Coastal Shelf Areas Research Articles

Spatial distribution of dissolved methane and its source in the western Arctic Ocean

Recent Arctic warming and decreasing sea-ice can promote the release of methane (CH4), a greenhouse gas, from the Arctic Ocean, thereby providing a strong climate feedback. However, the dynamics of dissolved CH4 in the Arctic Ocean remain uncertain, especially in western areas. This report describes the horizontal and vertical distributions of concentration and stable carbon isotope ratio (δ13C value) of CH4 in the western Arctic Ocean. Surface layer samples used for this study were supersaturated with CH4 in comparison to the atmosphere. Especially high CH4 concentrations (up to 10.3 nmol kg−1) were observed at stations in the continental shelf area. At the bottom layer of the shelf stations, the CH4 concentration was higher (up to 55.9 nmol kg−1). Its δ13C value was lower (down to − 63.8‰) than in the surface layer, which suggests that CH4 in the shelf water is produced mainly by methanogens in sediment. At deeper stations in the Canada Basin (seafloor > 300 m depth), the maxima of CH4 concentration were detected at depths of 10–50 m and 100–200 m, although δ13C values were lowest at 50 m depth. The shallower CH4 maximum coincided with the DO maximum, suggesting CH4 production by plankton activity or sinking particles. The deeper CH4 maximum corresponded to the nutrient maximum, suggesting horizontal advection of shelf water from the coastal shelf area. From the results, we were able to confirm that the dynamics of dissolved CH4 in the western Arctic Ocean in summer 2012 varied with area and depth.

Clay mineralogy and source-to-sink transport processes of Changjiang River sediments in the estuarine and inner shelf areas of the East China Sea

We examined the source-to-sink sediment transport processes from the Changjiang River to the estuarine coastal shelf area by analyzing the clay mineral assemblages in suspended sediment samples from the Changjiang River catchment and surface samples from the estuarine coastal shelf area following the impoundment of the Three Gorges Dam (TGD) in 2003. The results indicate that the clay mineral compositions throughout the study area are dominated by illite, with less abundant kaolinite and chlorite and scarce smectite. The clay minerals display distinct differences in the tributaries and exhibit obvious changes in the trunk stream compared with the periods before 2003, and the source of sediment has largely shifted to the mid- to lower reaches of the river after 2003. Spatially, the clay mineral assemblages in the estuarine area define two compositionally distinct provinces. Province I covers the mud area of the Changjiang River estuary and the Zhe-Min coastal region, where sediment is primarily supplied by the Changjiang River. Province II includes part of the Changjiang River estuary and the southeastern portion of the study area, where the sediment is composed of terrestrial material from the Changjiang River and re-suspended material from the Huanghe River carried by the Jiangsu coastal current. Moreover, the other smaller rivers in China (including the Oujiang and Minjiang rivers of mainland China and the rivers of West Taiwan) also contribut sediments to the estuarine and inner shelf areas. In general, the clay mineral assemblages in the Changjiang River estuarine area are have mainly been controlled by sediment supplied from upstream of the Changjiang River tributaries. However, since the completion of the TGD in 2003, the mid- to downstream tributaries have become the main source of sediments from the Changjiang catchment into the East China Sea. These analyses further demonstrate that the coastal currents and the decrease in the sediment load of the river have the greatest impacts on the distribution and transport of clay minerals assemblages in the sediments.

Pockmark morphology and turbulent buoyant plumes at a submarine spring

The input flow of groundwater from the seabed to the coastal ocean, known as Submarine Groundwater Discharge (SGD), has been only recently recognized as an important component of continental margin systems. It potentially impacts physical, chemical and biological marine dynamics. Independently of its specific nature (seepage, submarine springs, etc.) or fluid chemical composition, a SGD is generally characterized by low flow rates, hence making its detection and quantification very difficult, and explaining why it has been somewhat neglected by the scientific community for a long time. Along with the growing interest for SGDs emerged the need for in-situ observations in order to characterize in details how these SGDs behave. In this work, we describe the morphology of a pockmark field, detected in the Southern Tyrrhenian Sea (Mediterranean Sea), and provide observational evidences of the presence of active submarine springs over the coastal shelf area. We describe the effect of the fluid seeps on the water column stratification close to the main plumes and in the neighbouring areas, providing quantitative estimates of the intensity of the turbulent mixing and discussing their potential impact on the seabed morphology and pockmark formation in the context of turbulent buoyant plumes analytical modelling.

