Shallow Seafloor Research Articles

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.

Monitoring Short-Term Morphobathymetric Change of Nearshore Seafloor Using Drone-Based Multispectral Imagery

Short-term changes in shallow bathymetry affect the coastal zone, and therefore their monitoring is an essential task in coastal planning projects. This study provides a novel approach for monitoring shallow bathymetry changes based on drone multispectral imagery. Particularly, we apply a shallow water inversion algorithm on two composite multispectral datasets, being acquired five months apart in a small Mediterranean sandy embayment (Chania, Greece). Initially, we perform radiometric corrections using proprietary software, and following that we combine the bands from standard and multispectral cameras, resulting in a six-band composite image suitable for applying the shallow water inversion algorithm. Bathymetry inversion results showed good correlation and low errors (<0.3 m) with sonar measurements collected with an uncrewed surface vehicle (USV). Bathymetry maps and true-color orthomosaics assist in identifying morphobathymetric features representing crescentic bars with rip channel systems. The temporal bathymetry and true-color data reveal important erosional and depositional patterns, which were developed under the impact of winter storms. Furthermore, bathymetric profiles show that the crescentic bar appears to migrate across and along-shore over the 5-months period. Drone-based multispectral imagery proves to be an important and cost-effective tool for shallow seafloor mapping and monitoring when it is combined with shallow water analytical models.

Shallow Water Bathymetry Mapping from ICESat-2 and Sentinel-2 Based on BP Neural Network Model

Accurate shallow water bathymetry data are essential for coastal construction and management, marine traffic, and shipping. With the development of remote sensing satellites and sensors, the satellite-derived bathymetry (SDB) method has been widely used for bathymetry in shallow water areas. However, traditional satellite bathymetry requires in-situ bathymetric data. Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) with the advanced high-resolution topographic laser altimeter system (ATLAS) provides a new technical tool and makes up for the shortcomings of traditional bathymetric methods in shallow waters. In this study, a new method is proposed to automatically detect photons reflected from the shallow seafloor with ICESat-2 altimetry data. Two satellite bathymetry models were trained, to obtain shallow water depth from Sentinel-2 satellite images. First, sea surface and seafloor signal photons from ICESat-2 were detected in the Oahu (in the U.S. Hawaiian Islands) and St. Thomas (in the U.S. Virgin Islands) sampling areas, to obtain water depths along the surface track. The results show that the RMSE is between 0.35 and 0.71 m and the R2 is greater than 0.92, when compared to the airborne LiDAR bathymetry (ALB) data in the field. Second, the ICESat-2 bathymetric points from Oahu Island are used to train the Back Propagation (BP) neural network model and obtain the SDB. The RMSE is between 0.97 and 1.43 m and the R2 is between 0.90 and 0.96, which are better than the multi-band ratio model with RMSE of 1.03–1.57 m and R2 of 0.89–0.95. The results show that the BP neural network model can effectively improve bathymetric accuracy, when compared to the traditional multi-band ratio model. This approach can obtain shallow water bathymetry more easily, without the in-situ bathymetric data. Therefore, it extends to a greater extent with the free ICESat-2 and Sentinel-2 satellite data for bathymetry in shallow water areas, such as coastal, island and inland water bodies.

The dynamics of wave and current supported gravity flows: Evaluation of drag coefficient and bulk Richardson number

Wave and current supported gravity flows (WCSGFs) carry substantial fine-grained sediment across gently sloping seabeds of coastal and shelf waters, representing a primary mechanism of sediment transport. An existing buoyancy-friction model, which considers the important parameters of drag coefficient (CD) and bulk Richardson number (Rib), has been widely used to analyze WCSGF dynamics. However, an improved validation of the two parameters on the basis of in situ measurements is still outstanding. Here, we analyze 23 short-lived WCSGF events that were recorded at the shallow sea floor of a muddy open coast to quantify the parameters. The CD values, as defined in the buoyancy-friction model, were found to be around 0.02 and 0.01 for early flood and ebb phases of the tide, respectively, which are significantly larger than the range of 0.001–0.006 reported elsewhere from in situ observations, numerical simulations and laboratory experiments. The calculations of Rib from our WCSGF observations were all close to the critical value of 0.25, which supports the use of such a value in WCSGF models. Large CD values of our observed WCSGFs imply that strong currents can dilute the high-concentration layer and slow down the movement of WCSGFs, which reveals a remarkable difference between WCSGFs and gravity flows supported by waves alone.

