Seafloor Stability Research Articles

A review of submarine landslides associated with rapid sedimentation

Submarine landslides are a critical geological process that significantly influences seafloor stability and geological evolution, making them a focal point of geological disaster research. Particularly, rapid sedimentation as a crucial preconditioning factor for submarine landslide initiation is increasingly recognized in contemporary research. However, no comprehensive review currently examines the correlation between rapid sedimentation and submarine landslides. To elucidate the interaction mechanisms between submarine landslides and rapid sedimentation and to provide a new theoretical foundation for the prediction and prevention of geological hazards, the study categorizes three primary sedimentation patterns in marine environments and establishes a detailed historical database of sedimentation-related submarine landslide events. Moreover, the paper systematically reviews and critiques the existing research methods and advancements in addressing rapid sedimentation-related submarine landslides, and a bibliometric analysis of recent research progress is conducted. Finally, the study highlights several challenges in the study of rapid sedimentation and proposes future recommendations, aiming to provide robust support for the in-depth investigation of submarine landslides.

How does Global Warming Influence Seafloor Stability?

How does Global Warming Influence Seafloor Stability?

The diverse morphology of pockmarks around Aotearoa New Zealand

Seafloor pockmarks are abundant around Aotearoa New Zealand, occurring across a diverse range of tectonic, sedimentological and geomorphological settings. Globally, the formation and source of pockmarks is widely researched because they: 1) have potential links to subsurface hydrocarbon systems, 2) can provide important habitats for benthic organisms and 3) may be indications of fluid escape pathways or areas of sediment disturbance, which influence seafloor stability and could pose a risk to infrastructure. Pockmarks are widely associated with fluid release (such as gas or water) from subsurface reservoirs. However, the formation of pockmarks, the processes that shape and modify their morphology over time, and the relative timing of these events, remains enigmatic. Here, we compile the first national database of over 30,000 pockmarks around Aotearoa New Zealand, allowing us to begin to comprehend the dynamic processes that shape and affect pockmarks by exploring regional and inter-regional patterns in pockmark geometry and seabed characteristics. This compilation reveals several significant trends, including a distinct lack of correlation between active seafloor seeps and pockmarks, and a strong association of pockmarks with mud-rich seafloor substrate. Furthermore, we highlight key knowledge gaps that require further investigation moving forward, including a lack of constraint on the timing of pockmark formation, and limited modelling of the processes involved in their formation.

Both Longitudinal and Transverse Extension Controlling Gas Migration Through Submarine Anticlinal Ridges, New Zealand's Southern Hikurangi Margin

AbstractSub‐seabed fluid flow, gas hydrate accumulation and seafloor methane seepage are tightly interwoven processes with implications for marine biodiversity, ocean chemistry and seafloor stability. We combine long‐offset seismic reflection data with high‐resolution seismic data to investigate shallow structural deformation and its relationship to focused gas migration and hydrate accumulation in the southern Hikurangi subduction wedge. Anticlines, effective traps for focusing free gas, are characterized by both normal faults and vertical zones of hydraulic fracturing within the hydrate stability zone. The normal faults form as a result of sediment layer folding and gravitational collapse of ridges during uplift. We document both longitudinal (ridge‐parallel) and transverse (ridge‐perpendicular) extensional structures (normal faults and elongated hydraulic fracture zones) in the sub‐seafloor of anticlinal ridges. Intriguingly, gas flow through ridges close to the deformation front of the wedge exploits longitudinal structures, while ridges further inboard are characterized by gas flow along transverse structures. This highlights pronounced changes in the shallow deformation of ridges in different parts of the wedge, associated with a switching of the least and intermediate principal stress directions. It is critical to understand these shallow stress fields because they control fluid flow patterns and methane seepage out of the seafloor.

