Shallow Water Depth Research Articles

Organic geochemistry, sedimentary environment, and organic matter enrichment of limestone-marlstone rhythms in the middle Permian northern Sichuan Basin, China

Limestone-marlstone (LM-MR) rhythms widely develop in the middle Permian Maokou Formation of Sichuan basin, and they are regarded as potential source rocks and important shale gas targets due to their high organic matter contents. In this study, LM-MR rhythms in the lower part of the Maokou Formation in the Changjianggou section of the Sichuan basin are selected for research because of their moderate thermal maturity of organic matter. A series of studies are implemented to investigate the organic geochemical characteristics, formation processes, organic matter enrichment mechanism, and hydrocarbon potentials of these LM-MR rhythms, from perspectives of sedimentology, organic petrology, and geochemistry. Organic matter in marlstones is suggested to be mainly contributed by cyanobacteria according to organic petrographic results, low regular steranes/17α-hopanes, and high mid-chain branched monomethyl alkanes (MMAs). In contrast, organic matter in limestones is suggested to mainly result from benthic algae. Marlstone samples have more negative carbon and oxygen isotopes and higher Fe/Mn ratios than the limestone samples, suggesting sedimentary environments with shallower water depth and stronger restrictions during marlstone deposition. Moreover, marlstone samples have lower ratios of gammacerane/C30 αβ hopane than limestone samples, suggesting lower water salinity during marlstone deposition. Furthermore, marlstone samples have higher TOC contents than limestone samples, and this is attributed to the less carbonate dilution, the suboxic-anoxic environment, and the increased productivity during marlstone deposition, all of which are initially controlled by the high-frequency relative sea-level change. The hydrocarbon potential analysis suggests that LM-MR rhythms in the study area are with high TOC contents, Type II organic matter, and moderate thermal maturity.

Evaluation of the Applicability of STIV to Wave Characteristic Measurement in the Swash Zone

The swash zone is an area that causes a change in the shape of a beach by generating sediment transport under the influence of intermittent waves, where wave run-up and run-down are infinitely repeated in the final stage of the shoaling process. However, the ability to predict the sediment transport is extremely poor despite the swash zone being an extremely important area in terms of offshore disaster prevention. In particular, many researchers are conducting studies on the development of various types of observation equipment and analysis techniques because the turbulent flow of active fluid dominates the sediment transport and is an extremely important parameter for the analysis of the transport mechanism. However, in flow velocity measurement, it is difficult to measure a quantitative representative flow velocity over time because the swash zone has a shallow water depth and an active turbulent flow. Expensive equipment and short-time measurement are also limitations. Therefore, the purpose of this study was to evaluate the applicability of nonintrusive space-time image velocimetry(STIV) to analyze the flow characteristics of fluid in the swash zone, such as the movement velocity and period of intermittent waves in the shoaling process. The prediction accuracy was improved by removing various noises included in the images with the introduction of artificial intelligence for immediate and accurate calculation of the representative flow velocity using images that can be obtained easily. Consequently, it was discovered that the spatial representative flow velocity occurring in the swash zone, change in the wave period according to the shoaling effect, rip current and surface velocity can be measured.

Boiling effects on trace element and sulfur isotope compositions of sulfides in shallow-marine hydrothermal systems: Evidence from Milos Island, Greece

Boiling is a crucial process triggering ore formation in magmatic-hydrothermal systems and controlling the enrichment of precious and rare metal(loid)s in epithermal-porphyry mineralizations. Steep physicochemical gradients during boiling of hydrothermal fluids at shallow water depths caused metal(loid) precipitation along a 3 km long Pb-Zn-Ag vein system on Milos Island in the South Aegean Volcanic Arc. We present new trace element and Pb, Sr, and S isotope data from sulfides providing insights into the diversity of mineralization processes in shallow-marine hydrothermal systems. Lead and Sr isotope compositions of sulfides and sulfates reflect the mixing of fluids that reacted with metamorphic basement and the volcanic host rocks, whereas some of the S were derived from seawater. Investigation of mineralized samples along the Kondaros-Vani fault zone revealed distinct chemical variations that represent a vertical profile through the boiling zone of a hydrothermal system. Boiling during fluid ascent at decreasing temperatures (230–150 °C) and sulfur fugacities triggered the precipitation of sulfides rich in Zn, Pb, Fe, Cu, Ag, Sb, and As, resulting in increasing Pb, Ag, and Sb contents with decreasing depth. A pyrite group with high Tl/Cu and low As/Sb ratios, as well as δ34SVCDT values reaching as low as −6‰, is interpreted as precipitating from high-Cl liquids that underwent vigorous boiling at deeper crustal levels subsequent to tectonic faulting. Condensation of vapor (high Hg, Bi, and As contents) and mixing with seawater (high Mo contents) in the shallow-marine subseafloor sequences at Vani caused Hg-Bi-Mo-As-rich mineralization similar to the active hydrothermal system at Milos.

