Slope Off Research Articles

A geoestatística como ferramenta de estudo da batimetria na vertente continental portuguesa

GEOSTATISTICS AS A TOOL FOR THE STUDY OF BATHYMETRY ON THE PORTUGUESE CONTINENTAL SLOPE. The increasing interest on deep-water resources by the commercial fishing fleet during the last 20 years led the Fisheries Research National Institute (presently INIAP-IPIMAR) to conduct seafloor topography studies, which aimed the identification of potential fishing grounds for deep-water species in the Portuguese continental slope.The present study lies in the scope of the above topography studies. The spatial variability of the seafloor depth from a particular area of the Portuguese continental slope off Figueira da Foz with about 665 sq. m. was investigated. A geostatistical analysis comprising spatial continuity, several estimation methods and its uncertainty was conducted. The sequential gaussian simulation method was applied to evaluate estimation values uncertainty.Residual kriging yielded the lower estimation errors. The higher uncertainty observed in the estimation results obtained by Ordinary Kriging suggests that the geostatistic model to be adopted for the study area should be non-stationary. However its implementation is more straightforward since it has less requirements. Based on the estimation and simulation results two digital maps were constructed for the study area, one on the bathymetry and the other on the trawl fishing uncertainty.

Benthic Foraminiferal Biofacies on the Shelf and Upper Continental Slope off North Kerala (Southwest India)

Abstract: This study examines benthic foraminifera (>63 μm) both qualitatively and quantitatively, from 19 closely spaced surficial sediment samples covering 30 to 200 m water depths across the shelf and upper continental slope off north Kerala (SW India). A total of 59 species are recorded. The major constituents of benthic foraminiferal assemblages in the study area are fursenkoinids, bolivinids, nonionids, rotaliids, elphidiids, buliminids, miliolids, gavilinellids, amphestiginids, bagginids, vaginulinids, uvigerinids and various agglutinated taxa. Cluster analysis using Bray Curtis similarity index defines four sample groups, each typified by a characteristic assemblage representing a biofacies. The major benthic foraminiferal biofacies identified are: Biofacies I, Fursenkoina-Nonion-Ammonia beccarii s.l. (30-40 m); Biofacies II, Fursenkoina-Nonion (40-55 m); Biofacies III, Bolivina robusta-Hanzawaia-Cancris-Amphistegina and miliolids (55-115 m) and Biofacies IV, Bolivina persiensis- Uvigerina-Bulimina-Fursenkoina and agglutinants (115-200 m). Relict foraminifera, most commonly represented by shallow-water benthic taxa are concentrated on the outer shelf. The relict assemblage appears to be a product of late Pleistocene low sea level. The foraminiferal biofacies have a good correspondence with the bathymetrically distributed three major lithofacies across the shelf and the upper continental slope. We studied the distribution pattern of individual taxa constituting the biofacies. The study demonstrates a relationship between the pattern of distribution of major benthic fauna and the sediment-size and organic carbon content across the inner shelf to upper slope. Additionally, certain taxa appear to be sensitive to various hydrographic parameters, such as, Bulimina marginata to temperature and Fursenkoina complanata and Uvigerina ex gr. U. semiornata to dissolved oxygen level.

OIL AND NATURAL GAS IN THE NORD AMERICA’S CONTINENTAL SLOPE

In the North America’s continental slope off Canada, Mexico and USA, there are 322 deepwater (222-3042 m) oil and gas fields. Their total reserves are equal to 7605 million tons of oil equivalent in the Pleistocene, Cenozoic and Mesozoic sands and sandstones at the depths of 879 to 10 692 m.

Sahel megadrought during Heinrich Stadial 1: evidence for a three-phase evolution of the low- and mid-level West African wind system