Salt leaching by freshwater and its impact on seafloor stability: An experimental investigation

Offshore freshened groundwater (OFG) has been documented in many continental margins worldwide. OFG systems are dynamic, expanding and contracting with falling and rise sea-levels. OFG has long been thought to be an important geomorphic agent in continental margins, either via active discharge at the seafloor, which can erode depressions, or the generation of excess pore pressure, which can deform sediments and cause slope failure. It has also been proposed that OFG flow can drive the loss of sediment shear strength via salt leaching, when seawater in pores is replaced by freshwater. Here, we measure changes in the geotechnical properties of seafloor clayey silt due to salt leaching using flushing experiments, and assess the implications of these changes on the stability of siliciclastic continental margins with 2D limit equilibrium modelling. We document a ∼ 50% decrease in undrained cohesive strength of seafloor sediment after flushing, as well as a decrease in its shear strength, bulk density, and moisture content, which is similar to that reported for subaerial quick clays undergoing salt leaching. When applied to a theoretical submarine domain 300 m wide by 100 m high, we estimate that salt leaching can trigger slope failure when the thickness of the flushed layer is >3.5 m or when the slope gradient is >3°. Such conditions are primarily satisfied on the continental slope or the shallow seafloor close to the shoreline. Salt leaching by OFG flow merits consideration as a potential mechanism destablising submarine sedimentary slopes.

Shear Modulus of a Carbonate Sand–Silt Mixture with THF Hydrate

The maximum shear modulus (Gmax) is an important factor determining soil deformation, and it is closely related to engineering safety and seafloor stability. In this study, a series of bender element tests was carried out to investigate the Gmax of a hydrate-bearing carbonate sand (CS)–silt mixture. The soil mixture adopted a CS:silt ratio of 1:4 by weight to mimic the fine-grained deposit of the South China Sea (SCS). Tetrahydrofuran (THF) was used to form the hydrate. Special specimen preparation procedures were adopted to form THF hydrate inside the intraparticle voids of the CS. The test results indicate that hydrate contributed to a significant part of the skeletal stiffness of the hydrate-bearing CS–silt mixture, and its Gmax at 5% hydrate saturation (Sh) was 4–6 times that of the host soil mixture. Such stiffness enhancement at a low Sh may be related to the cementation hydrate morphology. However, the Gmax of the hydrate-bearing CS–silt mixture was also sensitive to the effective stress for an Sh ranging between 5% and 31%, implying that the frame-supporting hydrate morphology also plays a key role in the skeletal stiffness of the soil mixture. Neither the existing cementation models nor the theoretical frame-supporting (i.e., Biot–Gassmann theory by Lee (BGTL)), could alone provide a satisfactory prediction of the test results. Thus, further theoretical study involving a combination of cementation and frame-supporting models is essential to understand the effects of complicated hydrate morphologies on the stiffness of soil with a substantial amount of intraparticle voids.

Characterizing bedforms in shallow seas as an integrative predictor of seafloor stability and the occurrence of macrozoobenthic species

AbstractIn soft‐bottom marine ecosystems, bedform variation is induced by wind‐ and tidal‐driven hydrodynamics. The resulting megaripples, sand waves and sandbanks form a spatially and temporally heterogeneous seafloor landscape. The strong physical forces imposed by the migration of these bedforms are important determinants for the occurrence of different macrozoobenthic species. Quantifying the effect of these forces can help in differentiating natural‐ and anthropogenically induced physical stressors. However, large‐scale mapping of seabed morphology at high resolution using multibeam echosounder is challenging, costly and time‐consuming, especially in shallow seas, prohibiting wide swaths. Instead, their bathymetry is typically studied using single‐beam transects that are interpolated to bathymetric grids with a relatively coarse resolution (20 m). However, this leaves out information on smaller scale (<20 m) bedforms that can be ecologically relevant. In the Dutch Wadden sea, a shallow tidal system, we characterized bedform variation at high resolution using single‐beam data for the first time. We calculated a 2‐D Terrain Ruggedness Index (TRI) at sub‐meter resolution along the single‐beam transects and interpolated the results to a full 3‐D grid. We then validated the result by relating TRI to independently modeled hydrodynamic parameters and to the distribution of macrozoobenthic species. We found that TRI successfully integrates the variation of tidal‐driven bed shear stress and wave‐driven orbital velocity. In addition, we found TRI to be a good predictor of the occurrence of macrozoobenthic species. The inferred small‐scale bedforms provide valuable information for separating the relative importance of natural dynamics versus anthropogenic disturbances such as dredging and bottom trawling activities. We discuss that by repurposing already available single‐beam data in this way, bedforms can be characterized at high resolution without the need for additional equipment or mapping campaigns, yielding novel input to decision‐making on marine management and conservation.

