Muddy Sand Research Articles

Processes and products of turbidity currents entering soft muddy substrates

New laboratory experiments reveal that cohesionless turbidity currents are able to enter cohesive soft muddy substrates without losing their shape. These intrabed currents are driven by bed shear stress exceeding bed cohesive strength, and by flow density exceeding bed den - sity. The flows produce unique turbidites with internal mud layers, mixed cohesive-nonco - hesive sediment layers, and flame and load structures. A depositional model for intrabed (I) turbidites is proposed, comprising, from base to top: I1�sand-bearing mud, with a scoured base, dispersed mud, and mud clasts; I2�muddy sand from the intrabed portion of the tur - bidity current; I3�sandy mud with a speckled appearance; and I4�mud-poor sand from the suprabed portion of the flow. Complete I1�I4 turbidites are inferred to dominate loca - tions in nature where the currents mix with the bed and deep erosional scours form, filled with deformed or chaotic sand-mud mixtures. Further downflow, base-missing I2�I4 and I4 sequences signify gradual deceleration, loss of erosivity, and termination of intrabed flow

Seasonal sedimentary processes of the macrotidal flat in Gomso Bay, Korea

ABSTRACT Kang, J., Woo, H.J., Lee, Y.-K., Son, Y.B., 2014. Seasonal sedimentary processes of the macrotidal flat in Gomso Bay, Korea. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 157–163, ISSN 0749-0208. To understand seasonal sedimentary processes, we analyzed sediment grain size, short-term sedimentation rates, and chemical tracers to determine sediment provenance in the Gomso tidal flat, which is located on the western coast of Korea. It is a funnel-shaped embayment with a mean tidal range of 4.3 m. Calculating the elevation from a digital elevation model (DEM), the landward inner bay was relatively higher than that of the outer bay. Sandy sediments generally dominated the outer bay, whereas sandy mud sediments were distributed in the inner bay. The middle bay consisted of muddy sand sediments. From February 2011 to February 2012, the net deposition rate in the inner tidal flat ...

Late Holocene Geomorphology of the Columbia River Estuary, Oregon and Washington, USA

Abundant river sediment supply and an open-water central bay area characterize the geomorphology of the large Columbia River estuary (~ 100 km in length). Lateral floodplains and marsh islands do constrict the uppermost reaches of the estuary, but the central axes of the lower estuary are dominated by shallow sand shoals (0–4 m water depth). A total of 58 vibracores are used to document the grain size and age (0–2,500 14CyrBP) of late Holocene deposits in the estuary. Sedimentation rates in stable floodplains (1.1 m ka-1) reflect rates of relative sea level rise (0.75 m ka-1). Sedimentation rates of muddy sand accretionary banks and prehistoric sand shoals (1.5–7 m ka-1) greatly exceed coeval rates of sea level rise, so they must represent short–term rates of vertical accretion resulting from channel lateral migration and associated cut and fill processes. The apparent paradox of unfilled accommodation space in the estuary is resolved by 1) winter wind–wave erosion of sand shoals to -3 m NAVD88 elevation and 2) asymmetric fluvial-tidal advection that results in net seaward transport of bed load in shallow tidal channels (> – 10 m NAVD88) and shallow subtidal shoals (> – 4 m NAVD88) during spring river flooding.

Trematode communities in cockles (Cerastoderma edule) of the Ria de Aveiro (Portugal): Influence of inorganic contamination

This work aims to assess the trematode parasites infecting the edible cockle Cerastoderma edule, collected in the Ria de Aveiro lagoon, one of the most relevant biodiversity hotspots of the Western Iberia, and evaluate the relationship between the observed patterns and environmental descriptors. A total of 11 of the 16 trematode species known to infect C. edule were identified, including Himasthla continua and Psilostomum brevicolle as new occurrences in this lagoon. Parvatrema minutum was the most abundant and dominant species. Species richness and prevalence were high. The relationship between trematode species abundance, intensity and prevalence, and also environmental variables, showed that most parasites preferred muddy sand areas with euhaline conditions in opposition to areas with contamination and/or distant from the lagoon entrance. This study highlighted the good ecological status of the ecosystem and the transitional biogeographic characteristics of the western Portuguese coast where northern and subtropical faunas can coexist.