Millennial-scale dry events in the Northern Hemisphere monsoon regions during the last Glacial period are commonly attributed to southward shifts of the Intertropical Convergence Zone (ITCZ) associated with an intensification of the northeasterly (NE) trade wind system during intervals of reduced Atlantic meridional overturning circulation (AMOC). Through the use of high-resolution last deglaciation pollen records from the continental slope off Senegal, our data show that one of the longest and most extreme droughts in the western Sahel history, which occurred during the North Atlantic Heinrich Stadial 1 (HS1), displayed a succession of three major phases. These phases progressed from an interval of maximum pollen representation of Saharan elements between ∼19 and 17.4 kyr BP indicating the onset of aridity and intensified NE trade winds, followed by a millennial interlude of reduced input of Saharan pollen and increased input of Sahelian pollen, to a final phase between ∼16.2 and 15 kyr BP that was characterized by a second maximum of Saharan pollen abundances. This change in the pollen assemblage indicates a mid-HS1 interlude of NE trade wind relaxation, occurring between two distinct trade wind maxima, along with an intensified mid-tropospheric African Easterly Jet (AEJ) indicating a substantial change in West African atmospheric processes. The pollen data thus suggest that although the NE trades have weakened, the Sahel drought remained severe during this time interval. Therefore, a simple strengthening of trade winds and a southward shift of the West African monsoon trough alone cannot fully explain millennial-scale Sahel droughts during periods of AMOC weakening. Instead, we suggest that an intensification of the AEJ is needed to explain the persistence of the drought during HS1. Simulations with the Community Climate System Model indicate that an intensified AEJ during periods of reduced AMOC affected the North African climate by enhancing moisture divergence over the West African realm, thereby extending the Sahel drought for about 4000 years.

A Model-Based Approach to Designing a Fishery-Independent Survey

This paper uses Generalized Additive Models to evaluate model-based designs for wildlife abundance surveys where substantial pre-existing data are available. This is often the case in fisheries with historical catch and effort data. Compared to conventional stratified design or design-based designs, our model-based designs can be both efficient and flexible, for example in allowing uneven sampling due to survey logistics, and providing a general framework to answer specific design questions. As an example, we describe the design and preliminary implementation of a trawl survey for eleven fish species along the continental slope off South-East Australia.

Demersal fish fauna of the continental slope off Nova Scotia, Canada, based on exploratory bottom trawl surveys in 1994�95

The demersal fish fauna at 900–1800 m depths off Nova Scotia, Canada, is described using data from exploratory bottom trawl surveys conducted in November 1994 and March 1995 by a commercial fishing trawler. Approximately 25 metric tons (39 000 specimens) of demersal fish belonging to at least 82 species were caught, 30% of which had not previously been recorded from the area. However, more than half the catch consisted of the two species Centroscyllium fabricii (black dogfish) and Coryphaenoides rupestris (roundnose grenadier). Catches were higher in the shallower depth strata fished and cluster analysis showed that depth was the primary factor determining species composition of catches. It is suspected, however, that the vessel fished less effectively at depths greater than about 1500 m, contributing to the reduction in catch quantities at these depths. The importance in catches of large bodied species, particularly Chimaeriformes and sharks, contrasts with results from surveys in adjacent areas. This likely reflects the greater fishing power of the vessel/gear used in present surveys rather than real differences in faunal composition. Catches of mesopelagic species during these surveys, and during an earlier deepwater trawling survey in this area, are also described.

Demersal fishes caught with bottom gillnets and baited gears at 500-2800m on the continental slope off Nova Scotia, Canada

Fishing trials with bottom fixed gears (primarily gillnets but also shrimp and crab traps and longlines) were conducted on the continental slope off Nova Scotia in August 1991. Fishing was conducted at several depths between 500 and 2800 m in two areas, one on the open slope south of Emerald Bank and the other at the mouth of The Gully, a large canyon. Catches by gillnets accounted for about 90% of the total and were predominated by deepwater chimaera (Hydrolagus affinis), black dogfish (Centroscyllium fabricii) and Portuguese shark (Centroscymnus coelolepis). The most frequently caught species in traps were snubnose eel (Simenchelys parasitica) and abyssal grenadier (Coryphaenoides armatus). Variations in catches by depth and area are described, and comparisons are made with catches from otter trawl surveys at similar depths with regard to the most prominent species caught and their size compositions.

