Continental Shelf Research Articles

The Underworld: Journeys to the Depths of the Ocean; Deep Water: From the Frilled Shark to the Dumbo Octopus and from the Continental Shelf to the Mariana Trench; A Window into the Ocean Twilight Zone: Twenty-four Days of Science at Sea

The Underworld: Journeys to the Depths of the Ocean; Deep Water: From the Frilled Shark to the Dumbo Octopus and from the Continental Shelf to the Mariana Trench; A Window into the Ocean Twilight Zone: Twenty-four Days of Science at Sea

:Deep Water: From the Frilled Shark to the Dumbo Octopus and From the Continental Shelf to the Mariana Trench

:<i>Deep Water: From the Frilled Shark to the Dumbo Octopus and From the Continental Shelf to the Mariana Trench</i>

Tidal Conversion into Vertical Normal Modes by Near-Critical Topography

Abstract The solution from linear theory for the barotropic-to-baroclinic tidal energy conversion into vertical modes is validated with numerical simulations and analytical results. The main result is the translation of the traditional critical slope condition into a modewise condition on the topographic height only. Our findings are then used for estimates of the global M2 tidal conversion into the first 10 vertical modes in the open ocean (excluding the continental shelves and slopes). We observe a rapid increase with mode number of the fraction of the World Ocean where linear theory is invalid. In terms of conversion, which is highly variable in space, this corresponds to an even more rapid increase with mode number of the fraction of the converted energy that is strongly affected by nonlinear effects. Out of the 373.6 GW of the globally integrated conversion into modes 1–10, only 241.7 GW occur in locations where linear theory is valid. While it represents 95% for mode 1, this fraction rapidly drops with mode number to reach 27% for mode 10. Moreover, for the conversion into a single mode, we show that capping the linear solution at supercritical topography is inappropriate. Hence, linear theory appears unfit to directly quantify the role played by high-mode internal tides in the internal wave energy budget.

Provenance, Intensity of Weathering, and Economic Potential of Placer-bearing Sediments on the Shelf off Quilon, Southwestern Coast, India

ABSTRACT In this study, the mineralogy and geochemistry of sediments of the continental shelf off Quilon, southwest coast, India is used as proxies to constrain their provenance, the intensity of weathering at source, and economic potential. The weathering index calculations were carried out on the sediments and the results showed that Chemical Alteration Index (CIA) (Average = 72.88), Chemical Index of Weathering (CIW) (Average = 92.72), and Plagioclase Alteration Index (PIA) (Average = 89.74) are high, indicating moderate to intense source area weathering. High source area weathering is also supported by high Th/U ratio (Average = 5.68) which is above the average crustal Th/U ratio of 3.8. The sediments are first-cycle sediments derived from the nearby source area, as evidenced by the identification of unaltered heavy minerals and rock-forming minerals, along with a high Average Composition Variability Index (ICV) (Average = 1.20). The high K2O/Na2O ratios (Average = 3.88), and LREE/HREE ratios (Average = 20), plotting of samples in the felsic field of various binary plots (SiO2 vs. Al2O3/TiO2, Zr vs. TiO2, Ce vs. La/Yb and La/Th vs. Hf binary plots) and normalized REE patterns with LREE enrichment and negative Eu anomaly indicate the sediments were primarily derived from Khondalites of the Trivandrum block and were transported by the Kallada and Ithikkara rivers. Average Σ REE, Th, and U concentrations are 222 ppm, 15.2 ppm, and 2.58 ppm respectively in the bulk sediments. The heavy mineral separates showed significantly increased concentrations of Σ REE (Average = 3009 ppm), Th (Average = 478 ppm), and U (Average = 30 ppm) due to the presence of monazite. Due to high Σ REE, Th, and U concentrations in the sediments due to monazite, present study recommend for further exploration of these sediments, which could contribute significantly to the resource base of the nation.

