Sea Floor Dynamics Research Articles

DIGITAL TWINNING IN THE OCEAN – CHALLENGES IN MULTIMODAL SENSING AND MULTISCALE FUSION BASED ON FAITHFUL VISUAL MODELS

Abstract. In engineering, machines are typically built after a careful conception and design process: All components of a system, their roles and the interaction between them is well understood, and often even digital models of the system exist before the actual hardware is built. This enables simulations and even feedback loops between the real-world system and a digital model, leading to a digital twin that allows better testing, prediction and understanding of complex effects. On the contrary, in Earth sciences, and particularly in ocean sciences, models exist only for certain aspects of the real world, of certain processes and of some interactions and dependencies between different “components” of the ocean. These individual models cover large temporal (seconds to millions of years) and spatial (millimetres to thousands of kilometres) scales, a variety of field data underpin them, and their results are represented in many different ways. A key to enabling digital twins in the oceans is fusion at different levels, in particular, fusion of data sources and modalities, fusion over different scales and fusion of differing representations. We outline these challenges and exemplify different envisioned digital twins employed in the oceans involving remote sensing, underwater photogrammetry and computer vision, focusing on optical aspects of the digital twinning process. In particular, we look at the holistic sensing scenarios of optical properties in coastal waters as well as seafloor dynamics at volcanic slopes and discuss road blockers for digital twins as well as potential solutions to increase and widen the use of digital twins.

Multiannual Seafloor Dynamics around a Subtidal Rocky Reef Habitat in the North Sea

Rocky reefs provide complex structures in the otherwise largely sand-dominated coastal North Sea. Therefore, these reefs are highly important natural habitats for the functioning of coastal ecosystems, as they provide shelter, refuge and nursery grounds for various mobile and sessile species. In the North Sea, the spatial distribution of these habitats has been intensively investigated over recent years. However, these studies generally provide static accounts of the current state of these reef systems, but limited data exist on the temporal variations in sediment dynamics at and around natural rocky reefs. In this study, we provide observations from a multiannual time series of hydroacoustic seafloor surveys conducted at an isolated rocky reef in the North Sea. We use multibeam bathymetry and side-scan sonar backscatter data in combination with video observations, sediment sampling, and sub-bottom profiler data to assess the long-term variations of the rocky reef system. The reef is located in water depths between 11 and 17 m with an areal extent of ~0.5 km2 and is surrounded by mobile sands. The topography of the rocky reef appears to create a distinct hydrodynamic system that permits mobile sands to settle or move into bathymetrical deeper parts of the reef. Our results suggest a very dynamic system surrounding the reef with large scale scouring, sediment reworking and transport, while the shallower central part of the reef remains stable over time. We demonstrate the importance of hydrodynamics and current scouring around reefs for the local variability in seafloor properties over time. These small-scale dynamics are likewise reflected in the spatial distribution of sessile species, which are less abundant in proximity to mobile sands. The hydroacoustic mapping and monitoring of seafloor dynamics at higher spatial and temporal resolutions presents an important future direction in the study of valuable coastal habitats.

Prediction of Changes in Seafloor Depths Based on Time Series of Bathymetry Observations: Dutch North Sea Case

Guaranteeing safety of navigation within the Netherlands Continental Shelf (NCS), while efficiently using its ocean mapping resources, is a key task of Netherlands Hydrographic Service (NLHS) and Rijkswaterstaat (RWS). Resurvey frequencies depend on seafloor dynamics and the aim of this research is to model the seafloor dynamics to predict changes in seafloor depth that would require resurveying. Characterisation of the seafloor dynamics is based on available time series of bathymetry data obtained from the acoustic remote sensing method of both single-beam echosounding (SBES) and multibeam echosounding (MBES). This time series is used to define a library of mathematical models describing the seafloor dynamics in relation to spatial and temporal changes in depth. An adaptive, functional model selection procedure is developed using a nodal analysis (0D) approach, based on statistical hypothesis testing using a combination of the Overall Model Test (OMT) statistic and Generalised Likelihood Ratio Test (GLRT). This approach ensures that each model has an equal chance of being selected, when more than one hypothesis is plausible for areas that exhibit varying seafloor dynamics. This ensures a more flexible and rigorous decision on the choice of the nominal model assumption. The addition of piecewise linear models to the library offers another characterisation of the trends in the nodal time series. This has led to an optimised model selection procedure and parameterisation of each nodal time series, which is used for the spatial and temporal predictions of the changes in the depths and associated uncertainties. The model selection results show that the models can detect the changes in the seafloor depths with spatial consistency and similarity, particularly in the shoaling areas where tidal sandwaves are present. The predicted changes in depths and uncertainties are translated into a probability risk-alert map by evaluating the probabilities of an indicator variable exceeding a certain decision threshold. This research can further support the decision-making process when optimising resurvey frequencies.