Seismic evidence for shallow gas‐escape features associated with a retreating gas hydrate zone offshore west Svalbard

Active gas venting occurs on the uppermost continental slope off west Svalbard, close to and upslope from the present‐day intersection of the base of methane hydrate stability (BMHS) with the seabed in about 400 m water depth in the inter‐fan region between the Kongsfjorden and Isfjorden cross‐shelf troughs. From an integrated analysis of high‐resolution, two‐dimensional, pre‐stack migrated seismic reflection profiles and multibeam bathymetric data, we map out a bottom simulating reflector (BSR) in the inter‐fan region and analyze the subsurface gas migration and accumulation. Gas seeps mostly occur in the zone from which the BMHS at the seabed has retreated over the recent past (1975–2008) as a consequence of a bottom water temperature rise of 1°C. The overall margin‐parallel alignment of the gas seeps is not related to fault‐controlled gas migration, as seismic evidence of faults is absent. There is no evidence for a BSR close to the gas flare region in the upper slope but numerous gas pockets exist directly below the predicted BMHS. While the contour following trend of the gas seeps could be a consequence of retreat of the landward limit of the BMHS and gas hydrate dissociation, the scattered distribution of seeps within the probable hydrate dissociation corridor and the occurrence of a cluster of seeps outside the predicted BMHS limit and near the shelf break indicate the role of lithological heterogeneity in focusing gas migration.

A new grenadier of the genus Nezumia (Pisces: Gadiformes: Macrouridae) from southern Japan

A new grenadier, Nezumia shinoharai, is described based on two specimens collected from the continental slope off thePacific coast of southern Japan, at depths of 627–700 m. The new species is clearly distinguished from all other congenersby the following combination of characters: pelvic fin rays 10; spinules on body scales needlelike, arranged in discreteparallel rows; underside of head almost completely naked with prominent sensory pores; teeth in broad cardiform bandsin both jaws; snout long (31–34% HL), conically pointed, protruding well beyond upper jaw; first dorsal fin uniformlydark with pale distal tip, second spinous ray not extremely elongated (height of first dorsal fin about equal to HL); no dark band encircling trunk.

Eastern boundary drainage of the North Atlantic subtropical gyre

The eastern boundary of the North Atlantic subtropical gyre (NASG) is an upwelling favorable region characterized by a mean southward flow. The Canary Upwelling Current (CUC) feeds from the interior ocean and flows south along the continental slope off NW Africa, effectively providing the eastern boundary condition for the NASG. We follow a joint approach using slope and deep-ocean data together with process-oriented modeling to investigate the characteristics and seasonal variability of the interior–coastal ocean connection, focusing on how much NASG interior water drains along the continental slope. First, the compiled sets of data show that interior central waters flow permanently between Madeira and the Iberian Peninsula at a rate of 2.5 ± 0.6 Sv (1 Sv = 106 m3 s-1 109 km s-1), with most of it reaching the slope and shelf regions north of the Canary Islands (1.5 ± 0.7 Sv). Most of the water entering the African slope and shelf regions escapes south between the easternmost Canary Islands and the African coast: In 18 out of 22 monthly realizations, the flow was southward (−0.9 ± 0.4 Sv) although an intense flow reversal occurred usually around November (1.7 ± 0.9 Sv), probably as the result of a late fall intensification of the CUC north of the Canary Islands followed by instability and offshore flow diversion. Secondly, we explore how the eastern boundary drainage may be specified in a process-oriented one-layer quasigeostrophic numerical model. Non-zero normal flow and constant potential vorticity are alternative eastern boundary conditions, consistent with the idea of anticyclonic vorticity induced at the boundary by coastal jets. These boundary conditions cause interior water to exit the domain at the boundary, as if recirculating through the coastal ocean, and induce substantial modifications to the shape of the eastern NASG. The best model estimate for the annual mean eastward flow north of Madeira is 3.9 Sv and at the boundary is 3.3 Sv. The water exiting at the boundary splits with 1 Sv flowing into the Strait of Gibraltar and the remaining 2.3 Sv continuing south along the coastal ocean until the latitude of Cape Ghir. The model also displays significant wind-induced seasonal variability, with a maximum connection between the interior and coastal oceans taking place in autumn and winter, in qualitative agreement with the observations.

