Steep Shelf Research Articles

Numerical Simulations of Internal Tide Dynamics in a Steep Submarine Canyon

Abstract This study investigates three-dimensional semidiurnal internal tide (IT) energetics in the vicinity of La Jolla Canyon, a steep shelf submarine canyon off the Southern California coast, with the Stanford Unstructured Nonhydrostatic Terrain-Following Adaptive Navier–Stokes Simulator (SUNTANS) numerical simulator. Numerical simulations show vertical structure and temporal phasing consistent with detailed field observations. ITs induce large (approximately 34 m from peak to peak) isotherm displacements and net onshore IT energy flux up to 200 W m−1. Although the net IT energy flux is onshore, the steep supercritical slope around the canyon results in strong reflection. The model provides the full life span of internal tides around the canyon, including internal tide generation, propagation, and dissipation. ITs propagate into the canyon from the south and are reflected back toward offshore from the canyon’s north side. In the inner part of the canyon, elevated mixing occurs in the middle layer due to an interaction between incident mode-1 ITs and reflected higher-mode ITs. The magnitude of IT flux, generation, and dissipation on the south side of the canyon are higher than those on the north side. An interference pattern in horizontal kinetic energy and available potential energy with a scale of approximately 20–50 km arises due to low-mode wave reflections. Our results provide new insight into IT dynamics associated with a small-scale canyon topography. Significance Statement Internal waves play an important role in ocean circulations and ecosystems. In particular, internal waves with frequencies of tides, known as internal tides, strongly enhance energy, heat, and mass transport in coastal oceans. This study presents internal tide dynamics in La Jolla Canyon, California, using a high-resolution numerical model. Model results show energy convergence in the canyon leading to internal tide energy dissipation and mixing. Some parts of internal tide energy reflect back offshore resulting in standing internal waves off California. This study provides new insights into internal tide dynamics and energy budgets in submarine canyons.

Quantifying Mechanisms Responsible for Extreme Coastal Water Levels and Flooding during Severe Tropical Cyclone Harold in Tonga, Southwest Pacific

The South Pacific region is characterised by steep shelves and fringing coral reef islands. The lack of wide continental shelves that can dissipate waves makes Pacific Island countries vulnerable to large waves that can enhance extreme total water levels triggered by tropical cyclones (TCs). In this study, hindcasts of the waves and storm surge induced by severe TC Harold in 2020 on Tongatapu, Tonga’s capital island, were examined using the state-of-the-art hydrodynamic and wave models ADCIRC and SWAN. The contributions of winds, atmospheric pressure, waves, and wave-radiation-stress-induced setup to extreme total water levels were analysed by running the models separately and two-way coupled. The atmospheric pressure deficit contributed uniformly to the total water levels (~25%), while the wind surge was prominent over the shallow shelf (more than 75%). Wave setup became significant at locations with narrow fringing reefs on the western side (more than 75%). Tides were dominant on the leeward coasts of the island (50–75%). Storm surge obtained from the coupled run without tide was comparable with the observation. The wave contribution to extreme total water levels and inundation was analysed using XBEACH in non-hydrostatic mode. The model (XBEACH) was able to reproduce coastal inundation when compared to the observed satellite imagery after the event on a particular coastal segment severely impacted by coastal flooding induced by TC Harold. The coupled ADCIRC+SWAN underestimated total water levels nearshore on the reef flat and consequently inundation extent as infragravity waves and swash motion are not resolved by these models. The suite of models (ADCIRC+SWAN+XBEACH) used in this study can be used to support the Tonga Meteorological Service Tropical Cyclone Early Warning System.

Features of the Field of Internal Waves on the Abkhazian Shelf of the Black Sea according to Remote Sensing Data and In Situ Measurements

The field of internal waves in the Black Sea is quite significant. The Black Sea shelf is of particular interest, but it has not been studied enough in some specific regions. For example, a narrow and steep shelf of Abkhazia has been poorly explored. Particularly unexplored are the actual parameters and causes of the generation of internal waves in this area. In this article, we have attempted to fill this gap by analyzing remote sensing data and in situ data. An analysis of a set of optical multispectral satellite images (Sentinel-2, Landsat-8) and a collection of sea-truth data of the shelf zone of Abkhazia was carried out to identify features of internal wave fields of this region. In situ data were acquired over 9 years using ADCP, CTD, and SVP probes and moored stations with point and line temperature sensors. It is shown that internal waves are widespread on the Abkhazian shelf. They appear as trains of short-period waves (as a rule, soliton-like). The quantitative parameters and features of internal waves are shown and analyzed. The form of manifestation and direction of internal wave trains’ travel depend on the mechanisms of their origin, among which are the transformations of inertial internal waves, generation by river plumes, and submesoscale structures. In general, the article is the most complete and relevant study of the field of internal waves on the shelf of Abkhazia.

