Showing papers on "Seabed published in 2018"

An Overview of Seabed Mining Including the Current State of Development, Environmental Impacts, and Knowledge Gaps

Abstract: Rising demand for minerals and metals, including for use in the technology sector, has led to a resurgence of interest in exploration of mineral resources located on the seabed. Such resources, whether seafloor massive (polymetallic) sulfides around hydrothermal vents, cobalt-rich crusts on the flanks of seamounts or fields of manganese (polymetallic) nodules on the abyssal plains, cannot be considered in isolation of the distinctive, in some cases unique, assemblages of marine species associated with the same habitats and structures. In addition to mineral deposits, there is interest in extracting methane from gas hydrates on continental slopes and rises. Many of the regions identified for future seabed mining are already recognised as vulnerable marine ecosystems. Since its inception in 1982, the International Seabed Authority (ISA), charged with regulating human activities on the deep-sea floor beyond the continental shelf, has issued 27 contracts for mineral exploration, encompassing a combined area of more than 1.4 million km2, and continues to develop rules for commercial mining. At the same time, some seabed mining operations are already taking place within continental shelf areas of nation states, generally at relatively shallow depths, and with others at advanced stages of planning. The first commercial enterprise, expected to target mineral-rich sulfides in deeper waters, at depths between 1,500 and 2,000 metres on the continental shelf of Papua New Guinea, is scheduled to begin early in 2019. In this review, we explore three broad aspects relating to the exploration and exploitation of seabed mineral resources: (1) the current state of development of such activities in areas both within and beyond national jurisdictions, (2) possible environmental impacts both close to and more distant from mining activities and (3) the uncertainties and gaps in scientific knowledge and understanding which render baseline and impact assessments particularly difficult for the deep sea. We also consider whether there are alternative approaches to the management of existing mineral reserves and resources, which may reduce incentives for seabed mining.

A comprehensive sediment dynamics study of a major mud belt system on the inner shelf along an energetic coast

Abstract: Globally mud areas on continental shelves are conduits for the dispersal of fluvial-sourced sediment. We address fundamental issues in sediment dynamics focusing on how mud is retained on the seabed on shallow inner shelves and what are the sources of mud. Through a process-based comprehensive study that integrates dynamics, provenance, and sedimentology, here we show that the key mechanism to keep mud on the seabed is the water-column stratification that forms a dynamic barrier in the vertical that restricts the upward mixing of suspended sediment. We studied the 1000 km-long mud belt that extends from the mouth of the Changjiang (Yangtze) River along the coast of Zhejiang and Fujian Provinces of China and ends on the west coast of Taiwan. This mud belt system is dynamically attached to the fluvial sources, of which the Changjiang River is the primary source. Winter is the constructive phase when active deposition takes place of fine-grained sediment carried mainly by the Changjiang plume driven by Zhe-Min Coastal Currents southwestward along the coast.

Abundance and richness of key Antarctic seafloor fauna correlates with modelled food availability

Abstract: Most seafloor communities at depths below the photosynthesis zone rely on food that sinks through the water column. However, the nature and strength of this pelagic–benthic coupling and its influence on the structure and diversity of seafloor communities is unclear, especially around Antarctica where ecological data are sparse. Here we show that the strength of pelagic–benthic coupling along the East Antarctic shelf depends on both physical processes and the types of benthic organisms considered. In an approach based on modelling food availability, we combine remotely sensed sea-surface chlorophyll-a, a regional ocean model and diatom abundances from sediment grabs with particle tracking and show that fluctuating seabed currents are crucial in the redistribution of surface productivity at the seafloor. The estimated availability of suspended food near the seafloor correlates strongly with the abundance of benthic suspension feeders, while the deposition of food particles correlates with decreasing suspension feeder richness and more abundant deposit feeders. The modelling framework, which can be modified for other regions, has broad applications in conservation and management, as it enables spatial predictions of key components of seafloor biodiversity over vast regions around Antarctica.

