Outer Shelf Off Research Articles

Cross-shelf transport off the northern Taiwan, East China Sea

Cross-shelf transport is an important material exchange process between the shelf seas and open seas. The distribution characteristics of phytoplankton, surface sediment particle size, minerals, etc., all indicate the possible existence of a cross-shelf transport conduit from the inner shelf to the outer shelf off northern Taiwan. This study confirmed cross-shelf transport off northern Taiwan based on remote sensing sea surface temperature (SST) data and heat flux data, which can be divided into two patterns: cold eddy and cold water tongue. Furthermore, combined with AVISO sea surface level data, CCMP wind field data, the dynamic formation factors of different morphological characteristics of cross-shelf transport are studied.Topographic changes mainly induce cross-shelf transport off northern Taiwan and occur at the curve of isobath to deep water. When there is a strong thermal front along the coast, cross-shelf transport is more likely to occur. Against the steady, strong northerly wind, the Zhe-Min coastal current's southward flow and Kuroshio's invasion strengthened the temperature front and facilitated cross-shelf transport. Typhoon strengthened cross-shelf transport over northern Taiwan after passing through the southern part of the study area. This research has important implications to find the channel and transporting pattern for material exchange between the East China Sea and the West Pacific, which will contribute to the study of ecological environmental protection and seabed muddy area formation.

Trophic structure of benthic communities in the Cabo Frio upwelling system (southeastern Brazilian shelf): a temporal study using stable isotope analysis

In Brazil, coastal upwelling is observed in 7 areas along the southeastern/southern region and is most pronounced near Cabo Frio, Rio de Janeiro. This region is exposed to moderate seasonal Ekman-driven upwelling that brings cold water with increased nutrient levels nearshore and is more frequent and intense during the austral spring and summer, primarily due to the prevalence of northeasterly winds. Our aim was to verify the influence of this upwelling on the benthic trophic structure of the inner and outer shelf off Cabo Frio through measurements of sta- ble nitrogen (δ 15 N) and carbon (δ 13 C) isotopes. We identified 1 main pathway of organic matter (OM) transfer from the base to the top of the food web, as observed from the tendencies of the δ 13 C and δ 15 N values. The isotopic signatures of benthic consumers exhibited temporal and spatial vari- ability, with no interaction between them. As the result of a time lag, only consumers (mostly deca- pod carnivores) appeared to reflect the assimilation of 15 N-depleted and 13 C-enriched OM pro- duced and deposited during strong upwelling that occurred 4 mo earlier. Therefore, the intensity and period of the upwelling phenomenon were important for detecting upwelling in benthic food webs. Lighter nitrogen and heavier carbon isotopes were found on the inner shelf. Consumers may have exploited OM of different quality on the inner and outer shelf due to differences in sed- iment, hydrodynamics, mineralisation and assimilation of 13 C-enriched microphytobenthos. Nev- ertheless, 4 trophic levels were estimated in the benthic communities of the continental shelf off the Cabo Frio upwelling system, independent of period or area.

Chapter 16 The continental shelf of eastern India

Abstract The eastern continental margin of India is aligned NE–SW due to the trend of the Eastern Ghat orographic belt in the northern part and almost north–south in the southern part. It is of passive type, the origin of which can be traced back to its separation from Antarctica around 127 Ma ago. The 2493 km-long shoreline of eastern India is fringed with a continental shelf with a variable width of 35 km off Tamil Nadu to 60 km off north Andhra Pradesh and 120 km around Digha. The shelf has a gentle slope in the northern sector and is moderately steep in the south. Net longshore sediment transport along the east coast is from south to north. The east continental shelf of India is characterized by four major deltas; the Ganges, Mahanadi, Krishna–Godavari and Cauvery. The interdeltaic shelves are generally sediment starved. The inner shelf is silty to clayey silt and sandy, whereas the outer shelf has carbonate sands with coral debris and shell fragments. The outer shelf is characterized by carbonate sands, lime mud and ooids in the north Andhra Pradesh sector off the Kakinada–Kalingapatnam sector. The outer shelf off Chennai–Mahabalipuram, Nagapattinam–Point Calimere is characterized by a thin veneer of authigenic sediment represented by verdine, glaucony and phosphatic sediment. In Tamil Nadu, the region between 10°N and 12°30′N is characterized by two megalineaments and associated tectonics, suggesting that the area is tectonically active. The 14 C dating of relict corals, from water depths of 120 m, off Karaikal indicates an age of 18 390±210 years BP, establishing the low sea-level position of the Last Glacial Maximum. The large volume of sediment input from the deltaic system of two major drainages, Krishna and Godavari, has a significant influence on the morphology and sedimentation of the continental shelf. The influence of glacio-eustasy is noticeable in outer shelf sediments along almost the entire length of the shelf.

