Wide Continental Shelf Research Articles

Bottom–up propagation of synoptic wind intensification and relaxation in the planktonic ecosystem of the South Senegalese Upwelling Sector

ABSTRACT Synoptic intensification or relaxation of upwelling favorable winds are major sources of variability in eastern boundary upwelling systems. This study aims to investigate their impact on the planktonic ecosystem of the South Senegalese Upwelling Sector (SSUS), located south of the Cape Verde peninsula over a wide and shallow continental shelf. Numerical experiments using a three-dimensional coupled physical–biogeochemical model with four plankton functional types simulated the response of the coastal planktonic ecosystem to idealized synoptic (∼10 days) wind intensification and relaxation of the same amplitude. We find that these perturbations induce spatio-temporal oscillations of plankton concentrations. Zooplankton response occurred with a time lag that manifests itself in space as an equatorward/downstream shift in distribution relative to phytoplankton. Overall, the transmission of the synoptic perturbation from the physics to zooplankton is characterized by a damping in relative anomalies. All these elements and the weakness of the asymmetries in the biogeochemical/planktonic ecosystem responses between intensification and relaxation events support the hypothesis that synoptic variability has limited impact on the climatological state of low–latitude upwelling systems such as the SSUS. PLAIN LANGUAGE SUMMARY In some coastal regions of the world such as off Senegal, winds preferentially blow alongshore and induce subsurface, cold and nutrient rich waters to rise to the surface layer and favor the development of plankton blooms. These so-called upwelling favorable winds are not steady. Their fluctuations produce dynamical and biogeochemical variability over a broad range of scales. Here, we studied the biogeochemical effect of 10-day (i.e. weather or synoptic) wind fluctuations over the southern Senegal continental shelf. We used a numerical model with a simplified planktonic ecosystem consisting of two phytoplankton and two zooplankton size classes. The wind perturbations modulate ocean physics, the enrichment of the sun-lit surface layer in nutrients and the planktonic ecosystem. The plankton’s response to wind fluctuations exhibited oscillations more complex and relatively less intense than those of the wind. The modest effect of the studied short-term wind fluctuations on plankton found in this study may be specific to low-latitude coastal oceans with wide continental shelves.

Opening of the North Atlantic Ocean and the rise of Scandinavian mountains

Large-scale topography is usually associated with tectonic plate boundaries, but the Scandinavian mountains (Scandes) are located far from any active tectonic setting, and their origin is unknown. We demonstrate that the Precambrian lithospheric structure of Fennoscandia controlled both the Cenozoic ocean opening and mountain rise in the North Atlantic region. Our new seismic receiver function analysis reveals a block of thick continental crust formed by Proterozoic crustal stacking. We propose that this block created a wide continental shelf bounded by two transform fault systems during continental breakup. This geometry resulted in the formation of lithospheric steps at the continental margin on both sides of the stacked crustal structure, coinciding with the highest Southern and Northern Scandes. We propose that edge-driven convection at these steps caused the mountain rise. This study presents a general model for the formation of high elevation behind passive margins.

Possible Indication of the Impact of the Storegga Slide Tsunami on the German North Sea Coast around 8150 cal BP