A Multispectral Bayesian Classification Method for Increased Acoustic Discrimination of Seabed Sediments Using Multi-Frequency Multibeam Backscatter Data

Abstract: Multi-frequency backscatter data collected from multibeam echosounders (MBESs) is increasingly becoming available. The ability to collect data at multiple frequencies at the same time is expected to allow for better discrimination between seabed sediments. We propose an extension of the Bayesian method for seabed classification to multi-frequency backscatter. By combining the information retrieved at single frequencies we produce a multispectral acoustic classification map, which allows us to distinguish more seabed environments. In this study we use three triple-frequency (100, 200, and 400 kHz) backscatter datasets acquired with an R2Sonic 2026 in the Bedford Basin, Canada in 2016 and 2017 and in the Patricia Bay, Canada in 2016. The results are threefold: (1) combining 100 and 400 kHz, in general, reveals the most additional information about the seabed; (2) the use of multiple frequencies allows for a better acoustic discrimination of seabed sediments than single-frequency data; and (3) the optimal frequency selection for acoustic sediment classification depends on the local seabed. However, a quantification of the benefit using multiple frequencies cannot clearly be determined based on the existing ground-truth data. Still, a qualitative comparison and a geological interpretation indicate an improved discrimination between different seabed environments using multi-frequency backscatter.

A multiscale approach to mapping seabed sediments.

Abstract: Benthic habitat maps, including maps of seabed sediments, have become critical spatial-decision support tools for marine ecological management and conservation. Despite the increasing recognition that environmental variables should be considered at multiple spatial scales, variables used in habitat mapping are often implemented at a single scale. The objective of this study was to evaluate the potential for using environmental variables at multiple scales for modelling and mapping seabed sediments. Sixteen environmental variables were derived from multibeam echosounder data collected near Qikiqtarjuaq, Nunavut, Canada at eight spatial scales ranging from 5 to 275 m, and were tested as predictor variables for modelling seabed sediment distributions. Using grain size data obtained from grab samples, we tested which scales of each predictor variable contributed most to sediment models. Results showed that the default scale was often not the best. Out of 129 potential scale-dependent variables, 11 were selected to model the additive log-ratio of mud and sand at five different scales, and 15 were selected to model the additive log-ratio of gravel and sand, also at five different scales. Boosted Regression Tree models that explained between 46.4 and 56.3% of statistical deviance produced multiscale predictions of mud, sand, and gravel that were correlated with cross-validated test data (Spearman’s ρmud = 0.77, ρsand = 0.71, ρgravel = 0.58). Predictions of individual size fractions were classified to produce a map of seabed sediments that is useful for marine spatial planning. Based on the scale-dependence of variables in this study, we concluded that spatial scale consideration is at least as important as variable selection in seabed mapping.

Video transects reveal that tidal sand waves affect the spatial distribution of benthic organisms and sand ripples

Abstract: The sandy seabed of shallow coastal shelf seas displays morphological patterns of various dimensions. The seabed also harbors a rich ecosystem. Increasing pressure from human-induced disturbances necessitates further study on drivers of benthic community distributions over morphological patterns. Moreover, a greater understanding of the sand ripple distribution over tidal sand waves may improve morphological model predictions. Here we analyzed the biotic abundance and ripple morphology in sand wave troughs and crests using video transects. We found that both the epibenthos and endobenthos are significantly more abundant in sand wave troughs, where ripples are less abundant and more irregularly shaped. Finally, we show that camera systems are relatively quick and effective tools to study biotic spatial patterns in relation to seabed morphology.

In situ observation of storm-wave-induced seabed deformation with a submarine landslide monitoring system

Abstract: Submarine landslides move large volumes of sediment and are often hazardous to offshore installations. Current research into submarine landslides mainly relies on marine surveying techniques. In contrast, in situ observations of the submarine landslide process, specifically seabed deformation, are sparse, and therefore restrict our understanding of submarine landslide mechanisms and the establishment of a disaster warning scheme. The submarine landslide monitoring (SLM) system, which has been designed to partly overcome these pitfalls, can monitor storm-wave-induced submarine landslides in situ and over a long time period. The SLM system comprises two parts: (1) a hydrodynamic monitoring tripod for recording hydrodynamic data (e.g., waves, tides, and currents) and (2) a shape accel array for recording seabed deformation at different depths. This study recorded the development of the SLM system and the results of in situ observation in the Yellow River Delta, China, during the boreal winter of 2014–2015. The results show an abrupt small-scale storm-wave-induced seabed shear deformation; the shear interface is in at least 1.5-m depth and the displacement of sediments at 1.23-m depth is more than 13 mm. The performance of the SLM system under real-time field monitoring conditions confirms the feasibility and stability of this approach.