Possible variation in methane flux caused by gas hydrate formation on the northeastern continental slope off Sakhalin Island, Russia

The Sakhalin Slope Gas Hydrate Project (SSGH) is an international collaborative effort by scientists from Japan, Korea, and Russia to investigate natural gas hydrates (GHs) that have accumulated on the continental slope off Sakhalin Island, Okhotsk Sea. From 2009 to 2011, field operations of the SSGH-09, -10, and -11 projects were conducted. GH-bearing and -free sediment cores were retrieved using steel hydro- and gravity corers. The concentrations of sulfate ions in sediment pore waters were measured to investigate sulfate concentration–depth profiles. Seventeen cores showed linear depth profiles of sulfate concentrations. In contrast, eight cores and two cores showed concave-up and -down profiles plausibly explained by sudden increase and decrease in methane flux from below, respectively, presumably caused by the formation of gas hydrate adjacent to the core sampling sites.

Observational evidence from direct current measurements for propagation of remotely forced waves on the shelf off the west coast of India

We use data from six Acoustic Doppler Current Profiler (ADCP) moorings deployed during March–September 2008 on the continental shelf and slope off Bhatkal, Goa, and Jaigarh on the central west coast of India to present evidence for poleward propagation of shelf or coastal‐trapped waves (CTWs). Wave propagation is seen on the shelf in the 20–40‐day, 10–14‐day, and 3–5‐day‐period bands. The lag from south to north indicates that remote forcing is important even at periods as short as 4 days. Using QuikSCAT wind data, we show that the contribution of remote forcing to the shelf West Indian Coastal Current (WICC) is significant even when the local alongshore wind is strong, as during the summer‐monsoon onset during May–June, and forces a strong local response that masks the effect of remote forcing. Forced wave calculations using CTW theory show that remote forcing of the WICC is present at all times, but is most striking when the local winds are weak, as during March–April. The CTW calculations show that the source region for the remote forcing may extend beyond the west coast into the Gulf of Mannar between India and Sri Lanka. On the slope, propagation is seen only at the 4‐day period. At higher periods, the slope WICC decorrelates rapidly along the coast, but upward phase propagation, implying downward propagation of energy associated with poleward propagation, is evident even at these higher periods.

Revision of the population structuring of Illex argentinus (Castellanos, 1960) and a new interpretation based on modelling the spatio‐temporal environmental suitability for spawning and nursery

AbstractDuring the last 30 years, interpretation of the population structuring of Illex argentinus evolved from a single to a six‐stock model, in which stocks are characterized by having distinct spawning seasons, areas and migration patterns. It is assumed that major spawning activity occurs in discrete pulses along the outer‐shelf/slope off Argentina/southern Brazil during late‐fall/winter and that early life stages develop near the Brazil‐Malvinas Confluence (BMC). Nevertheless, there are inconsistencies in the delimitation of stocks and the interpretation of their life histories. Here, we developed a model based on records of seawater temperature and chlorophyll‐a concentration to establish the quality of different areas of the species’ geographic distribution as spawning and nursery grounds. Whereas the outer‐shelf/slope domain south of 37°S presents marked seasonal restrictions for embryonic development and breeding, coastal areas within 44°–28°S are highly suitable most of the year. Squid in pre‐spawning aggregations at the outer‐shelf/slope south of 41°S would invest less energy to reach equally suitable spawning areas at the coast than at the BMC. Findings of paralarvae, stranded juveniles and mature squid in coastal areas off northern Patagonia support the predictions of the model. Based on these results, the population structuring and reproductive–migration patterns are reviewed and a new interpretation is proposed: Illex argentinus spawns not only in discrete seasons and areas on the slope and mid/outer shelf, but also conducts quasi‐permanent spawning migrations to coastal areas, resulting in a more continuous pattern of recruitment than previously considered and in weak stock differentiation.

Two new gurnards of the genus Pterygotrigla (Scorpaeniformes: Triglidae) from Australian seas

In recent years there has been considerable research activity devoted to the triglid fauna of Australia, and included within this work has been an investigation of the genus Pterygotrigla. This fauna is considered to be amongst the most diverse of any region and multiple undescribed taxa occur in these seas. Two new species belonging to the subgenus Pterygotrigla are described from northern Australia. Pterygotrigla (Pterygotrigla) gomoni sp. nov. occurs in the eastern Indian Ocean on the upper continental slope off Western Australia, and P. (Pterygotrigla) saumarez sp. nov. occurs in the south-west Pacific on the outer continental shelf and upper slope off central and southern Queensland. These species differ markedly from each other in morphology, and are distinguished from other members of the subgenus by a combination of morphometrics, meristics, morphology of the head (in particular elements of its spination), shape and distribution of the dorsal-fin bucklers, and pectoral-fin coloration.