Controls of Submarine Canyons Connected to Shore during the LGM Sea-Level Rise: Examples from Taiwan

During the Last Glacial Maximum (LGM) (~20,000 years ago), the sea level was lower than that during the present day by 120 m and the heads of most submarine canyons were close or connected to the coastline or shore, with enhanced terrestrial sediment input due to direct connections with river mouths. This was followed by a relative sea-level rise of 120 m and the migration of coastlines landward. As a result, the heads of some paleo-submarine canyons were no longer near river mouths or connected to the shore. Such canyons became inactive due to the lack of terrestrial sediment input. Only 4% of the world’s submarine canyons reach the coastline and remain active today. Among 13 submarine canyons off the shore of Taiwan, we identified seven (n = 7, 54%) that remain connected to the shore and are active during the present-day highstand. The purpose of this study is to determine the key controls of canyon heads that remain connected to the shore with terrestrial sediment input during the Holocene sea-level rise. As a result of high uplift rates, narrow coastal range, steep gradients, frequent earthquakes, and typhoon development in the Taiwan mountain belt, Taiwan has the highest-yield river and sediment supply. This has led to the transportation of large volumes of sediment to the surrounding deep seas. Narrow steep shelves and large sediment volumes associated with small mountain rivers are the main controls involved in the development of shore-connected canyons on the active Taiwan margin. Shore-connected canyons are present in greater numbers in the major earthquake zone on the eastern Taiwan margin. Frequent earthquake events are another significant factor in the occurrence of shore-connected canyons in the Taiwan region.

Assessing Spatio‐Temporal Variability of Free Gas in Surficial Cohesive Sediments Using Tidal Pressure Fluctuations

AbstractFrom a geohazard assessment perspective, the distribution, content, and dynamics of free gas in surficial sediment were addressed by imaging and monitoring the upper 15 m beneath the shelf offshore Nice, France. Based on high‐resolution seismic data covering three sites where pore pressure was recorded over three and a half years, the presence of free gas was determined in the upper 2.75–14.75 m of cohesive, silty clay. Seismic velocity changes delineate two layers with gas volume fractions ranging from 0.12% to 1.89%. By considering the tidal response recorded by eight pore pressure sensors, estimates of gas volume fractions vary from 0.26% to more than 9.4% on a spatio‐temporal scale which cannot be achieved with seismic data. To depict spatio‐temporal patterns three types of free gas occurrence (FGO) were distinguished. Type 3 FGO uniquely showed sawtooth fluctuations in overpressure of 27%–45% of the hydrostatic effective stress as evidence of bubble growth and rise. The other two types showed long‐term overpressure trends indicative of a situation whereby bubble growth has ceased. Type 1 FGOs are distinguished from type 2 by their gas volume fraction lower than 9.4% and ratios of overpressure to hydrostatic effective stress lower than 0.3. Values higher than this threshold are considered sufficient for shear failure to initiate from the steep shelf edge (>20°). Beyond site‐specific insights, the distinction of FGO from their overpressure levels yields testable implications for the dynamics of methane in sediments.