Visualization of shallow subseafloor fluid migration in a shallow gas hydrate field using high-resolution acoustic mapping and ground-truthing and their implications on the formation process: a case study of the Sakata Knoll on the eastern margin of the Sea of Japan

Acoustic mapping enables an understanding of the surface distribution of shallow gas hydrate (GH) and related products. Acoustically characteristic materials such as fluid-seepage-related methane-derived authigenic carbonate and/or shallow GHs, may be widely distributed beneath the shallow seafloor of the Sakata Knoll. High-amplitude reflectors over the knoll are the top of gas-bearing permeable layers and connect to the reverse fault at the foot of the knoll. Shallow GH and bacterial mats were observed at the high-amplitude layer cut by depression and/or the locally disturbed seafloor. Acoustic blanking zones observed on the sub-bottom profiler sections are current gas migration routes from the depth to the seafloor. Optical observations indicate that fluid seepage is not active in the current seafloor, and it is not necessarily observed above the acoustic blanking zones or shallow faults reaching the seafloor. In the Sakata Knoll, the tectonically formed reverse fault and gas-bearing permeable layers play more important roles in fluid migration from depth to the summit area of the knoll compared to acoustic blanking and shallow faults. The depression at the summit area of the Sakata Knoll was formed by the dissociation of a shallow GH at around the last glacial maximum. Limited fluid seepage is currently witnessed within and around the depression and it is less extensive than that in the past. Such knolls, with tectonically formed large faults and an anticline are abundant in the area and they can be good reservoirs for shallow GH along the eastern margin of the Sea of Japan.

Estuaries as coastal reactors: importance of shallow seafloor habitats for primary productivity and nutrient transformation, and impacts of sea level rise

ABSTRACT Estuaries are hotspots of primary productivity and nutrient transformation that contribute to food webs and ecosystem functioning locally and in adjacent ecosystems. The depth-dependence of nutrient transformation and primary productivity rates in the water column and the seafloor were investigated, and the lateral transport of solutes and materials across the estuary mouth were quantified. Using an estuary dominated by shallow soft-sediment habitats as a case study, the effects of sea level rise (SLR) on productivity and nutrient transformation processes were projected. The estuary was a net importer of dissolved nutrients from the coast, and a net exporter of suspended sediments and chlorophyll a, supporting the notion that estuaries are important nutrient transformation reactors. A significant depth (and light) effect on productivity indicates that increasing stressors associated with climate change that reduce light at the seafloor (SLR and increased turbidity) will negatively impact estuarine productivity. Intertidal and shallow subtidal benthic habitats were responsible for most of the productivity of the estuary and this is likely to be consistent for other shallow estuaries globally. SLR and anthropogenic intervention that prevents landward migration (i.e. seawalls and armouring), will result in the loss of these habitats and their significant contributions to adjacent coastal ecosystems.

Application of SPH method in the study of ship capsizing induced by large-scale rising bubble

In some sea areas, the rising storm of bubbles including the release of large-scale methane gas and eruption of submarine volcanoes will pose a potential threat to the safety of ships. In the former case, a large amount of rising methane gas may generate some large-scale bubbles which may cause sailing ships to capsize. When the size of the bubble is close to the characteristic length of the ship, one single bubble could sink a ship. For the latter, the initial radii of the bubbles formed by the eruption of shallow seafloor volcanoes maybe tens even hundreds of meters, which also largely increases the risk of maritime transportation. In this paper, a 2D smoothed particle hydrodynamics (SPH) model is used to investigate the interaction between a ship and large-scale bubbles. Firstly, the adopted SPH model is validated by comparing with other numerical results. Then, the coupling interaction between a bubble and free-surface is simulated. Finally, under different conditions of different bubble sizes, bubble depths and horizontal distances, interactions between a ship and one or two large-scale rising bubbles are simulated and discussed, aiming to provide a reference for the investigation of ship capsizing caused by large-scale rising bubbles.

Gravity-driven sediment flows on the shallow sea floor of a muddy open coast

Wave and/or current supported gravity-driven sediment flows (GDSFs) cause substantial sediment movement across the continental shelf, contributing to morphological evolution in many regions worldwide. However, they appear to occur episodically and ephemerally and, therefore, it remains a challenge to document them in detail with in situ measurements. Here we present solid evidence of frequent generation of such flows over the shallow sea floor of a muddy open coast. They were triggered by wave resuspension and/or sediment settling from the overlying water column, maintained by wave- and/or current-induced bed stress, and terminated due to upward spreading of bottom sediment within the high concentration layer. Randomly selected GDSF events were analyzed to realize parameterization with a buoyancy-friction model; the resultant bed drag coefficient for two of the cases is higher than the value of 0.003, which is attributed to the additional drag at the interface between the overlying flow and the moving GDSFs. Although short-lived, the observed GDSF events in the three different seasons indicate that they occur more frequently than previously thought.