Reconstruction of time-varying tidal flat topography using optical remote sensing imageries

Tidal flats (TFs) occupy approximately 7% of the total coastal shelf areas worldwide. However, TFs are unavailable in most global digital elevation models (DEMs) due to water-impermeable nature of existing remote sensing approaches (e.g., radar used for WorldDEM™ and Shuttle Radar Topography Mission DEM and optical stereo-pairs used for ASTER Global Digital Elevation Map Version 2). However, this problem can be circumvented using remote sensing imageries to observe land exposure at different tidal heights during each revisit. This work exploits Landsat-4/-5/-7/-8 Thematic Mapper (TM)/Enhanced TM Plus/Operational Land Imager imageries to reconstruct topography of a TF, namely, Hsiang-Shan Wetland in Taiwan, to unveil its formation and temporal changes since the 1980s. We first classify water areas by applying modified normalized difference water index to each Landsat image and normalize chances of water exposure to create an inundation probability map. This map is then scaled by tidal amplitudes extracted from DTU10 tide model to convert the probabilities into actual elevations. After building DEM at intertidal zone, a water level-area curve is established, and accuracy of DEM is validated by sea level (SL) at the timing of each Landsat snapshot. A 22-year (1992–2013) dataset composed of 227 Landsat scenes are analyzed and compared with tide gauge data. Root-mean-square differences of SL reaches 48cm with a correlation coefficient of 0.93, indicating that the present technique is useful for constructing accurate coastal DEMs, and that products can be utilized for estimating instant SL. This study shows the possibility of exploring evolution of intertidal zones using an archive of optical remote sensing imageries. The technique developed in the present study potentially helps in quantifying SL from the start of optical remote sensing era.

Spatial and seasonal variations of iron speciation in surface waters of the Subantarctic front and the Otago Continental Shelf

Here we present the longest time series of iron speciation along a coastal shelf area. A time series of a surface transect across the Otago Continental Shelf and into the high-nutrient low-chlorophyll (HNLC) Subantarctic surface water (SAW) was undertaken between 2000 and 2008 to measure the dissolved iron (DFe) concentration, organic iron-complexation parameters, as well as macronutrient concentrations and general hydrographic data. The study area contains three distinct water types: 1) neritic water; 2) modified subtropical water (STW); and 3) Subantarctic surface water (SAW). Variations in nutrient concentrations in the study area indicated that SAW is the predominant source of nitrate and phosphate to the shelf. Dissolved iron (DFe) concentrations dropped seawards from several nanomolar to sub-nanomolar levels. The dissolved iron was fully complexed with strong organic ligands (L1) in all three water types, and the ligand concentrations decreased seawards. However the ratio between [L1]/[DFe], a measure of the ligand saturation, increased from <2 in the neritic water to 10 in SAW. Seasonal trends in DFe and the L1 concentrations were only significant in SAW, where DFe was lowest in early spring and late autumn and highest in summer. The seasonal DFe maximum coincided with [L1] minimum, as well as minimum concentrations of macronutrients, indicating a shift from iron limitation in spring with potential siderophore production to probably silica limitation in summer. This study provides new insight on the driving forces of iron speciation in a coastal shelf area and HNLC waters.