ABOUT LITHODYNAMICAL PROCESSES ALONG THE KOLA FIORD COAST OF THE BARENTS SEA

This article is devoted to the study of the basic laws of the formation, structure, mor­phology and composition of sediments of accumulative landforms at the mouths of small rivers on the northern (fjord) coast of the Kola Peninsula washed by the Barents Sea waters. Along the northern Kola fjord Coast of Barentz Sea, the development peculiarities of lithodynamical processes are geographical location, expansion of old strong rocks with slowly rates of cliffs retreat and with severe hydrodynamical re­gime. As a result of this alongshore drift flows cannot develop. Normal exchange by sediment with adjoining regions of a sea shores are absent. Abrasive-denudation process is dominant, and it intensifies the sediment deficit within narrow coastal zone of the Sea. In addition, coastal deposition of the shore origin is unlikely along the entire distance by absolute Murmansk shoreline. The main reason is the strong impact of water dynamic and very steep shelf slope. They throw off sedimentary material to deep bottom without sphere of positive waves (surf) impulse for big debris of local rocks. That is why within the Northern coast of the Kola Peninsula the most important lithodynamical action all time were separate hearth’s sites of deposition and short flows of migration. All time the fiord tops have location of sediment debris which undergo wave rubbing, and during sea-ice time discard shore sediment take place. A detailed analysis in the mouths of the Teriberka, Voronya and Kharlovka rivers where modern sandy terraces of the coastal-marine genesis lie have been carried out. It was noticed that the influenced of strong igneous and crystallized rocks and active phys­ical weathering after the fast melting of snow from the catchment area carries away fine-grained material into the rivers (fractions ≤ 3 mm). These materials which are composed mainly of Alluvials leads to the construction of terraces. The productive vector of the wind flow on the coast with an average long-term wind speed ≈ 8 m/s is directed from land to sea. The smallest sediments are blown into the sea from the surface of the terraces. In contrast to the eastern shores of the Barents and White Seas, these sediments form sandy tidal dehydrations. Silt debris composed of fine-grained material of (fractions ≤ 1 mm) dominates here. The main part of alluvium ≥ 1.0 mm is retained in river channels. It should be noted that on the studied coast, an average of 15.36% of these fractions erupt into the composition of coastal terraces. The smallest debris and part of the sand goes down to a depth along steep underwater slope (up to i10 ≤ 0,15). On the frontal surface slope of the terraces, fractions of 1.0-0.25 mm on the beach dominate (of which the content is 0.5-1.0 mm = 38.93%). All sand fractions account for 88.78%, including coastal dunes. The dune relief forms are small and are in the germinal state. They are characteristics of the mouths of the rivers where they exist.

Internal Waves Study on a Narrow Steep Shelf of the Black Sea Using the Spatial Antenna of Line Temperature Sensors

The results of investigations into internal waves on a narrow steep shelf of the northeastern coast of the Black Sea are presented here. To measure the parameters of internal waves, the spatial antenna of three autonomous line temperature sensors were equipped in the depth range of 17 to 27 m. In observations that lasted for 10 days, near-inertial internal waves with a period close to 17 h and short-period internal waves with periods of 2–8 min, regularly approaching the coast, were revealed. The wave amplitudes were 4–8 m for inertial waves and 0.5–4 m for short-period internal waves. It was determined that most of the short-period internal waves approached from the southeast direction, from Cape Kodor. A large number of short waves reflected from the coast were also recorded. The intensification of short-period waves with inertial periodicity and the belonging of trains of short waves to crests of inertial waves were identified. In general, it was shown that the internal wave field at a narrow shelf significantly differs in its features from analogs of ordinary shelves of the Black Sea.

Oceanic sediment accumulation rates predicted via machine learning algorithm: towards sediment characterization on a global scale

Observed vertical sediment accumulation rates (n = 1031) were gathered from ~ 55 years of peer reviewed literature. Original methods of rate calculation include long-term isotope geochronology (14C, 210Pb, and 137Cs), pollen analysis, horizon markers, and box coring. These observations are used to create a database of global, contemporary vertical sediment accumulation rates. Rates were converted to cm year−1, paired with the observation’s longitude and latitude, and placed into a machine learning–based Global Predictive Seabed Model (GPSM). GPSM finds correlations between the data and established global “predictors” (quantities known or estimable everywhere, e.g., distance from coastline and river mouths). The result, using a k-nearest neighbor (k-NN) algorithm, is a 5-arc-minute global map of predicted benthic vertical sediment accumulation rates. The map generated provides a global reference for vertical sedimentation from coastal to abyssal depths. Areas of highest sedimentation, ~ 3–8 cm year−1, are generally river mouth proximal coastal zones draining relatively large areas with high maximum elevations and with wide, shallow continental shelves (e.g., the Gulf of Mexico and the Amazon Delta), with rates falling exponentially towards the deepest parts of the oceans. The exception is Oceania, which displays significant vertical sedimentation over a large area without draining the large drainage basins seen in other regions. Coastal zones with relatively small drainage basins and steep shelves display vertical sedimentation of ~ 1 cm year−1, which is limited to the near shore when compared with shallow, wide margins (e.g., the western coasts of North and South America). Abyssal depth rates are functionally zero at the time scale examined (~ 10−4 cm year−1) and increase one order of magnitude near the Mid-Atlantic Ridge and at the Galapagos Triple Junction.