Phytoplankton blooms and its influencing environmental factors in the southern Yellow Sea

Phytoplankton blooms are a widespread phenomenon in oceans that support the food webs and ecosystem. In this study, we analyzed temporal and spatial variations of phytoplankton blooms in the southern Yellow Sea (SYS) during 2003–2018, using the 8-day satellite datasets. An empirical orthogonal function (EOF) analysis was performed to evaluate seasonal variability of phytoplankton blooms in the SYS. The first two EOF modes explained the main features of phytoplankton blooms in the SYS. The first EOF mode and the second EOF mode accounted for 49% and 10% of the total spatial variance of blooms in the SYS, respectively. The indices of phytoplankton phenology (bloom timing) revealed distinct phytoplankton growth patterns/regimes in different subregions of the SYS. Specifically, the coastal area of Shandong Peninsula (CASP) showed significantly higher biomass and longer duration of phytoplankton blooms from fall to following early spring, which spanned an average of about 20 weeks. However, spring and fall blooms were observed in the central area of the SYS (CASYS). The spring bloom was more than twice the size of the fall bloom. By exploring the relationships between the phytoplankton biomass and the physical and biological parameters between these two regions, we established that a relatively stable shallow water depth and sufficient nutrients concentrations were the mechanisms driving phytoplankton blooms.

SATELLITE-DERIVED BATHYMETRY USING CONVOLUTIONAL NEURAL NETWORKS AND MULTISPECTRAL SENTINEL-2 IMAGES

Abstract. Satellite-Derived Bathymetry (SDB) has been used in many applications related to coastal management. SDB can efficiently fill data gaps obtained from traditional measurements with echo sounding. However, it still requires numerous training data, which is not available in many areas. Furthermore, the accuracy problem still arises considering the linear model could not address the non-relationship between reflectance and depth due to bottom variations and noise. Convolutional Neural Networks (CNN) offers the ability to capture the connection between neighbouring pixels and the non-linear relationship. These CNN characteristics make it compelling to be used for shallow water depth extraction. We investigate the accuracy of different architectures using different window sizes and band combinations. We use Sentinel-2 Level 2A images to provide reflectance values, and Lidar and Multi Beam Echo Sounder (MBES) datasets are used as depth references to train and test the model. A set of Sentinel-2 and in-situ depth subimage pairs are extracted to perform CNN training. The model is compared to the linear transform and applied to two other study areas. Resulting accuracy ranges from 1.3 m to 1.94 m, and the coefficient of determination reaches 0.94. The SDB model generated using a window size of 9x9 indicates compatibility with the reference depths, especially at areas deeper than 15 m. The addition of both short wave infrared bands to the four visible bands in training improves the overall accuracy of SDB. The implementation of the pre-trained model to other study areas provides similar results depending on the water conditions.

Extreme guanotrophication by phosphorus in contradiction with the productivity of alkaline soda pan ecosystems

