Chenier plain development in northern Sierra Leone, West Africa

The coast of northern Sierra Leone, between the Scarcies and Mellacoree estuaries, forms a prograding muddy Holocene plain comprising two separate, as yet undated, clusters of sand cheniers. The general coastal morpho-stratigraphy and available data on regional climatic and sealevel history suggest that alternations between muddy progradation and chenier cluster formation have been controlled by a probable conjunction of several factors, including the local geomorphic framework, a climatically induced change in mud supply, and relative sealevel oscillations since the middle Holocene. The inner chenier cluster was apparently elaborated under conditions of efficient wave action over a deep, largely unfilled Mellacoree estuary and at a time of reduced mud availability. It peters out seaward, probably in response to several factors, including an increase in mud supply from the upland, a negative sealevel pulse resulting in higher rates of mud export from accreted inner tidal flats, increased muddy sedimentation over a shallower nearshore zone, and attenuated wave action as a result of energy capture by accreted estuarine shoals. The outer cluster, which partly fringes the coast, was formed in a mayor embayment representing the terminus of longshore sand drift. It is suggested that its formation may have been favoured by a higher sealevel resulting in efficient winnowing of sands from subtidal muds, a greater propensity for accretion of tidal flats landwards and, consequently, a lower mud supply to the foreshore.

Although the timing of muddy progradation and chenier formation has not been determined, the foregoing interpretation suggests a complex sequence of events, reflecting as much the influence of local morphodynamic factors as external factors.     

Fisheries and failing states: The case of Sierra Leone

Marine fisheries play an important role in the economy of Sierra Leone, supporting livelihoods and contributing significantly to food security. This paper looks in detail at how the performance of fisheries was impacted by the ten year civil war, an event which contributed to the country's reputation for being a “failed state”. The paper focuses mainly on the artisanal fisheries sector, which employs the majority of the country's coastal population, and demonstrates how the conflict caused major social dislocation to fishing communities as well as reducing the productive capacity of the fleet. The paper concludes with a discussion of the policy challenges now facing Sierra Leone, particularly the prevention of resource looting through illegal fishing of the offshore stocks and the development of strategies to enable the potential wealth of these fisheries to be captured.     

西菲律宾海比科尔陆架深海沉积物古菌多样性研究

通过构建16S rRNA 基因文库, 对西太平洋西菲律宾海东板比科尔陆架5 个不同层位沉积物样品中的古菌的多样性进行了研究, 并获得了465 个有效克隆63 个OTUs （Operational Taxonomic Units）。通过16S rRNA 序列与GenBank 已知序列的同源性比较及构建系统进化树的结果显示...     

Tectonically induced deformation of deep-sea sediments off Washington and northern Oregon: Mechanical consolidation

Convergent motion of the North American and Juan de Fuca plates has resulted in deformation of Cascadia Basin sediments and accretion of these deposits to the North American continental margin. The accreted deposits, which occur as anticlinal ridges and thrust blocks, constitute the lower continental slope or borderland off Washington and northern Oregon. Over the past 2.0 m.y., approximately 30 km of this deformed material has been added to the lower slope, removing undeformed deposits from Cascadia Basin at a rate of 2.3–2.9 cm/yr.Near-surface sediments involved in this accretionary process are mechanically consolidated: muds?tones dredged from the lower slope exhibit physical properties (water contents, 20–47%; void ratios, 0.4–1.2; preconsolidation pressures, 0.8–8.2 MPa) which differ significantly from properties of similar, but undeformed sediments (water contents, 50–250%; void ratios, 1.1–1.9). While some consolidation may be attributable to prior burial ($\text{< 2.0}\text{MPa}$) or carbonate precipitation, neither mechanism can wholly account for the values observed. It appears that most of the consolidation has occurred in response to tectonically induced overpressures.Initial consolidation occurs rapidly across a narrow ($\text{< 3}\text{km}$) front, defined by the base of the continental slope. Further consolidation and dewatering appears to take place, at a much reduced rate, over the entire width of the lower slope.Development of foliation is nearly ubiquitous in the deformed mudstones. This property limits the strength of the deposits (shear strengths, 90–416 kPa) and movement along these planes probably accommodates much of the strain after initial consolidation.The physical properties characteristic of Washington—Oregon deformed sediments may represent limiting values for mechanical consolidation of near-surface terrigenous sediments under horizontal stress.     

