Side-scan Sonar Imagery Research Articles

Geomorphology, sedimentary processes and development of the Zenisu deep-sea channel, northern Philippine Sea

The Zenisu deep-sea channel originates on the Izu-Ogasawara island arc, and disappears in the Shikoku Basin of the Philippine Sea. The geomorphology, sedimentary processes, and the development of the Zenisu deep-sea channel were investigated on the basis of swath bathymetry, side-scan sonar imagery, submersible observations, and seismic data. The deep-sea channel can be divided into three segments according to the downslope gradient and channel orientation. They are the Zenisu Canyon, the E-W fan channel, and the trough-axis channel. The sediment fill is characterized by turbidite and debrite deposition and blocky-hummocky avalanche deposits on the flanks of the Zenisu Ridge. In the Zenisu Canyon and the Zenisu deep-sea channel, sediment transport by turbidity currents generates sediment waves (dunes) observed during the Shinkai 6500 dive 371. The development of the Zenisu Canyon is controlled by a N-S shear fault, whereas the trough-axis channel is controlled by basin subsidence associated with the Zenisu Ridge. The E-W fan channel was probably affected by the E-W fault and the basement morphology.

A new synthetic index and a protocol for monitoring the status ofPosidonia oceanica meadows: a case study at Sanremo (Ligurian Sea, NW Mediterranean)

1. The status of a Posidonia oceanica meadow in front of the town of Sanremo, Italy, was studied through a combined use of benthic mapping and synthetic indices. 2. Mapping was accomplished by integrating side scan sonar imagery and data collected by scuba diving along transects placed perpendicularly to the coastline. A thematic map (scale 1:5000) was produced. Extent of the meadow, occurrence of dead matte areas, and morphology of the lower limit (with new definition) are all described. 3. Two synthetic environmental indices were applied to transect data in order to quantify the status of the meadow: the Conservation Index and the Substitution Index. The former is related to the proportion of dead matte; the latter is a novel index measuring the amount of replacement of the ‘constructional’ seagrass P. oceanica by the ‘non-constructional’ seagrass Cymodocea nodosa. The potential of a ‘phase shift’ in Ligurian Sea seagrass meadows is discussed. 4. The approach here proposed, based on detailed mapping plus synthetic indices, may provide immediate information to evaluate the state of Mediterranean Posidonia oceanica for monitoring and management. Copyright © 2006 John Wiley & Sons, Ltd.

Quantitative textural analyses of TOBI sonar imagery along the Almería Canyon, Almería Margin, Alborán Sea, SE Spain

Abstract Hydrocarbon exploration and the ongoing assessment of potential seismic risks are the main drivers behind the high-resolution mapping of continental margins. The large volume of literature devoted to turbidite systems in particular shows the importance of detailed descriptions of their characteristics and overall geomorphological variability, primarily through the distribution of sediment facies. These descriptions rely mainly on acoustic measurements, which are notoriously difficult to interpret. Textural analyses quantify the second-order statistics of sonar imagery, detecting and quantifying details invisible to the human eye. We show the potential (and limitations) of this approach using high-resolution (6 m) towed ocean bottom instrument (TOBI) sidescan sonar imagery acquired in the Alborán Sea, south of Almería, Spain, during the high resolution imaging of Tsunamigenic structures (HITS) 2001 programme. The imagery is co-registered with EM-12 multibeam bathymetry, topographic parametric sonar (TOPAS) sub-bottom profiles, and localized ground truthing. Our study focuses on the Almería Canyon, a meandering channel system more than 57 km long and transporting large amounts of sediments from the coast down to the Alborán Trough, c . 1700 m deep. Textural analyses quantify the variations of sediment processes along the slope of the Almería Canyon. They agree with the results of previous studies, and they can be used to provide new insights in the dynamics and evolution of the canyon.

