Benthic boundary layer processes in coastal environments: An introduction

This special issue ofGeo-Marine Letters Benthic Boundary Layer Processes in Coastal Environments includes 20 papers devoted to results of recent near-shore experiments supported by the Coastal Benthic Boundary Layer (CBBL) program. Experiments were conducted in gas-rich muddy sediments of Eckernförde Bay of the Baltic Sea and on relict sandy sediments of the West Florida Sand Sheet. In this introductory paper we present scientific justification for experiments and summarize preliminary results.     

Manoeuvring prediction of pusher barge in deep and shallow water

This paper presents an experimental investigation on the manoeuvring characteristics of a pusher-barge system for deep (H/d>3) and shallow water (H/d=1.3) condition. Since, the operation of pusher-barge mainly concentrates on confined waters, there is a need to predict and analyze the manoeuvring characteristic of the system for a safe and acceptable performance. A time domain simulation programme was developed for this purpose. A series of model experiments were carried out to determine the hydrodynamic coefficients using a planar motion mechanism (PMM). The time domain simulation shows the manoeuvring characteristic in the form of turning circle trajectories and zig-zag manoeuvre based on the hydrodynamic coefficients, which were derived based on experimental results. The manoeuvring characteristics in shallow and deep water conditions were compared through the simulation results. A comparison of simulation results based on experimental and empirical driven coefficients for both conditions shows that the experimental coefficients gave better manoeuvring characteristics for both turning circle trajectories and zig-zag manoeuvre.     

Nonlinear saturation-based whitecapping dissipation in SWAN for deep and shallow water

This study investigates the effectiveness of a revised whitecapping source term in the spectral wind wave model SWAN (Simulating WAves Nearshore) that is local in frequency space, nonlinear with respect to the variance density and weakly dependent on the wave age. It is investigated whether this alternative whitecapping expression is able to correct the tendency towards underprediction of period measures that has been identified in the default SWAN model. This whitecapping expression is combined with an alternative wind input source term that is more accurate for young waves than the default expression. The shallow water source terms of bottom friction, depth-induced breaking and triad interaction are left unaltered. It is demonstrated that this alternative source term combination yields improved agreement with fetch- and depth-limited growth curves. Moreover, it is shown, by means of a field case over a shelf sea, that the investigated model corrects the erroneous overprediction of wind-sea energy displayed by the default model under combined swell-sea conditions. For a selection of field cases recorded at two shallow lakes, the investigated model generally improves the agreement with observed spectra and integral parameters. The improvement is most notable in the prediction of period measures.     

Fine-grained sediments in deep water: An overview of processes and facies models

There appears to be a continuum of processes affecting the transport and deposition of fine-grained sediments in the deep sea. This results in a facies continuum within which we can recognize three broadly different facies groups: turbidites, contourites, and pelagites/hemipelagites. Several distinct facies models can be defined for each group on the basis of their chief structural, textural, and compositional attributes.     

Effects of fine grain size on distribution of Mn in shallow and deep water Black Sea sediments: A comparison between oxic and anoxic depositional environments

The uppermost 5–6 cm of the sediments (between 8 and 2248 m water depths) were studied to understand the effects of varying redox conditions on the Mn distribution in the recent sediments of the Black Sea. It was found that most Mn concentrations are consistent with the average abundance in crustal and/or sedimentary rocks. There exist no important differences between Mn concentrations in oxic (shallower water; <70 m) and anoxic (deeper water; 120 m) sediments. Previously reported Mn-enrichment above the Black Sea oxic/anoxic interface, due to the peculiar redox cycling, shows no significant contribution of Mn to the bottom sediments. A marked relationship between total Mn concentrations and clay/mud contents at shelf depth along the southern Black Sea margin indicates increased accumulation of Mn in association with the fine-grained particles and eastward water circulation.     

The Black Sea basins structure and history: New model based on new deep penetration regional seismic data. Part 1: Basins structure and fill

This work is based upon results of interpretation of about 8872 km-long regional seismic lines acquired in 2011 within the international project Geology Without Limits in the Black Sea. The seismic lines cover nearly the entire Black Sea Basins, including Russia, Turkey, Ukraine, Romania and Bulgaria sectors. A new map of acoustic basement relief and a new tectonic structure scheme are constructed for the Black Sea Basins. The basement of the Black Sea includes areas with oceanic crust and areas with highly rifted continental crust. A chain of buried seamounts, which were interpreted as submarine volcanoes of Late Cretaceous (Santonian to Campanian) age, has been identified to the north of the Turkish coast. On the Shatsky Ridge, probable volcanoes of Albian age have also been recognized. Synorogenic turbidite sequences of Paleocene, Eocene and Oligocene ages have been mapped. In the Cenozoic, numerous compressional and transpressional structures were formed in different parts of the Black Sea Basin. During the Pleistocene–Quaternary, turbidites, mass-transport deposits and leveed channels were formed in the distal part of the Danube Delta.     