Waterbirds as nutrient vectors can cause high phosphorus loading in shallow inland aquatic ecosystems. The main goal of this study was to determine the causal relationships between the characteristic physico-chemical properties of intermittent (temporary) alkaline soda pan (playa) ecosystems and specific (surface and volume-related) P loading of waterbirds by in situ field investigation, estimation as well as laboratory experiments using standard methods. In addition, our aim was to estimate the contribution of groundwater and precipitation to the total phosphorus pool of soda pans in Hungary.The estimated high specific external P loading of waterbirds (mean: 185 mg P/m2/y, 3.32 mg P/L/year) can explain the majority of the hypertrophic TP pool (mean: 5.17 mg/L, 64%) in soda pans, which is mediated by large-bodied herbivorous (e.g. geese and ducks) and medium-bodied omnivorous (e.g. gulls) waterbirds, who are important external nutrient importers and major phosphorus source. The results also confirm the hypothesis that groundwater (3%) and precipitation (5%) together account for a smaller estimated (8% in this study) contribution to the hypertrophic TP pool in soda pans, while the contribution of waterbirds (64% in this study) to the TP is much higher (64–100%). In this study, the remaining part of TP (maximum 28%) pool can be explained by internal P sources.Soda pans are characterized by physical and chemical characteristics coupled with high densities of waterbirds, as biotic mediators of external P sources, which together cause the maintenance of high concentrations of P-forms. The extreme guanotrophication by high P loading of herbivorous waterbirds causing a hypertrophic state is in contradiction with the limited primary production of natural soda pans. This unique phenomenon can be explained by the multiple impact of prevailing extreme physico-chemical drivers (intermittent hydrological cycle, shallow water depth, high turbidity, salinity, alkalinity) and by the specific nutrient cycle of these alkaline soda ecosystems.

Ultra-long period and small-amplitude tsunami generated following the July 2020 Alaska Mw7.8 tsunamigenic earthquake

The July 2020 Mw7.8 Alaska tsunamigenic earthquake was a fresh call for potential large tsunamis associated with the Aleutian subduction zone. The second largest ever-recorded earthquake worldwide (Mw 9.2) occurred in this zone in 1964 indicating its massive earthquake and tsunami risk. Our analysis of the July 2020 tsunami revealed that it involves very long period waves (51–64 min) which is unusual for an Mw7.8 earthquake. The tsunami coastal amplitude was small (~0.5 m) which is much smaller than that usually expected from a tsunamigenic earthquake of this size. Here, through numerical simulations and spectral analyses, we explain the ultra-long period and small amplitude waves of the tsunami. Our analysis using an analytical equation showed that the ultra-long period of the tsunami (51–64 min) can be reproduced using the shallow water depth around the source region (100–200 m) and the length of the coseismic deformation area (~100 km). By comparing the coastal amplitude of this event with four other similar-size and similar-mechanism earthquakes (thrust Mw7.8), we attribute the relatively small coastal amplitude of this tsunami to the deep focal depth of the earthquake (28 km) and the extremely shallow water around the source region.

Feasibility studies of a novel spar-type floating wind turbine for moderate water depths: Hydrodynamic perspective with model test

Spar-type platforms have been widely applied in ocean engineering and first get commercial applications in the offshore wind industry. The spar-type platform is typically deployed in deep waters whereas shallow and moderate water depths are regarded as more ideal regions for deploying floating wind turbines (FWT). However, the deep draft of spar-type platform limits its use in these regions, encountering great challenges. This paper aims to seek a solution for deploying spar-type FWTs in moderate water depths of about 100 m. To this end, a novel Stepped Short Spar for Shallow water is developed by Shanghai Jiao Tong University (SJTU) with a draft of 50 m, abbreviated as SJTU-S4. Then, a model system is designed and manufactured at a scaling ratio of 1:50. An integrated experimental apparatus is constructed and comprehensive model tests are carried out. The hydrodynamic characteristics of the proposed SJTU-S4 FWT are objectively evaluated referring to those of the typical OC3 Hywind FWT. The model test results indicate that the SJTU-S4 FWT has reasonable hydrodynamic performance in water depth of 100 m. This study suggests that the application of SJTU-S4 FWT concept is feasible and can be an alternative in offshore wind exploitation in moderate water depths.