Geochemistry of trace metals in shelf sediments affected by seasonal and permanent low oxygen conditions off central Chile, SE Pacific (∼36°S)

Trace metals (Cd, U, Co, Ni, Cu, Ba, Fe, Mn), total organic carbon (TOC) and C and N stable isotope signatures (δ 13C and δ 15N) were determined in short sediments cores from the inner and outer shelf off Concepción, Chile (∼36°S). The objectives were to establish the effect of environmental conditions on trace metal distributions at two shelf sites, one affected by seasonal oxygenation and the other by permanent low oxygen conditions due to the presence of the oxygen minimum zone (OMZ). We evaluate trace metals as proxies of past changes in primary productivity and the bottom water oxygen regime. Concentrations of pore water sulfides and NH 4 + were also measured as indicators of the main diagenetic pathways at each site. Our results for the inner shelf (seasonal suboxia) suggest that the oxidative state of the sediments responds to seasonal pulses of organic matter and that seasonal oxygenation develops during high and low primary productivity in the water column. Here, positive fluxes (to the water column) estimated from pore water concentrations of several elements were observed (Ba, Co, Ni, Fe and Mn). The less reduced environment at this site produces authigenic enrichment of Cu associated with the formation of oxides in the oxic surface sediment layer, and the reduction of U within deeper sediment sections occur consistently with negative estimated pore water fluxes. In the outer shelf sediments (permanent suboxia, OMZ site), negative fluxes (to the sediment) were estimated for all elements, but these sediments showed authigenic enrichments only for Cd, Cu and U. The short oxygenation period during the winter season did not affect the accumulation of these metals on the shelf. The distribution of Cu, Cd and U have been preserved within the sediments and the authigenic accumulation rates estimated showed a decrease from the deep sections of the core to the surface sediments. This could be explained by a gradual decrease in the strength of the OMZ in the last 100 years, in combination with periods of strong oxygenations during El Niño events.

Characteristics of sediments and their dispersal systems along the shelf and slope of an active forearc margin, eastern Hokkaido, northern Japan

This study revealed sediment distributions and their dispersal systems in a shelf-slope setting along an active forearc margin off eastern Hokkaido, northern Japan, where the Pacific Plate is being subducted beneath the Okhotsk (North American) Plate at the 7 km deep Kuril Trench. The findings were based on analyses of the textures and structures of surface sediments, components of the sampled sand fractions, high-resolution acoustic reflection records, and sediment budgets. The studied margin has a 20–30 km wide shelf and steep (5–10°) slope marked by faults and folds associated with transpressional tectonics. The shelf sediments are classified into gravelly coarse to fine sand from the shelf edge to the uppermost slope (110–1000 m water depth), gravel and coarse sand on the shelf off Hanasaki, fine sand on the middle-outer shelf off Kiritappu, and very fine sand on the shelf off Akkeshi. The coarse sediments on the eastern shelf off Hanasaki and the shelf margin are interpreted to represent lag deposits from coastal erosion associated with a post-glacial transgression across topographic highs. The two depocenters of shelf sediments subsequent to the Last Glacial Maximum are situated in the middle–outer shelf off Kiritappu and in the inner–middle shelf off Akkeshi; these are interpreted as transgressive–highstand sequences whose locations were controlled by ridges on the shelf margin and topographic highs on the shelf. The shelf sediment thickness reaches ∼ 20 m, and the total volume is estimated at ca. 6 km 3. The slope sediments are grouped into medium–fine sand on the uppermost and upper slopes (150–2000 m water depth) and mud-dominated sediments on the upper to middle slope (1000–3000 m water depth). The sandy sediments on the slope at water depths greater than 1000 m are regarded as being derived from the shelf break by gravitational redistribution. The occurrence of diamictite-like sediments around gullies that incise into the slope indicates that mass movement and sediment gravity flows play an important role in the redistribution of sediments upon the slope. On the upper–middle slope, hemipelagic fallout and advection of turbid water from the shelf edge provide a supply of suspended material. The main contributor of sediment supply to the area is coastal erosion (1.81 Mt y − 1 ), along with a lesser contribution from fluvial input (0.10 Mt y − 1 ). During the latest Quaternary, the rates of sediment accumulation upon the shelf are estimated to have been ∼ 0.47 Mt y − 1 . The remainder of the material transported into the sea is probably redistributed to deeper parts of the slope via gravitational sediment flows or transported outside of the study area by alongshore and tidal currents. The studied shelf that kept less than a quarter of the mass from land is comparable with other active margin shelves of New Zealand and California, although they are fed by high sediment-laden rivers.