First evidence of (paleo)pockmarks in the Bass Strait, offshore SE Australia: A forced regression modulated shallow plumbing system

Pockmarks are morphological expressions of seabed fluid escape along continental margins. Identifying the underlying controls on their formation and spatial distribution is crucial for understanding substrate fluid plumbing systems and has important implications for hydrocarbon exploration, seafloor stability and seabed release of greenhouse gasses. Here, we use 3D seismic reflection dataset and a machine learning approach to present the first evidence for paleo-pockmarks in the Bass Strait, southeast Australia. The paleo-pockmarks are identified in the Bass Basin, within an interval between 250 and 310 m below present-day seafloor, corresponding to the Miocene carbonate-dominated Torquay Group. The paleo-pockmarks have depths ranging from ∼29 to 74 m and areas between ∼0.01 and 0.8 km2, with diameters varying between ∼0.1 and 1.1 km. The absence of an underlying seal-bypass system such as pipes and faults associated with these paleo-pockmarks discounts a deeper thermogenic source or a potential magmatic-driven fluid system. Rather a biogenic fluid system derived from the degradation of organic-rich layers and pore water expelled during early-stage compaction is hypothesised to drive paleo-pockmark formation. The seismic interval comprising these paleo-pockmarks demonstrates a distinctive seaward progradation and stepping-down configuration, indicating a forced regression. We propose this resulted in the destabilization of hydrostatic pressure triggering the creation of the paleo-pockmarks.

Toxicological issues of conventional gas hydrate inhibitors

AbstractGas hydrates formation is a major challenge to effective hydrocarbons flow in pipelines; thus, flow assurance is a remedy. The use of gas hydrate inhibitors is commonly used as an effective method to inhibit gas hydrate formation. This study focuses on the toxicological issues of conventional gas hydrate inhibitors. The study evidence reveals that conventional chemical additives that ensure effective flow assurance are said to contribute significantly to environmental change and damage. This is attributed to their toxicity nature that poses a great environmental risk by such chemical inhibitors; more so thermodynamic inhibitors due to their dissociation ability that affects seafloor stability. This leads to the release of methane (and associated gases) and other toxic components into the column water. However, little is known about the actual toxicity of frequently used gas hydrate inhibitors. This calls for an in‐depth study into such and also the development of greener inhibitors to replace the existing ones.

A machine-learning derived model of seafloor sediment accumulation

Previous studies regarding the depositional pattern and quantity of accumulated seafloor sediment tend to be regional, limited in scope and involving costly and time-consuming geologic field campaigns and laboratory work. Presented herein is a global map of predicted modern (postindustrial, 20th and 21st century) oceanic mass accumulation rates of 5-arc-minute pitch and in log10-space, trained on observed marine mass accumulation rates from 43 peer reviewed sources (n = 1744) and predicted using a k-nearest neighbor geospatial algorithm. The resultant model predicts ~3.3 × 104 Mt. yr−1 of sediment accumulating onto the sea floor (R2 = 0.88). Most sediment accumulates proximal to major river outlets and deltas. Continental regions with the highest sediment accumulation are Asia and Oceania. This model is the first of its kind to predict the rate and quantity of sediment accumulating on to the ocean floor, globally, using decades of regional real-world observations. The generated global map of modern, benthic mass accumulation rates also serves to highlight areas of interest for future study in related fields, such as sediment dynamics and seafloor stability.