Deformation and submarine landsliding caused by seamount subduction beneath the Costa Rica continental margin — new insights from high-resolution sidescan sonar data

Abstract Subduction of seamounts at destructive sedimented plate margins results in spectacular deformation of the overriding plate. High-resolution sidescan sonar imagery from the Costa Rica margin show the tracks of five individual seamounts, of which four are described in this paper. These were subducted at various times during the last 690 ka and each represents a different stage in the subduction process. Each subducted seamount leaves a parallel-sided depression in its wake, that can be traced for up to 55 km landward of the deformation front. This wake is created by deformation and uplift of the continental slope as the seamount passes beneath it, followed by collapse due to landsliding as support for the uplifted area is withdrawn. Areas of uplift above seamounts are characterized by complex normal and strike-slip fault patterns. Collapse of the uplift along the trailing edge of the seamount creates a zone of slope failure (landsliding) that migrates upslope (or landward) with the seamount. Landslide processes are dominated by debris flow, but also include sliding of coherent blocks and debris avalanche. Erosion occurs by repeated landslides, which produce a series of overlapping debris flows. Downslope sediment transport typically extends over limited distances, resulting in partial ‘backfilling’ of the scar as its headwall moves up slope. The amount of margin material disrupted by seamount subduction is four to five times the volume of the subducting seamount, of which about three quarters seems to be recycled downslope, backfilling the scar, and nearly one quarter is subducted with the seamount.

The Nyk Slide—morphology, progression, and age of a partly buried submarine slide offshore northern Norway

The late Weichselian Nyk Slide offshore northern Norway was investigated using high resolution seismic data, side-scan sonar imagery, and gravity cores. The mapable extent of the slide is ∼2200 km 2, but parts of the slide were buried by glacigenic debris flows, and parts were removed by the younger Trænadjupet Slide. The exposed parts of the Nyk Slide exhibit extensional and, somewhat anomalously, compressional ridges in the upper slide area. We suggest that the two distinct morphologic zones originate from the same slide event. The compressional ridges are related to a particular large-scale funnel-shape morphology defined by the Vøring Plateau slope facing the continental slope, and were generated as the slide reached the foot of the Vøring Plateau slope. The extensional ridges were generated on the continental slope as sediments were transported downslope and subjected to stretching. Dating of the slide deposits indicate an age of ≥16.3 ka BP, synchronous with a time that the Fennoscandian Ice Sheet was fluctuating near the shelf edge. Nearby sediment cores and seismic reflection data reveal that the slide was both preceded and succeeded by episodes of deposition of glacigenic debris flows. The glide plane is located within the same unit of contouritic sediments as the glide plane of the Trænadjupet Slide, and the Nyk Slide, was probably triggered by loading and unloading of a fluctuating ice-front. The full extent of the slide is yet unknown, but the existing data suggest that the headwall continues upslope and follows the shelf edge outside the Trænadjupet Trough, implying that the Nyk Slide influenced an area substantially larger than the exposed slide deposits.

Focused magmatism versus amagmatic spreading along the ultra‐slow spreading Southwest Indian Ridge: Evidence from TOBI side scan sonar imagery

The analysis of the Towed Ocean Bottom Instrument (TOBI) side scan sonar images along the Southwest Indian Ridge between 63°40′E and 65°40′E reveals strong focusing of magmatic activity and long amagmatic accretionary ridge segments. Fresh‐looking volcanic terrains are observed at distinct locations along the axis separated by highly tectonized and sedimented terrains of an along‐axis extent as much as 82 km. The largest tectonized section corresponds to a dramatically thin crust area with moderate magnetization anomalies. We suggest that seafloor spreading is mainly amagmatic in this tectonized section of the Southwest Indian Ridge with upper mantle rocks exposed at the seafloor. Amagmatic accretionary ridge segments of such dimensions are quite distinct from what is observed at the Mid‐Atlantic Ridge but are also recognized at the Gakkel Ridge and may thus be characteristic of ultra‐slow spreading ridges. The correlation between the distribution of fresh‐looking volcanic terrains, the occurrence of shallow areas crowned by axial volcanic ridges, and high magnetization values suggests a shallow segmentation of the ridge mainly related to variation in the thickness and/or the intrinsic magnetization of the basaltic source layer. By contrast, strong along‐axis variations of the gravity‐derived crustal thickness are shrunken in length relative to this shallow segmentation of the ridge and occur only beneath the elevated segments. Adjacent to these elevated segments, small bathymetric swells with fresh‐looking volcanic constructions do not correspond to thicker crust areas. This suggests a highly focused melt supply beneath the elevated segments which may feed volcanic constructions up to 60 km from the center of these segments by shallow lateral melt migration in the crust, probably through large dikes. Neither the ultra‐slow spreading rate nor the ridge obliquity explains the variation of the magmatic vigor along the ridge. Mantle source heterogeneities together with lower mantle temperatures beneath the easternmost Southwest Indian Ridge could partly control its segmentation.