Historic and current distribution patterns, and minimum abundance of killer whales (Orcinus orca) in the north-west Atlantic

This study represents the first comprehensive examination of the distribution and abundance of killer whales (Orcinus orca) in the north-west Atlantic. Based on a collation of sightings data and a multi-year photographic catalogue of killer whales, 836 sighting events have been recorded between 1758 and 2012, with most occurring in the last ten years. Killer whales were most commonly observed during June–September in Newfoundland/Labrador, Canada. Most sightings were made close to shore, although many occurred beyond coastal shelf areas and in water depths in excess of 3000 m. Relatively fewer sightings were recorded on the Scotian Shelf, in the Gulf of St Lawrence or the north-eastern USA, despite appreciable aerial and vessel-based cetacean survey effort. In the north-west Atlantic, killer whales have been sighted both alone and in groups, with group sizes ranging from 2 to 30 whales (rarely more than 15, although an aggregation of 100 was reported 43 years ago). Groups usually comprised 2–6 individuals. Based on photographic records, there are at least 67 identified killer whales in the northwest Atlantic; this is an underestimate, since a large portion of our image collection was not of sufficient quality to be considered in the analysis, and many of the whales do not have easily discernible markings. The discovery curve of newly-identified whales has not plateaued, suggesting that there are more whales to identify. These data allow us to better understand the ecology of these killer whales, and provide a baseline against which population changes and distribution patterns can be assessed.

The role of juveniles in structuring demersal assemblages in trawled fishing grounds

The capture of large amounts of small, immature fish of commercial species is a serious problem particularly in multispecies fisheries. Moreover, considerable and increasing interest is being devoted by fishery scientists to identify the distribution and habitat needs of species throughout their life cycle. To elucidate species composition, the abundance of juveniles in the demersal assemblages and the role of different life history (juvenile and adult) stages of target species in structuring demersal communities, two bottom trawl surveys were carried out during the autumn 2003 and 2004. Multivariate analyses were performed on density indices of adults and juveniles life stages of 30 target species and total density indices for the remainder of the catch species. Juveniles represent more than 61% of the total catch in both the years investigated and their abundance and spatial distribution was strictly related to the sea bottom biocoenotic features. Most juveniles were concentrated in the coastal shelf area and in particular in the hauls performed on the Coastal Terrigenous Mud biocoenosis (CTM). The demersal assemblages located in the slope stratum showed, in general, a lower concentration of juvenile specimens; however, some facies of the Bathyal Mud biocoenosis that characterizes the deep layer of our study area showed a very high percentage of juveniles. This information improves our understanding of ecosystem functioning and represents a useful basis for providing advice on the management of multispecies demersal fisheries within an ecosystem approach.

Global multi-scale segmentation of continental and coastal waters from the watersheds to the continental margins

Abstract. Past characterizations of the land–ocean continuum were constructed either from a continental perspective through an analysis of watershed river basin properties (COSCATs: COastal Segmentation and related CATchments) or from an oceanic perspective, through a regionalization of the proximal and distal continental margins (LMEs: large marine ecosystems). Here, we present a global-scale coastal segmentation, composed of three consistent levels, that includes the whole aquatic continuum with its riverine, estuarine and shelf sea components. Our work delineates comprehensive ensembles by harmonizing previous segmentations and typologies in order to retain the most important physical characteristics of both the land and shelf areas. The proposed multi-scale segmentation results in a distribution of global exorheic watersheds, estuaries and continental shelf seas among 45 major zones (MARCATS: MARgins and CATchments Segmentation) and 149 sub-units (COSCATs). Geographic and hydrologic parameters such as the surface area, volume and freshwater residence time are calculated for each coastal unit as well as different hypsometric profiles. Our analysis provides detailed insights into the distributions of coastal and continental shelf areas and how they connect with incoming riverine fluxes. The segmentation is also used to re-evaluate the global estuarine CO2 flux at the air–water interface combining global and regional average emission rates derived from local studies.

