Spatial heterogeneity of acoustic bottom backscattering model parameters and predicted results

Depositional patterns of sand ridges in tide-dominated shallow water environments: Yellow Sea Coast and South Sea of Korea

Sediment transport and deposition in flood-tidal deltas: Insight from the Texas Coast

The eastern Yellow Sea is characterized by a number of tidal sand ridges which occur as a series of linear sediment bodies in the shelf (shelf sand ridges) and as a group of individual sand bodies in the nearshore (nearshore sand ridges). The shelf sand ridges are present in water depths of 50–90 m and show large, elongate shapes with a length up to 200 km. In contrast, the nearshore sand ridges are much smaller in size (up to 34 km length) and occur in water depths shallower than about 30 m. Detailed interpretation of seismic and lithologic data, using radiocarbon dating to constrain the ages of the ridges, has shown that the regional sea-level changes played a major role in the existence and development of different morphologic features of these sand ridges. The shelf sand ridges developed mainly during the postglacial transgression (ca. 14,000–9,500 yr B.P.), possibly during episodes of stillstand or very slow rise of sea level. The shape of the shelf sand ridges more or less results from an erosional process dominantly acting during the postglacial transgression. The substratum of the shelf sand ridges consists of the regressive or lowstand deltaic mud deposits, probably formed prior to the last glacial maximum (> ca. 17,000 yr B.P.). In contrast, the nearshore sand ridges have undergone tidal action during the recent highstand of sea level (ca. < 7,000 yr B.P.) and show a typical modern geomorphology of erosional sand ridges. The substratum of the nearshore sand ridges consists of remnants of the last interglacial tidal deposits. Large dunes indicate a strong hydrodynamic influence on the entire ridge surface at present.

Seafloor response to flow in a southern hemisphere sand-ridge field: Argentine inner shelf

Lateral collapses and active sedimentary processes on the northwestern flank of Stromboli volcano, Italy

Sediment dynamics over sand ridges on a tideless mid-outer continental shelf

Sediment and associated macrofauna of an industrial extraction site off Dieppe have been monitored during a 10-year period. The original heterogeneous substrate of the shingle bank, characterized by gravels and coarse sands, was progressively dominated by fine sands deposited in dredging tracks. The maximum impact on benthic macrofauna was a reduction by 80% for species richness and 90% for both abundance and biomass. The structure of the community changed from one of coarse sands with Branchiostoma lanceolatum to one of fine sands with Ophelia borealis, Nephtys cirrosa, and Spiophanes bombyx, with local dominance of the opportunistic, sessile Pomatoceros triqueter on bare shingles. Impact of overflowing sands on benthic macrofauna in the surrounding deposition area proved equally large as in the dredged area. Early stages of recolonization were studied from 1995 to 1997 after cessation of dredging. Species richness has been fully restored after 16 months, while densities and biomass were still 40% and 25%, respectively, lower than in reference stations after 28 months. Nevertheless, community structure differed from the initial one corresponding to the new type of sediment. Impact within and around the dredging site was classified according to three levels. Exploration of a former experimental site (CNEXO) dredged in the 1970s provided an example of long-term restoration.

Understanding the dynamics and fate of methane (CH4) release from oceanic seepages on margins and shelves into the water column, and quantifying the budget of its total discharge at different spatial and temporal scales, currently represents a major scientific undertaking. Previous works on the fate of methane escaping from the seafloor underlined the challenge in both, estimating its concentration distribution and identifying gradients. In April 2019, the Envri Methane Cruise has been conducted onboard the R/V Mare Nigrum in the Western Black Sea to investigate two shallow methane seep sites at ∼120 m and ∼55 m water depth. Dissolved CH4 measurements were conducted with two continuous in-situ sensors: a membrane inlet laser spectrometer (MILS) and a commercial methane sensor (METS) from Franatech GmbH. Additionally, discrete water samples were collected from CTD-Rosette deployment and standard laboratory methane analysis was performed by gas chromatography coupled with either purge-and-trap or headspace techniques. The resulting vertical profiles (from both in situ and discrete water sample measurements) of dissolved methane concentration follow an expected exponential dissolution function at both sites. At the deeper site, high dissolved methane concentrations are detected up to ∼45 m from the seabed, while at the sea surface dissolved methane was in equilibrium with the atmospheric concentration. At the shallower site, sea surface CH4 concentrations were four times higher than the expected equilibrium value. Our results seem to support that methane may be transferred from the sea to the atmosphere, depending on local water depths. In accordance with previous studies, the shallower the water, the more likely is a sea-to-atmosphere transport of methane. High spatial resolution surface data also support this hypothesis. Well localized methane enriched waters were found near the surface at both sites, but their locations appear to be decoupled with the ones of the seafloor seepages. This highlights the need of better understanding the processes responsible for the transport and transformation of the dissolved methane in the water column, especially in stratified water masses like in the Black Sea.