Sidescan Sonar Imagery of the Winter Marginal Ice Zone Obtained from an AUV

Abstract The first Arctic under-ice sidescan sonar imagery from an autonomous underwater vehicle (AUV) has been obtained in the winter marginal ice zone of the East Greenland Current at 73°00′N, 11°47′W, using a Maridan Martin 150 vehicle operated from R/V Lance. First-year, multiyear, brash, and frazil ice can be discriminated, with the underside of multiyear ice appearing smooth as compared to the rough underside detected by submarine-borne sidescan in the Arctic Basin, implying downstream bottom melt. The ice draft profile was obtained from the vertical part of the sidescan beam, and the probability density function of ice thickness was derived and found to agree well with upward sonar results from this region obtained in 1987 from a British submarine.

Coastal and nearshore morphology, bedforms and sediment transport pathways at Teignmouth (UK)

The beach and nearshore area of Teignmouth (UK) has been the object of a multidisciplinary research project, COAST3D, carried out by 11 European institutions. This paper emphasises the sedimentological and morphological investigations of the intertidal and nearshore area using a variety of techniques. From the results presented, it is clear that the Teignmouth site is very dynamic. Sediment mobility is high during fair-weather conditions owing to the strong jet-like current flows associated with the presence of the narrow estuary mouth. The beach at Teignmouth can be categorised as composite with a dissipative low-tide terrace and a reflective upper beach. From surficial sediment sampling, selective transport processes were identified, imposed by the high current velocities at the estuary mouth. This process also leads to the formation of characteristic depositional lobes with poorly sorted sediment. From digital side-scan sonar mosaicing, cross-shore and longshore-oriented bedforms could be observed, with their characteristics being dependent on the pre-survey hydro-meteo conditions. Although the nearshore system is dominated by the jet-outflow, littoral drift processes can also be important. They proved to be important for beach morphological changes leading to a rythmic alongshore beach behaviour which was attributed to a reversal of the littoral drift imposed by a period of larger waves. Moreover, the influence of waves and associated longshore currents cannot be neglected as they form the link between different sedimentary environments. From combined side-scan sonar imagery and sediment trend analysis, residual sediment transport directions were identified. These showed that fair-weather and storm-dominated processes are needed to explain the textural and morphological differentiation at this site. Further analysis will be required to identify the different time-scales involved in producing these changes.

Physical Impacts of Marine Aggregate Dredging on Seabed Resources in Coastal Deposits

A detailed study of the seabed surrounding dredge pits created during the mining of marine aggregate from a small licence off the south coast of the United Kingdom (“Licence Area 122/3”) has been completed. Over 350 km of high-resolution sidescan sonar imagery and 177 sediment samples have been obtained over a study area extending 10 km either side of the dredge zone (representing one full tidal excursion) in order to identify far-field effects on both physical and biological resources of the seabed.The physical results presented here for Area 122/3 clearly show that the physical impact of dredging (without screening) on the seabed is limited to a zone within approximately 300 m downtide of the dredge area. This will generally be within the dredge licence boundary due to operational procedures. There is no evidence of suspended sediments falling to the seabed beyond this zone and causing significant changes, which may be manifested as infilling of small pits by fine sediments, siltation within crevices or development of migratory sand ripples. However, there is some statistical evidence from grab sampling that surface sediments have a greater sand fraction within the excursion track of the plume, than those sediments either side. Despite this small change in seabed particle size distribution, the benthic communities do not exhibit a detectable impact, as reported in the accompanying paper by Newell et al. (2002).Analysis of ADCP backscatter data supports recent evidence for development of a near bed benthic boundary plume some 2–4 metres thick and a few tens of metres wide which extends beyond the limits of the dredge activity. On an extraction licence undertaking cargo screening, this near bed plume may exceed 4.5 kilometres downtide. Such a phenomenon provides a potential mechanism for impacting physical and benthic resources well beyond the dredge licence boundary and requires further investigation.