Large-scale seismic seafloor stability analysis in the South China Sea

Earthquakes are considered as the major trigger of submarine landslides. This study analyzes the large-scale seismic seafloor stability in the South China Sea (SCS), taking into consideration the earthquake probability and spatial distribution of seismicity and seafloor soil properties. For this cause, the geographical, seismic, and seafloor soil characteristics are first determined. The digital elevation model (DEM) of the study area is established using the geographic information system (GIS). Large-scale Peak Ground Acceleration (PGA) maps under different exceedance probabilities (EPs) are derived using the Chinese Probability Seismic Hazard Analysis (CPSHA) method. Three-dimension continuous models for seafloor soil properties are obtained using a proposed best-fit distribution-based Kriging method. With this information, the large-scale seafloor stability under different EPs is evaluated with an infinite slope model in the study area of the SCS. The proposed large-scale seismic seafloor stability is validated against recorded earthquake-induced submarine landslides. The analysis shows that the seafloor stability is found to be highly nonuniform, and the study area can be divided into three main parts based on their distinct seafloor stabilities. This study sheds new light on the large-scale seafloor stability, and can assist the development of ocean engineering projects in the SCS.

Physical Properties of Gas Hydrate-Bearing Pressure Core Sediments in the South China Sea

Gas hydrates are a potential future energy resource and are widely distributed in marine sediments and permafrost areas. The physical properties and mechanical behavior of gas hydrate-bearing sediments are of great significance to seafloor stability and platform safety. In 2013, a large number of pressure cores were recovered during China’s second gas hydrate drilling expedition in the South China Sea. In this study, we determined the gas hydrate distribution, saturation, physical properties, and mechanical behavior of the gas hydrate-bearing sediments by conducting Multi-Sensor Core Logger measurements and triaxial and permeability tests. Disseminated gas hydrates, gas hydrate veins, and gas hydrate slabs were observed in the sediments. The gas hydrate distribution and saturation are spatially heterogeneous, with gas hydrate saturations of 0%–55.3%. The peak deviatoric stress of the gas hydrate-bearing sediments is 0.14–1.62 MPa under a 0.15–2.3 MPa effective confining stress. The permeability is 0.006– 0.095 × 10 − 3 μ m 2 , and it decreases with increasing gas hydrate saturation and burial depth.

Geotechnical Characterization of Marine Unconsolidated Sediments for Cable Routing between the Islands of Madeira and Porto Santo

Pombo, J.; Rodrigues, A., and Lapa, N., 2020. Geotechnical characterization of marine unconsolidated sediments for cable routing between the islands of Madeira and Porto Santo. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1245–1251. Coconut Creek (Florida), ISSN 0749-0208.The national economic sustainability strategy encourages the use of oceanic territory and resources for the reduction of dependency on fossil fuels. Submarine cables are subject to several risks, especially those associated with the seafloor stability processes, which are not described by the normal characterization of the marine sedimentary layer. As such, to ensure the safety and economic success of any new project it is essential to know the properties of the seabed, in particular, the geotechnical ones. Only in this way it will be possible to estimate and mitigate the natural hazards effects (e.g. submarine movements and landslides) or excessive burial of the electric power cable that could compromise the integrity of the infrastructure. If unavoidable, protection actions and measures should be taken, when submarine cables cross any rocky area, or highest slope area. In this paper, we will present the results of the geotechnical characterization of the marine soils along a corridor between Madeira and Porto Santo Islands. Unconsolidated sediments were collected in both islands and results of laboratory analysis (grain size, water content, carbonate content, Atterberg limits and triaxial tests) indicate the occurrence of frequent downslope mass movements in both islands.

Lessons learned from the monitoring of turbidity currents and guidance for future platform designs

Abstract Turbidity currents transport globally significant volumes of sediment and organic carbon into the deep-sea and pose a hazard to critical infrastructure. Despite advances in technology, their powerful nature often damages expensive instruments placed in their path. These challenges mean that turbidity currents have only been measured in a few locations worldwide, in relatively shallow water depths (<<2 km). Here, we share lessons from recent field deployments about how to design the platforms on which instruments are deployed. First, we show how monitoring platforms have been affected by turbidity currents including instability, displacement, tumbling and damage. Second, we relate these issues to specifics of the platform design, such as exposure of large surface area instruments within a flow and inadequate anchoring or seafloor support. Third, we provide recommended modifications to improve design by simplifying mooring configurations, minimizing surface area and enhancing seafloor stability. Finally, we highlight novel multi-point moorings that avoid interaction between the instruments and the flow, and flow-resilient seafloor platforms with innovative engineering design features, such as feet and ballast that can be ejected. Our experience will provide guidance for future deployments, so that more detailed insights can be provided into turbidity current behaviour, in a wider range of settings.