A significant portion of a proposed Exxon pipeline gathering system offshore Mobile Bay, Alabama would, be constructed in sandy soils. If the pipeline trench were mechanically backfilled the soil density is expected to be initially relatively loose and susceptible to wave-induced liquefaction. If the sandy soil surrounding the pipe liquefies, and the density of the pipe is less than the density of the liquefied soil, the pipe would potentially float to 'the surface of the seabed. However, a significant storm event is required to cause liquefaction. Weather conditions milder than the liquefaction event generate smaller waves that, over time, densify the backfilled material. Subsequent to this type of "preshearing", much larger wave action is required to cause liquefaction. Eventually the soil is densified to a state that liquefaction ceases to be a concern. In order to evaluate the time required for natural conditions to create a densified non-liquefiable backfilled material, a test was conducted offshore Mobile Bay, Alabama, in the Gulf of Mexico in about 30-ft. water depth. Analysis of the test data indicates that soil densification occurs rapidly and significantly reduces the risk of liquefaction and subsequent pipeline flotation. To simulate actual pipeline conditions, a 100-ft test trench was constructed and a 40-ft long, 30 in. diameter closed end section of pipeline was installed. After the trench was constructed and the pipe installed the trench was backfilled and a series of minicone penetrometer tests were conducted to determine soil density. The subsequent minicone test series were performed in the trench over the next five month period. The inferred changes in soil density were used to evaluate the liquefaction resistance of the soil based on laboratory tests and correlations with liquefaction events. INTRODUCTION Liquefaction from wave action results from a surface wave imparting a pressure 'wave' onto the seafloor as shown in Figure 1. This pressure pulse will create oscillatory changes in the shear stresses in the seabed. If the density of the soil is sufficiently low, this shearing can cause excess pore water pressures to develop. If the permeability of the seabed is such that full dissipation of excess pore water pressure is prevented, excess pore pressure buildup can eventually cause liquefaction or fluidization of the seabed. The principal concern for the Mobile Bay project was the likelihood of the clean, fine sands becoming less dense during the trench excavation and backfilling, if the mechanical backfilling option was selected. This soil type is historically identified as having a high liquefaction potential. For an initial study, medium dense soil conditions with a relative density of about 45 to 55 percent were assumed to be typical, based on available information and judgment. The results indicated that for sand at this density, liquefaction could 'occur for storm events with a 10-year return period in water depths ranging from 12 to 47 feet. Figure 2 shows the location of potential problem areas.

The results of three decades of seafloor research provide the most reliable information on the importance of water depth in massive sulfide formation. Available data from over 130 occurrences show that water depths of seafloor vent sites vary with plate tectonic setting and the regional magmatic and volcanic environment. The shallowest hydrothermal systems in subduction-related settings are hosted by arc volcanoes. These shallow vent sites have a number of features in common with subaerial epithermal systems. Massive sulfide occurrences in arc-related rifts, the most likely setting for many ancient analogs, are generally restricted to water depths from ~700 to 2,000 m, with rifts developing within old arc crust at the deeper end of this range. Back-arc spreading centers proximal to arcs host massive sulfide deposits at depths of ~1,500 to 2,000 m. The deepest hydrothermal systems occur along mature back-arc spreading centers distal to volcanic arcs where water depths range from ~2,000 to 3,700 m. These deeper vent sites probably represent the best modern analogues of ophiolite-hosted massive sulfide deposits. Boiling of the hydrothermal fluids is common at volcanic arcs and in arc-related rifts. In these environments, elevated magmatic gas contents of the hydrothermal fluids can contribute to the widespread occurrence of phase separation and associated gas loss. By contrast, the high ambient pressures in deep marine hydrothermal systems along mature back-arc spreading centers prevent fluids from boiling during their ascent to the seafloor. Boiling controls the maximum temperature at which hydrothermal fluids discharge at the seafloor and, therefore, influences the metal content of seafloor sulfide deposits. Copper-rich massive sulfides typically occur at water depths exceeding ~1,000 m, whereas Zn- and Pb-rich occurrences may form at any water depth. Boiling can be an important control on Ag and Au grades but is not the only factor controlling precious metal enrichment in massive sulfides. Shallow marine hot spring deposits can be highly enriched in trace metals such as As, Hg, and Sb. Submarine volcanic arc and back-arc settings are geologically complex and significant variations in water depth can occur over short distances. Paleoenvironmental reconstruction of these environments in ancient volcanic terranes is hampered by the lack of unequivocal volcanological or sedimentological criteria that indicate water depth. The relationships established here using modern seafloor observations provide important constraints on the paleoenvironmental setting of ancient volcanic-hosted massive sulfide deposits.