Spatially quantitative seafloor habitat mapping: example from the northern South Carolina inner continental shelf

Naturally occurring hard bottom areas provide the geological substrate that can support diverse assemblages of sessile benthic organisms, which in turn, attract many reef-dwelling fish species. Alternatively, defining the location and extent of bottom sand bodies is relevant for potential nourishment projects as well as to ensure that transient sediment does not affect reef habitats, particularly in sediment-starved continental margins. Furthermore, defining sediment transport pathways documents the effects these mobile bedforms have on proximal reef habitats. Thematic mapping of these substrates is therefore crucial in safeguarding critical habitats and offshore resources of coastal nations. This study presents the results of a spatially quantitative mapping approach based on classification of sidescan-sonar imagery. By using bottom video for image-to-ground control, digital image textural features for pattern recognition, and an artificial neural network for rapid, quantitative, multivariable decision-making, this approach resulted in recognition rates of hard bottom as high as 87%. The recognition of sand bottom was less successful (31%). This approach was applied to a large (686 km 2), high-quality, 2-m resolution sidescan-sonar mosaic of the northern South Carolina inner continental shelf. Results of this analysis indicate that both surficial sand and hard bottoms of variable extent are present over the study area. In total, 59% of the imaged area was covered by hard bottom, while 41% was covered by sand. Qualitative spatial correlation between bottom type and bathymetry appears possible from comparison of our interpretive map and available bathymetry. Hard bottom areas tend to be located on flat, low-lying areas, and sandy bottoms tend to reside on areas of positive relief. Published bio-erosion rates were used to calculate the potential sediment input from the mapped hard bottom areas rendering sediment volumes that may be as high as 0.8 million m 3/yr for this portion of the South Carolina coast.

Short-term dynamics and maintenance processes of headland-associated sandbanks: Shambles Bank, English Channel, UK

The short-term (over a spring tidal cycle) dynamics of a headland-associated sandbank (Shambles Bank, English Channel) are investigated, by means of field measurements (synchronous data, using an acoustic Doppler current profiler (ADCP) and repeated side-scan sonar imagery) and two-dimensional (2D) hydrodynamic and sediment transport models. The dynamics of the bank are described in terms of along- and cross-bank velocity components, sand transport pathways and bedform asymmetries. The results showed the occurrence of a net bedload convergent zone along the crest of the bank as a result of anti-clockwise veering of sediment transport towards the crest. This transport pattern is suggested to be the result of the dynamic interplay between two distinct tidally related processes acting over the sandbank, during each phase of the tidal cycle: (a) the formation and different stages of a transient tidal eddy that drives bedload movement during the flood phase of the tide and (b) the bottom-friction induced by the presence of the sandbank that governs the bedload transport dynamics during the ebb phase of the tidal cycle.