Habitat shifts of the vulnerable crinoid Leptometra phalangium under climate change scenarios

Crinoid beds of Leptometra phalangium (Müller, 1841) have been identified as sensitive habitats by the General Fisheries Commission for the Mediterranean (GFCM) due to their high vulnerability to bottom trawl fisheries. Poorly resilient to physical damage, L. phalangium has been listed as a potential indicator of Vulnerable Marine Ecosystems (VMEs) in the Mediterranean Sea. If fishing activities represent the main cause of habitat destruction for this species, the ongoing changes in climate conditions may rapidly exacerbate the process. In this study, we developed an ensemble Species Distribution Modeling framework to predict the potential habitat of L. phalangium for present-days in the Mediterranean Sea, and used the model to infer potential changes in its spatial distribution by 2050 under two different climate scenarios (IPCC Representative Concentration Pathways RCP2.6 and RCP8.5). True presence-absence records were used and correlated to a parsimonious set of environmental predictors considered as important drivers of benthic species distribution. In present conditions, L. phalangium seems to be widely distributed along the continental slopes of the western and central Mediterranean. This crinoid is often described as confined to the continental shelf-break (100–200 m), but our results show that it can be found over a wider depth range, between 100 and 500 m. Our predictions obtained for the mid-21st century indicate an important habitat loss for L. phalangium under future climate conditions, mainly in the central and southern basins. Declines of 50 to 70 % in its suitable habitat were predicted under RCP2.6 and RCP8.5 compared to present-day predictions. Climate refugia (i.e., areas where environmental conditions remain suitable for the species in the future) were restricted to the northwestern basin (e.g., Gulf of Lion, the Catalan Sea, the Balearic Sea, Ligurian Sea) and in the southern Adriatic Sea. Provided by a robust statistical framework, climate refugia predictions, along with uncertainty assessments, must support the identification of priority areas for the conservation of VME indicator species by governmental bodies and regional management organizations.

AUV Observations of Langmuir Turbulence in a Stratified Shelf Sea

Abstract Measurements collected by a Remote Environmental Monitoring Units (REMUS) 600 autonomous underwater vehicle (AUV) off the coast of southern California demonstrate large-scale coherent wave-driven vortices, consistent with Langmuir turbulence (LT), and played a dominant role in structuring turbulent dissipation within the oceanic surface boundary layer. During a 10-h period with sustained wind speeds of 10 m s−1, Langmuir circulations were limited to the upper third of the surface mixed layer by persistent stratification within the water column. The ensemble-averaged circulation, calculated using conditional averaging of acoustic Doppler dual current profile (AD2CP) velocity profiles using elevated backscattering intensity associated with subsurface bubble clouds, indicates that LT vortex pairs were characterized by an energetic downwelling zone flanked by broader, weaker upwelling regions with vertical velocity magnitudes similar to previous numerical studies of LT. Horizontally distributed microstructure estimates of turbulent kinetic energy dissipation rates were lognormally distributed near the surface in the wave mixing layer with the majority of values falling between wall layer scaling and wave transport layer scaling. Partitioning dissipation rates between downwelling centers and ambient conditions suggests that LT may play a dominant role in elevating dissipation rates in the ocean surface boundary layer (OSBL) by redistributing wave-breaking turbulence.

Датирование современной осадочной толщи краевых бассейнов Восточного Мурмана (Кольский полуостров) на примере губ Ярнышная и Зеленецкая

The authors have studied sediment genesis in the Kola Peninsula bays, the most popular and developed part of the coastal shelf by the population. The main tool used is the method of radioisotope chronology of sedimentary layers based on changes in the specific activity of the natural isotope 210Pb and technogenic 137Cs. It has been revealed that the marginal basins of Murman are characterized by monotonous sedimentation in recent centuries. The average sedimentation rate is 0.15—0.17 cm/year. The sedimentary sequence is composed of silty-pelite-sized material. It has been established that the role of ice rafting in the formation of bottom sediment in coastal bays has decreased over the past centuries.