Ensemble Mapping and Change Analysis of the Seafloor Sediment Distribution in the Sylt Outer Reef, German North Sea from 2016 to 2018

Recent studies on seafloor mapping have presented different modelling methods for the automatic classification of seafloor sediments. However, most of these studies have applied these models to seafloor data with appropriate numbers of ground-truth samples and without consideration of the imbalances in the ground-truth datasets. In this study, we aim to address these issues by conducting class-specific predictions using ensemble modelling to map seafloor sediment distributions with minimal ground-truth data combined with hydroacoustic datasets. The resulting class-specific maps were then assembled into a sediment classification map, in which the most probable class was assigned to the appropriate location. Our approach was able to predict sediment classes without bias to the class with more ground-truth data and produced reliable seafloor sediment distributions maps that can be used for seafloor monitoring. The methods presented can also be used for other underwater exploration studies with minimal ground-truth data. Sediment shifts of a heterogenous seafloor in the Sylt Outer Reef, German North Sea were also assessed to understand the sediment dynamics in the marine conservation area during two different short timescales: 2016–2018 (17 months) and 2018–2019 (4 months). The analyses of the sediment shifts showed that the western area of the Sylt Outer Reef experienced sediment fluctuations but the morphology of the bedform features was relatively stable. The results provided information on the seafloor dynamics, which can assist in the management of the marine conservation area.

The stratigraphic evolution of a submarine channel: linking seafloor dynamics to depositional products

ABSTRACT We investigate the relationship between the cross-sectional geomorphic expression of a submarine channel as observed on the seafloor and the stratigraphic product of long-lived erosion, bypass, and sediment deposition. Specifically, by reconstructing the time–space evolution of an individual channel fill (i.e., channel element) exposed in outcrop, we establish a genetic link between thick-bedded channel-element-axis sandstone to thinly interbedded channel-element-margin deposits. Although the bounding surface between axis sandstone and margin thin beds is sharply defined, it is composed of a series of geomorphic surface segments of various ages; as such, the composite stratigraphic surface (∼ 17 m relief) was formed from numerous incision events that repeatedly sculpted the conduit. By demonstrating the origin of the stratigraphic surface, we conclude that geomorphic surfaces with 2–7 m of erosional relief were largely responsible for the observed intra-channel-element architecture (and ultimately, the composite 17-m-thick element). The widely documented channel element axis-to-margin architecture is a product of submarine-channel thalweg dynamics, primarily recording interactions between the seafloor and the basal high-concentration layers of channelized turbidity currents.

Early Silurian tholeiitic-boninitic Mailisu ophiolite, South Tianshan, Kyrgyzstan: a geochemical record of subduction initiation

ABSTRACTOphiolite assemblages of the South Tianshan fold-and-thrust-belt (STS) track the sea floor dynamics of the late Cambrian to Carboniferous Turkestan Ocean in the western Central Asian Orogenic Belt. We interpret new geochronological and geochemical data for the Mailisu ophiolite from western Kyrgyzstan as evidence for an early Silurian incipient subduction zone outboard the South Chatkal Arc of the Middle Tianshan. Igneous zircons from a leucogabbro yielded a U-Pb age of 440 ± 6 Ma indicating one of the oldest sea floor fragments in the STS. Metabasalt samples have N- to E-MORB trace element patterns and initial εNd values of ~5 to 9 support melting of depleted asthenosphere comprising a minor OIB component. Cross-cutting diabase dykes show geochemical characteristics of high-Ca basaltic boninites. Their low REE abundances and highly fractionated middle-heavy REE patterns indicate sources that were more melt-depleted than those of the metabasalts. Concave upwards LREE patterns are compatible with a residual peridotite source, fertilised by small-degree OIB-like melts with εNd of ~5 to 6. Positive Ba, Sr, and Pb anomalies support the involvement of slab-derived hydrous fluids that probably facilitated the melting of the residual mantle. The association of MORB-like metabasalts with younger dykes of boninite affinity suggests subduction initiation for the origin of the Mailisu ophiolite. Our data elucidate one of the major yet little understood episodes in the Paleozoic history of the Turkestan Ocean. Future identification of similar rock assemblages will be helpful to understand the insufficiently constrained history of ocean floor subduction in the Paleozoic Oceans of the western CAOB.