Petrography and genesis of Quaternary coastal gypcrete in North Kuwait, Arabian Gulf

Gypcrete crusts cover Late Quaternary playa deposits within the northern coastal flat of Kuwait Bay, Arabian Gulf. This study provides insight into the genesis of the gypcrete crust associated with the fluvial playa in an arid coastal area. The playa sediments are mostly muddy sand rich in diagenetic gypsum. The latter is concentrated at the top part of this playa where the gypcretic crust is developed. Field investigation, megascopic, microscopic and nanoscopic examination, and XRD analysis revealed the occurrence of several types of gypcretes, namely massive hard crust, hard clustered gypsum roses, gypseous rhyzocrete, and powdery crust. With the exception of the last type, all gypcretes are formed largely of gypsum with subordinate amounts of bassanite. Two genetic types of bassanite are recognised; these are replacive and precipitative. Replacive bassanite is the most common and is mainly represented by topotactic alteration of gypsum, whereas the precipitative mode is mostly restricted to the powdery crumbled crust and root cavity-fill. The micromorphology of both types of bassanite is discussed. It is suggested that the stability of bassanite is attributed to the elevated ambient temperatures in the presence of highly saline brine and organic acids.

Benthic nitrogen fluxes and fractionation of nitrate in the Mauritanian oxygen minimum zone (Eastern Tropical North Atlantic)

We present sedimentary geochemical data and in situ benthic flux measurements of dissolved inorganic nitrogen (DIN: NO3−, NO2−, NH4+) and oxygen (O2) from 7 sites with variable sand content along 18°N offshore Mauritania (NW Africa). Bottom water O2 concentrations at the shallowest station were hypoxic (42μM) and increased to 125μM at the deepest site (1113m). Total oxygen uptake rates were highest on the shelf (−10.3mmolO2m−2d−1) and decreased quasi-exponentially with water depth to −3.2mmolO2m−2d−1. Average denitrification rates estimated from a flux balance decreased with water depth from 2.2 to 0.2mmolNm−2d−1. Overall, the sediments acted as net sink for DIN. Observed increases in δ15NNO3 and δ18ONO3 in the benthic chamber deployed on the shelf, characterized by muddy sand, were used to calculate apparent benthic nitrate fractionation factors of 8.0‰ (15εapp) and 14.1‰ (18εapp). Measurements of δ15NNO2 further demonstrated that the sediments acted as a source of 15N depleted NO2−. These observations were analyzed using an isotope box model that considered denitrification and nitrification of NH4+ and NO2−. The principal findings were that (i) net benthic 14N/15N fractionation (εDEN) was 12.9±1.7‰, (ii) inverse fractionation during nitrite oxidation leads to an efflux of isotopically light NO2− (−22±1.9‰), and (iii) direct coupling between nitrification and denitrification in the sediment is negligible. Previously reported εDEN for fine-grained sediments are much lower (4–8‰). We speculate that high benthic nitrate fractionation is driven by a combination of enhanced porewater–seawater exchange in permeable sediments and the hypoxic, high productivity environment. Although not without uncertainties, the results presented could have important implications for understanding the current state of the marine N cycle.

Occurrence of diagenetic alunites within karst cavity infill of the Dammam Formation, Ahmadi, Kuwait: an indicator of hydrocarbon gas seeps

Alunite minerals occur as white powdery lumps and laminated coloured deposits within cavity and solution channel infill of the palaeokarst zone of the Upper Eocene Dammam Formation. This formation is exposed in a quarry located on the Al Ahmadi ridge within the Greater Burgan oil field in southern Kuwait. Field occurrences and sedimentary structures of the alunite deposits were described. Collected samples were petrographically described, and their mineralogy and geochemistry were determined using X-ray diffraction and X-ray fluorescence, respectively. Microfabrics were investigated using SEM, revealing that they are primarily composed of fibrous alunogen (hydrous aluminium sulphate) and pseudo-cubical K-alunite (hydrous potassium aluminium sulphate). Their mode of occurrence suggests a hypogenetic origin, where sulphide gases associated with hydrocarbon gases reacted with an Al-rich solution leached from clay minerals and feldspars of the cavity-fill muddy sand sediments. The hydrocarbon gases may have seeped from sub-surface petroliferous formations within the Greater Burgan oil field along vertical fractures. This study suggests that these acidic seeps may have played a role in the development of the palaeokarst zone of the Dammam Formation.