Assessment of tidal current potential in the Amapá’s inner continental shelf (Eastern Amazonia - Brazil)

Assessment of tidal current potential in the Amapá’s inner continental shelf (Eastern Amazonia - Brazil)

Large Porewater‐Derived Carbon Outwelling Across Mangrove Seascapes Revealed by Radium Isotopes

AbstractMangrove‐dominated coastlines have high carbon sequestration capacity, but it remains unclear whether tidally outwelled carbon is transformed within the coastal ocean or exported offshore. Here, we used radium isotopes (224Ra and 223Ra) to investigate carbon outwelling in two mangrove seascapes in Brazil across multiple spatial scales. We sampled porewaters to define the source composition, mangrove creek waters to resolve tidal cycles, and cross‐shelf transects to trace outwelling in coastal seascapes. Radium isotopes were positively correlated with dissolved inorganic (DIC), organic (DOC) and particulate organic (POC) carbon across the seascapes. DIC was the primary form of carbon (mean ± SD), representing 85% of the total carbon pool as bicarbonate (75 ± 11%), carbonate (6 ± 5%), and CO2 (4 ± 9%). DOC and POC accounted for 10 ± 6% and 5 ± 6% of total carbon, respectively. Although mangrove waters emitted CO2 to the atmosphere (38–143 mmol m−2 d−1), both bays and continental shelves were a CO2 sink (−2.5 to −0.5 mmol m−2 d−1) associated to chlorophyll‐a enrichments (r2 = 0.86). Total carbon outwelled from mangroves were 3–4 times higher than soil carbon burial at both mangrove sites. Bicarbonate export (27–72 mmol m−2 d−1) to the continental shelf was the major fate of carbon outwelling, more than doubling the perceived capacity of mangrove soil to sequester carbon. Hence, disregarding outwelling as a blue carbon sink mechanism would lead to underestimated assessments of how mangroves capture CO2 and help to mitigate climate change.

Holocene to present-day coastal landscapes of Bar al Hikman (Oman): Neolithic waypoints on the shores of the Arabian Sea

Dramatic climatic and environmental changes over the last 12,000 years have significantly impacted Arabian coastal stratigraphy and human populations. The Bar Al Hikman peninsula (BAH), the largest low-lying area (1000 km2) along the Arabian Sea coast of Oman is a monsoon storm-dominated carbonate-evaporite system, where late Neolithic artifacts suggest human presence from at least 5.75–5.05 ka before the present (BP). Despite its archaeological significance at the crossroads of important Neolithic coastal sites in Dhofar, Masirah, and Ja'alan coast, paleoenvironmental interpretation, timing, and expression of relative sea level (RSL) changes in this coastal area remain poorly understood. Fortunately, the extensive size and arid climate of the area have preserved carbonate-dominated Mid-Late Holocene coastal geomorphologies exceptionally well, making this area an excellent geological archive to study past coastal environments, thereby understanding how environmental changes at BAH influenced Neolithic human mobility and settlement patterns. This study reconstructs the paleoenvironment of BAH over the last 12,000 years using a combination of field and remote sensing techniques, including satellite imagery, digital elevation models, bathymetric data, GIA modeling, and Sea Level Index Points to trace RSL history. The Holocene transgression began flooding the extensive continental shelf offshore BAH around 10 ka BP, progressively separating Masirah from the mainland through channel formation between 9.5 and 8.0 ka BP. It then inundated the present-day BAH peninsula around 7.7 ka BP, reaching a highstand of 2.5–3.2 m above present sea level by 6.0 ka BP (Mid-Holocene Highstand; MHHS), before gradually declining to the current level. Human presence documented at BAH (5.75–5.05 ka BP) coincides with the end of the mid-Holocene highstand, and the onset of the regression. This relative sea level history is contemporaneous to the aridification of Arabia, a period of transition from mangrove-dominated intertidal settings to coral reef and carbonate coastal barriers. The disappearance of mangrove-like gastropods around 5.4 cal ka BP and the subsequent appearance of significant coral fragments in the sediment indicates a critical change in terms of environmental settings with less nutrients and a warmer sea surface temperature). Comparing the findings at BAH to well-established nearby Neolithic sites on Masirah Island, the scarcity of prehistoric remains at BAH suggests that during the Mid-Holocene Highstand (MHHS), the small, rocky, paleo-low-lying islands at BAH may have served as waypoints and shelters between Masirah and the mainland.