River Plume Liftoff Dynamics and Surface Expressions.

The water surface expression of liftoff and its dependence on discharge are examined using numerical simulations with the Regional Ocean Modeling System (ROMS). Liftoff is the process by which buoyant river water separates from the bed and flows over denser saltwater. During low‐discharge conditions liftoff occurs in the river and is accompanied by a change in the surface slope. During high‐discharge conditions liftoff occurs outside the mouth and generates a ridge on the water surface. The location and height of the ridge can be described by analytical equations in terms of discharge, shelf slope, and river mouth aspect ratio. The offshore distance and height of the ridge are proportional to the river discharge and vary inversely with river mouth aspect ratio. For steep shelf slopes liftoff occurs close to the river mouth and generates a large ridge. The ridge is modified, but not eliminated, by the presence of tides. The water surface slope change at the ridge peak is large enough to be detected by the upcoming Surface Water and Ocean Topography (SWOT) altimeter and can be used to identify the liftoff location during high discharge. However, during low discharge the water surface slope change at the liftoff location is too small to be detected by SWOT. These results indicate that remote measurements of the presence or absence of the ridge may be useful to distinguish between low and high flows, and remote measurements of the ridge location or height could be used to estimate freshwater discharge.

Holocene Transgressive Sands on the Nearshore of a Micro-tidal Shelf

Lee, H.J.; Lee, S.H.; Baek, Y.S., and Lim, H.S., 2020. Holocene transgressive sands on the nearshore of a micro-tidal shelf. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 553-557. Coconut Creek (Florida), ISSN 0749-0208.A beach retreat by the Holocene sea-level rise is commonly observed in shelf environments. To investigate into a consequence of the beach retreat in a micro-tidal shelf, surface and cored sediments were sampled from a coastal area around a stream in the eastern coast of Korea and were analyzed texturally. A coastal stratigraphy is constructed with three units representing fluvial sediments, eolian sands and beach sands in an ascending order. This stratigraphy results from the Holocene transgression. The textural characteristics of the units are compared with those of the surficial samples to determine the origin of surficial sands. As a result, the nearshore sands in the study area are most probably the erosional remnants of the antecedent eolian and beach sands. This study exemplifies the Holocene transgression over a narrow, steep shelf whereby active shoreface erosion left portions of beach sediments behind in small estuarine environments.

Forest Stands in Phyllitido-Aceretum Community Occurring on Limestone Rocks and Shelves – Comparison of their Structure in Strictly Protected and Managed Forests

The aim of the study was to determine the structure of forest stands in rare Phyllitido-Aceretum communities, which grow in extremely harsh conditions – steep limestone slopes and rock shelves. Permanent research plots were established in five well-developed patches of Phyllitido-Aceretum communities from Krakow-Czestochowa Upland (Poland). Two of the plots were located in strictly protected areas of the Ojcow National Park, whereas another three were established in the managed forests of Olkusz Forest District. The species composition of the forest stands was determined, including all dead and live trees and their diameters at breast height were measured. Moreover, the horizontal coordinates of each tree were measured to determine the spatial pattern of the trees. Ripley's L function was used for the spatial pattern analysis.The research showed that the patches of Phyllitido-Aceretum communities are characterized by rich species composition of tree layers; nine species were found. There was no significant difference in the species composition between the living and dead trees and the shares of particular species were similar. The density and share of live trees in the phytocoenoses located within the strict protection zone were significantly lower than in managed forests. On the contrary, the same parameters when calculated for dead trees were significantly higher in phytocoenoses located in protected zones than in managed forests. The spatial distribution of the trees in all the analysed plots (except one) displayed a clumped pattern, which is observed in various spatial scales and reflects the habitat mosaics and life strategies of species typical for Phyllitido-Aceretum community.