DNA/RNA metabarcoding and morphological analysis of epipelagic copepod communities in the Izu Ridge off the southern coast of Japan

AbstractThe Izu Ridge off the southern coast of Japan is composed of many seamounts and knolls and is an important fishery ground and hatchery for commercially important fish. As pelagic copepods link lower and higher trophic levels, we investigated the epipelagic copepods in the Izu Ridge using 28S DNA/RNA metabarcoding and microscopic approaches. In comparison with off-ridge regions, a low water temperature and high chlorophyll a concentration were observed in the Izu Ridge, possibly attributed to high levels of nutrients supplied by strong turbulences around the complex topography. We observed unique copepod communities with a large total biomass in the Izu Ridge, owing to the increased dominance of large copepod species. rRNA:rDNA (28S ratio) of these large species mainly showed positive correlations with the chlorophyll a concentration, suggesting a rapid response to the food supply. Migrating copepods were less dominant around the Izu Ridge with a shallow water depth. Higher species diversity and more rare species were detected in the Izu Ridge, which might be maintained by complex environment conditions including temporal and spatial variations in the concentration of chlorophyll a. Thus, the combined approach characterized copepod community structure and diversity supporting the rich pelagic ecosystems in the Izu Ridge.

Wave Dissipation and Sediment Transport Patterns during Shoreface Nourishment towards Equilibrium

Implementing shoreface nourishment is an effective method to protect sandy beaches. A better understanding of the equilibrium mechanism of shoreface nourishments is necessary for coastal engineering designs and constructions. Two experiments on the beach profile equilibrium of the shoreface nourishment are carried out under mild wave conditions on the reflective and intermediate beach. It is observed that the shoreface nourishment increases local wave height and strengthens wave nonlinearity by its shallow water depth. The most intense wave breaking dissipation has been found on the crest of the shoreface nourishment, and the distribution of wave energy dissipation rate is more uniform on the quasi-equilibrium profile than that on the initial profile. A process-based numerical model is used to reproduce bed profile evolution successfully. On that basis, it is found that onshore bedload transport is the primary cause for the onshore migration of the shoreface nourishment. The magnitude of bedload transport decreases during the evolution of the shoreface nourishment towards equilibrium. The most intense sediment transport rate occurs over the shoreface nourishment or in front of the shoreline, depending on the ’lee effect’ of the nourishment. Furthermore, the effects of incident wave height, wave period, and sea-level rise on the equilibrium profile of the shoreface nourishment under mild wave conditions are analyzed.

Freak wave in high-order weakly nonlinear wave evolution with bottom topography change

In deep-water, quasi-resonant four-wave interaction gives rise to the occurrence probability of extreme wave height, which leads to freak wave as a natural disaster. As water wave enters shallow water, four-wave interaction becomes weak and wave shoaling makes this effect become complicated. This study conducts a Monte Carlo simulation in modified Nonlinear Schrödinger equation considering bottom topography change, and gives spatial evolution of high-order nonlinearity in wave train from statistical moment. The result indicates, slope angle plays an important role in a shallow water depth, and steep slope brings about a significant enhancement of extreme event in distribution of maximum wave height and surface elevation.

On using the finescale parameterization and Thorpe scales to estimate turbulence from glider data

AbstractTurbulence plays a key role in many oceanic processes, but a shortage of turbulence observations impedes its exploration. Parameterizations of turbulence applied to readily-available CTD data can be useful in expanding our understanding of the space-time variability of turbulence. Typically tested and applied to shipboard data, these parameterizations have not been rigorously tested on data collected by underwater gliders, which show potential to observe turbulence in conditions that ships cannot. Using data from a 10-day glider survey in a coastal shelf environment, we compare estimates of turbulent dissipation from the finescale parameterization and Thorpe scale method to those estimated from microstructure observations collected on the same glider platform. We find that the finescale parameterization captures the magnitude and statistical distribution of dissipation, but cannot resolve spatiotemporal features in this relatively shallow water depth. In contrast, the Thorpe scale method more successfully characterizes the spatiotemporal distribution of turbulence; however, the magnitude of dissipation is overestimated, largely due to limitations on the detectable density overturn size imposed by the typical glider CTD sampling frequency of 0.5 Hz and CTD noise. Despite these limitations, turbulence parameterizations provide a viable opportunity to use CTD data collected by the multitude of gliders sampling the ocean to develop greater insight into the space-time variability of ocean turbulence and the role of turbulence in oceanic processes.