Age and growth of larval Sardinella aurita (Pisces: Clupeidae) from northeastern Venezuela, determined by otolith microstructure analysis

Age and growth of Sardinella aurita larvae were analyzed based on daily growth increments in sagitta otoliths. Larvae were obtained from ichthyoplankton samples collected from the outer shelf off Margarita Island and from Mochima Bay (northeastern Venezuela), between March 1999 and January 2001. Total length, standard length (SL) and height of larvae were measured and the otoliths were examined under a light microscope. Larval size ranged from 1.35 to 15.90 mm SL, the Margarita Island larvae being signicantly larger than those from Mochima (t = 18.69; P < 0.001). The three morphological parameters were significantly and positively correlated among themselves and with otolith diameter and age, and fitted to a linear regression model; hence, size growth as a function of age was not different between the localities studied (SL = 2.65 + 1.10 × age; r2 = 0.78, P < 0.001). Growth rates per locality were also determined by backcalculating sizes, with no significant differences (F = 2.60, P > 0.05) between them. The mean backcalculated growth rate was 0.99 mm day–1 and it was not significantly different from the mean growth rate obtained from fitted length-age data (F = 3.32, P > 0.05). Morphological changes in otoliths were evident from the mean daily increment widths, with wider increments in larval otoliths from Margarita Island (H = 42.54, P < 0.001).

Large‐scale elongated gas blowouts along the U.S. Atlantic margin

In May 2000 we surveyed a series of en echelon, asymmetric depressions along the outer shelf off Virginia and North Carolina using high‐resolution chirp and side‐scan sonar. The features, which are elongated parallel to the shelf edge and have steep landward walls, are ∼4 km long, 1 km wide, and up to 50 m deep. On the basis of internal stratal geometry interpreted from chirp profiles, the depressions do not appear to result from simple, down‐to‐the‐east, normal displacement along deep‐seated faults or structure. Rather, the depressions seem to have been excavated primarily by gas expulsion, creating large‐scale asymmetric gas escape structures that have been termed “gas blowouts.” Gas appears to have been trapped beneath a shelf edge delta that is a few tens of meters thick and exhibits internal soft sediment deformation suggestive of progressive downslope (seaward) creep. These new data suggest the blowouts occurred when thin‐skinned deformation and creep of the surficial deltaic sediment layers combined with updip/upslope gas migration, ultimately leading to gas pressure in excess of the overburden. The location of the expulsion craters along the shelf edge and their elongated, asymmetric shapes strongly suggests a causal relation between the downslope creep of the delta and the expulsion event. We suggest a positive feedback between upward migration of gas‐rich fluids through the low‐stand delta and the downslope creep processes. While the complex interplay between differential permeability, overpressure, and upslope fluid migration remains poorly understood, we suggest such interactions may play an important role in controlling slope stability.