Mineralogical and geochemical studies of glacial sediments from Schirmacher Oasis, East Antarctica

Schirmacher Oasis, East Antarctica hosts loose unconsolidated sediments that consist dominantly of sand-silt admixtures with minor clay. These sediments were collected from different glacial environments such as polar ice sheets, inland lakes, exposed bedrock, and coastal shelf areas. Two sediment fractions (coarse 0.25–0.125 mm and fine 0.125–0.063 mm) were separated and chemically analyzed. Mineralogically, the mixture is composed dominantly of quartz and feldspar, and a large variety of heavy minerals: zircon, tourmaline, rutile, garnet, hornblende, hypersthene, enstatite, kyanite, sillimanite, andalusite, zoisite, lawsonite, chlorite, spinel, topaz, and opaques. The clay minerals i.e., chlorite, illite, smectite, kaolinite, and vermiculite constitute a small fraction. There is a mineralogical control over the observed geochemical patterns and anomalies of these sediments. The rare earth element and incompatible trace element spidergrams show relative enrichment in the coarse fraction as against fine fraction, indicating enrichment of the carrier minerals of these elements (mostly heavy minerals) in the coarse fraction. The overall geochemistry of the sediments classifies them as greywacke as per Lindsey, Fe-sand to arenite as per Herron, and sodic sandstones as per Blatt et al. These sediments show quartzose sedimentary provenance, but influence of other unknown sources is also discernible. Transporting agencies for the sediments are commonly meltwater channels and wind. However, transport under influence of gravity, glacier action and sea waves are also envisaged. The sediments have undergone low degrees of chemical weathering ranging between incipient and moderate types.

Mercury sources and fate in the Gulf of Maine

Most human exposure to mercury (Hg) in the United States is from consuming marine fish and shellfish. The Gulf of Maine is a complex marine ecosystem comprising twelve physioregions, including the Bay of Fundy, coastal shelf areas and deeper basins that contain highly productive fishing grounds. Here we review available data on spatial and temporal Hg trends to better understand the drivers of human and biological exposures. Atmospheric Hg deposition from U.S. and Canadian sources has declined since the mid-1990s in concert with emissions reductions and deposition from global sources has increased. Oceanographic circulation is the dominant source of total Hg inputs to the entire Gulf of Maine region (59%), followed by atmospheric deposition (28%), wastewater/industrial sources (8%) and rivers (5%). Resuspension of sediments increases MeHg inputs to overlying waters, raising concerns about benthic trawling activities in shelf regions. In the near coastal areas, elevated sediment and mussel Hg levels are co-located in urban embayments and near large historical point sources. Temporal patterns in sentinel species (mussels and birds) have in some cases declined in response to localized point source mercury reductions but overall Hg trends do not show consistent declines. For example, levels of Hg have either declined or remained stable in eggs from four seabird species collected in the Bay of Fundy since 1972. Quantitatively linking Hg exposures from fish harvested from the Gulf of Maine to human health risks is challenging at this time because no data are available on the geographic origin of seafood consumed by coastal residents. In addition, there is virtually no information on Hg levels in commercial species for offshore regions of the Gulf of Maine where some of the most productive fisheries are located. Both of these data gaps should be priorities for future research.

Grain-size characteristics of deposits derived from different glacigenic environments of the Schirmacher Oasis, East Antarctica

Sediment samples have been collected from the Schirmacher Oasis and adjoining area in East Antarctica; these areas consist of polar ice, ice-free area, lakes and the coastal shelf area. The 37 samples have been analysed for their grain-size parameters and statistical relationships. The oasis is characterised by ongoing glacial processes, including deposition and erosion of the sediments by ice, meltwater and winds, thus influencing the sediments in various ways. Basic statistical grain-size parameters like graphic mean, standard deviation, skewness and kurtosis have been calculated for the four units. The sediments of all areas are almost all poorly to very poorly sorted, fine-skewed to near-symmetrical and platykurtic to leptokurtic in nature. Bivariate plots between the grain-size parameters have been interpreted. They do, apart from a few exceptions, not show any trend suggesting relationships between the sediments of the four landscape types. Both t- and F-tests have been applied on the samples, and the phi values and grain-size parameters have been analysed; these show that the group variances of the samples are not significant, but that the phi-values are significant.