Records and Descriptions of Epitoniidae (Orthogastropoda: Epitonioidea) from the Deep Sea off Northeastern Brazil and a Checklist ofEpitoniumandOpaliafrom the Atlantic Coast of South America

A total of six genera and 10 species of marine gastropods belonging to the family Epitoniidae were collected from dredges of the continental slope off Brazil during the development of the REVIZEE (Live Resources of the Economic Exclusive Zone) Program. These species, referable to the generaAlora, Amaea, Cycloscala, Epitonium, Gregorioiscala,andOpalia,are reported from bathyal depths off northeastern Brazil.Alorasp.,Gregorioiscala pimentain. sp., andOpalia revizeen. sp. are species heretofore unknown to science. A list of the species ofEpitoniumandOpaliafrom the Atlantic coast of South America is presented based primarily on data from the literature. In addition, an overview of the biodiversity and distribution of the genera studied is presented for the Atlantic Ocean.

Sediment waves on the South China Sea Slope off southwestern Taiwan: Implications for the intrusion of the Northern Pacific Deep Water into the South China Sea

Using an integrated multi-beam bathymetry, high-resolution seismic profile, piston core, and AMS 14C dating data set, the current study identified two sediment wave fields, fields 1 and 2, on the South China Sea Slope off southwestern Taiwan. Field 1 is located in the lower slope, and sediment waves within it are overall oriented perpendicular to the direction of down-slope gravity flows and canyon axis. Geometries, morphology, and internal seismic reflection configurations suggest that the sediment waves in field 1 underwent significant up-slope migration. Field 2, in contrast, is located more basinward, on the continental rise. Instead of having asymmetrical morphology and discontinuous reflections as observed in field 1, the sediment waves in field 2 show more symmetrical morphology and continuous reflections that can be traced from one wave to another, suggesting that vertical aggradation is more active and predominant than up-slope migration.Three sediment wave evolution stages, stage 1 through stage 3, are identified in both field 1 and field 2. During stage 1, the sediment waves are built upon a regional unconformity that separates the underlying mass-transport complexes from the overlying sediment waves. In both of these two fields, there is progressive development of the sediment waves and increase in wave dimensions from the oldest stage 1 to the youngest stage 3, even though up-slope migration is dominant in field 1 whereas vertical aggradation is predominant in field 2 throughout these three stages.The integrated data and the depositional model show that the upper slope of the study area is strongly dissected and eroded by down-slope gravity flows. The net result of strong erosion is that significant amounts of sediment are transported further basinward into the lower slope by gravity flows and/or turbidity currents. The interactions of these currents with bottom (contour) currents induced by the intrusion of the Northern Pacific Deep Water into the South China Sea and preexisting wavy topography in the lower slope result in the up-slope migrating sediment waves in field 1 and the contourites as observed from cores TS01 and TS02. Further basinward on the continental rise, turbidity currents are waned and diluted, whereas along-slope bottom (contour) currents are vigorous and most likely dominate over the diluted turbidity currents, resulting in the vertically aggraded sediment waves in field 2.The results from this study also provide the further evidence for the intrusion of the Northern Pacific Deep Water into the South China Sea and suggest that this intrusion has probably existed and been capable of affecting sedimentation in South China Sea at least since Quaternary.