Chlorophyll distribution in continental shelf sediments off West Palm Beach,Florida and west end,Bahamas

Measurements were made of chlorophyll-a and phaeophytin-a in calcareous sediments along transects off the east coast of Florida (75–190 m) and the west coast of Grand Bahama Island (170–300 m). Solvent partitioning showed that chlorophyll-a concentrations never exceeded 0·1 mg m^?2 at either location, most as degradation products. Total pigment concentrations (chlorophyll and phaeopigments) ranged from 0·18–1·83 mg m^?2 in sediments off Grand Bahama Island and 2·50 to 20·65 mg m^?2 off West Palm Beach. Pigments, expressed per gram dry weight of sediments, increased with depth across the Florida Continental Shelf. This is probably due to differences in sediment character between near-shore and off-shore sediments.     

Particle transport from the West African shelves of Liberia and Sierra Leone to the deep sea: A chemical approach

The chemical and mineralogical composition of suspended particle suites has been used to assess off-shelf transport on the West African shelves of Liberia and Sierra Leone. Using the ratios of Si/Al, Fe/Al, Mg/Al and Mn/Al as tracers, it was possible to detect shelf-derived materials in slope waters. In the majority of cases, these inputs could not have been detected using particle mass or light scattering measurements and could only be measured by using the chemical signatures of the particulate matter. At the time of sampling, the suspensate which had been moved seaward over the slope was detected adjacent to submarine canyons and highly turbid areas on the outer shelf. The chemical and mineralogical composition of the suspensates, and sediments in the adjacent eastern Atlantic basin are similar to those found in the water column seaward of the West African continental shelves and yet distinct from the Sahelian dust which is considered the major source of sedimentary material for the tropical and semi-tropical areas of the deep eastern basin. These findings suggest that materials from this shelf area could be a more important input to the deep sea than was previously realised.     

西非深水浮筒和穿梭油轮水动力性能分析

应用三维势流理论及非线性时域耦合分析方法,在西非双方向涌浪和波浪条件下对深水浮筒及穿梭油轮水动力性能进行了数值计算,并与水池模型试验对比。对单浮筒在作业、生存海况下的运动响应进行了准确预报,同时对浮筒和穿梭油轮连接原油外输作业时的波频运动、平面运动给出了合理分析;得到原油外输装置在西非深水海域环境下的水动力响应特性,为我国参与西非深水海洋油气开采提供技术建议。     

Wintering seabirds in West Africa: foraging hotspots off Western Sahara and Mauritania driven by upwelling and fisheries

Cold-water upwelling supports abundant and diverse faunas. Upwelling off Mauritania has been highlighted as being important for seabirds, but very few systematic offshore surveys have been conducted in that region. Mauritanian waters are increasingly targeted by commercial fisheries along the shelf break and, with the likelihood of future exploitation of mineral oils on the continental shelf, the absence of information on seabirds is worrying. This paper describes the distribution of wintering seabirds in the context of fisheries and hydrography. The avifauna was dominated by surface-feeding and shallow plunge-diving, often planktivorous, seabirds, originating from West Palaearctic breeding grounds (Arctic, subarctic and temperate zones). Many seabirds were associated with fishing trawlers around the shelf break, but the fleets were more evenly distributed than the birds, with certain hydrographical parameters influencing or overruling the attraction of trawlers by seabirds. The results support the belief that West African waters are of prime importance for seabirds.     