An explorative Bayesian analysis of functional dependencies in emergency management systems

AbstractThe study of emergency or crisis management practices acquires strategical relevance for resilient decision‐making under uncertainty. The assessment of system resilience is an asset to identify potential design or operational improvements of a complex socio‐technical system, such as an Emergency Management (EM) system. This research aims at analyzing the functional properties of an EM system recurring to a novel integration of the Functional Resonance Analysis Method (FRAM) and Bayesian Belief Networks (BBN). The FRAM is used to model and display the actors and the interactions in the system, while the BBN, dynamically updated when new data becomes available, supports a complementary quantitative assessment. The methodology is iterated in the analysis of an EM procedure, issued by a second‐line Emergency Response organization for Oil and Gas (O&amp;G) operators in Norwegian continental shelf. The results of the study show that the proposed stochastic methodology compensates the drawbacks of traditional FRAM modeling, via the outcomes of BBN quantitative analyses. The findings, contextualized in EM, can be transferred to different socio‐technical contexts, both military and civil ones.

Tectonically-controlled biofacies distribution in the Eocene Foraminiferal Limestone (Pag, Croatia): A quantitative-based palaeontological analysis

This paper analyses the lower to middle Eocene carbonate succession exposed at the Island of Pag (Croatia), in the External Dinarides, providing its palaeoenvironmental, palaeobathymetric, and stratigraphic reconstruction. A total of 125 samples have been collected within the Foraminiferal Limestone and the overlying Transitional Beds units cropping out in the sites of Vrčići and Pag for the quantitative analysis of the skeletal assemblage (point counting) and the foraminiferal association (area counting), including both large benthic and small benthic foraminifera. Further samples were collected from the overlying Dalmatian Flysch, to constrain the age of the top of the succession using calcareous nannofossil biostratigraphy. On these bases, the Foraminiferal Limestone has been dated from the Ypresian to the middle Eocene (late Lutetian/early Bartonian). Thanks to the analysis of the skeletal and foraminiferal assemblages, and the use of multivariate statistics, seven main biofacies were identified within the Foraminiferal Limestone and one biofacies within the Transitional Beds. The porcelaneous and agglutinated benthic foraminifera biofacies (BF1) indicates a well-illuminated, oligotrophic to mesotrophic, shallow water, lagoonal environment. The hyaline SBF and encrusting benthic foraminifera biofacies (BF2) developed in a shallow water, inner-ramp environment, and is related to a vegetated seafloor. The nummulitid biofacies (BF3) indicates a moderately high energy, shallow water environment, whereas the comminuted bioclasts and nummulitid biofacies (BF4) corresponds to a low-energy, shallow water environment, and both deposited in inner-to-middle ramp settings. The nummulitid and orthophragminid biofacies (BF5) indicates a moderate energy environment, deposited in middle ramp settings. The nummulitid and serpulid biofacies (BF6) consists of transported material from the inner ramp deposited in middle ramp settings. The orthophragminid and nummulitid biofacies (BF7) indicates a below-wave base, outer shelf setting and the planktic foraminifera biofacies (BF8), recorded in the Transitional Beds, indicates a hemipelagic environment. Based on the foraminiferal counting, quantitative parameters such as the orthophragminids/nummulitids ratio, the planktic/benthic foraminifera ratio, and the hyaline/porcelaneous foraminifera ratio were calculated. These parameters indicate that the succession formed along a distally steepened ramp profile, showing a progressively tectonically-controlled deepening of the depositional environment, culminating with the final drowning of the carbonate ramp. Quantification serves as a crucial instrument for a precise and reliable palaeoenvironmental reconstruction, allowing the comparison amongst different successions.