Insights into seafloor litter spatiotemporal dynamics in urbanized shallow Mediterranean bays. An optimized monitoring protocol using towed underwater cameras

Monitoring of marine litter at the sea surface, the beaches and the seafloor is essential to understanding their sources, pathways and sinks and design effective clean-up programs or increase public awareness for reducing litter waste. Up until today, seafloor litter is the least exploited component of marine litter. Although the protocols for recording and assessing seafloor litter in the deep-sea environments are currently being actively defined and practiced, shallow seafloor litter survey protocols are still notably under-developed. Moreover, trawling for fishing, which is the main means for collecting seafloor litter data, needs to be phased out in the coming years due to its high environmental footprint and be replaced by less destructive ways based on underwater imagery. In this paper we propose an integrated approach for assessing in detail the spatiotemporal distribution and composition of seafloor litter in shallow coastal environments, using common towed underwater cameras. Effort has been put to correctly estimating spatial litter densities regarding the true coverage of the visualized area, which was efficiently extracted through photogrammetric reconstruction of the seafloor. Interpretation of the spatial distribution of litter was aided by auxiliary bathymetric and swath sonar backscatter datasets, to determine the seabed geomorphological features that control their dispersion and composition. Local geo-morphology, along with any reported coastal anthropogenic activity, are correlated to seafloor litter densities to investigate the temporal and spatial dynamics that control their distribution and temporal trends in Syros Island, Cyclades, Greece. There, in the context of LIFE DEBAG project, monitoring of an urbanized shallow bay for 3 consecutive years has been performed to assess the impact of an intensive local awareness raising campaign to the local environment. A significant reduction of litter densities under the impact of this campaign has been documented, while links between the seafloor litter transport dynamics and the seabed micro- and macro-topography were made evident. Monitoring litter densities on the seafloor of urbanized shallow bays proved to be a prospective way of tracking marine litter pressures on the local marine environment.

Ice front retreat reconfigures meltwater-driven gyres modulating ocean heat delivery to an Antarctic ice shelf

Pine Island Ice Shelf (PIIS) buttresses the Pine Island Glacier, the key contributor to sea-level rise. PIIS has thinned owing to ocean-driven melting, and its calving front has retreated, leading to buttressing loss. PIIS melting depends primarily on the thermocline variability in its front. Furthermore, local ocean circulation shifts adjust heat transport within Pine Island Bay (PIB), yet oceanic processes underlying the ice front retreat remain unclear. Here, we report a PIB double-gyre that moves with the PIIS calving front and hypothesise that it controls ocean heat input towards PIIS. Glacial melt generates cyclonic and anticyclonic gyres near and off PIIS, and meltwater outflows converge into the anticyclonic gyre with a deep-convex-downward thermocline. The double-gyre migrated eastward as the calving front retreated, placing the anticyclonic gyre over a shallow seafloor ridge, reducing the ocean heat input towards PIIS. Reconfigurations of meltwater-driven gyres associated with moving ice boundaries might be crucial in modulating ocean heat delivery to glacial ice.

Shallow seafloor seismic wave monitoring using 3-component fiber optic interferometric accelerometer

A fiber optic interferometric seismic monitoring system developed by us was deployed on the shallow seafloor near the Wuchang oceanic monitoring station off the Hainan Island, in the South China Sea, in 2017. In the past three plus years, it has recorded more than 40 teleseismic activities of magnitude 6.0+. We analyze these shallow seafloor observed seismic signals monitored by the fiber optic interferometric three-component accelerometer from the perspective of analyzing the underwater ambient seismic noise and the attenuation of the noise in the shallow seafloor seismic signals. The ambient seismic noise produced by wind, wave, shipping, and on-shore manmade activities admixed in the seismic signal is analyzed and discussed in detail. And the attenuation of such noise in the shallow seafloor seismic data using a hybrid denoising algorithm is demonstrated by applying it to three representative seismic events having different distances and magnitudes, with results showing that the noise in the shallow seafloor seismic signal is effectively reduced, and the ratio of the maximum acceleration value of seismic wave to the noise level is improved by more than 32 dB, rendering the seismic waves clearly observable.