Late Quaternary inner shelf deposits in response to late Pleistocene–Holocene sea level changes: Nakdong River, SE Korea

The high-resolution seismic profiles from the inner shelf off the Nakdong River in SE Korea can be directly correlated with the lithologic characteristics in the borehole data. These correlations offer an opportunity to document the evolution of the inner shelf deposits with sediment supplies in response to the late Quaternary sea level changes. The inner shelf sequence in this area consists of four sedimentary units, which comprise a set of lowstand, transgressive, and highstand systems tracts deposited since the last glacial maximum: incised-channel fill (SU1), estuary (SU2), sand veneer (SU3), and subaqueous delta (SU4). The lowermost unit (SU1), which overlies the sequence boundary, is interpreted as fluvial deposits formed during the last glacial period and the early stage of transgression, and belongs to the lowstand to transgressive systems tract. The lower middle unit (SU2) lying below the ravinement surface represents a paralic component that consists of estuarine sandy mud or muddy sand developed between approximately 13 and 8 cal ka BP, whereas the upper middle unit (SU3) above the ravinement surface corresponds to a marine component that consists of sand veneer produced by the shelf erosion during the ensuing sea-level rise. These two units (SU2 and SU3) belong to the transgressive deposits. The uppermost unit (SU4), regarded as the highstand systems tract, formed the Nakdong subaqueous delta including the proximal and distal systems developed after the highstand sea level at approximately 6 cal ka BP. The lateral transition from the proximal to distal facies suggests a prograding delta system in the Nakdong River.

Impact of sand extraction from the bottom of the southern Baltic Sea on the relief and sediments of the seabed

Investigations of the geological structure and seabed dynamics as well as the morphological and sedimentological effects of sand extraction generated by different mining techniques were carried out in Polish waters of the Baltic Sea, NW of the Gulf of Gdańsk, at a water depth of 15–17m. Three research cruises took place: just before, directly after and 11months after dredging operations. Seismoacoustic profiling, a multibeam echosounder, a side-scan sonar, a 3m vibro-corer and a box-corer were used during the research cruises. The grain size distribution and 137Cs content of the sand samples were determined. Marine shells were dated by the AMS14C technique and pollen analyses were carried out on samples of muddy sands lying below the marine sand. A 2 to 4.5m thick layer of marine sands lies on the boulder till and locally on late Pleistocene ice margin lake deposits. The 137Cs content indicates that the 0.4–0.8m thick sand layer is mobile during storms.After the dredging operations, four pits with diameters from 80 to 120m, depths from 3 to 4.5m and slopes with gradients up to 30–55° were measured. Several smaller irregularly shaped pits and double furrows 30–150m in length and 0. 3–0.5m in depth were found. The sonar mosaic also shows a 50–100m buffer zone of fine sand around the pits which flowed over the dredger’s side with water and settled on the bottom.During one year after the dredging operation the furrows generated by trailer suction hopper dredging as well as the fine sand cover around the pits disappeared completely. The four post-dredging pits left by stationary suction dredging were shallower by 2–2.5m, their diameters increased by 40–50m, the gradient of the slopes was reduced by up to 5–10°, and the total volume was only about 3.5% smaller than directly after dredging.