Analysis and extreme event prediction of waves in the eastern shelf seas of China based on a 44-year hindcast

Analysis and extreme event prediction of regional waves are crucial for the design, spatial layout and protection of ocean engineering structures. In this study, the regional wave characteristic is analysed in the eastern shelf seas of China based on a 44-year wave hindcast. The spatial variation of peak period is smaller than that of significant wave height, which may be affected by swell propagation, and the seasonal distribution differences of these parameters can generally be explained by changes in the East Asian monsoon and are significantly affected by tropical cyclones. Both local wind wave and remotely generated swell should be considered when designing ocean structures. In addition, the joint distribution characteristics of wave period and height reveal a natural regional classification of representative wave states, which can be used to avoid deploying structures near classification boundaries. The extreme wave events are characterized by their number, duration and intensity, highlighting areas susceptible to extreme waves. Meanwhile, the extreme event is predicted, and its return patterns are highly consistent with weather system characteristics such as tropical cyclone tracks. The results can provide a significant basis for the design and protection of ocean engineering structures.

Storm‐Driven Warm Inflow Toward Ice Shelf Cavities—An Idealized Study of the Southern Weddell Sea Continental Shelf System

AbstractSudden peaks in south‐westward wind strength (storms) have been observed to drive pulses of enhanced southward currents on the continental shelf east of the Filchner Trough. However, the link between wind and southward flow is not persistent, and it is uncertain which conditions favor the wind‐driven pulses that typically bring modified Warm Deep Water southward toward the front of the Filchner Ice Shelf. We run a set of experiments in an idealized numerical model setup and find that storms induce a net southward “warm” (Θ &gt; −1.5°C) volume transport in the trough region throughout the year. This is mainly explained by enhanced barotropic circulation on the shelf. The greatest storm‐driven increase in southward heat transport occurs during summer and fall, with an exceptionally large increase in November and December due to storm‐enhanced circulation on the shelf and seasonally varying heat content availability south of the shelf break. Analysis of ERA5‐data shows that the number of storm days (wind speed &gt;10 m s−1) per year in the region co‐vary with SAM. The positive trend in SAM can hence be expected to further enhance the importance of storm‐driven southward heat transport toward the Filchner Ice Shelf cavity, which may have consequences for the basal melting.

From Bottom‐Water Production to Warm Water Intrusions—The Cenozoic History of Bottom‐Current Evolution Offshore the Denman‐Shackleton Region, East Antarctica

AbstractThe deep glacial trough and hinterland of the Denman Glacier (East Antarctica) makes the area around the Shackleton ice shelf sensitive to ice loss due to warmer deep water intruding onto the continental shelf in the near future. In addition, the configuration of the ocean currents offshore is an important factor in priming the local and regional vulnerability to warm water intrusions. Here, we use reflection seismic data sets from the Bruce Rise offshore the Denman‐Shackleton region to investigate the Cenozoic history of the ocean bottom current configurations offshore and their influence on the Cenozoic sedimentation patterns of the Denman‐Shackleton region. On the Bruce Rise, sediment drift building, and erosional features indicate three distinct ocean current configurations, (a) the production of dense shelf waters in times of a smaller East Antarctic Ice Sheet (EAIS), (b) periods dominated by a strong Antarctic Slope Current (ASC) and (c) periods with a weak ASC. During the early establishment of the EAIS, the Denman‐Shackleton area contributed to the production of Antarctic Bottom Water, a process which stabilizes the regional ice sheet. With a growing icesheet, the ASC strengthened representing an effective barrier between the continental shelf and the warmer water masses of the deeper ocean during most of the times of an extended EAIS. The transition in the paleoceanographic setting from a strong, erosive ASC, toward a weak ASC increases the vulnerability of the Denman‐Shackleton continental shelf to deep water intrusions as we are observing today.

Textural and geochemical evaluation of the surface sediments from the continental shelf of Southwestern Bay of Bengal, India

Textural and geochemical evaluation of the surface sediments from the continental shelf of Southwestern Bay of Bengal, India

Disentangling bottom-up and top-down controls on fish consumption of key prey in the Northeast US Shelf ecosystem