The Storegga slide tsunami (SST) at ca. 8100 ± 100–250 cal BP is known to be the largest tsunami that affected the North Sea during the entire Holocene. Geological traces of tsunami landfall were discovered along the coasts of Norway, Scotland, England, Denmark, the Faroes and Shetland Islands. So far, the German North Sea coast has been considered as being well protected due to the wide continental shelf and predominant shallow water depths, both assumed to dissipate tsunami wave energy significantly, thus hindering SST propagation dynamics. The objective of our research was to clarify if the SST reached the German Bight and if corresponding sediment markers can be found. Our research was based on the in-depth investigation of a 5 m long section of the research core Garding-2 from Eiderstedt Peninsula near Garding in North Frisia known from a previous study. For this, we newly recovered sediment core Garding-2A at exactly the same coring location as core Garding-2. Additionally, high-resolution Direct Push sensing data were collected to gain undisturbed stratigraphic information. Multi-proxy analyses of sediment material (grain size, geochemical, geochronological and microfaunal data) were carried out to reconstruct palaeoenvironmental and palaeogeographical conditions. We identified a high-energy event layer with sedimentological (e.g., erosional unconformity, rip-up clasts, fining-upward), microfaunal (e.g., strongly mixed foraminiferal assemblage) and other features typical of tsunami influence and identical in age with the SST, dated to ca. 8.15 ka cal BP. The event layer was deposited at or maximum ca. 1–1.5 m below the local contemporary relative sea level and several tens of kilometers inland from the coastline within the palaeo-Eider estuarine system beyond the reach of storm surges. Tsunami facies and geochronological data correspond well with SST signatures identified on the nearby island of Rømø. SST candidate deposits identified at Garding represent the southernmost indications of this event in the southeastern North Sea. They give evidence, for the first time, of high-energy tsunami landfall along the German North Sea coast and tsunami impact related to the Storegga slide. SST deposits seem to have been subsequently reworked and redeposited over centuries until the site was affected by the Holocene marine transgression around 7 ka cal BP (7.3–6.5 ka cal BP). Moreover, the transgression initiated energetically and ecologically stable shallow marine conditions within an Eider-related tidal channel, lasting several millennia. It is suggested that the SST was not essentially weakened across the shallow continental shelf of the North Sea, but rather caused tsunami run-up of several meters (Rømø Island) or largely intruded estuarine systems tens of kilometers inland (North Frisia, this study). We, therefore, assume that the southern North Sea coast was generally affected by the SST but sedimentary signals have not yet been identified or have been misinterpreted. Our findings suggest that the German North Sea coast is not protected from tsunami events, as assumed so far, but that tsunamis are also a phenomenon in this region.

Depositional control on the fate of reactive iron in shelf sediments since the last deglaciation: A case study of the East China Sea

The East China Sea (ECS) is located between the Eurasian continent and the Pacific Ocean with a wide continental shelf, which acts as a potential source of reactive iron in the Western Pacific. However, the source and fate of reactive iron in continental shelf sediments of the ECS remain poorly constrained. Here, we examined the influence of the depositional environment on the fate of reactive iron on the continental shelf of the ECS since the last deglaciation. The contents of redox-sensitive elements (U and Mo) indicate that the sediments in the ECS inner shelf have primarily deposited in oxic and suboxic environments since 18.5 ka. The ratio of reactive iron to total iron (FeHR/FeT) ranges from 0.24 to 0.41, and the ratio of total iron to aluminum (FeT/Al) is approximately 0.55 ± 0.11. These ratios suggest that the majority of reactive iron is derived from fine-grained terrestrial sediments discharged by the Changjiang River. The contents of Fepy and Fecarb exhibit opposite trends with depth in the core, indicating competition between carbonate (bicarbonate) ions and sulfide ions for ferrous ions. This competition is primarily controlled by the depositional environment and redox state since 18.5 ka. The Fecarb is the dominant iron speciation throughout the core sediments, but its abundance declined since 13.2 ka when the ECS inner shelf was influenced by seawater transgression due to deglacial sea-level rise. The Fepy content reached its maximum when the ECS inner shelf was fully flooded. Our study highlights the depositional control on the source-sink processes of reactive iron, providing new insights into the fate of reactive iron on continental shelves in response to environmental evolution.

Forcing conditions of cross-shelf plumes on a wide continental shelf, Winyah Bay, South Atlantic Bight

Buoyant cross-shelf river plumes can extend far offshore through the combined effect of buoyancy and wind forcing, creating a critical land-ocean link in global biogeochemical cycles. On the Carolinas continental shelf, cross-shelf plume structure has been analyzed using satellite imagery, with forcing conditions represented by an estuarine Richardson number, wind stress, and alongshore pressure gradient. Three distinct cross-shelf plume patterns emerged, each occurring under an upwelling-favorable wind: (1) The separated plume, when a single filament of buoyant water spreads offshore (a prototypical cross-shelf plume structure); (2) The upwind-curving plume, which turns against the wind at some offshore distance and is created by stronger buoyancy forcing; and (3) The multi-lobe plume, which is partially trapped by the coast with multiple streaks protruding offshore and is created by stronger wind forcing, and further aided by a coincident alongshore pressure gradient force. The latter two regimes represent a low-wind, high discharge limit and a strong-wind limit of cross-shelf plumes. High-resolution satellite images reveal rich submesoscale variability associated with each plume type. Results suggest plume transport extends farthest offshore in low-energy separated plumes through a balance of weak buoyancy and weak wind forcing.