An in-situ study in the Xijiang River basin revealed adverse effects of total dissolved gas supersaturation on fish.

The subtidal, depth‐average momentum balances in 12 m and 27 m water depth are investigated using observations from 2001 to 2007 of water velocity, temperature, and density; bottom pressure; surface gravity waves; and wind stress. In the fluctuating across‐shelf momentum budget, the dominant terms are surface wind stress, pressure gradient, and Coriolis acceleration. The balance is a combination of (1) the geostrophic balance expected at midshelf sites and (2) the coastal setup and setdown balance driven by the across‐shelf wind stress expected where surface and bottom boundary layers overlap. At the 12 m site, the estimated wave radiation stress gradient due to surface gravity wave shoaling is also large but is uncorrelated with the observed pressure gradient. A simple model suggests the wave radiation stress gradient is balanced by an across‐shelf pressure gradient with a spatial scale too small to resolve with this mooring array. In the fluctuating along‐shelf momentum balance, the dominant terms are surface wind stress, pressure gradient, and bottom stress at the shallower site, but the other estimated terms are not negligible. Our results support the Grant and Madsen (1986) formulation for wave‐induced bottom stress. The fluctuating along‐shelf pressure gradient is mainly a local sea level response to wind forcing, not a remotely generated pressure gradient. A strong correlation between along‐shelf velocity and along‐shelf wind stress at the shallower site is captured by a simple steady model of imbalance between wind stress and pressure gradient balanced by linear bottom drag.

Diversity of phytophilous macroinvertebrates in polycultures of semi-intensively managed fishponds

Spatial variability of domoic acid concentration in king scallops Pecten maximus off the southeast coast of Ireland

Abstrct:The core SYS-0702was recovered in the distal part of the tidal sand ridges in the South Yellow Sea,off the Jiangsu coast at a water depth of 32m,and the uppermost 18.64mof the core corresponds to the distal deposits of the sand ridges.Based on lithologic features,grain-size distribution pattern,AMS14C dating data and 210Pb data of the core sediments,in combination with the high-resolution shallow seismic profile passing through the core site,this paper reveals the sedimentary characteristics of the distal sand ridges,formation time,and the influence of the old Yellow River on the sedimentation of the sand ridges when the river discharged into the South Yellow Sea from the Jiangsu coast during 1128to 1855.The research results show that the distal sand ridges have been keeping active since about 2.5cal.kaBP,and their internal reflections dip to the east and northeast downlapping onto the basal boundary of the sand ridges.The dip angles of the internal foresets are generally less than 0.2°,mostly convex in shape and tangent to the erosive surface of the sand ridges.The core section of the distal sand ridges roughly coarsens upward,and in the sandy sediments the saltation load is higher than 80%and the suspension load less than 20%,with a good sorting.During 1128—1855when the old Yellow River entered the South Yellow Sea from the Jiangsu coast,sedimentation rates in the distal sand ridges increased significantly,from 0.28～0.77cm/a before the river shifting up to 1.10cm/a during the event.But the rate decreased to 0.23cm/a after the old Yellow River going back to the Bohai sea in 1855.According to previous researches,the paleo-Yangtze river estuary and Dongtai were supposed to be the vertex of the radial tidal sand ridges in about 8kaBP and 6kaBP,respectively.The progradation of the sand ridges to the core site from the paleo-estuary during 8～2.5kaBP and from Dongtai during 6～2.5kaBP are about 61.7m/a and 65.9m/a,respectively.The progradation further towards the sea from the distal ridge,however,has reduced to about 4m/a since about 2.5cal.kaBP,which is considered to be caused by weakened tidal action and declined sediment supply in the deeper water.