Seismic and sequence stratigraphy of the central western continental margin of India: late-Quaternary evolution

Seismic and sequence stratigraphic architecture of the central western continental margin of India (between Coondapur and south of Mangalore) has been investigated with shallow seismic data. Seismic stratigraphic analysis defined nine seismic units, that are configured in a major type-1 depositional sequence possibly related to fourth-order eustatic sea-level changes, comprising regressive, lowstand, transgressive and highstand systems tracts. The late-Quaternary evolution of the continental margin took place under the influence of an asymmetric relative fourth-order sea-level cycle punctuated by higher frequency cycles. These cycles of minor order were characterised by rapid sea-level rises and gradual sea-level falls that generated depositional sequences spanning different time scales. During the regressive periods, dipping strata were developed, while erosional surfaces and incised valleys were formed during the lowstands of sea level. Terraces, v-shaped depressions, lagoon-like structures observed on the outer continental shelf are the result of the transgressive period. In the study area we have recognised a complex erosional surface that records a long time span during the relative sea-level fall (regressive period) and the following sea-level lowstand and has been reworked during the last transgression. We also infer that sedimentation processes changed from siliciclastic sedimentation to carbonate sedimentation and again to siliciclastic sedimentation, marking an important phase in the late-Quaternary evolution of the western continental shelf of India. We attribute this to an abrupt climate change at the end of the oxygen isotope stage 2, between the Last Glacial Maximum and the Bølling–Allerod event (14 000 yr BP). This sensitive climate change (warming) favoured the formation of reefs at various depths on the shelf, besides the development of Fifty Fathom Flat, a carbonate platform on the outer shelf off Bombay developed prior to 8300 yr BP. The highstand systems tracts were deposited after the sea level reached its present position.

Capelin (Mallotus villosus) in the Iceland–East Greenland–Jan Mayen ecosystem

The stock of capelin, Mallotus villosus (Müller), that inhabits the area between Iceland, East Greenland, and the island of Jan Mayen spawns in shallow coastal water south and west of Iceland. Juveniles grow up over the continental shelf off north Iceland and off East Greenland west of the Denmark Strait. The main feeding area of adults is the Iceland Sea, the oceanic area from about 68 to 72°N, between the Jan Mayen Ridge and the East Greenland continental shelf. After the feeding season, the adult stock assembles over the outer shelf off north Iceland and migrates to the spawning grounds along the south and west coasts from December to March. The main oceanographic features of Icelandic waters and the Iceland Sea are described and capelin migrations related to the distribution of water masses and the ocean current systems in the area. In the past two decades there have been large variations in capelin migrations and catchability, especially during the feeding season. However, these variations can only be explained in part by the available environmental data. Year-class abundance appears to be determined by survival during the first winter, in tune with the greater environmental variability off north Iceland than south and west of Iceland, where these capelin spawn and the larvae start drifting. Adult growth is positively related to the flow of Atlantic water into the area north of Iceland, indicating improved feeding conditions in the Iceland Sea when the Irminger Current is strong. There can be large interannual variations in number and weight-at-age in the adult stock. The main predators are whales, seabirds, and fish, especially cod. The combined annual removal by predators is estimated to have been 2.1–3.4 million tonnes in the early 1990s. The mean weight-at-age of cod aged 5–8 years dropped by up to 25–30% when capelin abundance was low in the early 1980s and 1990s. The relatively low mean weight of cod in the past 3 years may well be due to changed distribution and migration of capelin, resulting in reduced access of cod to this most important item in their diet.

Glaciomarine sedimentation and its paleoclimatic implications on the Antarctic Peninsula shelf over the last 15 000 years