Seafloor Methane Seepage Related to Salt Diapirism in the Northwestern Part of the German North Sea

This study focuses on seafloor methane seep sites and their distribution in the northwestern part of the German North Sea. Methane seepage is a common phenomenon along marine shelves and known to occur in the North Sea, but proof of their existence was lacking in the study area. Using a ship-based multibeam echosounder we detected a minimum of 166 flares that are indicative for free gas releases from the seafloor in the German “Entenschnabel” area, which are not related to morphologic expressions at the seafloor. However, a group of small depressions was detected lacking water column anomalies but with indications of dissolved fluid release. Spatial analysis revealed that flares were not randomly distributed but show a relation to locations of subsurface salt diapirs. More than 60% of all flares were found in the vicinity of the salt diapir “Berta”. Dissolved methane concentrations of ∼100 nM in bottom waters were ten times the background value in the “Entenschnabel” area (CH4< 10 nM), supporting the finding of enhanced seepage activity in this part of our study area. Furthermore, locations of flares were often related to acoustic blanking and high amplitude reflections in sediment profiler echograms, most prominently observed at location Berta. These hydroacoustic signatures are interpreted to result from increased free gas concentrations in the sediments. Electromagnetic seabed mapping depicts local sediment conductivity anomalies below a flare cluster at Berta, which can be explained by small amounts of free gas in the sediment. In our area of interest, ten abandoned well sites were included in our mapping campaign, but flare observations were spatially not related to these wells. Naturally seeping methane is presumably transported to the seafloor along sub-vertical faults, which have formed concurrently to the updoming salt. Due to the shallow water depths of 30 to 50 m in the study area, flares were observed to reach close to the sea surface and a slight oversaturation of surface waters with methane in the flare-rich northeastern part of the working area indicates that part of the released methane through seepage may contribute to the atmospheric inventory.

Technology Focus: Extended-Reach and Complex Wells (May 2021)

In selecting papers for this feature, reviewer Stéphane Menand of Helmerich and Payne has identified a trio of papers that investigates new approaches toward familiar issues encountered when drilling complex well types. Whether considering the customization of drilling approaches in Middle Eastern carbonate reservoirs, implementing a collaborative work flow in tackling high-tortuosity wells offshore Western Australia, or researching the ability of a fibrous material to effect hole cleaning as opposed to polymeric sweeps, the authors of these papers understand that technical expertise may not be completely realized if it is not applied to problems in original ways. In carbonate reservoirs, the goal of drilling extended-reach wells is set against the geological makeup of such formations, the complexity of which adds significant uncertainty to geosteering and well placement. The authors of paper SPE 203335 develop a work flow that makes possible the customization of drilling scenarios through an emphasis on mechanical specific energy, as well as the use of an optimized borehole-assembly design. The work flow helped deliver what the authors write is the longest well in the Middle East offshore Abu Dhabi. In a similar vein, the authors of paper SPE 202251 describe a challenging scenario involving an ultraextended-reach well in a mature field offshore Western Australia. The project overcame shallow water depth and a high tortuosity requirement by implementing an integrated plan that used a reservoir-mapping-while-drilling service. The authors stress that this technology, coupled with active collaboration between specialists, town, and rig site, allowed the project to achieve the desired oil-column thickness with zero collision incidents. Highly deviated wells often face problems resulting from ineffective hole cleaning. Paper SPE 203147 studies the properties of a fibrous material when compared with the hole-cleaning performance of common polymeric pills. The authors write that the fibrous material proved effective, in part because of a unique characteristic in which a spiderweb-like network of fibers is created that does not allow cuttings to settle easily in complex wells. In addition, the material is environmentally friendly. All three papers approach well- established problems in the critical industry sector of extended-reach drilling with innovation and confidence. Enjoy the papers and be sure to search SPE’s OnePetro online library for more fresh approaches to the technical challenges posed by these well types. Recommended additional reading at OnePetro: www.onepetro.org. SPE 196410 - Analysis of Friction-Reduction System During Drilling Operation at a High-Inclination Well on Field X by Rizqiana Mudhoffar, Tanri Abeng University, et al. SPE 197257 - Successful Management of Collision Risk in an Extended-Reach Well by Manchukarn Naknaka, Mubadala Petroleum, et al. SPE 202730 - Challenges in Drilling and Completion of Extended-Reach-Drilling Wells With Landing Point Departure of More Than 10,000 ft in Light/Slim Casing Design by Nitheesh Kumar Unnikrishnan, Abu Dhabi National Oil Company, et al.