High-frequency multibeam echo-sounder measurements of seafloor backscatter in shallow water: Part 1 – Data acquisition and processing

Abstract Methods for processing multibeam echo-sounder (MBES) bathymetry data are well established, how-ever, there is no universal and established approach to processing and interpretation of MBES backscatter data. The aim of this paper is to (1) detail the different backscatter logging methods implemented in modern high-frequency MBES systems used primarily in shallow water; (2) describe and suggest appropriate methods of data processing; and (3) discuss likely errors and artefacts expected from MBES backscatter processing. These issues are illustrated using data collected with a Reson SeaBat MBES in coastal shelf areas of Australia, although the methods and errors discussed are relevant to other high-frequency shallow-water MBES systems. Production of backscatter mosaics and analysis of backscatter for seafloor classification using MBES data are considered in Part 2 – Mosaic production, analysis and classification (Parnum and Gavrilov, 2011), published alongside this paper.

Occurrence and distribution of Carangidae larvae (Teleostei, Perciformes) from the Southwest Atlantic Ocean, Brazil (12-23°S)

The objective of this study was to identify larvae species composition, distribution and abundance of carangids along the Brazilian coastal shelf area in order to provide background information on the reproductive community patterns. All carangid larvae used in this study are from the larval fish collection of the Zooplankton and Ichthyoplankton Integrated Laboratory of Federal University of Rio de Janeiro – Brazil (DZUFRJ). Carangid larvae were collected in 13 surveys (nine in the dry season and four during the rainy season) between 1998 and 2008, from Bahia (12°S) to Rio de Janeiro (23°S), comprising 158 sampling stations (45 coastal; 98 oceanic; 15 over seamounts). A bongo net with 330 and 500-μm mesh size was towed at a speed of about two knots, obliquely (surface-bottom-surface) from a maximum depth of 200 m. Samples were collected both during the day and at night and combined in the analysis. A total of 1321 carangid larvae were identified representing 14 species: Decapterus punctatus (56.7%), Chloroscombrus chrysurus (16.8%), Caranx crysos (8.4%), Selar crumenophthalmus (6.3%), Trachurus lathami (6.3%), Caranx latus (2.4%), Carangoides bartholomaei (1.3%), Oligoplites spp. (0.6%), Pseudocaranx dentex (0.3%), Seriola dumerili (0.2%), Elagatis bipinnulata (0.2%), Selene setapinnis (0.2%), Trachinotus carolinus (0.2%) and Selene vomer (0.1%). There was no significant difference in carangid density (P = 0.17) or in species composition (P = 0.09) among the coastal, oceanic and seamount stations. Although there was a significant difference in larval densities between the dry and rainy periods (P = 0.000264), the community composition did not vary significantly (P = 0.47). Therefore, seasonality (rainy and dry periods) seems to be the main factor affecting carangid densities although the community composition is not influenced by sampling periods or by the gradient coast-ocean.

Seasonal and spatial variability of sediment oxygen fluxes in the Beobsan intertidal flat of Taean Bay, mid-western Korean Peninsula

Core incubation and oxygen microelectrode measurements were performed at two stations in the Beobsan intertidal flat of Taean Bay on the mid-western Korean Peninsula. Total and diffusive fluxes were determined for four different seasons (August and October 2006 and February and May 2007). Diffusive oxygen fluxes ranged from 7.62 to 11.4 mmol m−2 d−1 at station S1 and 8.85 to 14.3 mmol m−2 d-1 at station S2. Total oxygen fluxes varied from 20.2 to 27.6 mmol m−2 d−1 at station S1 and from 18.7 to 31.7 mmol m−2 d−1 at station S2. Total oxygen fluxes were 2∶3 times higher than diffusive oxygen fluxes, a result that agrees well with previous studies of the coastal and continental shelf area. The difference between the total and diffusive fluxes is caused by the bio-irrigation activities of macrofauna living in the sediments, which enhance the ventilation of the sediment and introduce oxygenated water to the deeper, anoxic sediment layer. The total oxygen flux at station S1 exhibited small seasonal variation while that at station 2 showed large seasonal variation. Organic carbon contents in the surface sediments correlated well with total oxygen flux, suggesting that the input flux of organic matter is a major factor controlling the seasonal variation in total oxygen flux. Annual benthic aerobic respiration rates were determined to be 8.8 and 9.1 mol C m−2 yr−1 at stations S1 and S2, respectively; these values are very similar to estimations for Aarhus Bay, Denmark and the southwestern lagoon of New Caledonia.