Analyses of sedimentological, geochemical and micropaleontological parameters from radiocarbon-dated sediment cores retrieved from the Antarctic Peninsula’s western continental shelf reveal a detailed paleoclimatic and/or paleoceanographic history over the last 15 000 radiocarbon years. Deglaciation of the outer shelf off Anvers Island commenced prior to at least 15 000 yr BP, marked by the deposition of distal glaciomarine diamicton (facies 2) beneath a floating ice shelf, and lasted for 3800 years with increasing diatom abundance and total organic carbon (TOC) over time. A return to colder conditions occurred between 12 800 and 11 600 yr BP with a drop in TOC content and diatom abundance, which is coincident with the Younger Dryas event in the North Atlantic region. At this time, an abrupt increase in percentage sea-ice taxa as well as in the ratio of ( Fragilariopsis curta+Fragilariopsis cylindrus)/ Thalassiosira antarctica suggests renewed ice-shelf advance. In contrast, the inner shelf was deglaciated somewhat later about 11 000 yr BP, that is, 3000 years after the outer shelf. Prior to 11 000 yr BP, deposition of proximal glaciomarine diamicton (facies 1) close to the grounding line under a floating ice shelf and/or persistent sea ice may have occurred on the inner shelf. After this date, deposition of distal glaciomarine diamicton (facies 2) followed. A climatic optimum is recognized between 6000 and 2500 yr BP, coinciding with a ‘mid-Holocene climatic optimum’ from several other Antarctic sites, e.g. the Palmer Deep. During this time, as the glacial system receded from the shelf, greatly enhanced primary productivity occurred in open marine conditions, resulting in the deposition of diatomaceous mud (facies 3) and causing post-depositional dissolution of calcareous benthic and planktonic foraminifers in sediment. Around 2500 yr BP (the onset of the Neoglacial), diatomaceous sandy mud (facies 4), characterized by a decrease in TOC and diatom abundance, reflects the formation of more extensive and seasonally persistent sea ice, as evidenced by an increase in percentage of sea-ice taxa and in the ratio of ( F. curta+F. cylindrus) /T. antarctica. Our results provide evidence of climatic change on the Antarctic Peninsula’s western shelf that helps to refine the existence and timing of late Pleistocene and Holocene millennial-scale climatic events in the Southern Hemisphere.

Distribution and abundance of thaliaceans and copepods off the southeastern U.S.A. during winter

The abundance and distribution of planktonic copepods and tunicates were studied in relation to physical and biological variables from late January to early February 1990 on the middle and outer shelf off South Carolina, U.S.A. The inner and part of the middle shelf were vertically mixed throughout the study period, the middle and outer shelf only after passage of a storm on 4 and 5 February. Subtidal currents at the 40 m isobath were highly correlated with local alongshore winds, whereas at the 75 m isobath the currents showed a mixed response to wind and Gulf Stream events. Chlorophyll concentrations in the study area usually surpassed 0.5 and reached levels of 3 μg l −1. Zooplankton was characterized by high abundances of doliolids (Tunicata, Thaliacea) and relatively low concentrations of calanoid and cyclopoid copepods. When doliolids were very abundant the vertical distribution of doliolids was inversely related to those of the calanoids. We hypothesized that this inverse relationship could be the result of (a) doliolids removing food particles, thus reducing food particle concentrations, and through that calanoid reproduction rates; (b) doliolids ingesting calanoid eggs and nauplii, thereby reducing copepod recruitment; (c) active avoidance of doliolids by downward migration of copepods; and (d) vertical shear between the upper and lower part of the stratified water column. At stations on the middle shelf the abundance of photo- and heterotrophic nanoplankton was inversely related to the abundance of doliolids. The frequency of occurrence of thaliacea in neritic regions is evaluated in relation to hydrographic regimes and the availability of seed populations. From previous and present observations we assume that thaliacea due to their high rates of feeding, individual and population growth could impact zooplankton communities two-fold: indirectly, by reducing phytoplankton concentrations rapidly thus affecting reproduction of certain calanoid taxa, and directly, by consuming copepod eggs.

Characteristics of the near-bottom suspended sediment field over the continental shelf off northern California based on optical attenuation measurements during STRESS and SMILE

Time-series measurements of current velocity, optical attenuation and surface wave intensity obtained during the Sediment Transport Events on Shelves and Slopes (STRESS) experiments, combined with shipboard measurements of conductivity, temperature and optical attenuation obtained during the Shelf Mixed Layer Experiment (SMILE), provide a description of the sediment concentration field over the central and outer shelf off northern California. The questions addressed are: (1) existence and characteristics of bottom nepheloid layers and their relationship to bottom mixed layers; (2) characteristics of temporal fluctuations in sediment concentration and their relationship to waves and currents; (3) spatial scales over which suspended sediment concentrations vary horizontally; and (4) vertical distribution of suspended sediment.