Community structure and decadal changes in macrozoobenthic assemblages in Lake Poyang, the largest freshwater lake in China

Lake Poyang is the largest freshwater lake in China and contains unique and diverse biota within the Yangtze floodplain ecosystem. However, knowledge of its macrozoobenthic assemblages remains inadequate. To characterize the current community structure of these assemblages and to portray their decadal changes, quarterly investigations were conducted at 15 sites from February to November 2012. A total of 42 taxa were recorded, and Corbicula fluminea, Limnoperna fortunei, Gammaridae sp., Nephtys polybranchia, Polypedilum scalaenum and Branchiura sowerbyi were found to dominate the community in terms of abundance. The bivalves Corbicula fluminea, Lamprotula rochechouarti, Arconaia lanceolata and Lamprotula caveata dominated the community in biomass due to their large body size. The mean abundance of the total macrozoobenthos varied from 48 to 920 ind·m-2, the mean biomass ranged from 28 to 428 g·m-2. The substrate type affected strongly the abundance, biomass, and diversity of the macrozoobenthos, with muddy sand substrates showing the highest values. Compared with historical data, remarkable changes were observed in the abundance of macrozoobenthos and the identity of the dominant species. The mean total abundance decreased from 724 ind·m-2 in 1992 to 228 ind·m-2 in 2012. The dominant species have shifted dramatically. Large unionids were dominant before 1998, whereas pollution-tolerant species (e.g., Branchiura sowerbyi) increased in dominance after 2008. Our findings should have implications for the conservation of the benthic biodiversity of this large Yangtze-connected lake.

퇴적물 오염기준을 이용한 금강 하구역 표층 퇴적물내 유기물 및 미량금속 오염 평가

【We evaluated contamination with organic matter and trace metals by analyzing grain size, ignition loss (IL), chemical oxygen demand (COD), acid volatile sulfide (AVS), and trace metals (Al, Fe, Cu, Pb, Zn, Cd, Ni, Cr, Mn, Hg, and As) in surface sediments at 28 stations around the Geum River estuary in July 2008. The surface sediments in the estuary were mainly composed of coarse sediment (sand and muddy sand), with mean grain size (Mz) ranging between $2-4{\O}$ . The high concentrations of IL, COD, and trace metals were mainly found at stations in front of the Gusan outer port and industrial complex, and near the Seocheon coast with relatively fine sediments. In addition, the concentrations of IL and all trace metals, except Pb and As, showed good positive correlations with Mz, indicating that the concentrations of organic matter and trace metals were mainly dependent on sediment grain size. The concentrations of COD, AVS, and trace metals in most sediments did not exceed the sediment quality guideline (SQGs). Although the sediments in the study region are not polluted with organic matter and trace metals, there are many point sources of pollutants, such as Gusan port and industrial complex, Janghang refinery, and a thermoelectric power plant around the Geum River estuary. Thus, the management of coastal environments through periodic monitoring of organic matter and trace metals is required in the future.】

The Biodiversity of Marine Gastropods of Thailand in the Late Decade.

This study is mainly based on the collection of marine gastropods along the east coast of the Gulf of Thailand which had been carried out along the coastline in 55 sites from the province of Chonburi to Trad during April 2005 – December 2009. As many habitats as possible were examined at each sites from sandy beaches, muddy sand, rocky shore, and coral reefs. A total of 306 species of gastropods were collected and had been classifi ed in53families 116genera.The most widespread species were Planaxis sulcatus (Planaxidae) and Polinices mammilla (Naticidae) found in 37 sites, followed by Echinolittorina malaccana (Littorinidae) in 35 sites. The highest diversity was 187 species in Trat whereas Koh Mark had the most abundance in this area. The lowest diversity was in Chanthaburi, 88 species, whereas Koh Nomsoa was the most abundant site.The diversity of gastropods in Chonburi was 152 species, whereas Koh Samaesarn and Koh Juang were the most abundant site. 137 species had been found in Rayong and Koh Munnai was the most abundant site. The data from this study had been compared with the resent studies in the late decade from the west coast of the Gulf of Thailand and Andaman Sea.The total gastropods in the late decade were 454 species 205 genera 69 families. (Keywords: marine gastropods, distribution, diversity, Gulf of Thailand)

The origin of muddy sand sediments associated with mud volcanism in the Horonobe area of northern Hokkaido, Japan

The origin of muddy sand and gas in muddy sand sediments in the Horonobe area of northern Hokkaido, Japan, was investigated by analyzing the mineralogical and chemical compositions of the sediments and the chemical/isotopic compositions of the gas. X‐ray fluorescence and X‐ray diffraction analyses indicate that chemically, the muddy sand is derived from a mixing of components from the Hakobuchi and overlying formations, and that the characteristic mineral of the muddy sand is heulandite, which, in the study area, has been detected only in the Hakobuchi Formation. These results suggest that the sediments ascended from depths of at least 2200–2400 m. The δ13CCH4 values and the methane/(ethane + propane) ratios of the gas indicate that the primary origin of the methane is by thermogenic decomposition of coal‐bearing beds in the Haboro or Hakobuchi formations, or further deep sources. This study provides new data on processes of onshore mud volcanism in Japan, and contributes to an understanding of processes of subsurface mass transport in regions of mud‐volcanic activity.