Abstract Exploited forage fishes serve a dual role in marine ecosystems by supporting directed fisheries and predator productivity, and thus both harvest and predatory removals should be accounted for when developing stock assessments and evaluating management trade-offs. Predator catch and stomach content data collected on the Northeast US Shelf from 1978 to 2019 by two fisheries-independent surveys were combined within multivariate spatiotemporal models to estimate time-series of consumptive removals during spring and fall for four commercially exploited prey; Atlantic herring (Clupea harengus), silver hake (Merluccius bilinearis), butterfish (Peprilus triacanthus), and longfin squid (Doryteuthis pealeii). Seasonal consumption trends were mostly synchronous for Atlantic herring and silver hake, asynchronous for butterfish and longfin squid, and predatory removals were generally greater during fall. Consumption has increased since the 1990s for all prey except Atlantic herring and butterfish during fall, which coincides with the widespread implementation of harvest constraints meant to rebuild predator and prey populations. These time-series were linked to hypothesized drivers using state-space regression models; prey availability (bottom-up; positive relationships) and commercial catch (top-down; primarily negative relationships) were the strongest predictors of consumption. Although the mechanisms underlying these relationships remain unresolved, these linkages highlight connections among the systemic drivers of productivity on the Northeast Shelf.

Mercury on the Norwegian Continental Shelf (NCS). What value to use for design?

Elemental mercury (Hg0) is a safety hazard for some materials and could therefore impact material specifications as well as general process design. Knowledge of Hg0 concentration in reservoir fluids is therefore desired early in the field development phase. In practice one must either rely on measurements from exploration wells or use some kind of model or analogues to estimate a Hg0 concentration in the reservoir fluid at question. The objective of this study was firstly to analyze available data from around 190 wells on the Norwegian Continental Shelf (NCS) to determine whether measurements from exploration wells were consistent with data from fields in operation. Secondly, the aim was to compose a workflow for determining which data to rely on when specifying a Hg concentration to use for new developments. Across NCS the scatter proved very large with no clear trends. Based on the present work it is recommended to trust the highest observed value from the gas phase measurements when evaluating Hg0 analyses from reservoir sampled fluids.

Inception of Constructional Submarine Conduit by Asymmetry Generated by Turbidity Current

Submarine conduits are features responsible for transporting clastic debris from continents to the deep ocean. While the architecture of conduits has been extensively studied, the process of their inception remains unclear. This study highlights the possibility that some conduits are initiated by depositional processes involving turbidity currents. Here, we present the results of eight experiments where gravity currents were allowed to develop their own pathways. The simulation tank represented natural scales of continental shelves, slopes, and basins. The initial experiments involved sediment-laden flows with low density (1–10% in volume). In first experiment runs (Series I), sediment deposition occurred primarily on the shelf and slope, resulting in an asymmetric transverse profile. This asymmetry facilitated subsequent conservative currents (1034 to 1070 kg/m3 due to salt dissolution) flowing alongside during the second series, resulting in the formation of a constructive submarine conduit. This feature is analogous to gully formations observed in various locations. This study correlates these findings with gully-like features and proposes a model where non-confined density flows can evolve into confined flows through the construction of asymmetric topography. An evolutionary model is proposed to explain the mechanism, which potentially elucidates the formation of many submarine conduits.

Cambrian (Fortunian to Jiangshanian) depositional environments of northwestern Mexico: Distribution and facies patterns along the western margin of Laurentia

The Cambrian sedimentary successions from Sonora, Mexico provides information about the sedimentary and geological evolution that occurred on the western margin of Laurentia. The sedimentary rocks from the Fortunian to Jiangshanian stages denote eustatic changes. Initially (Fortunian to Stage 3), shallow water marine clastic-carbonate deposits are found in northwest Sonora. These deposits comprise tidal flats and lagoons with storm deposits and reefs dominated by archaeocyaths. The Stage 4 to Wuliuan sedimentary succession represents the most documented Cambrian transition in Mexico, which includes deposits from the shoreline - beach conditions, lagoon and restricted inner platform with low diversity of polymerid trilobites, to an outer platform - deep shelf margin with abundant agnostids. On the other hand, the sedimentary environments during the Drumian to Guzhangian changed from an outer shelf marine environment to a shallow water marine platform. And finally, rocks from the Paibian to Jiangshanian denote an outer shelf transition to a middle to outer shelf margin.