Geophysical constraints on the surface distribution of authigenic carbonates across the Hydrate Ridge region, Cascadia margin

On active tectonic margins methane-rich pore fluids are expelled during the sediment compaction and dewatering that accompany accretionary wedge development. Once these fluids reach the shallow subsurface they become oxidized and precipitate cold seep authigenic carbonates. Faults or high-porosity stratigraphic horizons can serve as conduits for fluid flow, which can be derived from deep within the wedge and/or, if at seafloor depths greater than ∼300 m, from the shallow source of methane and water contained in subsurface and surface gas hydrates. The distribution of fluid expulsion sites can be mapped regionally using sidescan sonar systems, which record the locations of surface and slightly buried authigenic carbonates due to their impedence contrast with the surrounding hemipelagic sediment. Hydrate Ridge lies within the gas hydrate stability field offshore central Oregon and during the last 15 years several studies have documented gas hydrate and cold seep carbonate occurrence in the region. In 1999, we collected deep-towed SeaMARC 30 sidescan sonar imagery across the Hydrate Ridge region to determine the spatial distribution of cold seep carbonates and their relationship to subsurface structure and the underlying gas hydrate system. High backscatter on the imagery is divided into three categories, (I) circular to blotchy with apparent surface roughness, (II) circular to blotchy with no apparent surface roughness, and (III) streaky to continuous with variable surface roughness. We interpret the distribution of high backscatter, as well as the locations of mud volcanoes and pockmarks, to indicate variations in the intensity and activity of fluid flow across the Hydrate Ridge region. Seafloor observations and sampling verify the acoustic signals across the survey area and aid in this interpretation. Subsurface structural mapping and swath bathymetry suggest the fluid venting is focused at the crests of anticlinal structures like Hydrate Ridge and the uplifts along the Daisy Bank fault zone. Geochemical parameters link authigenic carbonates on Hydrate Ridge to the underlying gas hydrate system and suggest that some of the carbonates have formed in equilibrium with fluids derived directly from the destabilization of gas hydrate. This suggests carbonates are formed not only from the methane in ascending fluids from depth, but also from the shallow source of methane released during the dissociation of gas hydrate. The decreased occurrence of high-backscatter patches and the dramatic reduction in pockmark fields, imaged on the eastern part of the survey, suggest gas hydrate near its upper stability limit may be easily destabilized and thus, responsible for these seafloor features. High backscatter along the left-lateral Daisy Bank fault suggests a long history of deep-seated fluid venting, probably unrelated to destabilized gas hydrate in the subsurface.

Holocene to modern fine-grained sedimentation on a macrotidal shoreface-to-inner-shelf setting (eastern Bay of the Seine, France)

This paper presents the results of an investigation on different timescales of the main mechanisms governing fine-grained sedimentation on a macrotidal sandy shoreface-to-inner-shelf setting with a supply of terrigenous sediment: the subtidal area of the southeastern Bay of the Seine (Calvados coast, France). Interpretation and calibration of side-scan sonar imagery clearly shows that compact clays crop out in water depths of 3–6 m. Radiocarbon dating and palynological studies of the material sampled using long cores from this subtidal area show that these relict sediments constitute the infilling of lateral valleys of the palaeo-Seine during the last 10 000 years. The lower parts of these deposits consist of compact clays that accumulated in a floodplain setting, later a salt marsh environment, and are succeeded by sediment with sand/mud couplets which formed in a tidal/estuarine system. The top of this sequence has been truncated by a wave erosion surface formed during the Holocene transgression. Today, the sediment accumulating is composed of fine sand, mixed with fine-grained sediment and sometimes temporarily covered by fresh mud. The more recent sedimentological data, compared with surveys in the 1960s–1970s, demonstrate both an increase in the erosion of the submerged earlier Holocene clays and an increase in the mud (silt+clay) content of the superficial sediments. On a seasonal timescale, the seafloor is affected by high-frequency variations in the nature of the contemporary sedimentary cover. Spatial and temporal observations of the seafloor composition have been undertaken during different seasons for four years (1998–2001) to study these sedimentation events. The sedimentation on the inner-shelf is at its maximum when veneers of fresh mud occur after some particular hydrological periods, i.e. sustained high-river outputs following several dry years (i.e. prolonged weak river flows), when significant volumes of mud have been stored within the Seine estuary. Such mud veneers result from: (1) the direct supply of river-born material, (2) the seaward shifting of the turbidity maximum, and (3) the resuspension of mud from the lower estuary (i.e. fluid mud and intertidal flats) under wind-waves, and have been termed ‘estuarine flood deposits’. On a longer timescale of at least the last decades, the southeastern Bay of the Seine is an area of erosion, but it is subjected to ephemeral fine-grained sedimentation on a seasonal timescale.