Community structure and trophic ecology of euphausiids in the Benguela ecosystem

Current knowledge of the community structure, life history strategies and trophodynamics of euphausiids in the Benguela ecosystem is synthesized. Three species dominate, Euphausia lucens over the shelf region of the southern Benguela, Euphausia hanseni over the outer shelf off Namibia and Nyctiphanes capensis in the neritic and shelf regions of the northern Benguela. Various models of interactions between biological processes and physical features are considered to explain likely mechanisms whereby these species can remain spatially segregated. All three species breed throughout the year, produce multiple broods and large numbers of eggs per brood and have high turnover rates. These strategies enable them to maintain high production in terms of both somatic growth and reproductive output throughout the year. Biomass of euphausiids in the northern Benguela is about double that of the west coast of the southern Benguela, consistent with similar geographical differences in phytoplankton abundance. Adult euphausiids are opportunistic omnivores and the larval stages are primarily herbivorous. Alternative feeding patterns are reflected in differences in the vertical migratory behaviour of various ontogenetic stages and ambient food type. Factors causing a switch from herbivory to carnivory are considered. Grazing of euphausiids is considered to play a minor role in phytoplankton losses, but euphausiids could exert considerable predatory impact on mesozooplankton and therefore compete directly with pelagic fish. Combined predation by various fish species has an appreciable impact on euphausiid production.

Présentation d'une carte bathymétrique au 1/25 000° du précontinent marseillais. (Au large de la zone limitée par la grotte Cosquer et l'habitat préhistorique de Carry le Rouet)

Bathymetric map (1/25 000°) of the outer shelf off Marseilles : From the Cosquer Cave to the prehistoric site of Carry le Rouet. A bathymetric map (1/25 000°) of the outer shelf off Marseilles (France) has been realized, using the plotting sheets from the Hydrographie and Oceanographic service of the French Marine (S.H.O.M). This map allows to find the main geologic characteristics of the observed area during the last glacial period, and, then, accessible to the prehistoric artists of the Cosquer Cave and the hunters of the prehistoric site of Carry le Rouet. The morphological outline drawn already shows the N.W.-S.E. and N.E.-S.W. trends of the submarine reliefs, non-structural slopes which are probably marine shorelines levels situated at -50 m, -90 m, and -100 m and, perhaps 130 m depht, some karstified areas if the multiple endoreic depressions identifiable on the map are interpreted as dolines. This presentation of the bathymétrie document is only a preliminary report since the morphostructural analysis of the maps still going and will, in the future, be correlated with others approaches and more particularly, geophysical and sedimentological data.

A new sandperch,Parapercis maritzi(Teleostei: Pmguipedidae), from South Africa

A new species of sandperch, Parapercis maritzi;, is descriptionbed from 12 specimens from the outer shelf off Transkei and Natal. The species differs from all other sandperches in its oolouration, particulars of its dentition, and meristics of its axial skeleton and squamation. It is one of the deep-water sand perches, and is found on open sandy-rubble bottoms of the outer shelf.

Sea whip goby,Bryaninops yongei, collected from outer shelf off Miyakojima, East China Sea

Sea whip goby,Bryaninops yongei, collected from outer shelf off Miyakojima, East China Sea

Halimeda bioherms of the northern Great Barrier Reef

The reefless tract directly behind the ribbon reefs on the outer shelf off Cooktown supports a luxuriant growth of Halimeda that, during the Holocene, has developed into bioherms. These mounded biodies of unconsolidated sediment have formed banks that vary in height between 2 and 20 m. Combined shallow, high-resolution seismic reflection profiles and side-scan sonar have diferentiated three areas of biohermal complexes behind the ribbon reefs of Cooktown. Observations by SCUBA and submersible plus the sedimentology of the bioherms indicate that they are in situ accumulations. Evidence from dating of cores suggests that the Halimeda bioherms began to grow about 10 000 years B.P. and their growth has continued to the present time, even though their tops are presently restricted to a depth of -20 m. It is suggested that the origin and morphology of the bioherms are related to a specific hydrodynamic phenomenon, involving jets of nutrient-rich, upwelled oceanic water intruding onto the outer shelf via the narrow passes between the ribbon reefs, and forming eddies behind the ribbons.

