughout the North Pacific from 1996 to 1999 as part of the Acoust. Thermometry of Ocean Climate (ATOC) project. A second source, located near Kauai, began transmitting in 1997. The Kauai acoustic transmissions are part of the North Pacific Acoust. Lab. (NPAL) project. A 6-year time series has been obtained with sampling six times per day every 4 days. The cabled-to-shore source and receivers provide near real-time data. Long-period changes in temperature are apparent in the time series from the Kauai source. The paths from Kauai toward the California coast show steady cooling. A path to the northwest showed warming until early 2003, when the ‘‘normal’’ annual cycle returned. Comparisons of the acoustic data to observations by altimetry and Argo floats are not definitive because of the great differences between data types. (J. Colosi, WHOI; B. Cornuelle, SIO; B. Dushaw, APL-UW; M. Dzieciuch, SIO; B. Howe, APL-UW; J. Mercer, APL-UW; R. Spindel, APL-UW; P. Worcester, SIO)

Redescription and distribution of two species of Syscenus (Crustacea, Isopoda, Aegidae) in the North Atlantic

The isopod species Syscenus atlanticus Kononenko, 1988, is recorded for the second time, from Bear Seamount off New England, western North Atlantic. The species, previously known only from the central North Atlantic, is redescribed, illustrated, and compared with the more common S. infelix Harger, 1881, which is also redescribed and illustrated. The distribution of S. infelix in a broad arc across the North Atlantic Ocean, based on 171 specimens from several museum collections, is discussed.

A revision of stalked barnacles (Cirripedia: Thoracica: Scalpellomorpha: Eolepadidae: Neolepadinae) associated with hydrothermalism, including a description of a new genus and species from a volcanic seamount off Papua New Guinea

In describing a new species of pedunculate barnacle that occurs in large numbers in sulphidic seep conditions on a volcanic seamount off Lihir Island, Papua New Guinea, it has been necessary to revise the Neolepadinae and erect two new genera. Capitulum and peduncle characters distinguish the new genera from the species ofNeolepas that occur at hydrothermal vents along the East Pacific Rise. The new genus and species from Lihir Island also differs fromNeolepas osheai described from the Kermadec Ridge, which, together with an undescribed species discovered on the Pacific-Antarctic Ridge, belongs to another new genus. The species discussed are all adapted to feed on fine particles, including bacteria.

A new genus and species of hermit crab (Decapoda: Anomura: Paguridae) from seamounts off south-eastern Tasmania, Australia

McLaughlin, P.A. 2003. A new genus and species of hermit crab (Decapoda: Anomura: Paguridae) from seamounts off south-eastern Tasmania, Australia. Memoirs of Museum Victoria 60(2): 229‐236. A new and highly variable species, representing a new genus of hermit crabs, is described and illustrated. In addition to the morphological changes in shape and armature of the chelipeds associated with growth in Bythiopagurus macrocolus, gen. nov., sp. nov., this species exhibits a singular development of the left chela that may be an adaptation to its specialised habitat among colonies of the stony coral Solenosmilia variabilis Duncan. The similarities seen among the genera Bythiopagurus, gen. nov., Michelopagurus McLaughlin and Icelopagurus McLaughlin appear the be superficial and convergent; Bythiopagurus appears phylogenetically related to the Pylopaguropsis group of genera characterised by 13 pairs of bi- or quadriserial gills.

The slip distribution of the 1946 Nankai earthquake estimated from tsunami inversion using a new plate model

Abstract A new model for the shape of the Philippine Sea Plate (PSP) subducting beneath southwest Japan was constructed by combining marine seismic survey results and seismicity data. This resulted in a plate model that is generally 3–5 km shallower than previous estimates, and contains new details of plate boundary structure near the Nankai Trough axis. We estimated the slip distribution of the 1946 Nankai earthquake by inverting tsunami waveforms using the new plate boundary model, and found that the large slip off Cape Muroto and seismic moment were reduced by about 10% from the previous estimate of Tanioka and Satake (2001a) . However, we also found reason to suspect the tide gauge closest to the epicenter (at Shimotsu), and this resulted in a much more substantial reduction in the amount of slip in the eastern segment of the rupture area. We also find that our new rupture model correlates well with features of the subducting plate boundary, such as a large subducting seamount off Cape Muroto ( Kodaira et al., 2000a ). This suggests that such features have an important effect on either earthquake rupture propagation or the interseismic accumulation of elastic strain energy, or both.