한국 서해안 강화 남부 갯벌 퇴적물 및 지형의 장기적인 변화

강화 남부 갯벌의 장기적인 퇴적환경 변화를 파악하기 위하여 1997년과 2011년 여름철 표층 퇴적물의 분포와 1998년 4월과 2013년 3월의 갯벌 고도를 비교하였다. 표층퇴적물은 1997년에 동부의 펄, 서부의 모래 펄 퇴적물이 우세하였으나, 2011년에는 동부의 선두리와 동검도 갯벌은 펄과 모래 펄, 중부의 동막리 갯벌은 모래 펄, 그리고 서부의 여차리와 장화리 갯벌은 모래와 펄 모래 퇴적물이 우세하게 분포하였다. 지난 14년 동안 펄 퇴적물 면적은 감소하였으나, 모래와 펄 혼합 퇴적물의 면적은 동쪽으로 확장되었다. 갯벌의 장기적인 지형 변화는 지난 15년 동안 동막리와 여차리 갯벌은 퇴적, 장화리 갯벌은 침식된 것으로 나타났다. 이러한 변화들은 1990년대 이후 강화 남부 갯벌 주변에 여러 종류의 인공구조물건설에 의하여 지역적인 수리적 변화가 영향을 주었을 것이다. Comparisons of surface sediment distributions in summer 1997 and 2011 and elevations on the tidal flats in April 1998 and March 2013 had been used for understanding the long-term changes of sedimentary environments at southern Kanghwa tidal flat, west coast of Korea. The mud sediments dominated in the eastern part and sandy mud sediments dominated in the western part of the tidal flat in 1997. In 2011, the surface sediments were dominant mud and sandy mud at Sunduri and Tonggum in the eastern part, sandy mud at Tongmakri in the middle part, and sand and muddy sand at Yeochari and Changhwari tidal flats in the western part. The area of mud sediments had decreased, but that of sand-mud mixed sediments extended to eastward tidal flat for 14 years. The long-term topographic changes showed that deposition occurred at Tongmakri and Yeochari and erosion occurred at Changhwari tidal flat during 15 years. These changes should be effected the local hydrodynamic changes by several constructions near the tidal flat since the 1990s.

전남 남부 도서갯벌 퇴적물의 유기물 및 미량금속 오염 평가

We measured the grain size, ignition loss (IL), chemical oxygen demand (COD), acid volatile sulfide (AVS) and trace metals (Al, Fe, Cu, Pb, Zn, Cd, Cr, Mn, Hg, and As) of intertidal surface sediment collected from 11 islands (62 stations) in the southern region of Jeollanam Province. The objective of this research was to evaluate the organic matter and trace metals contaminations of sediments from coastal island tidal flats. Surface sediment texture was characterized as follows: mud, sandy silt, muddy sand, and slightly gravelly sand facies. The finer sediments are mainly dominated in the northern part of each island. The concentrations of IL, COD, AVS and some trace metals (Al, Fe, Zn, Cr, Cu, and Hg) were higher in the northwestern part of Wan Island and the area between Gogeum and Sinji Islands, and were associated with relatively finer sediment, as compared to other locations. The concentrations of Mn, Pb, Cd, and As were higher in the northwestern and southeastern parts of Geoguem and Pyungil Islands, but were not correlated with mean grain size. Based on sediment quality guidelines (SQGs), the concentrations of trace metals were lower than the values of effect range low (ERL), used in United States, and threshold effects level (TEL), used in Korea, with exception of As. Similarly, the intertidal sediments were moderately contaminated with As, based on the the enrichment factor (EF) and the geoaccumulation index (). The high concentration of As in intertidal sediments from this study region may be due to the input of naturally or artificially contaminated submarine groundwater, contaminated waste from seaweed aquaculture operations and/or land-based seaweed processing facilities. Further studies are needed to identify the sources of As in this study region, and to determine the effects of As contamination on coastal ecosystem.