True Volumes of Slope Failure Estimated From a Quaternary Mass‐Transport Deposit in the Northern South China Sea

Abstract: Submarine slope failure can mobilize large amounts of seafloor sediment, as shown in varied offshore locations around the world. Submarine landslide volumes are usually estimated by mapping their tops and bases on seismic data. However, two essential components of the total volume of failed sediments are overlooked in most estimates: a) the volume of sub-seismic turbidites generated during slope failure and b) the volume of shear compaction occurring during the emplacement of failed sediment. In this study, the true volume of a large submarine landslide in the northern South China Sea is estimated using seismic, multibeam bathymetry and ODP/IODP well data. The submarine landslide was evacuated on the continental slope and deposited in an ocean basin connected to the slope through a narrow moat. This particular character of the sea floor provides an opportunity to estimate the amount of strata remobilized by slope instability. The imaged volume of the studied landslide is ~1035±64 km3, ~406±28 km3 on the slope and ~629±36 km3 in the ocean basin. The volume of sub-seismic turbidites is ~86 km3 (median value) and the volume of shear compaction is ~100 km3, which are ~8.6% and ~9.7% of the landslide volume imaged on seismic data, respectively. This study highlights that the original volume of the failed sediments is significantly larger than that estimated using seismic and bathymetric data. Volume loss related to the generation of landslide-related turbidites and shear compaction must be considered when estimating the total volume of failed strata in the submarine realm.

The Impact of Acoustic Imaging Geometry on the Fidelity of Seabed Bathymetric Models

Abstract: Attributes derived from digital bathymetric models (DBM) are a powerful means of analyzing seabed characteristics. Those models however are inherently constrained by the method of seabed sampling. Most bathymetric models are derived by collating a number of discrete corridors of multibeam sonar data. Within each corridor the data are collected over a wide range of distances, azimuths and elevation angles and thus the quality varies significantly. That variability therefore becomes imprinted into the DBM. Subsequent users of the DBM, unfamiliar with the original acquisition geometry, may potentially misinterpret such variability as attributes of the seabed. This paper examines the impact on accuracy and resolution of the resultant derived model as a function of the imaging geometry. This can be broken down into the range, angle, azimuth, density and overlap attributes. These attributes in turn are impacted by the sonar configuration including beam widths, beam spacing, bottom detection algorithms, stabilization strategies, platform speed and stability. Superimposed over the imaging geometry are residual effects due to imperfect integration of ancillary sensors. As the platform (normally a surface vessel), is moving with characteristic motions resulting from the ocean wave spectrum, periodic residuals in the seafloor can become imprinted that may again be misinterpreted as geomorphological information.

Disconnected submarine lobes as a record of stepped slope evolution over multiple sea-level cycles

Abstract: The effects of abrupt changes in slope angle and orientation on turbidity current behavior have been investigated in numerous physical and numerical experiments and examined in outcrop, subsurface, and modern systems. However, the long-term impact of subtle and evolving seabed topography on the stratigraphic architecture of deep-water systems requires fine-scale observations and extensive 3-D constraints. This study focuses on the Permian Laingsburg and Fort Brown formations, where multiple large sand-rich systems (Units A–F) have been mapped from entrenched slope valleys, through channel-levee systems, to basin-floor lobe complexes over a 2500 km2 area. Here, we investigate three thinner (typically <5 m in thickness) and less extensive sand-rich packages, Units A/B, B/C, and D/E, between the large-scale systems. Typically, these sand-rich units are sharp-based and topped, and contain scours and mudstone clast conglomerates that indicate deposition from high-energy turbidity currents. The mapped thickness and facies distribution suggest a lobate form. These distinctive units were deposited in similar spatial positions within the basin-fill and suggest similar accommodation patterns on the slope and basin floor prior to the larger systems (B, C, and E). Stratigraphically, these thin units represent the first sand deposition following ­major periods of shut-down in sediment supply, and are interpreted as marking a partial re-establishment of sand delivery pathways creating “disconnected lobes” that are fed mainly by flows sourced from failures on the shelf and upper slope rather than major feeder channel-levee systems. Thickness and facies patterns throughout the deep-water stratigraphy suggest seabed topography was present early in the basin formation and maintained persistently in a similar area to ultimately form a stepped slope profile. The stepped slope profile evolved through three key stages of development: Phase 1, where sediment supply exceeds deformation rate (likely caused by differential subsidence); Phase 2, where sediment supply is on average equal to deformation rate; and Phase 3, where deformation rate outpaces sediment supply. This study demonstrates that smaller systems are a sensitive record of evolving seabed topography and they can consequently be used to recreate more accurate paleotopographic profiles.