Novel three-dimensional numerical model for residual seabed response to natural loadings near a single pile

The evaluation of seafloor stability is of vital importance for engineers involved in the protection of marine foundations. Hence, it has attracted considerable attention in the field of offshore geotechnical engineering. In this investigation, a novel three-dimensional (3D) numerical model of fluid–seabed-structure interactions (FSSI) is proposed within the framework of OpenFOAM to explore the residual soil dynamic response around a single pile. Different from previous studies, three new features are added in the present model. (i) The wave–current interactions are taken into consideration, (ii) the source term of pore pressure generations is redefined, and (iii) both the oscillatory pore pressure and residual pore pressure are considered while judging the soil liquefaction potential. After verified by previous laboratory experiments available in literature, the novel model is applied to investigate the residual soil response around the single pile induced by natural loadings. The numerical results confirm that the novel 3D model can provide sufficiently accurate predictions of fluid–seabed-structure interactions. Moreover, wave/current characteristics, soil properties, and pile diameter can significantly affect the fluid-induced residual pore pressures along the vicinity of the single pile.

Fluid escape features as relevant players in the enhancement of seafloor stability?

AbstractFluid migration within the sedimentary column contributes significantly to slope failure and pockmark formation and can be an effective triggering mechanism to generate submarine landslides. Pockmarks are thus commonly listed among geohazards. Contrary to these accepted notions, we propose here an alternative view of pockmarks with an example from the Eastern Niger Submarine Delta: Pockmarks and associated chimneys may increase or modify the shear strength of sedimentary layers and locally enhance seafloor stability. The analysis of two 3D seismic volumes shows that a landslide deposit divides into two branches around a cluster of three pockmark chimneys, interpreted to impede its further development. The morphological characteristics of a slide constrained by fluid seepage features show the potential role of fluid escape in marine sediment strengthening.

Large-scale seafloor stability evaluation of the northern continental slope of South China Sea

With the continuous expansion of energy demand, the deep-water continental slope in the northern South China Sea has become one of the significant offshore oil and gas exploration regions. The frequent occurrence of marine geological hazards in this region, especially submarine landslides, can cause serious damage to engineering facilities. However, there have been few studies on the stability of the northern continental slope of the South China Sea; these studies mainly focused on a specific submarine slope or small-range evaluation, resulting in a lack of large-scale and quantitative understanding. Hence, considering the variation in the physical and mechanical properties of marine soils with depth, formulas for calculating the safety factor of submarine slopes by an infinite sliding model are established, and the factors affecting slope stability such as soil properties, slope gradient and horizontal seismic action are systematically investigated. Using GIS techniques, the terrain slope gradients and a historical seismic database of the northern South China Sea are obtained. Combined with soil mechanical parameters, a regional stability evaluation of the northern continental slope is carried out. Furthermore, the distribution of risk zones is given. On the whole, under strong seismic action, large-scale submarine slope instability occurs and must be highly considered when assessing risk. This achievement is of great significance to engineering sites, route selection and engineering risk assessment.