Sedimentary and water column processes in the Oyster Grounds: a potentially hypoxic region of the North Sea

The purpose of this research was to investigate the potential causes of low oxygen levels in the bottom water of the Oyster Grounds region of the shallow southern North Sea, an area which provides suitable conditions for low oxygen levels to develop. At the end of the summer stratified period, relevant biogeochemical processes were investigated using a combination of sedimentary and water column rate measurements. Phytoplankton nitrate and ammonium uptake was measured throughout the water column using (15)N labelled isotopes and showed ammonium uptake dominated in the upper and bottom mixed layer with a maximum 294.4 micromol N m(-3)h(-1). In the deep chlorophyll maximum at the thermocline, primary production was dominated by nitrate uptake, with an average of 35.0 micromol N m(-3)h(-1), relative to ammonium uptake, with an average of 24.6 micromol N m(-3)h(-1). This high relative nitrate uptake will in part result in exportable new production to the isolated bottom mixed layer and sediments, as opposed to regenerated ammonium driven uptake. This biomass export was indicated by significant benthic oxygen consumption rates in the stratified region (782-1275 micromol O(2)m(-2)h(-1)micromol N m(-3)h(-1)) long after the end of the spring bloom. The sediments were also an active net source of nitrate, ammonium, phosphate and silicate into the bottom mixed layer of 4.4, 8.4, 2.3 and 68.8 micromol m(-2)h(-1), respectively. The export of new production within the thermocline to the bottom mixed layer and the consequent sediment oxygen consumption in the isolated bottom mixed layer in the Oyster Grounds are expected to have contributed to the low bottom water oxygen concentrations of 2.07 mg l(-1) (64.7 micromol l(-1)) measured. The long stratified period associated with this low oxygen is predicted to occur more regularly in the future and continued monitoring of this ecologically important region is therefore essential if the causes of these potentially damaging low oxygen levels are to be fully understood.     

南海深地震探测的重要科学进程:回顾和展望

南海是西太平洋最大的边缘海之一,研究其深部地壳的结构对深入认识南海共轭边缘的构造属性、深海盆形成演化历史、含油气盆地的形成机制均具有十分重要的科学意义。南海地壳结构的深地震探测从构造区域上可分为南海北部陆缘、南海南部陆缘、南海中部深海盆等几个海域,在探测技术上经历了声纳浮标、双船扩展剖面(expanding spread profile,ESP)、海底地震仪(ocean bottom seismometer,OBS)探测3个阶段。特别是近20年OBS探测蓬勃发展,从南海北部、发展到南海南部、再到南海中部,从二维直线探测到三维网格探测。这些探测和研究得到了宝贵的深部地壳结构信息,为南海的形成演化理论提供了重要依据,同时也推动了国产OBS的应用和人才队伍的培养。而最新完成的深海盆三维OBS探测标志了一个新的历史阶段,具有非常深远的科学意义。     

Observations and measurements of wave-induced drift of surface inextensible film in deep and shallow waters

The present study investigates the drift of two-dimensional floating surface films by both deep and shallow water waves in the laboratory. The focus is on the observations and measurements on the drift behavior and to clarify some outstanding issues. Thin polyethylene sheets with reflective markers were attached to simulate the inextensible surface films. Upon the initiation of a wave train, two infrared cameras recorded continuously the instantaneous position of the markers at 50 Hz. The temporal variation of the drift velocity was then determined by processing the recorded data. In all experiments, the drift velocity increased quickly in the beginning and reached a quasi-steady mean value. The magnitude of the surface drift typically increased with the longitudinal length of the polyethylene sheet until the sheet length was approximately the same as the wavelength. Lengthening the sheet length further did not lead to significant changes in the drift velocity. Under the shallow water condition, the wave-induced drift velocity increased linearly with the Ursell number and with a lower water depth.     