Geological Responses to Urbanization of the Naples Bay Estuarine System, Southwestern Florida, USA

The Naples–Dollar Bay Estuarine System (NDBES), situated in southwestern Florida, has undergone extensive modifications caused directly and indirectly by anthropogenic influences. These alterations include: (1) the substitution of mangrove-forested shorelines with concrete bulkheads and installation of residential canals; (2) installation of a regionally extensive navigational channel; and (3) canalization of the watershed, resulting in annexation of a heavily altered drainage basin ten times the size of the pre-alteration basin and with a significantly different soil and bedrock. The NDBES consists of northern Naples Bay, southern Naples Bay, and Dollar Bay, whose shorelines range from highly developed to undeveloped, respectively. This project explored the geological response of the system to these alterations using data from side-scan sonar, sediment grab samples, and vibracores. In highly urbanized northern Naples Bay, benthic substrates consist primarily of muddy sand with few oyster reefs. Southern Naples Bay and Dollar Bay, however, consist of coarser sediment, and are characterized by extensive mangrove shorelines and numerous fringing oyster reefs. The impact of anthropogenic alterations has significantly shifted sediment distributions in northern Naples Bay from a relatively coarser to a relatively fine grained substrate; to a lesser degree in southern Naples Bay, and Dollar Bay, this transition has not taken place due to the general lack of anthropogenic modifications made to this part of the system.

전남 남부 반폐쇄적인 내만 갯벌 퇴적물의 지화학적 특성

To understand the geochemical characteristics of intertidal sediment in a semi-enclosed bay, we measured various geochemical parameters, including grain size, ignition loss (IL), chemical oxygen demand (COD), acid volatile sulfide (AVS), and trace metals (Al, Fe, Cu, Pb, Zn, Cd, Hg, and As), in intertidal sediment from three bays (Deukryang Bay, Yeoja Bay, and Gamak Bay) in the southern region of Jeollanam Province. The intertidal sediment in Deukryang Bay consisted of various sedimentary types, such as sand, gravelly muddy sand, mud, and silt, whereas the intertidal sediments in Yeoja and Gamak Bays were composed mainly of mud. The concentrations of IL, COD, AVS and trace metals in the intertidal sediments of the three study regions were relatively high near areas affected by input of stream waters and/or shellfish farming waste. The concentrations of organic matter and trace metals in Gamak Bay were much higher than those in Deukryang and Yeoja Bays, which appears to be due to the influence of anthropogenic pollutants, originating from the city and the industrial complex near Gamak Bay. The evaluation results of organic matter and metal pollution using the sediment quality guidelines showed that the intertidal sediments in the three study regions were not polluted in terms of organic matter and trace metals. In future, sustainable management for sources of organic matter and trace metal is necessary to conserve a healthy benthic ecosystem in intertidal sediments.

The performance, application and integration of various seabed classification systems suitable for mapping Posidonia oceanica (L.) Delile meadows

In the context of current global environmental changes, mapping and monitoring seagrass meadows have become highly important for management and preservation of coastal zone ecosystems. The purpose of this research was to determine the numerical precision of various cost-effective benthic habitat mapping techniques and their suitability for mapping and monitoring of Posidonia oceanica meadows in the Croatian Adriatic. We selected ultra-high resolution aerial imagery, single-beam echo sounder (SBES) seabed classification system from Quester Tangent Co. (QTC), and surface based underwater videography as affordable, non-destructive and simple to use systems for data acquisition. The ultra-high resolution digital imagery was capable of detecting P. oceanica meadows up to 4m depth with 94% accuracy, from 4m to 12.5m depth the accuracy dropped to app. 76%, and from 12.5 to 20m the system was only capable of distinguishing seabed biota from substrata, though with 97% accuracy. The results of the QTC system showed over 90% detection accuracy for Cymodocea nodosa covered seabed, excellent separation capabilities (>92%) of different sediment types (slightly gravelly sand, gravelly muddy sand and slightly gravelly muddy sand) and reasonable accuracy for mapping underwater vegetation regardless of the bathymetric span. The system proved incapable of separating P. oceanica from dense macroalgae on the same type of substratum. Surface-based underwater videography demonstrated great potential for estimating P. oceanica cover in a sampled region using either a single human rater or a computer estimate. The consistency between two human scorers in evaluating P. oceanica bottom coverage was near perfect (>98%) and high between digital and human scorers (80%). The results indicate that although the selected systems are suitable for mapping seagrasses, they all display limitations in either detection accuracy or spatial coverage, which leads to a conclusion that suitable system integration is essential for producing high quality seagrass spatial distribution maps.