Marine litter on the shallow seafloor at Natura 2000 sites of the Central Eastern Adriatic Sea

Shallow seafloor marine litter (ML) is the least studied component of ML, and a better understanding of its distribution and accumulation patterns is needed to develop efficient management practices. Abundance and composition of seafloor ML were quantified in two eastern Adriatic Natura 2000 sites (Vis Island and Pakleni Islands), applying a structured sampling design to test whether the intensity of nautical tourism or exposure to prevailing winds and currents are significant drivers determining the ML distribution on shallow, coastal seafloor. ML was widespread and present in considerable amounts across the entire studied area, regardless of the remoteness from most known potential ML sources. Tourism intensity and exposure seem not to be significant predictors of ML accumulation in the investigated Natura 2000 sites. ML abundances displayed large small-scale spatial variation, while for ML composition at a larger scale, regional differences between the two Natura 2000 sites were detected. The Pakleni Islands are mainly polluted by glass items, suggesting a local vessel-based source. The ML composition on Vis Island, with a predominance of plastics, is more determined by long-distance transport. The study indicates the complexity and multi-layeredness of processes driving the patterns of seafloor ML accumulation. Hence, it is suggested to mitigate the issue by addressing the root causes: inadequate waste management and lack of public awareness.

Hormone-mediated foraging strategies in an uncertain environment: Insights into the at-sea behavior of a marine predator.

Hormones are extensively known to be physiological mediators of energy mobilization and allow animals to adjust behavioral performance in response to their environment, especially within a foraging context.Few studies, however, have narrowed focus toward the consistency of hormonal patterns and their impact on individual foraging behavior. Describing these relationships can further our understanding of how individuals cope with heterogeneous environments and exploit different ecological niches.To address this, we measured between‐ and within‐individual variation of basal cortisol (CORT), thyroid hormone T3, and testosterone (TEST) levels in wild adult female Galápagos sea lions (Zalophus wollebaeki) and analyzed how these hormones may be associated with foraging strategies. In this marine predator, females exhibit one of three spatially and temporally distinct foraging patterns (i.e., “benthic,” “pelagic,” and “night” divers) within diverse habitat types.Night divers differentiated from other strategies by having lower T3 levels. Considering metabolic costs, night divers may represent an energetically conservative strategy with shorter dive durations, depths, and descent rates to exploit prey which migrate up the water column based on vertical diel patterns.Intriguingly, CORT and TEST levels were highest in benthic divers, a strategy characterized by congregating around limited, shallow seafloors to specialize on confined yet reliable prey. This pattern may reflect hormone‐mediated behavioral responses to specific risks in these habitats, such as high competition with conspecifics, prey predictability, or greater risks of predation.Overall, our study highlights the collective effects of hormonal and ecological variation on marine foraging. In doing so, we provide insights into how mechanistic constraints and environmental pressures may facilitate individual specialization in adaptive behavior in wild populations.

Geological and seafloor investigations of the cause of partial submergence of Yokoshima Island, Nagasaki, Japan

Between 1950 and 1980, about half of Yokoshima Island disappeared beneath the East China Sea. The island lies about 1 km off the coast of Nagasaki and was mined for coal during the late 19th and early 20th centuries. We analyzed the geology of the island and surrounding seafloor to investigate how it became submerged. Two low-cost techniques were used to view underwater structures: a commercial drone fitted with a video camera in waters shallower than 5 m and an echo sounder and side-scan sonar system designed for amateur fishers in waters deeper than 5 m. We showed that the partial submergence of the island could be interpreted as a result of subsidence. Subsidence and northward tilting of sedimentary rocks north of the island, probably caused by collapse of abandoned subsea mine workings, created an extensional regime south of the island that led to the formation of steep cliffs and collapse of the southern side of the island. Our study also shows that low-cost commercially available technology can be used for rapid scientific investigations in shallow water areas. • Submergence of half of an island in a 30 year period was investigated. • Low-cost shallow seafloor investigation techniques helped geological analysis. • Subsidence was associated with uneven settlement into abandoned coal mine workings.