Unravelling evidence for global climate change in Mississippian carbonate strata from the Derbyshire and North Wales Platforms, UK

The Mississippian Derbyshire and North Wales carbonate platforms were formed in similar tectonic settings within the Pennine and East Irish Sea Basin, respectively. The Derbyshire Platform was surrounded by sub-basins to the north, west and south whilst the North Wales Platform, 130 km west, had a simpler land-attached geometry. Comparison of these age-equivalent platforms allows the controls on sedimentation, at an important juncture in Earth history, to be evaluated. Both platforms are dominated by moderate-to-high-energy, laterally discontinuous facies, with weak evidence for facies cyclicity, suggesting multiple controls on deposition. Influx of siliciclastic mud on the North Wales Platform led to perturbations in carbonate accumulation; along with abundant palaeosols and coal beds this implies a more humid climate, or shallower water depths compared to the Derbyshire Platform. On both platforms, exposure surfaces can rarely be correlated over >500 m except for a regionally correlative palaeokarstic surface at the Asbian–Brigantian boundary. This exposure event appears to coincide with a significant regional facies change. Given the lack of evidence for ordering and cyclicity within the strata, the Asbian–Brigantian boundary may mark a significant event that could reflect the onset of a transitional climate, prior to the second glaciation event in the Late Palaeozoic Ice Age.

Shallow Water Bathymetry Mapping of Xinji Island Based on Multispectral Satellite Image using Deep Learning

Nearshore bathymetry is a basic parameter of the ocean, which is crucial to the research and management of coastal zones. Previous studies have demonstrated that remote sensing techniques can be employed in estimating bathymetric information. In this paper, we propose a deep belief network with data perturbation (DBN-DP) algorithm for shallow water depth inversion from high resolution multispectral data, and applying it in Xinji Island of Malacca Strait and Yongxing Island in China. Results show that the DBN-DP method can produce more accurate water depth estimations than other traditional methods particularly for deeper water, which reaches 1.2 m of mean absolute error (MAE) and 12.8% of mean relative error (MRE) in Xinji Island. Most of the estimated bathymetry meet the category of zone of confidence C level defined by the International Hydrographic Organization. These findings are encouraging for employing deep learning in bathymetry, which may become a novel approach for bathymetric inversion in the future.

Habitat use and activity patterns of female Deacon Rockfish (Sebastes diaconus) at seasonal scales and in response to episodic hypoxia

We combined a high-resolution acoustic telemetry array with presence/absence receivers to conduct a preliminary study of the seasonal movements, activity patterns, and habitat associations of the newly described Deacon Rockfish (Sebastes diaconus). Eleven mature female Deacon Rockfish were tagged and monitored during an 11-month period, at a nearshore rocky reef off Seal Rock, Oregon, USA, an area of recurring seasonal hypoxia (defined as dissolved oxygen concentration [DO] < 2 mg l−1). Two tags were detected leaving the study area by day 35, indicating predation or emigration. Three tags became inactive within the array, indicating tag loss or fish death. Six “resident” fish inhabited the array for 246–326 days. Resident fish exhibited high site fidelity, small home ranges (mean 95% KDE = 4907 m2), and consistent activity patterns for the duration of the summertime high-resolution array (5 months), except during seasonal hypoxia. Resident fish were strongly diurnal in summer, with high levels of daytime activity above the bottom in relatively rugose habitat, followed by nighttime rest periods in deeper, less rugose habitat. During summertime hypoxia, resident fish exhibited less daytime activity during daytime hours with no rest periods at night, inhabited shallower water depths, and moved well away from their core activity areas on long, erratic forays. During the winter, diel patterns were less evident with higher activity levels at night (than in the summer) and lower activity levels in the day (than in the summer). We propose that some Deacon Rockfish continuously inhabit nearshore reefs throughout the year, but that daily/seasonal movement patterns, seasonally occurring hypoxia, and prey preferences for planktonic organisms influence relocation.