Sand ridges off Sarasota, Florida: A complex facies boundary on a low-energy inner shelf environment

The innermost shelf off Sarasota, Florida was mapped using sidescan-sonar imagery, seismic-reflection profiles, surface sediment samples, and short cores to define the transition between an onshore siliciclastic sand province and an offshore carbonate province and to identify the processes controlling the distribution of these distinctive facies. The transition between these facies is abrupt and closely tied to the morphology of the inner shelf. A series of low-relief nearly shore-normal ridges characterize the inner shelf. Stratigraphically, the ridges are separated from the underlying Pleistocene and Tertiary carbonate strata by the Holocene ravinement surface. While surficial sediment is fine to very-fine siliciclastic sand on the southeastern sides of the ridges and shell hash covers their northwestern sides, the cores of these Holocene deposits are a mixture of both of these facies. Along the southeastern edges of the ridges the facies boundary coincides with the discontinuity that separates the ridge deposits from the underlying strata. The transition from siliciclastic to carbonate sediment on the northwestern sides of the ridges is equally abrupt, but it falls along the crests of the ridges rather than at their edges. Here the facies transition lies within the Holocene deposit, and appears to be the result of sediment reworking by modern processes. This facies distribution primarily appears to result from south-flowing currents generated during winter storms that winnow the fine siliciclastic sediment from the troughs and steeper northwestern sides of the ridges. A coarse shell lag is left armoring the steeper northwestern sides of the ridges, and the fine sediment is deposited on the gentler southeastern sides of the ridges. This pronounced partitioning of the surficial sediment appears to be the result of the siliciclastic sand being winnowed and transported by these currents while the carbonate shell hash falls below the threshold of sediment movement and is left as a lag. The resulting facies boundaries on this low-energy, sediment-starved inner continental shelf are of two origins which both are tied to the remarkably subtle ridge morphology. Along the southeastern sides of the ridges the facies boundary coincides with a stratigraphic discontinuity that separates Holocene from the older deposits while the transition along the northwestern sides of the ridges is within the Holocene deposit and is the result of sediment redistribution by modern processes.

Mud volcanism in the Gulf of Cadiz: results from the TTR-10 cruise

A new deep water mud volcano field (between 2000 and 3500 m water depth) was discovered in the deep South Portuguese margin, as well as several new mud volcanoes in the South Spanish and Western Moroccan margins of the Gulf of Cadiz, during the TTR-10 (Training Through Research, UNESCO/IOC) cruise, in July/August 2000. This work followed the discovery of a large mud volcano field in the Gulf of Cadiz, first investigated during the TTR-9 cruise [Gardner (2001) Geophys. Res. Lett. 28, 339–342; Kenyon et al. (2000) IOC, Technical series no. 56]. The discoveries were made based on a SEAMAP side-scan sonar mosaic and multibeam bathymetry (SEABEAM) collected in the area by the Naval Research Laboratory (NRL), Washington, DC, USA, in 1992, kindly released for this purpose. Single-channel seismics, long-range side-scan sonar (OKEAN), TV-controlled grab, hull-mounted 3.5-kHz profiler and coring were used to investigate several seafloor features observed on the side-scan sonar imagery, in the South Iberia, Spanish and Moroccan margins of the Gulf of Cadiz, which were confirmed to be mud volcanoes. The typical structures related to fluid venting in the Gulf of Cadiz are essentially represented by conical mud volcanoes with diameters ranging from several tens of meters to 4 km and heights that can reach 200 m. Some of these structures appear to be aligned along major conjugate NE–SW and NW–SE trending faults that can be identified on the side-scan sonar imagery. The new field discovered in the South Portuguese margin is the deepest in the Gulf of Cadiz area and includes three new mud volcanoes – Bonjardim, Olenin and Carlos Ribeiro – which seem to be quite active, with near-surface gas hydrate occurrence and a high saturation in H 2S and hydrocarbon gases (mainly methane) in the mud breccia and overlying pelagic sediments. Gas hydrates were recovered from the Bonjardim mud volcano. An intensely gassified mud breccia, with one fragment of semi-consolidated claystone with a thin bituminous veneer at the surface, was recovered from the Carlos Ribeiro mud volcano. The fauna recovered consists mainly of pogonophoran worms belonging to several species and undetermined species of Foraminifera. Three new mud volcanoes were also discovered in the NW Moroccan margin: Rabat, Student and Jesus Baraza. These show a richer fauna that includes several species of molluscs, polychaetes, pogonophoran worms, crustaceans, echinoderms and some fragments of dead coral ( Madrepora and Lophelia). Carbonate crusts were recovered from the Student mud volcano. The Ginsburg mud volcano, discovered during the previous TTR-9 cruise, was revisited and gas hydrates recovered once again. A new mud volcano was also discovered in the Spanish margin, Tasyo, where evidence was found of coral build-ups on the hard substratum of the mud volcanic edifice.