Variability of the distribution of the transparency and suspended matter: Concentration in the surface layer of the northwestern part of the Black Sea in the summer period

Particular features of the distribution of the transparency and particulate matter content, their variability, and their interdependence in the surface water layer (0–5 m) over the northwestern shelf and in the adjacent abyssal part of the Black Sea in the summer were considered on the basis of long-term simultaneous optical, biological, and hydrological observations (1979–1993). In the shelf regions with different river discharges and in the waters of the open part of the sea, the distributions of the transparency, the total particulate matter, and its organic components (organic carbon, nitrogen, and chlorophyll a), as well as the relative content of particulate organic carbon in the total amount of the particulate matter and the content of chlorophyll a in the particulate organic carbon, were considered. The distributions of the transparency and particulate matter and their dependence on the water dynamics are in good agreement. It was demonstrated that extreme anthropogenic eutrophication influences the western and northern coastal shelf areas. The water transparency and particulate organic matter distributions in the central shelf area subjected to the influence of transformed river water and the water properties of the southern part of the shelf, which is influenced by the waters of the open sea, were determined according to the particular structure of the phytoplankton, its abundance, and the processes of its production and destruction.

Distribution and air–sea exchange of reduced sulphur gases in European coastal waters

Recent research work provided the basis for a much improved understanding of the open ocean sources of dimethylsulphide (DMS), carbonyl sulphide (COS), and carbon disulphide (CS 2) to the atmosphere. Coastal ocean emissions remained less well constrained, arguably because of the inherently high variability of coastal and estuarine systems. This article presents a review of the spatial and seasonal distributions of DMS, COS and CS 2 together with a preliminary assessment of their respective emissions from European coastal waters. Our data compilation considers winter/summer scenarios and explicitly distinguishes shelf waters and estuarine systems. For DMS, our findings indicate similar emission rates for shelf waters and estuarine systems and an annual DMS source of approximately 16 Gmol a −1. Our annual emission estimates for COS and CS 2 are 0.26 Gmol a −1 and 0.16 Gmol a −1, respectively, and suggest significant contributions from estuarine systems to total shelf emissions in both cases (COS: 27%; CS 2: 19%). On this basis we recommend that estuarine COS and CS 2 emissions should be considered in future global emission estimates. Comparisons of our emission estimates to previous estimates of global marine sulphur gas emissions indicate disproportionally high contributions from European coastal waters. Given that emission rates from European shelves may be similar to those of other coastal shelf areas, our findings suggest that the coastal marine sources of DMS, COS and CS 2 may have been underestimated in previous work.

JELLYFISH AGGREGATIONS AND LEATHERBACK TURTLE FORAGING PATTERNS IN A TEMPERATE COASTAL ENVIRONMENT

Leatherback turtles (Dermochelys coriacea) are obligate predators of gelatinous zooplankton. However, the spatial relationship between predator and prey remains poorly understood beyond sporadic and localized reports. To examine how jellyfish (Phylum Cnidaria: Orders Semaeostomeae and Rhizostomeae) might drive the broad-scale distribution of this wide ranging species, we employed aerial surveys to map jellyfish throughout a temperate coastal shelf area bordering the northeast Atlantic. Previously unknown, consistent aggregations of Rhizostoma octopus extending over tens of square kilometers were identified in distinct coastal "hotspots" during consecutive years (2003-2005). Examination of retrospective sightings data (>50 yr) suggested that 22.5% of leatherback distribution could be explained by these hotspots, with the inference that these coastal features may be sufficiently consistent in space and time to drive long-term foraging associations.