Widespread overspill from a saline density‐current channel and its interaction with topography on the south‐west Black Sea shelf

AbstractSeaward of the Bosphorus Strait, the south‐west Black Sea shelf is dominated by the world's largest channel network maintained by a quasi‐continuous saline (ca 35 → 31 psu) underflow. Calculations indicate that >85% of the initial discharge of ca 104 m3s−1 spills overbank before the shelf edge. This paper documents interaction of the overspill with sea bed topography using multibeam bathymetry, echo‐sounder images of the water column, conductivity–temperature–depth profiles and sediment cores. Overbank spill is widespread, particularly through crevasse channels and on the middle shelf where confinement by channel banks is negligible. Towards the outer shelf, the wind‐driven Rim Current advects mud along the shelf, contributing to levée successions and deposition on stoss sides of elongate transverse ridges. Echo‐sounder profiles reveal metre‐scale eddies over megaflutes, and breaking lee waves and internal hydraulic jumps over ridges. Megaflutes reach 600 m long and 7 m deep, yet form where the underflow, outside the flute, is no thicker than ca 2 to 5 m. Two types of elongate seaward‐facing ridges are recognized. Type 1 ridges, 2 to 5 m high, consist of bivalve‐rich muddy sand in low‐angle (3·5° to 6°) cross‐sets created by the underflow. Type 2 ridges, ca 5 m high, have crests up to 2 km long and a buried wedge‐shaped foundation (the ‘ridge‐core’) comprised of facies similar to Type 1 ridges. These ridge‐cores are blanketed on the landward side by stratified muds, and are capped by obliquely oriented ribs supporting a diverse benthic community. This facies distribution is interpreted to result from stoss‐side and lee‐side velocity and turbulence fluctuations induced by internal hydraulic jumps and breaking lee waves in overspilling portions of the underflow. Experimental results published by W.H. Snyder and co‐workers effectively explain ridge evolution and flow across the ridges, and therefore can be applied with confidence to less easily studied deep‐marine settings swept by turbidity currents.

Temporal changes in the structure of a slope suprabenthic community from the Bay of Biscay (NE Atlantic Ocean)

The suprabenthic community of the upper slope off Arcachon (site A at about 400m depth on a muddy sand substratum) was sampled monthly from February 1991 to January 1992 with a suprabenthic sled towed over the sea bottom. The fauna collected in the 0–50cm water layer above the bottom was classified into 9 major groups and 109 species (56 amphipods, 12 mysids, 10 isopods, 10 decapods, 9 cumaceans, 6 euphausiids, 4 fishes, 1 lophogastrid and 1 tanaid). The total abundance of the community fluctuated between a maximum of 3199 ind./100m2 in July and a minimum of 82 ind./100m2 in November, with an annual mean value of 969±601 ind./100m2. The community structure was mainly affected by the temporal abundance fluctuations of the asellote isopod Munnopsurus atlanticus. This species was numerically dominant during the first part of the year and showed a drastic decrease in August, followed by the dominance of the mysids Erythrops neapolitana or Parapseudomma calloplura in autumn and early winter. Such structural changes in the dominance of major taxa are discussed with respect to the feeding behaviour of species and food availability in the near-bottom environment. We conclude that the population dynamics of M. atlanticus in the upper bathyal was mainly governed by the seasonal development of its major prey, the benthic foraminifers, favoured by the spring phytodetritus deposition on the sea floor.