Benthic Habitat Mapping in the Tortugas Region, Florida

Concern about declining trends in coral reef habitats and reef fish stocks in the Florida Keys contributed to the implementation of a network of no-take marine protected areas in 1997. In support of the efforts of the Dry Tortugas National Park and Florida Keys National Marine Sanctuary to implement additional no-take areas in the Tortugas region in 2001, we expanded the scale of our fisheries independent monitoring program for coral reef fishes in the region. To provide a foundation for the habitat-based, stratified random sampling design of the program, we created a digital benthic habitat map of coral reef and hard-bottom habitats in a geographic information system by synthesizing data from bathymetric surveys, side-scan sonar imagery, aerial photogrammetry, existing habitat maps, and in situ visual surveys. Existing habitat maps prior to 1999 were limited to shallow-water (< 20 m depth) soft-sediment, coral reef, and hard-bottom habitats within Dry Tortugas National Park and did not include deeper areas such as the Tortugas Bank, now partially contained within no-take marine protected area boundaries. From diver observations made during the 1999 survey, we developed a classification scheme based on habitat relief and patchiness to describe nine hard-bottom and coral reef habitats encountered from 1-33 m depth. We provide estimates of area by habitat type for no-take marine protected areas in the Tortugas region. Updated information on the spatial distribution and characteristics of benthic habitats will be used to guide future monitoring, assessment, and management activities in the region. Significant data gaps still exist for the western area of the Florida Keys National Marine Sanctuary and are a priority for future research.

Atypical heat-flow near gas hydrate irregularities and cold seeps in the Baikal Rift Zone

In this paper, we address the irregular behaviour and geometry of the gas hydrate stability zone (HSZ) inferred from reflection seismic data in relation to heat-flow measurements. The study area lies in the hanging wall of the Posolsky fault in the Southern Baikal Basin (SBB). Side-scan sonar imagery already revealed an undulating antithetic active fault structure and several isolated active vent structures. Remarkably, these fluid discharge structures occur only where the base of the hydrate stability zone (BHSZ), as inferred from seismic reflection profiles, is fluctuating and discontinuous, independent of lake floor morphology. The correlation between the interpreted BHSZ and heat-flow data across the Malenki seep is reasonable. On a seismic profile south of the fluid escape features, the BHSZ is expressed as an oscillatory, but continuous reflection, and shows poor correlation with heat-flow measurements. In nearly all cases, measured heat-flow exceeds inferred heat-flow. Additionally, the local inferred minima are anomalously low compared to the expected background values in the SBB. These observations suggest that the present-day hydrate accumulation and its (meta-)stability are more complicated than originally suspected. The limited area of these anomalies, their amplitudes and their occurrence in the immediate vicinity of faults and fluid escape features suggest that fluid convection cells disturb local gas hydrate stability conditions.