Sedimentology of rift climax deep water systems; Lower Rudeis Formation, Hammam Faraun Fault Block, Suez Rift, Egypt

In most marine rift basins, subsidence outpaces sedimentation during rift climax times. Typically this results in sediment-starved hangingwall depocentres dominated by deep-marine mudstones, with subordinate local development of coarser clastics in the immediate hangingwall derived from restricted catchments on the immediate footwall scarp. To highlight the spatial variability of rift climax facies and the controls upon them, we have investigated the detailed three-dimensional geometry and facies relationships of the extremely well exposed Miocene, rift climax Lower Rudeis Formation in the immediate hangingwall to the Thal Fault Zone, Suez Rift, Egypt. Detailed sedimentological analyses allows the Lower Rudeis Formation to be divided into two contemporaneous depositional systems, (1) a laterally continuous slope system comprising, hangingwall restricted (< 250 m wide) slope apron, slope slumps, fault scarp degradation complex and laterally extensive lower slope-to-basinal siltstones, and (2) a localized submarine fan complex up to 1 km wide and extending at least 2 km basinward of the fault zone. Interpretation of individual facies, facies relationships and their spatial variability indicate that deposition in the immediate hangingwall to the Thal Fault occurred via a range of submarine concentrated density flows, surge-like turbidity flows, mass wasting and hemipelagic processes. Major controls on the spatial variability and stratigraphic architecture of the depositional systems identified reflect the influence of the steep footwall physiography, accommodation and drainage evolution associated with the growth of the Thal Fault. The under-filled nature of the hangingwall depocentre combined with the steep footwall gradient result in a steep fault-controlled basin margin characterised by either slope bypass or erosion, with limited coastal plain or shelf area. Sediment supply to the slope apron deposits is controlled in part by the evolution and size of small footwall drainage catchments. In contrast, the localized submarine fan is interpreted to have been fed by a larger, antecedent drainage network. The structural style of the immediate footwall is also believed to exert a control on facies development and stratigraphic evolution. In particular, fault scarp degradation is enhanced by fault propagation folding which creates basinward-dipping bedding planes in the pre-rift footwall strata that large pre-rift blocks slide on.

Acoustic seabed classification of marine habitats: studies in the western coastal-shelf area of Portugal

Abstract Two single-beam, seabed-classification systems, QTC VIEW Series IV and QTC VIEW Series V, were used to identify and map biosedimentary gradients in a mid-shelf area off Western Portugal. The survey area has a moderate slope, a depth ranging from 30 to 90 m along a 3.5-km axis perpendicular to the shoreline, and is characterized by smooth sedimentary and biological gradients. Ground truth for sediment grain size and macrofaunal communities was based on grab sampling at 20 sites. The sedimentary and biological data were analysed using classification and ordination techniques. The acoustic data were analysed with qtc impact software and classified into acoustic classes. The affinity groups obtained in each data set were mapped using a Geographics Information System. All showed good agreement and identified prevailing gradients along a northwest–southeast direction. Three acoustic classes were identified, corresponding to the predominant sediment types, namely fine sand with low silt and clay content, silty, very fine sand, and mud. A close relationship with benthic communities was also verified, although less marked because benthic communities continuously change along the northwest–southeast gradient. Overall, the acoustic system coupled with ground-truthing data was able to discriminate and characterize the various benthic biotopes in the survey area.

Use of the Acoustic Doppler Current Profiler (ADCP) in the study of the circulation of the Adriatic Sea

Abstract. The results of basin-wide vessel-mounted ADCP measurements carried out from May 1995 through February 1996 in the Adriatic Sea are analysed in order to characterise the tidal flow, the steady current field and some specific sub-basin scale features. The M2 tide shows the amphidromic point close to the location predicted from theory. The K1 presents an almost constant phase structure that increases in the northern part from east to west during summer. The circulation in the Italian coastal shelf area is highly variable, due to the local wind forcing and pulses of the Po River discharge. The propagation of the signal associated with the latter, can be described in terms of hybrid internal Kelvin waves, revealed also from the upwelling events. It is also shown that the bottom density-driven current draining the bottom layer of the northern Adriatic is, to a large extent, a time-dependent feature with a temporal scale on the order of days.Key words. Oceanography: general (descriptive and regional oceanography; marginal and semi-enclosed seas) Oceanography: physical (currents)