Impact of Seabed Resuspension on Oxygen and Nitrogen Dynamics in the Northern Gulf of Mexico: A Numerical Modeling Study

Abstract: Resuspension affects water quality in coastal environments by entraining seabed organic matter into the water column, which can increase remineralization, alter seabed fluxes, decrease water clarity, and affect oxygen and nutrient dynamics. Nearly all numerical models of water column biogeochemistry, however, simplify seabed and bottom boundary layer processes and neglect resuspension. Here we implemented HydroBioSed, a coupled hydrodynamic-sediment transport-biogeochemical model to examine the role of resuspension in regulating oxygen and nitrogen dynamics on timescales of a day to a month. The model was implemented for the northern Gulf of Mexico, where the extent of summertime hypoxia is sensitive to seabed and bottom boundary layer processes. Results indicated that particulate organic matter remineralization in the bottom water column increased by an order of magnitude during resuspension events. This increased sediment oxygen consumption and ammonium production, which were defined as the sum of seabed fluxes of oxygen and ammonium, plus oxygen consumption and ammonium production in the water column due to resuspended organic matter. The increases in remineralization impacted biogeochemical dynamics to a greater extent than resuspension-induced seabed fluxes and oxidation of reduced chemical species. The effect of resuspension on bottom water biogeochemistry increased with particulate organic matter availability, which was modulated by sediment transport patterns. Overall, when averaged over the shelf and on timescales of a month in the numerical model, cycles of erosion and deposition accounted for about two thirds of sediment oxygen consumption and almost all of the sediment ammonium production. Plain Language Summary In coastal waters, oxygen and nitrogen levels affect the health of fish and other organisms. In the Gulf of Mexico, for example, low-oxygen regions called hypoxic areas or \"dead zones\" form in the summertime near the seabed in \"bottom water\". It can be difficult to understand and quantify variations in bottomwater oxygen and nitrogen levels, however, because: (1) water quality there is affected by many different physical and biological processes; and (2) observational studies are limited by cost, safety and technological advances. To complement previous observational studies, this paper used a new numerical modeling approach that accounts for many physical and biological processes in the seabed and water. Specifically, we used the model to evaluate how resuspension, especially the entrainment of organic matter from the seabed into the water, affected oxygen and nitrogen levels in the Northern Gulf of Mexico. Model results indicated that resuspension increased the decomposition of organic matter, decreasing oxygen levels and increasing ammonium (a form of nitrogen) levels in bottom water. This effect was largest in regions with abundant seabed organic matter and frequent resuspension. These modeling results can help scientists and environmental managers understand how resuspension affects oxygen and nitrogen levels in bottom waters.

Nonlinear standing wave-induced liquefaction in loosely deposited seabed

Abstract: Wave-induced residual liquefaction in loose seabed floor brings great risk to the stability of offshore structures in extreme climates. Understanding the characteristics of wave-induced residual liquefaction due to pore pressure buildup in loose seabed is meaningful for engineers involved in the design of offshore structures. In this study, standing wave-induced residual liquefaction is investigated deeply and comprehensively adopting a validated integrated numerical model. The time history of standing wave-induced pore pressure, effective stress, shear stress, lateral pressure coefficient $$K_0,$$ stress angle, and displacement of seabed surface are all quantitatively demonstrated. The variation process of progressive liquefaction, stress path, as well as the stress-strain relation also are illustrated in detail. It is shown that the integrated numerical model FSSI–CAS 2D (FSSI: fluid–structures–seabed interaction, CAS: Chinese Academy of Sciences) incorporating the PZIII soil model can effectively and precisely capture a series of nonlinear dynamic response characteristics of loose seabed floors under standing wave loading. The computational results further confirm that the wave-induced liquefaction in loose seabed soil is progressive downward, initiating at the seabed surface. In addition, it is found that two physical processes, including vertical distribution of oscillatory pore pressure and time history of stress angle possibly could be used to judge the occurrence of wave-induced residual liquefaction in loose seabeds. Furthermore, it is also found that the progressive liquefaction process is significantly affected by wave height, permeability and saturation of seabed soil.

Seabed Morphology and Sedimentary Regimes defining Fishing Grounds along the Eastern Brazilian Shelf