Seafloor Classification in a Sand Wave Environment on the Dutch Continental Shelf Using Multibeam Echosounder Backscatter Data

High resolution maps of sandy seafloors are valuable to understand seafloor dynamics, plan engineering projects, and create detailed benthic habitat maps. This paper presents multibeam echosounder backscatter classification results of the Brown Bank area of the North Sea. We apply the Bayesian classification method in a megaripple and sand wave area with significant slopes. Prior to the classification, corrections are implemented to account for the slopes. This includes corrections on the backscatter value and its corresponding incident angle. A trade-off in classification resolutions is found. A higher geo-acoustic resolution is obtained at the price of losing spatial resolution, however, the Bayesian classification method remains robust with respect to these trade-off decisions. The classification results are compared to grab sample particle size analysis and classified video footage. In non-distinctive sedimentary environments, the acoustic classes are not attributed to only the mean grain size of the grab samples but to the full spectrum of the grain sizes. Finally, we show the Bayesian classification results can be used to characterize the sedimentary composition of megaripples. Coarser sediments were found in the troughs and on the crests, finer sediments on the stoss slopes and a mixture of sediments on the lee slopes.

Impact of dense bottom water on a continental shelf: An example from the SW Adriatic margin

Dense shelf waters have been recognised as a key factor in transporting sediment and organic matter across continental slopes; however, the effect of their circulation on continental shelves is still overlooked. The Adriatic Sea is one of the areas in the Mediterranean where dense shelf water forms and flows as a bottom-hugging gravity current along the continental margin. A fraction of this dense water passes beyond the shelf break and flows downslope through cascading, while a large (lighter and faster) portion remains on the shelf, with a principal vein flowing between 80 and 120 m water depth.This paper presents the 3D shape, internal architecture and sediment facies of a variety of large-scale sedimentary features detected on the continental shelf and at the shelf edge, between 50 and 300 m water depth. These features include giant comet marks, large flute-shaped scours, muddy sediment waves, very large dunes with superimposed bedforms, active and relict sand ridges and shelf edge contourites. Both erosional and depositional features show an overall internal geometry and orientation consistent with water masses flowing southward, and are located where flows become accelerated by topographic constraints and deviated by the curvature of the shelf edge.In this study, the morphology and seismic stratigraphic structure of large-scale bedforms are investigated in an effort to place constraints on bedform evolution and preservation, especially as predictive tools for sand resource management, and to bring new evidence of the interaction between unidirectional hydrodynamic processes and seafloor dynamics on the shelf.

Improving a bathymetric resurvey policy with observed sea floor dynamics

Bathymetric resurveying in shallow seas is a costly process with limited resources, yet necessary for adequate nautical charts and therefore crucial for safe navigation. An important factor in an efficient resurvey policy is the type and size of sea floor dynamics. We propose four indicators, which we use in a procedure to recommend changes to the resurvey policies of the Belgian and Netherlands Continental Shelf (BNLCS). These shallow regions are characterised by a sandy sea floor covered with rhythmic patterns. The proposed indicators follow from the estimates for sea floor dynamics, as given by applying the statistical method of deformation analysis. We present a concept for the shallowest likely depth surface, on which we base two of the indicators. The other two indicators may act as a warning: they quantify the potentially missed dynamics, which makes the procedure more robust in case of complicated morphology. Finally, applying our procedure to the BNLCS, we show clear differences in recommended resurvey priority between the five analysed regions, which currently have equal resurvey frequencies.