Geomorphometric analysis of seabed pockmarks, offshore western Niger Delta: A case study of the Freeman Field

The morphometry and spatial distribution of seabed pockmarks have been used as proxies for subsurface conditions and local hydrodynamics. We have characterized and analyzed the distribution of seabed pockmarks in the Freeman Field, offshore western Niger Delta using a high-resolution 3D seismic data to understand the relationships between pockmarks and their controlling factors. We identified a total of 684 pockmarks in the Freeman Field at water depths between 1461 and 2395 m. The pockmarks are circular, elliptical, and elongated in plan view, having U-, V-, and W-shaped geometries in cross-sectional view. The average length, width, and depth of the pockmarks are 210, 111, and 15 m, respectively. More than 80% of the pockmarks were randomly distributed. From statistical analysis, most of the pockmarks occurred within a water depth range of 1600–2100 m. The randomly distributed pockmarks occurred at variable water depths whereas the pockmarks that were aligned along fault planes occurred at shallow water depths (approximately 1400–1700 m). However, those confined within the canyon occurred at deeper water depths (approximately 1700–2400 m). Our results show no correlation between the water depth and any of the pockmark dimensions; therefore, we hypothesize that changes in water depth had no effect on any of the pockmark dimensions because the Freeman Field is located at water depths greater than 1000 m where the current velocity range is lower (0.2 and 0.42 m/s). Hence, pockmark dimensions were comparatively uniform throughout the study area. We suggest that the variation in pockmark morphometry is linked to seafloor currents and the activity history of the pockmarks whereas the spatial distribution is linked to structural and stratigraphic discontinuities. Furthermore, our results give insights to the factors that should be considered during risk assessment before hydrocarbon exploration and production.

Assessing Marginal Shallow-Water Bathymetric Information Content of Lidar Sounding Attribute Data and Derived Seafloor Geomorphometry

Shallow-water depth estimates from airborne lidar data might be improved by using sounding attribute data (SAD) and ocean geomorphometry derived from lidar soundings. Moreover, an accurate derivation of geomorphometry would be beneficial to other applications. The SAD examined here included routinely collected variables such as sounding intensity and fore/aft scan direction. Ocean-floor geomorphometry was described by slope, orientation, and pulse orthogonality that were derived from the depth estimates of bathymetry soundings using spatial extrapolation and interpolation. Four data case studies (CSs) located near Key West, Florida (United States) were the testbed for this study. To identify bathymetry soundings in lidar point clouds, extreme gradient boosting (XGB) models were fitted for all seven possible combinations of three variable suites—SAD, derived geomorphometry, and sounding depth. R2 values for the best models were between 0.6 and 0.99, and global accuracy values were between 85% and 95%. Lidar depth alone had the strongest relationship to bathymetry for all but the shallowest CS, but the SAD provided demonstrable model improvements for all CSs. The derived geomorphometry variables contained little bathymetric information. Whereas the SAD showed promise for improving the extraction of bathymetry from lidar point clouds, the derived geomorphometry variables do not appear to describe geomorphometry well.

Abundance of a recently discovered Alaskan rhodolith bed in a shallow, seagrass-dominated lagoon

Abstract Rhodoliths are important foundation species of the benthic photic zone but are poorly known and rarely studied in Alaska. A bed of Lithothamnion soriferum rhodoliths was discovered in 2008 in Kinzarof Lagoon, Alaska, a shallow-water embayment dominated by eelgrass (Zostera marina). Rhodolith spatial extent and biomass were estimated to assess trends and environmental factors that may influence rhodolith distribution and abundance during four years spread over a 12-year period (2008–2010, and 2019). Presence and biomass of rhodoliths were negatively associated with percent eelgrass cover. Biomass of rhodoliths also decreased with increased water temperature. Rhodoliths occurred in two primary areas of the lagoon, a 182 ha core area in a shallow water (mean tide depth of −0.03 m MLLW) tidal channel with low eelgrass density, and a 22 ha outlying area at shallower water depths (&gt;0.2 m MLLW) with moderate to high eelgrass cover. There was no apparent trend in rhodolith biomass over the study period despite wide variation in mean annual estimates. This study establishes a baseline for continued investigations and monitoring of this important benthic resource in Alaska.