Abstract: Shelf morphology and sedimentary regimes are influenced by processes operating at different temporal and spatial scales and are important records of sea level changes and sediment supply and/or carbonate production. The northern continental shelf of Espirito Santo (Brazil) contains evidence of different sedimentary regimes that distribute diverse and complex marine habitats. Herein, seabed morphology, acoustic images of the seafloor (side scan sonar and sub-bottom profiler), and sediment samples were used to investigate the influence of sedimentary regimes on physical marine habitat distribution. Seabed mapping is also integrated with available data on fisheries to understand the influence of shelf morphology and sedimentology in the usage of distinct fishing gears. The results indicate five morpho-sedimentary facies: terrigenous mud, terrigenous sand, rhodolith beds, carbonate gravel with rhodoliths, and hardground. Through an integrated analysis of the geomorphology and sedimentary distribution, two morpho-sedimentary domains were identified: a sediment-fed shelf adjacent to the Doce River associated with a major mud depocenter and a delta front morphology characterized by gentle slopes and low terrain ruggedness, and a sediment-starved shelf dominated by carbonate sedimentation showing an irregular morphology associated with higher slopes and terrain ruggedness. These contrasting morpho-sedimentary domains are a result of sedimentary responses to sea level fluctuation during Late Quaternary, specially, during the deglaciation processes after the Last Glacial Maximum. The morphological and sedimentary contrasts along the area define the physical habitat distribution. The sediment supply regime area is associated with a terrigenous fine/muddy sedimentation bed, which control the local morphology and favors coastal and delta front progradation. This physical habitat is a well-known shrimp-fishing ground where intense trawling takes place, as well as gillnet fisheries targeting weakfish and croakers. The accommodation regime or low sediment influx area is characterized by carbonate sedimentation associated with hardgrounds and rhodolith beds. In contrast, this physical habitat with scarce sediment supply, facilitates extensive benthic colonization by crustose coralline algae (CCA), which is primarily associated to line fisheries, longlines, and spearfishing. Rhodoliths show a high diversity of CCA and the occurrence of an endemic kelp species. Long-term processes such as relative sea level fluctuations and sediment supply are a legacy for the distribution of benthic habitats, and their resulting morphology can be a surrogate for predicting fishing activities or a first-base analysis for marine spatial planning. Available low-resolution bathymetric datasets can be a powerful tool, if applied with caution and in a regional scale approach. Here, terrain variables (terrain slope and ruggedness) derived from an extensive available (low-resolution and interpolated) bathymetric dataset distinguished two contrasting morphological domains characterized by rugged and smooth/flat seabeds.

Wave generation of gravity-driven sediment flows on a predominantly sandy seabed

Abstract: Wave-supported gravity flows (WSGFs) generate rates of sediment flux far exceeding other cross-shelf transport processes, contributing disproportionately to shelf morphology and net cross-shelf fluxes of sediment in many regions worldwide. However, the conditions deemed necessary for the formation of WSGF limit them to a narrow set of shelf conditions; they have been observed exclusively in regions where the seabed consists of very fine-grained sediment and typically co-occur with nearby river flood events. Here we document the occurrence of a WSGF event on a predominantly sandy seabed and in the absence of a preceding river flood. Our measurements confirm that the dynamics are governed by the friction-buoyancy balance observed in other WSGF and that WSGF can form in mixed grain-size environments and transport high concentrations of sand. The occurrence of WSGF on a predominantly sandy seabed suggests that they may occur under a much wider range of conditions and, given the global prevalence of sandy shelves, they may be a more frequent and more ubiquitous feature of shelf dynamics than previously thought.

Wave-induced liquefaction of the seabed around a single pile considering pile–soil interaction

Abstract: The analysis of wave-induced liquefaction of seabeds is of profound importance for offshore structures such as wind turbines and oil platforms. The seabed around pile foundations is subjected to tw...

Modelling the distribution of hard seabed using calibrated multibeam acoustic backscatter data in a tropical, macrotidal embayment: Darwin Harbour, Australia

Abstract: Spatial information on the distribution of seabed substrate types in high use coastal areas is essential to support their effective management and environmental monitoring. For Darwin Harbour, a rapidly developing port in northern Australia, the distribution of hard substrate is poorly documented but known to influence the location and composition of important benthic biological communities (corals, sponges). In this study, we use angular backscatter response curves to model the distribution of hard seabed in the subtidal areas of Darwin Harbour. The angular backscatter response curve data were extracted from multibeam sonar data and analysed against backscatter intensity for sites observed from seabed video to be representative of “hard” seabed. Data from these sites were consolidated into an “average curve”, which became a reference curve that was in turn compared to all other angular backscatter response curves using the Kolmogorov–Smirnov goodness-of-fit. The output was used to generate interpolated spatial predictions of the probability of hard seabed (p-hard) and derived hard seabed parameters for the mapped area of Darwin Harbour. The results agree well with the ground truth data with an overall classification accuracy of 75% and an area under curve measure of 0.79, and with modelled bed shear stress for the Harbour. Limitations of this technique are discussed with attention to discrepancies between the video and acoustic results, such as in areas where sediment forms a veneer over hard substrate.