Surficial sediments of the wave-dominated Orange River Delta and the adjacent continental margin off south-western Africa

The textural and compositional characteristics of the surficial shelf sediments north and south of the Orange River Delta are reviewed and compared. Sediments are fractionated and dispersed both north- and southwards of the Orange River mouth by wave action, longshore drift and subsurface currents. The mean grain size decreases both offshore and southwards in response to decreased wave influence at the seabed and the competence of the weak poleward undercurrent respectively. The increasing dominance of marine biogenic components in sediments south of the prodelta indicates a greater marine influence, modifying previous inferences that the Namaqualand mudbelt is primarily derived from the southward transport of Orange River sediments. A sharp distinction can be drawn between sediments of the Orange Shelf to the south and the Walvis Shelf to the north. Foraminiferarich deposits that dominate the Orange middle shelf and slope indicate that upwelling is an inner-shelf phenomenon. On the Walvis Shelf, foraminiferal sediments are confined to the slope and outer shelf. Fish debris is more common in Walvis Shelf sediments. Although phosphorite and glauconite sands often occur together in the same deposits on the Orange Shelf, the two minerals are concentrated in separate deposits on the Walvis Shelf.     

On the formation of laminated sediments on the continental margin off Pakistan: the effects of sediment provenance and sediment redistribution

The sedimentary processes and sediment sources contributing to the formation of laminated sediments along the upper slope off Pakistan are unravelled using inorganic bulk sediment geochemistry of 43 surface cores from the Pakistani continental margin and additional geochemical and Pb and Nd-isotope data for different types of layers. An important process everywhere along the margin is redeposition of fluvial-derived detritus from the shelf onto the slope. This process is of considerably higher intensity along the Makran margin than on the Indus margin. Trace element enrichment related to early diagenesis or surface productivity, which is commonly detectable in bulk sediment composition, is swamped by the high clastic supply in the Makran region, but may be observed in the Indus region.Four types of layers are found in the laminated sediment cores from the upper slope. They reflect different mechanisms of deposition and different sediment sources. An alternating pattern of olive-grey and black layers results from downslope redeposition of fluvial material over most of the year, to which organic matter from sea surface production is added during the late summer monsoon season. Distinctive white to grey coloured layers along the Makran slope originate from large scale expulsion of sediments from the Makran accretionary wedge through mud volcanoes on the shelf, subsequent erosion by waves, and downslope redeposition. These layers may dominate the sedimentary record within the Makran accretionary wedge, but are absent on the Indus margin. Occasional red coloured turbidites, which probably represent larger floods on the Indus plain, contribute to this mixture of varying sedimentary processes and sediment sources along the Pakistani continental slope.     

Back-calculation and time of hyaline ring formation in the otoliths of the pilchard off South West Africa

The frequency of hyaline and opaque edges, back-calculation, and examination of otoliths bearing a natural tag were used to determine the time of ring formation for the pilchard off South West Africa. The extent of the error in the back-calculation method caused by seasonal variation in the relationship between otolith radius and fish length is assessed. The times at which hyaline rings were laid down between 1970 and 1981 are documented. Formation of hyaline rings is related to sea surface temperature (p < 0,001), prominent rings forming when the temperature is colder than the annual average. Generally, a prominent hyaline ring is formed in September and secondary rings in May and December.     

Physical and elastic properties of marine sediments off Bombay,India

Abstract

Keeping in view the paucity of information as to the nature of the marine sediments from the continental shelf adjoining the Indian subcontinent, a number of shallow seismic surveys were carried out, nearshore and offshore Bombay between 18°45'N and 21°00'N. Representative core samples preserving their natural state were also retrieved from the region in the water depths ranging from 5 to 70 m for the determination of physical properties in the laboratory. Data on the physical, acoustic, and elastic properties of the sediment cores are reported for the first time. Useful individual least‐squares relations are presented for acoustic impedance, reflection coefficient, and bulk modulus against density; for the dependence of rigidity and bulk moduli on the constrained modulus; for the association between impedance and field sediment velocity against P‐velocity; and for rigidity against Poisson's ratio. Results indicate that the bulk modulus and Young's modulus are higher for silty clay and clayey silt samples than for the clay samples. Similarly, the acoustic impedance, reflection coefficient, and constrained modulus of silty clay and clayey silt are also higher than clay. The results are found to be comparable to the North Atlantic and Bay of Bengal sediments.     