Stratigraphic architecture, shelf-edge delta and constraints on the development of the Late Oligocene to Early Miocene continental margin prism, the Pearl River Mouth Basin, northern South China Sea

The early-stage shelf-margin prism in the Pearl River Mouth Basin (PRMB), northern South China Sea, formed from the Late Oligocene to Early Miocene, comprises a set of shelf-edge delta and slope fan wedges of the Zhuhai Formation and records the development of the continental slope in response to the interaction of sea-level change, tectonic subsidence and sediment supply. Based on integrated analysis of seismic, well logging and core data, the stratigraphic architecture, depositional systems and controlling processes are documented. The Zhuhai Formation is a composite sequence bounded by regional unconformity, and can be further divided into five sequences (CS3-1-CS3-5). The depositional genetic units (transgression, ascending normal regression and descending forced regression) are identified within sequences based on variation in shelf edge trajectories. The shelf deltaic clinoforms are characterized by oblique-tangential reflections in seismic profiles with a thickness of 100–150 m. In contrast, the shelf-margin delta and slope clinoforms generally are 300–1200 m thick and mainly display as sigmoid or sigmoid-tangential seismic reflections. The delta deposits usually consist of thick beds of delta front deposits and prodelta slump and deformed sediments, and are associated with sandy slope fan systems. Eleven delta complexes are distinguished by tracking the distribution of the deltaic clinoforms in each sequence, and these deltaic deposits tend to extend along their strike direction which may be related to the reworking of strong waves and coastal currents. Slope fan systems consist mainly of turbidite channel fills, frontal splay and debris flow deposits, and generally develop along the slope in CS3-3-CS3-5, which may be triggered by steep shelf margin slope and fault activity. The stratigraphic pattern of the shelf margin prism and depositional evolution of the deltaic systems from the Late Oligocene to Early Miocene were controlled by the interaction of sea level changes, sediment supply and tectonic subsidence. The ascending trajectory from the CS3-1 (30 Ma) to the top of the CS3-5 indicates that the relative sea level continued to rise generally but the shelf margin prograded basinward rapidly due to the great amount of sediment supply and lower subsidence rate. Tectonic uplift of the source area and strengthened physical erosion during this period may be the reason for the large sediment supply.

Seabed Gradient Controlling Onshore Transport Rates of Surf Sand during Beach Retreat by Sea Level Rise

A simple relationship is proposed for the onshore transport rates of surf-zone sand to evaluate the beach retreat caused by sea level rise. It suggests that the preservation potential of surf sand is proportional inversely to the seabed gradient during beach retreat. According to this relationship, the erosional remnants of surf sand would be more readily developed on a gentler shelf collectively as transgressive sand sheets. This finding may explain the previous studies regarding the Korean shelves that proposed that the Holocene transgressive sand sheets (HTSS) occur not in the steep eastern shelf but in the gentle western shelf. In line with such presence/absence of the HTSS are the results from some coastal seismic profiles obtained in the present study. The profiles indicate that sand deposits are restricted within the nearshore in the eastern coast, whereas they are persistently traceable to the offshore HTSS in the western coast. Tide is proven to have a negligible influence on the total duration of surf-zone processes. This study may be useful in predicting the consequences of the beach retreat that takes place worldwide as sea levels rise as a result of global warming.

Spatial variability of internal waves in an open bay with a narrow steep shelf in the Pacific off NW Mexico

Small scale spatial patterns (<10km) in nearshore internal wave fields are rarely reported on, yet can have a large impact on nearshore mixing and productivity. In this study, the spatial pattern of internal wave characteristics were explored in Todos Santos Bay, Baja California (Mexico), using time series of temperature and currents from moored and towed thermistor chains and acoustic profiling current meters, as well as cross-shore transects with a towed undulating CTD system. Spectra of temperature and currents showed significant spatial variability within the bay, with the northern sector dominated by the internal tidally-forced semidiurnal signal, and the southern sector dominated by wind-forced, subinertial, baroclinic, diurnal fluctuations, which decreased with distance from shore. Semidiurnal internal tidal waves were generated by the barotropic tide at various sites on the continental slope to the west of the bay. They traveled toward the NE and reached the observation site in the northern part of the bay, after bouncing once or twice off the surface and the bottom. Despite the narrowness of the shelf, the semidiurnal internal tides at this site presented a first-mode structure, although not completely formed at times. On average, the semidiurnal internal waves had a ~9km wavelength, traveled in the form of an arc, and propagated with a phase velocity of ~20cm/s. When they reached shallow waters near the coast, they disintegrated rapidly into groups of short, nonlinear internal waves, with 15–20m amplitudes, 5–20min periods, and 50–200m wavelengths. The spatial patterns found in this study are most likely due to variability in distance from generation sites, complex bottom topography, and small scale (<10km) spatial variability in meteorological conditions such as winds.