Spatial differences in sand wave dynamics between the Amsterdam and the Rotterdam region in the Southern North Sea

The optimization of the bathymetric resurvey policy of the Netherlands Hydrographic Service requires insight into sea floor dynamics in the Southern North Sea. To study the spatial variations in sea floor dynamics, the bathymetric archives of the Netherlands Hydrographic Service are analyzed using deformation analysis, a statistical and innovative approach for bathymetric data. Based on the uncertainty of the data, our implementation of deformation analysis selects the significant spatial and temporal parameters, and provides estimates and their uncertainties for those parameters. We focus on sand wave areas in the regions of Rotterdam and of Amsterdam. In those areas, dredging takes place to guarantee a minimum depth. The results reveal a difference in sand wave migration between the two regions, over the past two decades. The dominant wavelengths of the sand waves vary within the regions, but we find a similar wavelength distribution for the two regions. We compare our results to earlier studies of the same sand wave areas in the Rotterdam region, showing similar migration rates, but different wavelengths. It is concluded, based on sand wave dynamics alone, that the Amsterdam region should be assigned a higher resurvey frequency than the Rotterdam region.

The estimation of sea floor dynamics from bathymetric surveys of a sand wave area

The analysis of series of offshore bathymetric surveys provides insight into the morphodynamics of the sea floor. This knowledge helps to improve resurvey policies for the maintenance of port approaches and nautical charting, and to validate morphodynamic models. We propose a method for such an analysis that is based on statistical testing theory, particularly aimed at the detection of dynamics in areas with tidal sand waves. The method explicitly considers the uncertainty of every measured depth value, resulting in parameter estimates for the sea floor dynamics, and their uncertainty. We apply the method to sea floor representations both with and without a sand wave pattern. A test scenario is set up, consisting of a survey of an existing area in the Southern North Sea, for which dynamics are simulated. The results show that the proposed method successfully detects different types of sea floor dynamics, leading to satisfactory estimates of the corresponding parameters.

Biological indicators of sedimentary dynamics in the central Gulf of Papua: Seasonal and decadal perspectives

Benthic and pelagic communities in the central Gulf of Papua, a major deltaic clinoform located on the south coast of Papua New Guinea, reflect the spatial and seasonal patterns in physical sedimentation conditions and the availability of food resources across this system. The distribution of terrigenous and marine organic substrates, the transport of riverine‐derived sediment, and seafloor dynamics are largely controlled by two seasons and wind patterns: the relatively quiescent NW Monsoon and more energetic SE Trades. The physically disturbed inner‐topset deposits (<20 m) are dominated at all times by bacterial biomass and have depressed numbers of macrofauna, with evidence of periodic recolonization recorded in sedimentary structures. The inner‐topset region also shows strong seasonality in the production and availability of labile organic matter and in bacterial activity. As measured by the concentrations of photopigments and microbial biomass in the upper 2 cm of surface sediments and in the water column, the comparatively quiescent NW Monsoon period is associated with net delivery of reactive substrates as labile organic matter to the bottom and net growth of the benthic microbial community, whereas the Transition period between the NW Monsoonal‐Winds (NW Monsoon) and SE Trade‐Winds (SE Trades), and the SE Trades, are periods of reactive substrate depletion in bottom deposits and decreased benthic bacterial and macroinfaunal biomass. In contrast to the inner‐topset, the less physically disturbed outer‐topset and foreset show relatively enhanced, but still low, abundances of large macrobenthos and possible evidence of variation related to food supply. The greatest observed seasonal variation in macrofauna occurs in the Umuda Valley off the Fly River, an apparent mobile sediment conduit.

Early diagenetic cycling, incineration, and burial of sedimentary organic carbon in the central Gulf of Papua (Papua New Guinea)

The clinoform complex of the Gulf of Papua represents a major deltaic system in Oceania. Two seasons largely control seafloor dynamics and sedimentary C cycling: the relatively quiescent NW monsoon, and the SE trades, characterized by remobilization and reoxidation of topset deposits. Surface sediments (∼20 cm) are reactive with ΣCO2 production fluxes ∼35–42 mmol m−2 d−1 at mangrove channel and topset sites during the monsoon, and ∼10–20 mmol m−2 d−1 on the foreset‐bottomset (>40 m). Fluxes decrease by a factor of ∼0.3 on the topset during the transition period and trades. The 13,14C isotopic compositions of pore water ΣCO2 reveal diagenetic fractionation, with dominant utilization of young (Δ14C = 1.4–31.1‰), terrestrial C substrates inshore (channels, topset δ13C = −29 to −25‰) and a progressive increase of young marine C sources seaward (outer topset, foreset; bottomset δ13C = −22.2 to −19.5). Remineralization patterns of terrestrial and marine Corg demonstrate cross‐shelf exchange. Multiple tracers show that a suboxic, mobile mud layer, ∼10–60 cm thick (usually ∼10–30 cm), characterizes the central gulf topset and Umuda Valley off the Fly River and unconformably overlies methanic deposits releasing old ΣCO2 (Δ14C = −159 to −229‰). Residual terrestrial Corg delivered to the bioturbated foreset continues to be remineralized slowly, generating ΣCO2 having net Δ14C = −270 within sediments deposited 100–200 years ago. The reactivity of Corg below ∼0.5 m in the foreset is ∼10–20 times lower than expected based on accumulation rates, reflecting loss of >50% of sedimentary Corg on the topset, which functions as a suboxic incinerator.