Distribution and nature of organic matter in recent sediments of Lake Nokoué, Benin (West Africa)

The distribution and nature of organic matter clearly identify the principle sectors of the lagoon which vary according to hydrological and sedimentological conditions. The central area frequently includes coarse ancient fluviatile sediments. The rare organic matter there offers aliphatic and nitrogenous characters which may be derived from both the origin of organic matter (algae and meiofauna) and the level of oxygenation prevailing during deposition. In the southern sector, highly oxygenated conditions resulting from sea currents generated by the Cotonou outlet could cause the elimination of labile compounds, and hence be responsible for the high degradation state of organic matter. In the western sector, still water conditions allow fine sediments to be deposited. This frequently leads to the accumulation of organic matter showing such reducing conditions that they allow the integration of sulphur in humic compounds and a decrease of their solubility. In the northern area, the contribution of terrestrial material supplied by the Ouémé and Sô rivers is clearly indicated by the low average percentages of hydrolyzable fraction, C/N ratios ranging between 9 and 15, while H/C and N/C ratios are low in humic acids.     

Distribution,provenance, and dispersal pattern of clay minerals in surface sediments,Bransfield Strait,Antarctica

Clay minerals of the surface sediments of Bransfield Strait, Antarctica, exhibit distinctive geographical distributions: kaolinite has the highest concentration near the shore of the South Shetland Islands in the northern strait (20%); chlorite, near Smith Island in the northwestern strait; illite, on the continental shelf off the Antarctic Peninsula in the southern strait (80%); and smectite, close to the Penguin and Bridgeman islands in the northeastern strait (25%). This distribution pattern, combined with hydrographic and climatic data for the strait, are used to infer clay mineral provenance and dispersal patterns.     

菲律宾海深海海底沉积物声学特性与物理性质相关关系

为研究深海海底沉积物声学特性与物理性质相关关系,于2016年11月在实验室对水深3164～5 592 m的菲律宾海深海海底沉积物柱状样品的声学特性进行测量,获取了沉积物声速、声速比、声阻抗、声阻抗指数等声学特性参数。结合沉积物的孔隙度和密度等物理性质参数,分析了海底沉积物声速、声速比、声阻抗、声阻抗指数与孔隙度、密度的相关关系,建立了该海域海底沉积物声学特性回归方程。研究结果表明:研究区深海数据与浅海回归方程符合度较差,与深海回归方程符合度较好;Hamilton校正方法有助于修正实验室测量引起的温度和压力误差,声速比与Hamilton方程符合度比声速好;声阻抗和声阻抗指数与物理性质参数的相关性优于声速和声速比。此外,研究认为由于海底沉积物的沉积环境较为复杂,其声学特性回归方程存在差异。由于上述差异的存在,在使用基于不同海域数据建立的回归方程进行海底沉积物声学特性预测时,应加以区别对待。该研究丰富了深海海底沉积物声学数据,对促进深海海底沉积物声学深入研究具有一定的借鉴意义。     

Marine ecosystem variability and human community responses: The example of Ghana,West Africa

This study describes variability in the marine ecosystem of Ghana, West Africa, on several temporal and spatial scales and discusses how the human communities using this ecosystem respond to this variability to cope socially and economically. Ghanaian marine waters are part of an upwelling system with strong seasonal and inter-annual variability. Much of this variability is forced at large spatial scales in the tropical Atlantic and by El Niño—Southern Oscillation events in the Pacific Ocean, which influence inter-annual variability of sea surface temperature and pelagic fish landings off Ghana. At decadal scales, Ghanaian marine waters experienced cool sea temperatures and low fishery landings during the 1960s, rapid warming and increases in fishery landings during the late 1970s and 1980s, and variable temperatures and fishery landings during the 1990s. In the late 1990s, pelagic and demersal fish populations appeared to be declining, partly due to over-fishing, although the per capita supply (domestic production plus net imports) of fish was kept high by increased imports. Artisanal fishers and fishing communities in Ghana have devised strategies to deal with variability on seasonal and inter-annual scales. These livelihood strategies include: (i) exploiting marine and terrestrial natural resources more intensively, initially at local scales but expanding to regional scales; (ii) ensuring multiple and diversified income sources; (iii) investing in social relationships and communities for support; and (iv) undertaking seasonal or permanent migrations. In addition, the national government imports fish to deal with shortages. However, these strategies may be less adapted to variability at decadal scales, and may not be sustainable when viewed at the larger scales of environmental change.     