Sedimentary processes and sediment dispersal in the southern Strait of Georgia,BC, Canada

This paper presents a review of sediment dispersal processes in the Strait of Georgia, based on marine geological studies. Sediment from the Fraser River is dispersed around the Strait through a variety of transport pathways. Most sand and coarser silt fractions settle out and are deposited within a few 100 m of the channel mouths. Both channelled and non-channelled gravity flows probably transport sediment downslope and onto the basin floor. Asymmetric tidal currents force a predominantly northward sediment drift, resulting in a reworked slope off Roberts Bank and a finer-grained depositional slope off Sturgeon Bank. Far-field sediment accumulation is controlled by local morphology and sediment dynamics. Multibeam mapping and seismic profiling reveal that some parts of the basin floor are characterized by bottom sediment reworking and erosion. Given the complexities of sediment dispersal and seafloor reworking, generalizations about sediment dispersal paths and sedimentation rates are difficult. Future understanding will be advanced by the cabled observatory, VENUS, which will enable near real-time monitoring of key processes.     

Sedimentary processes in the Stromboli Canyon and Marsili Basin, SE Tyrrhenian Sea: results from side-scan sonar surveys

 Sedimentary processes in the Stromboli Canyon and in the Marsili Basin are studied on the basis of side-scan sonographs. The basin margins are characterized by slump scars, gullies, channels, and large debrites on the Calabrian slope and by straight chutes of fast downslope sediment transport and blocky–hummocky avalanche deposits on the flanks of the Stromboli volcano. In the Stromboli Canyon and in minor deep-sea channels, sediment transport by turbidity currents generates sediment waves. Between the basin margins and the abyssal plain, the outcropping volcanic basement traps part of the sediment coming from the marginal areas. The abyssal plain is characterized by low relief lobes and ponded sediments.     

Wave loads on rubble mound breakwater crown walls in deep and shallow water wave conditions

The semi-empirical formulae by Pedersen (1996) for wave loads on vertical front faces of stiff crown walls are based on model tests with deep and intermediate water wave conditions. A new series of model tests performed at the same test facility as used by Pedersen has revealed that the formulae by Pedersen overpredict the loads in shallow water wave conditions. This paper presents a modification/expansion of the formulae to cover loads in both deep and shallow water wave conditions. The modification is based on a series of 162 physical model tests on typical rubble mound breakwaters with crown wall superstructures. The implementation of shallow water wave conditions in the formulae is done by modifying the term for wave run-up to be dependent on the incident wave height distribution. Moreover, the adjusted formulae provide more accurate estimates of the wave loads on free walls without front armour protection. Pressure transducers with very high eigen-frequencies were used in the present model tests as opposed to the transducers applied by Pedersen which in some cases seem to have been affected by dynamic amplifications.     

The link between tectonics and sedimentation in asymmetric extensional basins: Inferences from the study of the Sarajevo-Zenica Basin

The coupled tectonic and depositional history of extensional basins is usually described in terms of stratigraphic sequences linked with the activity of normal faults. This depositional-kinematic interplay is less understood in basins bounded by major extensional detachments or normal fault systems associated with significant exhumation of footwalls. Of particular interest is the link between tectonics and sedimentation during the migration of normal faulting in time and space across the basin. One area where such coupled depositional-kinematic history can be optimally studied is the Late Oligocene - Miocene Sarajevo-Zenica Basin, located in the Dinarides Mountains of Bosnia and Herzegovina. This intra-montane basin recorded Oligocene – Pliocene sedimentation in an endemic and isolated lake environment. We use field kinematic and sedimentological mapping in outcrops correlated with existing local and regional studies to derive a high-resolution evolutionary model of the basin. The novel results demonstrate a close correlation between moments of normal faulting and high-order sedimentological cycles, while the overall extensional basin was filled by a largely uni-directional sediment supply from the neighbouring mountain chain. The migration in time and space of listric NE-dipping normal faults was associated with a gradual shift of the sedimentological environment. Transgressive-regressive cycles reflect sequential displacements on normal faults and their footwall exhumation, defining a new sedimentological model for such basins. This Early - Middle Miocene extension affected the central part of the Dinarides and was associated with the larger opening of the neighbouring Pannonian Basin. The extension was preceded and followed by two phases of contraction. The Oligocene - Early Miocene thrusting took place during the final stages of the Dinarides collision, while the post-Middle Miocene contraction is correlated with the regional indentation of the Adriatic continental unit. This latter phase inverted the extensional basin by reactivating the inherited basal listric detachment.