Steep Shelf Stabilization of the Coastal Bransfield Current: Linear Stability Analysis

Abstract In situ measurements obtained during the 2010 COUPLING cruise were analyzed in order to fully characterize the velocity structure of the coastal Bransfield Current. An idealized two-layer shallow-water model was used to investigate the various instability processes of the realistic current along the coastal shelf. Particularly studied is how the topographic parameter To (ratio between the shelf slope and the isopycnal slope of the surface current) impacts the growth and the wavelength of the unstable perturbations. For small bottom slopes, when the evolution of the coastal current is controlled by the baroclinic instability, the increase of the topographic parameter To yields a selection of smaller unstable wavelengths. The growth rates increase with small values of To. For larger values of To (To ≳ 10, which is relevant for the coastal Bransfield Current), the baroclinic instability is strongly dampened and the horizontal shear instability becomes the dominant one. In this steep shelf regime, the unstable growth rate and the wavelength selection of the baroclinic coastal current remains almost constant and weakly affected by the amplitude of the bottom velocity or the exact value of the shelf slope. Hence, the linear stability analysis of an idealized Bransfield Current predicts a typical growth time of 7.7 days and an alongshore scale of 47 km all along the South Shetland Island shelf. The fact that these large growth times are identical to the typical transit time of water parcels along the shelf may explain why the current does not exhibit any unstable meanders.

Anomalous variation of the ocean’s inertial oscillations at the Hawaii shelf

The data of acoustic Doppler profilometers placed at the edge of the steep shelf of Oahu Island, Hawaii, are analyzed for the currents. The specific character of inertial oscillations is revealed in the region: strong elongation of inertial orbits, a large amount of anomalous (cyclonic) current rotation, and sharp weakening in the divergence layer of the background (low-frequency) oscillations, the strong variation of which results in significant deviations of the effective inertial frequency from its local geographical value. It is suggested that cyclonic rotation of the inertial currents is related to a strong decrease in the effective oscillation frequency of the oceanic waves on the shelf.

Island slopes and jumbled shell beds

Shell beds pose questions of provenance, but determining the degree of pre-depositional transport of bioclasts can be speculative. Bioclastic beds deposited on a broad, shallow continental shelf may consist of a minimally transported accumulation. In contrast, islands may have steep shelves and accumulate disharmonious bioclastic assemblages in deeper water. The latter are a signature of the stratigraphic record on many islands. Herein, we contrast differing examples of jumbled shell beds preserved around islands. The Lower Permian Mengkarang Formation of Sumatra preserves deep-water facies at the base of a volcanic section. Nektonic cephalopods overlie a fine-grained ash containing a diverse association of terrestrial pollen. The Lower Miocene Montpelier Formation of Jamaica yields allochthonous packstones and slide blocks of reef limestone embedded in chalk; the ichthyofauna indicates deposition in ≥200 m water depth. Abundant evidence that deposition occurred in a deep-water turbidite basin flanking a volcanic island is found in the Middle Miocene Grand Bay Formation of Carriacou, the Grenadines. Diverse, deeper water taxa are preserved in association with land snails, hermatypic scleractinians, algal balls and planktic pteropods. The famous Upper Pliocene Bowden shell beds, Jamaica, preserve terrestrial, shallow-water, deep-water and planktic taxa.

Dynamics of the summer shelf circulation and transient upwelling off Ningaloo Reef, Western Australia

The shelf circulation off Ningaloo Reef near the North West Cape of Western Australia is driven by complex interactions between the southward flowing Leeuwin Current and wind‐driven currents that episodically reverse the coastal flow toward the north. The presence of these northward (equatorward) wind‐driven currents is thought to make this section of coast one of the few locations along Western Australia to experience periodic coastal upwelling. We used a combination of field observations and numerical modeling to investigate the summer circulation and upwelling dynamics along Ningaloo Reef. We analyzed current and temperature profiles from moorings at four sites across the shelf and used two Regional Ocean Modeling System (ROMS) sub‐models: (1) a coarser model of northwestern Australia forced by a global ocean model and (2) a nested fine‐scale model of the Ningaloo region. This nesting significantly improved model skill as it included the offshore mesoscale dynamics that strongly influenced the shelf circulation off Ningaloo. The field observations revealed several northward flow reversals, accompanied by cooling of the coastal waters adjacent to Ningaloo, which were associated with strong northward wind events. Analysis of the coastal heat budget revealed that cooling events were primarily driven by upwelling, whereas warming of coastal waters during relaxation events resulted mostly from along‐shelf advection of warm water from the north. Due to the combined effects of its relatively steep (~1/50 slope) shelf and strong summer stratification, upwelled water was sourced from the interior of the water column, likely influencing the sources and fluxes of nutrients to Ningaloo Reef.