Mineral provinces and matter provenance of the surficial sediments in the western Philippine Sea: implication for modern sedimentation in West Pacific marginal basin

The characteristics and distribution patterns of detrital minerals (0.063~0.125 mm) in marine sediments provide a significant indicator for the identification of the origin of sediment.The detrital mineral composition of 219 surface sediment samples was analysed to identify the distribution of sediments within the western Philippine Sea. The area can be divided into three mineral provinces: (I) province east of the Philippine Trench, the detrital minerals in this province are mainly composed of calcareous or siliceous organisms, with the addition of volcanogenic minerals from an adjacent island arc; (II) middle mineral province, clastic minerals including feldspar, quartz and colorless volcanic glass, sourced from seamounts with intermediate-acid volcanic rock, or erupting intermediate-acid volcano; (III) province west of the Palau—Kyūshū Ridge, the matter provenance within this province is complex; the small quantity of feldspar and quartz may be sourced from seamounts or erupting volcano with intermediate-acid composition, with a component of volcanic scoria sourced from a volcano erupting on the Palau—Kyūshū Ridge. it is suggested that, (1) Biogenic debris of the study area is closely related to water depth, with the amount of biogenic debris controlled by carbonate lysocline. (2) Volcaniclastic matter derived from the adjacent island arc can be entrained by oceanic currents and transported towards the abyssal basin over a short distance. The weathering products of volcanic rocks of the submarine plateau (e.g.,Benham Plateau) and adjacent ridges provide an important source of detrital sedimentation, and the influence scope of them is constrained by the intensity of submarine weathering. (3) Terrigenous sediments from the continent of Asia and the adjacent Philippine island arc have little influence on the sedimentation of this study area, and the felsic mineral component is probably sourced from volcanic seamounts of intermediate-acid composition.     

The nature and maximum extent of glacial sediments off the west coast of Wales

Offshore drilling and coring together with high-resolution seismic surveys have established that the southern Irish Sea was extensively glaciated during the Devensian stage. Ice flowed southwards from Scotland and overran the western tip of the Lleyn peninsula, before competing with local Welsh ice in Cardigan Bay. It eventually terminated its advance at the southern entrance to St. Georges Channel. To date no glacial sediments have been found on the sea floor to the south of this limit.During deglaciation, the shallower regions of the Irish Sea area underwent a short but intensive period of erosion which produced numerous meltwater channels. These subsequently acted as traps for the deposition of estuarine sediments associated with the early stages of the Flandrian Sea Transgression. The reworking of glacial drifts by marine action occurred primarily during the passage of the Flandrian surf zone and in most areas has now ceased.     

Seasonal variations of the deep-sea macrobenthos communities in the coastal and bathyal zones off sanriku,northeastern Japan

Seasonal variations of macrobenthos communities were analyzed over six years at two fixed stations at the depths of 500 m and 1,000 m in the bathyal zone off Sanriku, northeastern Japan, and were compared with those of the 80 m station established at the mouth of Otsuchi Bay, northeastern Japan. Significant seasonal variations of macrobenthos density were detected at the 80 m and 500 m stations. While the density increased in late spring and decreased until July at the 80 m station, it increased between May and August, and decreased in September at the 500 m station. At the 1,000 m station, no seasonal variation of macrobenthos abundance could be detected. The seasonal variations of the density observed at the 80 m and 500 m stations are probably related to that of the supply of organic materials derived from surface phytoplankton. Differences in the patterns of seasonal variations among the three stations may reflect the different periods of food supply to the sea floor. Significant seasonal variation in the feeding structure of the polychaete communities could be detected at the 80 m and 1,000 m stations.