紀伊半島最南部，ドローン測量による海陸統合型の標高データの取得と海岸・浅海底の地形分類

紀伊半島最南部，ドローン測量による海陸統合型の標高データの取得と海岸・浅海底の地形分類

Characteristics of shear waves in shallow seafloor sediment in the Middle Southern Yellow Sea

The shear wave characteristics of shallow seafloor sediment are vital to marine acoustic study and ocean engineering exploration. We analyzed the shear wave speed and the physical properties of the southern Yellow Sea sediment based on the Grain-Shearing (GS) model. The results show that: (1) The shear wave speed of the high-porosity sediment is lower than that of the low-porosity sediment; (2) The shear wave speed of the fine-grained sediment is lower than that of the coarse-grained sediment; (3) The burial depth of the sediment has a significant influence on the shear wave speed of the seafloor sediment. The GS model agrees well with the measurement data from the southern Yellow Sea. However, the quantitative analysis of the impact of input parameters on the GS model requires further research.

Ancient Oil as a Source of Carbonaceous Matter in 1.88-Billion-Year-Old Gunflint Stromatolites and Microfossils

The 1.88-billion-year-old Gunflint carbonaceous microfossils are renowned for their exceptional morphological and chemical preservation, attributed to early and rapid entombment in amorphous silica. The carbonaceous matter lining and partly filling filamentous and spherical structures is interpreted to be indigenous, representing thermally altered relicts of cellular material (i.e., kerogen). Here we show that stromatolitic black cherts from the Gunflint Formation, Schreiber Beach, Ontario, Canada, were saturated in syn-sedimentary oil. The thermally altered oil (pyrobitumen), which occurs in the stromatolites and intercolumn sediments, fills pores and fractures, and coats detrital and diagenetic grain surfaces. The occurrence of detrital bitumen grains in the stromatolites points to the proximity of shallow seafloor oil seeps and hence the possible existence of chemosynthetic microbes degrading hydrocarbons. We suggest that hydrocarbons that migrated through the silicifying stromatolites infiltrated semi-hollow microbial molds that formed following silica nucleation on the walls or sheaths of decayed cells. Upon heating, the hydrocarbons were transformed to nanoporous pyrobitumen, retarding silica recrystallization and enhancing detailed preservation of the carbon-rich microfossils. Hydrocarbon infiltration of silicified microbes offers a new explanation for the preservation of the Gunflint microfossils and may have played a role in the formation of some of Earth's oldest microfossils.

Sky-high to the sea floor

Seafloor Bathymetry The ICESat-2 satellite, launched in 2018, was developed primarily to measure the surface elevation of polar ice sheets, but it can do much more than that. One of its other capabilities is determining water depths to as much as 40 meters. Thomas et al. used this ability, combined with remote optical imagery, to map the shallow seafloor in coastal areas of Florida, Crete, and Bermuda. This work demonstrates the viability of using ICESat-2 to help make global, space-borne estimates of nearshore bathymetry in shallow, clear water environments. Geophys. Res. Lett. 10.1029/2020GL092170 (2021).

Glacial melt disturbance shifts community metabolism of an Antarctic seafloor ecosystem from net autotrophy to heterotrophy

Climate change-induced glacial melt affects benthic ecosystems along the West Antarctic Peninsula, but current understanding of the effects on benthic primary production and respiration is limited. Here we demonstrate with a series of in situ community metabolism measurements that climate-related glacial melt disturbance shifts benthic communities from net autotrophy to heterotrophy. With little glacial melt disturbance (during cold El Niño spring 2015), clear waters enabled high benthic microalgal production, resulting in net autotrophic benthic communities. In contrast, water column turbidity caused by increased glacial melt run-off (summer 2015 and warm La Niña spring 2016) limited benthic microalgal production and turned the benthic communities net heterotrophic. Ongoing accelerations in glacial melt and run-off may steer shallow Antarctic seafloor ecosystems towards net heterotrophy, altering the metabolic balance of benthic communities and potentially impacting the carbon balance and food webs at the Antarctic seafloor.

Seagrasses provide a novel ecosystem service by trapping marine plastics

There is strong evidence that the seafloor constitutes a final sink for plastics from land sources. There is also evidence that part of the plastics lying on the shallow seafloor are washed up back to the shoreline. However, little is known on the natural trapping processes leading to such landwards return. Here we investigate microplastics and larger plastic debris within beached seagrass remains including balls (aegagropilae) made of natural aggregates of vegetal fibers intertwined by seawater motion. We found up to 1470 plastic items per kg of plant material, which were mainly composed of negatively buoyant polymer filaments and fibers. Our findings show that seagrass meadows promote plastic debris trapping and aggregation with natural lignocellulosic fibers, which are then ejected and escape the coastal ocean. Our results show how seagrasses, one of the key ecosystems on Earth in terms of provision of goods and services, also counteract marine plastic pollution. In view of our findings, the regression of seagrass meadows in some marine regions acquires a new dimension.