Current modification of a mass failure deposit on the continental shelf, North Canterbury, New Zealand

The Holocene mud blanket on the nearly flat (0.4°), middle to outer shelf off Canterbury contains a 255 km 2 zone of ridges and gullies exhibiting up to 10 m relief. Ridges are mantled by cohesive mud in various stages of dissection whereas gullies mainly have a flat, sandy floor. Morphologic and seismic evidence suggests that the topography initially formed by mass failure along an unconsolidated sand horizon which presumably acted as a failure plane. A number of mechanisms are readily available to favour such movement: earthquake shock, sediment overloading, subsurface instability due to gas, and wave loading. Since formation the topography has been modified by currents which have eroded mud from gullies as well as locally scouring ridge crests. Current meter measurements from 1 m above a ridge crest indicate that erosion most likely occurs under storm conditions when currents reach a laboratory-determined threshold speed of μ 100 = 37 cm s −1. In contrast, undissected crests may be resistant to erosion as a consequence of biological stabilization by dense colonies of chaetopterid worm tubes or associated biota.

Erythrosquilla Megalops, a Remarkable New Stomatopod from the Western Indian Ocean

Erythrosquilla megalops, known from one specimen taken on the outer shelf off Somalia in depths of 75 to 175 meters, is assigned to a new family in the Superfamily Lysiosquilloidea. It is the only lysiosquilloid with a distinct median carina on the telson. Among the stomatopods taken by the R/V Anton Bruun during the International Indian Ocean Expedition (IIOE) is one specimen which represents a new family, genus, and species. It is characterized below. The holotype has been deposited in the National Museum of Natural History, Washington (USNM). Measurements and indices are defined in Manning (1969). Erythrosquillidae, new family Definition. -Size large, total length more than 100 mm. Body loosely articulated, depressed. Rostral plate triangular, unarmed. Ischiomeral articulation of claw terminal. Dactylus of claw not inflated basally, with teeth on opposable margin. Propodi of third and fourth maxillipeds broad, beaded ventrally. Endopods of walking legs slender. Telson with distinct median carina and 2 intermediate marginal denticles. Submedian teeth oftelson with movable apices. Uropodal endopod lacking proximal fold on outer margin. Includes only the following genus:

Acoustic masking in sediments due to gases on the western continental shelf of India

Surveys carried out on the western continental shelf of India indicate that the inner and middle shelf to a depth of 50–60 m is covered by acoustically transparent clays. On the shelf off Bombay, the clays are thin near the shore where they overlie rocks or sands, and increase to 20–35 m on the middle shelf, beyond which they thin out at a depth of 50–60 m where relict oolitic sands are exposed. The shallow seismic and echosounding records of these clays at places are marked by turbidity which may be categorised as (1) extending over a distance of 50–60 km and covering large areas of over 2500 km 2 and (2) resembling isolated inverted “U” shaped features. The turbidity appears to rise from the underlying reflectors to within 5–10 m of the sea bed. These features are very prominent in two areas, viz. on the middle shelf off Bombay and on the inner shelf. The shallow bore holes drilled and the cores collected in the area show bubbles or fissures in the clays of the turbid zone. The gas predominantly consists of methane with terraces of higher hydrocarbons. The composition of the gas and the occurrence of the gassy sediments on the middle shelf and in the vicinity of the estuaries and rivers favours their biogenic nature. The common depth of occurrence of 5–10 m may represent the boundary between the sulfate-reducing zone and the underlying methane-producing zone. Perhaps the gases were formed by the degradation of large quantities of organic matter produced by increased productivity, or because of higher influx of organic matter in the recent past. However, the location of some of the occurrences in the vicinity of an oil and gas field indicate that the gases in the sediments of the outer shelf off Bombay may be petrogenic and those on the middle and inner shelf off Bombay, Konkan, Karnataka and Kerala may be biogenic.