Gas hydrate within the Winona Basin, offshore western Canada

In western Canada gas hydrates have been thought to exist primarily in the Cascadia accretionary prism off southern Vancouver Island, British Columbia (BC). We present evidence for the existence of gas hydrate in folds and ridges of the Winona Basin up to 40 km seaward from the foot of the continental slope off northern Vancouver Island. The occurrence of a bottom-simulating reflector (BSR) observed in a number of vintage seismic reflection profiles is strongly correlated to faulted, and folded sedimentary ridges and buried folds. The observed tectonic structures of the Winona Basin are within the rapidly evolving Juan de Fuca – Cascadia – Queen Charlotte triple junction off BC. Re-processing of multi-channel data imaged mildly to strongly deformed sediments; the BSR is confined to sediments with stronger deformation. Changes in the amplitude character of sediment-reflections above and below the depth of the base of gas hydrate stability zone were also used as an indicator for the presence of gas hydrate. Additionally, regional amplitude and frequency reduction below some strong BSR occurrences may indicate free gas accumulations. Gas hydrate formation in the Winona Basin appears strongly constrained to folds and ridges and thus correlated to deeper-routed fluid-advection regimes. Methane production from in situ microbial activities as a source of gas to form gas hydrates, as proposed to be a major contributor for gas hydrates within the accretionary prism to the south, appears to be insufficient to produce the widespread gas hydrate occurrences in the Winona Basin. Potential reasons for the lack of sufficient in situ gas production may be that sedimentation rates are 5–100 times higher than those in the accretionary prism so that available organic carbon moves too quickly through the gas hydrate stability field. The confinement of BSRs to ridges and folds within the Winona Basin results in an areal extent of gas hydrate occurrences that is a factor of five less than what is expected from regional gas hydrate stability field mapping using water-depth (pressure) as the only controlling factor only.

Meridional and zonal changes in water properties along the continental slope off central and northern Chile

The Humboldt-09 cruise covered a narrow meridional band along the Chilean continental slope (44–23º S). Here we use physical and biochemical data from a long meridional section (4000 km) and three short zonal sections (100 km) to describe the distribution of the different water masses found in this region. Six water masses were identified: Subantarctic Water (SAAW), Summer Subantarctic Water (SSAW), Subtropical Water (STW), Equatorial Subsurface Water (ESSW), Antarctic Intermediate Water (AAIW), and Pacific Deep Water (PDW). For the first time, a novel set of source water mass properties (or water types) is introduced for SSAW, and nutrient and dissolved oxygen water types are proposed for all the water masses. Optimum multiparameter (OMP) analysis was used through an iterative process to obtain a sound definition of the water types that minimizes the residuals of the method. Both the classic OMP and the quasi-extended OMP models reproduced the data rather well. Finally, the spatial distribution of the different water masses was calculated with the quasi-extended OMP, which is not influenced by the respiration of organic matter. The distribution of the different water masses is presented over the meridional and zonal transects and in property-property diagrams. A smooth meridional transition from subantarctic to tropical and equatorial water masses is observed in this area. This transition takes place in surface, central, and intermediate waters over distances of the order of 1000 km. The meridional transition contrasts with the abrupt zonal changes found in the cross-slope direction, which are of comparable magnitude but over distances of the order of 100 km. Both AAIW and SAAW (fresh and well oxygenated) partially mix with the hypoxic ESSW and, therefore, play an important role in the ventilation of the southern part of the oxygen minimum zone.

Depth and range tracking of sperm whales in the Gulf of Alaska using a two-element vertical array, satellite and bioacoustic tags

A two-element vertical array was deployed between August, 15 and 17 2010, on the continental slope off Southeast Alaska, in 1200 m water depth. The instruments were attached to a longline fishing anchorline, deployed at 300 m depth, close to the sound-speed minimum of the deep water profile. The anchorline also served as a decoy, attracting seven depredating sperm whales to the area. Three animals were tagged with a satellite tag and one of them was tagged with both a satellite and bioacoustic “BProbe” tag. Both tags recorded dive depth information. Relative arrival times of surface- and bottom-reflected paths are used to estimate animal range and depth on a single hydrophone, and compared with tagging results. The two-element array is then used to estimate vertical arrival angles of the direct and surface-reflected paths to determine whether range and depth localization can occur without the use of bottom multipath. This data will be useful in determining whether long-range tracking of sperm whales is possible using a single compact instrument deployment. Potential applications include observing what ranges whales are willing to travel to depredate. [Work conducted under the SEASWAP program, supported by the National Oceanic and Atmospheric Administration and the North Pacific Research Board.]

Primer registro de Florometra magellanica (Bell, 1882) (Echinodermata: Crinoidea) para el Perú

Florometra magellanica (Bell, 1882) (Echinodermata: Crinoidea) is reported for the first time from Peru. The species was collected in the continental slope (360 – 814 m) off Piura Department, Northern Peru (3°38.67’S, 81° 2.73’W). Reference material has been deposited in the Colección Cientifica del Instituto del mar del Perú (IMARPE), Lima, Peru and in the Colección Nacional de Equinodermos, Universidad Nacional Autonoma de Mexico, Mexico City.