Wave-induced sea floor dynamics

This article reviews recent developments in the interaction between water waves, seabed, and coastal structures. The scope of the review covers theoretical approaches and physical modeling for the wave-induced soil response, wave damping, and sea floor instability around coastal structures. Important advances in this field of study are summarized and scope for future studies is suggested. There are 208 references cited in this review article.

Multiple active spreading centres in the hot North Fiji Basin (Southwest Pacific): a possible model for Archaean seafloor dynamics?

The North Fiji Basin (NFB), in the Southwest Pacific, is one of the most evolved back-arc basins. Within the basin, active spreading is taking place along several ridges, some of which display the characteristics of spreading ridges observed in mature large oceans characterized by a range of spreading rates (segmented accretion centres, organized magnetic lineations and propagators, for example). However, the great cumulate length of the spreading centres of the NFB relative to the Basin's surface makes it an unique example among the world's oceanic basins. We have reviewed a comprehensive body of geological, geophysical and geochemical evidence (multiple spreading centres of various trends, high heat flow, high geoid and slow upper mantle seismic velocities) which collectively demonstrate that a hot upper mantle underlies the North Fiji Basin. Active upper mantle convection is the key process controlling the large crustal and lithospheric production in the NFB, which would be thus largely independent from the evolution of even the largest structural features, such as the Vitiaz and New Hebrides subduction zones. Petrological studies of the Archaean greenstone belts have led to a similar picture for the entire Earth during the Archaean: numerous short active ridges connected by unstable triple junctions would have, at this time, provided an efficient convective mechanism allowing for a large heat dissipation. We propose that the presently active processes in the North Fiji basin are modern analogues to Archaean seafloor dynamics.

An underwater camera-TV system for the study of sea floor profile

A ship deployed remote control underwater camera closed-circuit TV system was developed for observation of sea floor dynamics and processes, microgeomorphology and sediment-water profile of siltclay sediment, water visibility, activities of benthic organisms, etc.

Teaching Plate Tectonics with the Aid Of a Model of Sea-Floor Dynamics

A model which demonstrates selected apsects of plate tectonics is described. The working physical model of the earth's crustal dynamics demonstrates (1) divergence at mid-oceanic ridges and subduction at continental margins, (2) acquisition of normal or reversed magnetic polarity by solidifying lava, and (3) continental rupture and collision. Construction and operation of the model are explained.

Sea-Floor Characteristics and Dynamics Affecting Geotechnical Properties at Shelf-Slope Breaks: ABSTRACT

Shelf-break geotechnical and sea-floor stability investigations are rare but such studies offer challenging scientific and engineering opportunities in a unique sea-floor environment. The uniqueness stems from the highly variable sea-floor properties, morphology, and dynamic processes. Some characteristics and processes at the break are nonexistent in nearshore environments where most of our understanding and experience exists. The geotechnical properties and stability of submarine deposits are strongly influenced by the nature of the geologic environment, dynamic processes, sediment type, and sea-floor slope. Sediment properties and sea-floor morphology at the break can be highly variable over relatively short horizontal and vertical distances. The outer shelves and slopes off Cape May, New Jersey, and the Mississippi delta are among the few geotechnically investigated continental margins. Off Cape May, mobile shelf sands contribute to sandy silts overlying the upper slope muds which occasionally exhibit structures and geochemical properties suggesting slope-creep processes. In contrast, the Mississippi delta's shelf-slope break is characterized by clays and silty clays that mobilize as mudflows which encroach upon the upper slope. Geotechnical properties such as shear strength and water content display striking differences between the two areas as a function of regionally differing dynamic processes, sedimentation rate, type, and texture. End_of_Article - Last_Page 899------------