Evolution of the Sardinia Channel (Western Mediterranean): new constraints from a diving survey on Cornacya seamount off SE Sardinia

Sarcya 1 dive explored a previously unknown 12 My old submerged volcano, labelled Cornacya. A well developed fracturation is characterised by the following directions: N 170 to N–S, N 20 to N 40, N 90 to N 120, N 50 to N 70, which corresponds to the fracturation pattern of the Sardinian margin. The sampled lavas exhibit features of shoshonitic suites of intermediate composition and include amphibole-and mica-bearing lamprophyric xenoliths which are geochemically similar to Ti-poor lamproites. Mica compositions reflect chemical exchanges between the lamprophyre and its shoshonitic host rock suggesting their simultaneous emplacement. Nd compositions of the Cornacya K-rich suite indicate that continental crust was largely involved in the genesis of these rocks. The spatial association of the lamprophyre with the shoshonitic rocks is geochemically similar to K-rich and TiO 2-poor igneous suites, emplaced in post-collisional settings. Among shoshonitic rocks, sample SAR 1-01 has been dated at 12.6±0.3 My using the 40Ar/ 39Ar method with a laser microprobe on single grains. The age of the Cornacya shoshonitic suite is similar to that of the Sisco lamprophyre from Corsica, which similarly is located on the western margin of the Tyrrhenian Sea. Thus, the Cornacya shoshonitic rocks and their lamprophyric xenolith and the Sisco lamprophyre could represent post-collisional suites emplaced during the lithospheric extension of the Corsica–Sardinia block, just after its rotation and before the Tyrrhenian sea opening. Drilling on the Sardinia margin (ODP Leg 107) shows that the upper levels of the present day margin (Hole 654) suffered tectonic subsidence before the lower part (Hole 652). The structure of this lower part is interpreted as the result of an eastward migration of the extension during Late Miocene and Early Pliocene times. Data of Cornacya volcano are in good agreement with this model and provide good chronological constraints for the beginning of the phenomenon.

Turning point filters: Analysis of sound propagation on a gyre-scale

Acoustic transmissions from Pioneer Seamount off California to a vertical array near Hawaii are analyzed using a technique which we call a turning-point filter. The observables, travel time and axial inclination, are interpreted in terms of the ocean sound–speed field. The method permits a uniform treatment of the arrival pattern, from the early ray-like arrivals to the late mode-like arrivals, including peak arrivals which cannot be identified as either rays or modes. An adiabatic range dependence is assumed, and Wentzel–Kramers–Brillouin–Jeffreys formalism is applied.

B23 ハワイ沖ロイヒ海底火山, 南リフトゾーンの円錐型溶岩丘について

B23 ハワイ沖ロイヒ海底火山, 南リフトゾーンの円錐型溶岩丘について

Description of a new species of the Pacific shrimp genus Paracrangon (Crustacea: Decapoda: Crangonidae) from southern Australia, with a key to the genus

Hanamura, Y., Wadley, V. and Taylor, J., 1999. Description of a new species of the Pacific shrimp genus Paracrangon (Crustacea: Decapoda: Crangonidae) from southern Australia, with a key to the genus. Memoirs of Museum Victoria 57: 311-317. A sixth species of the genus Paracrangon, P. australis sp. nov. (Crustacea: Decapoda: Crangonidae), is reported from a seamount off southern Australia and from Macquarie Island. The Australian species is unique in having four-five teeth on the dorsal median margin of the carapace, which is associated with a distinct reticulate structure on the supraventral part. All species of the genus recorded to date have four or fewer teeth on the dorsal median margin of the carapace. The present record is the first occurrence of the genus in the southwest Pacific and greatly extends its known geographical range. A key for identification of all species is presented with data on geographical and bathymetric ranges.