Textural analyses of sidescan sonar imagery from two mound provinces in the Porcupine Seabight

Large mound structures have been discovered in the Porcupine Seabight (Northeast Atlantic) at a depth of 500–1200 m, associated with the growth of cold-water deep-sea coral species such as Lophelia pertusa (L.) or Madrepora oculata (L.). During the Training Through Research cruise in 1997, high-resolution OREtech sidescan sonar data were acquired over two provinces of these structures. This article focuses on the presentation and quantitative interpretation of representative sections of these sidescan data from areas around the Belgica and Hovland mounds. Several image analysis tools were used, but texture analysis, based on grey level co-occurrence matrices, gave the best results. Entropy and homogeneity indices were calculated, and the resulting images made it possible to discriminate between different seabed features on a quantitative basis. Mounds, moats and background sediments could be delineated accurately, and the image textures could be linked to the actual seafloor appearance through core descriptions and deep-towed video data. A major difference was found in the acoustic returns from the two provinces studied: the Belgica province shows much rougher textures. This is due to an actual difference in seabed roughness, caused by a difference in bottom currents and sediment dynamics in the two areas. The combined effect of the northward-directed North Atlantic slope current and superimposed internal waves and tides appears to be much stronger in the Belgica province. The reported difference in current strength might well influence the growth of the deep-water coral species in both areas.

TOBI sidescan sonar imagery of the very slow-spreading Southwest Indian Ridge: evidence for along-axis magma distribution

New deep tow sidescan sonar data from the Southwest Indian Ridge reveal complex volcanic/tectonic interrelationships in the axial zone of this ultra-slow spreading ridge. While some constructional volcanic features resemble examples documented at the slow-spreading Mid-Atlantic Ridge, such as axial volcanic ridges, hummocky and smooth lava flows, their distribution and dimensions differ markedly. The largest axial volcanic ridges occur at segment centres, but fresh-looking volcanic constructions also occur at segment ends and in the deep basins marking the non-transform discontinuities. The orientations of the dominant fault population and main volcanic ridges are controlled by tectonic processes such as orthogonal extension in the sections of the ridge perpendicular to the spreading direction and transtensional extension in the obliquely spreading sections of the ridge. Minor faults and small volcanic ridges striking parallel to the axis in the oblique part of the ridge are not controlled by these extensional regimes. This observation suggests that the ridge axis acts as a zone of weakness and that magmatic processes, with associated fractures opening in response to magma pressure, may control local emplacements of axial volcanic ridges at obliquely spreading ridges. This non-systematic pattern of ridge characteristics suggests an along-axis variation between focused and distributed magmatic supply, a model which is supported by our interpretation of low-amplitude mantle Bouguer anomalies calculated for the area. We propose that a change of the axial segmentation pattern, from two segments to the present-day three segments, may have introduced additional instability into the crustal accretion process.

Characterization and recognition of deep-water channel-lobe transition zones

The channel-lobe transition zone (CLTZ) is an important, but commonly overlooked, element of many deep-water turbidite systems. Recognizing this zone is difficult in both modern and ancient environments and depends largely on the quality and resolution of the data obtained. In this article, three case studies of modern CLTZs are presented, largely based on high-resolution side-scan sonar imagery. These data are then compared to other well-defined CLTZs, both modern and ancient, and the common characteristics identified. CLTZs occur at canyon/channel mouths and are commonly associated with a break of slope. Most sediment bypasses this zone, and consequently only coarse sands and gravels are deposited, although these are commonly patchily distributed and extensively reworked. The CLTZ is characterized by abundant erosional features, including isolated spoon- and chevron-shaped scours up to 20 m deep, 2 km wide, and 2.5 km long. In areas of more widespread erosion, these merge to form amalgamated scours several kilometers across. Depositional bed forms include sediment waves with wavelengths of 1-2 km and wave heights up to 4 m. The presence or absence of a CLTZ has important implications for hydrocarbon exploration and development, especially in terms of the connectivity between sandy channel-fill and lobe facies.