Shelf Impact on Buoyant Coastal Current Instabilities

Abstract The impact of shelf slope on the linear stability of buoyant coastal currents and on the nonlinear formation of coastal meanders and eddies is investigated. The authors consider a simplified two-layer stratification in cylindrical geometry where a buoyant surface current flows along the coast above a denser water, with a flat bottom or steep shelves. Simulations were performed using the Nucleus for European Modelling of the Ocean (NEMO) ocean global circulation model. The initial state of these simulations was defined according to laboratory experiments performed in the same configuration. Comparisons between laboratory and numerical results highlight the role of momentum diffusion and of the initial perturbations amplitude. The authors’ results confirm that the topographic parameter To (ratio between the shelf slope and the isopycnal slope of the current) is the relevant parameter to quantify the shelf impact on the linear and nonlinear dynamics of the surface current. When the evolution of the buoyant coastal current is controlled by the baroclinic instability, the increase of To yields a selection of smaller unstable wavelengths and a decrease of the unstable growth rates. For finite values of To, a complete stabilization of the surface current can be reached. The typical radius of the first eddies generated by the coastal current is set by the linear stage of the baroclinic instability. However, secondary nonlinear processes may lead to larger or smaller structures. The authors exhibit a new dynamical sequence, leading to the formation of submesoscale cyclonic eddies over a steep shelf by splitting of mesoscale eddies. These cyclonic eddies trap and transport water masses and may play an important role in the cross-shelf exchanges.

The late Cretaceous to Holocene sequence stratigraphy of a sheared passive upper continental margin, northern KwaZulu-Natal, South Africa

High resolution single-channel seismic data reveal seven seismic units (A–G) from the narrow and steep upper portions of the sheared passive continental margin of northern KwaZulu-Natal. Several characteristic internal geometries are evident: onlapping sheet-like (Unit C), aggradational/progradational (Unit A and Facies D3), and progradational (Unit B, Facies D1/2 and Facies G1). On the basis of internal geometry and bounding unconformities, lowstand (Units B, Facies D3 and lowermost Unit G), possible transgressive (Unit C), highstand (upper Unit G) and falling stage (Facies D1 and D2) systems tracts are delineated. Units E and F are not reconcilable due to acoustic opacity. Overall, five incompletely preserved sequences are observed. Sequence one comprises Unit A of suspected Middle Maastrichtian age, separated from the overlying sequence by a subaerial unconformity surface SB1. Units B and C form sequence two and span the late Cretaceous (Late Maastrichtian) to mid–late Palaeocene. Two major hiatuses ensued (Angus and Jimmy) until mid Pliocene times when Unit D was deposited during shelf margin advance linked to hinterland uplift. These falling stage systems tract deposits are separated from latest Pliocene/early Pleistocene lowstand shelf edge deltaic deposits by a subaerial unconformity marking the deposition of sequence four (Facies D3). The age of Unit E is uncertain. Unit F represents a series of shorelines formed during Oxygen Isotope Stage (OIS) 5a to 2 on the regressive and transgressive limbs preceding and following the Last Glacial Maximum (LGM). The final partial sequence preserved is comprised of Unit G formed during the OIS 2 transgression to present mean sea level and subsequent development of the contemporary highstand wedge. In terms of stratigraphic architecture, this study shows an evolution from a ramp-style margin with a poorly defined shelf break to that of a shelf break profile with a narrow and steep shelf. In this respect, the narrowness of this shelf cannot be accounted for by the inheritance of shearing effects. Rather, the stratigraphic architecture reflects a period of slow and protracted subsidence with strong sediment dispersal, later punctuated by hinterland uplift. This formed the late Pliocene shelf margin clinoforms of Unit D and established the shelf break position at the offlap break of this wedge. The resulting narrow shelf may play a significant role in the preservation of later sequences, particularly in light of the continued uplift of the margin causing off-shelf sediment bypass where TST and HST sequences may not be preserved. This is reflected in the isolated preservation of shoreline and shallow marine deposits of the Pleistocene Units E and F.