Strength characteristics of sediments from a gas hydrate deposit in the Krishna–Godavari Basin on the eastern margin of India

Knowledge of the strength of sediments overlying sub-seafloor gas-hydrate deposits is crucial for predicting borehole and seafloor stability during hydrate extraction. Ideally, sediment strength should be determined along a continuous downhole profile from the seafloor to the hydrate reservoir, but few such profiles have been obtained. In this study we used cores retrieved at Site NGHP-02-23 in the Krishna–Godavari Basin in unconfined penetration tests on split cores and in triaxial deformation experiments on hydrate-free sediment samples. Although penetrometer tests identified relatively low strength (70–250 kPa) likely due to hydrate dissociation in the hydrate-bearing interval 90–300 m below seafloor (mbsf), sediment strength exceeded 350 kPa in the intervals 140–170 and 250–270 mbsf, each of which lies just above a zone of high gas-hydrate concentration. The average stress ratio of triaxial strength at 4% axial strain to effective mean stress ( q ε=4% / p ’) of hydrate-free silts was about 0.67 throughout the Hole. An exception to this trend was in fine sands from 280 mbsf in the deeper gas-hydrate zone, where the stress ratios were greater than 1.0. The stress ratios of hydrate-bearing sediments in the deeper gas-hydrate zone that were reported from the pressure-core measurements were far greater than those of any hydrate-free sediments for a given effective mean stress. The high-strength intervals in silty sediments we identified by the penetration tests could be associated with zones of high hydrate concentration. Because high-strength layers in fine-grained silty sediments commonly exhibit lower permeability than sandy layers (potential hydrate-host), they may act as seals that assist the precipitation of hydrate below them. • Depth trend of strength is defined by a stress ratio of ∼0.67 • High-strength intervals are identified above hydrate concentration zones. • High-strength layers may act as seals to assist the precipitation of hydrate.

Putting the Deep Biosphere and Gas Hydrates on the Map

ABSTRACTMicrobial processes in the deep biosphere affect marine sediments, such as the formation of gas hydrate deposits. Gas hydrate deposits offer a large source of natural gas with the potential to augment energy reserves and affect climate and seafloor stability. Despite the significant interdependence between life and geology in the ocean, coverage of the deep biosphere is generally missing in most introductory oceanography textbooks, so there is a need for instructional materials on this important topic. In response to this need, a course module on the deep biosphere with a focus on gas hydrate deposits was created. The module uses Google Earth (Google, Mountain View, CA) to support inquiry-based activities that demonstrate the interaction of the deep biosphere with geology. The module was tried as both a series of in-class exercises and as an out-of-class assignment in an introductory, undergraduate oceanography course. The students took short, preactivity and postactivity quizzes to determine the effectiveness of the module in improving student knowledge about gas hydrates. The module was effective at increasing student knowledge about the basic environmental and biological controls on the formation of gas hydrates on the seafloor. Students showed a consistently low initial comprehension of the content related to gas hydrates, but most (>80%) of the students increased their quiz scores for all module activities. This module on gas hydrate deposits increases the available teaching resources focused on the deep biosphere for geoscience educators.

Imaging methane hydrates growth dynamics in porous media using synchrotron X‐ray computed microtomography

AbstractCommercial‐scale methane (CH4) extraction from natural hydrate deposits remains a challenge due to, among other factors, a poor understanding of hydrate‐host sediment interactions under low‐temperature and high‐pressure conditions that are conducive to their existence. We report the use of synchrotron X‐ray computed microtomography (CMT) to image, for the first time, time‐resolved pore‐scale methane CH4 hydrate growth from an aqueous solution containing 5 wt % barium chloride (BaCl2) and pressurized CH4 hosted in glass beads, all contained in an aluminum cell with an effective volume of 3.5 mL. Multiple two‐dimensional (2‐D) cross‐sectional images show CH4 hydrates, with 7.5 µm resolution, distributed in patches throughout the system without dependence on distance from the cell walls. The time‐resolved three‐dimensional (3‐D) images, constructed from the 2‐D slices, exhibited pore‐filling hydrate formation from dissolved CH4 gas, similar to natural CH4 hydrates (sI) in the marine environment. Furthermore, the 3‐D images show that the aqueous phase was the wetting phase of the glass beads, i.e., the host and the formed hydrate were separated by an aqueous layer. These results provide some fundamental understanding of the nucleation phenomenon of gas hydrate formation at the pore scale. Pore‐filling CH4 hydrate growth is likely to result in a reduced bulk modulus, and thus, could affect seafloor stability during the reverse phenomenon, i.e., dissociation of natural hydrate deposits.