A systemic and comprehensive assessment of coastal hazard changes: method and application to France and its overseas territories

Abstract. In the context of climate change, height and frequency variations in extreme sea levels (ESLs) are studied using deterministic and probabilistic approaches. However, this type of approach does not highlight the dynamic effects (waves, currents) generated by metocean events (storms, cyclones, long swells, and tsunamis) beyond their effects on sea levels. In particular, ESL estimates are calculated by considering the main determining physical factors but cannot include all the effects of these factors. Ultimately, this can lead to confusion between ESL and hazard. This article proposes a systemic assessment method to analyze coastal hazard changes at regional scales, integrating parameters influencing sea levels, as well as factors describing the geomorphological context (length and shape of the coast, width of the continental shelf), metocean events, and the marine environment (e.g., coral reef state and sea ice extent). French mainland and overseas territories were selected to apply the method. The present study highlights the need to consider not only the sea level variability, but also the current and future characteristics of metocean events. The long, concave coasts bordered by a wide continental shelf appear particularly sensitive to variations in the intensity or trajectory of metocean events. Coral reef degradation in the tropics and the decrease in seasonal sea ice extent in the polar regions can also significantly change the nearshore hydrodynamics and impacts on the shoreline. These results help us to predict the types of hazard (shoreline erosion, rapid submersion, and/or permanent flooding) that will increase the most in different coastal zones.

Multi-scale free oscillations and resonances over the continental shelf of the East China Sea from the 2011 Tohoku-Oki tsunami

A number of coastal tide gauges and offshore Deep-ocean Assessment and Reporting of Tsunamis (DART) buoys across the Pacific Basin recorded persistent oscillations excited by the 2011 Tohoku-Oki tsunami. The hazardous persistent oscillations prompted authorities to maintain a tsunami warning for dozens of hours. Tsunami waves reached the eastern China coast about five hours after the earthquake. The duration of detected prolonged oscillations was more than 80 h. In this study, we accurately reproduced tsunami generation and its evolution over the shallow and wide continental shelf of the East China Sea (ECS). For resonance analysis, we used an advanced numerical model that can consider simultaneously the effects of kinematic rupture process, horizontal displacement on tsunami generation, and Earth elasticity, seawater compressibility, and wave dispersion on tsunami propagation. The consistency of the simulated results was validated with both coastal and offshore tsunami records. In addition, we performed a combined spectral analysis of the observed tsunami records, background noise, and synthetic tsunami wave fields to identify tsunami oscillation modes and estimate their spatial distribution, including the spectral amplitude and phase angles. Eight main oscillation modes of the 2011 Tohoku tsunami were determined with periods of 24, 28, 37, 47, 57, 93, 136, and 157 min. They were all constrained within the 200-m isobaths for dozens of hours. The 57 min oscillations mode had the highest energy amplification across most of eastern China's coast. The spatial patterns of the resonance oscillations were used to propose hazard-prone areas. Based on the comprehensive spectral analysis results, the local and shelf topography properties played a key role in the dominant resonance modes of 136, 57, and 37 min. Identification of the shelf resonance oscillations coupled with the nearshore basin oscillations can provide valuable information for coastal tsunami hazard mitigation.

Satellite-estimated air-sea CO2 fluxes in the Bohai Sea, Yellow Sea, and East China Sea: Patterns and variations during 2003–2019

The Bohai Sea (BS), Yellow Sea (YS), and East China Sea (ECS) together form one of the largest marginal sea systems in the world, including enclosed and semi-enclosed ocean margins and a wide continental shelf influenced by the Changjiang River and the strong western boundary current (Kuroshio). Based on in situ seawater pCO2 data collected on 51 cruises/legs over the past two decades, a satellite retrieval algorithm for seawater pCO2 was developed by combining the semi-mechanistic algorithm and machine learning method (MeSAA-ML-ECS). MeSAA-ML-ECS introduced semi-analytical parameters, including the temperature-dependent seawater pCO2 (pCO2,therm) and upwelling index (UISST), to characterise the combined effect of atmospheric CO2 forcing, thermodynamic effects, and multiple mixing processes on seawater pCO2. The best-selected machine learning algorithm is XGBoost. The satellite-derived pCO2 achieved excellent performance in this complicated marginal sea, with low root mean square error (RMSE = 20 μatm) and mean absolute percentage deviation (APD = 4.12 %) for independent in situ validation dataset. During 2003–2019, the annual average CO2 sinks in the BS, YS, ECS, and entire study area were 0.16 ± 0.26, 3.85 ± 0.68, 14.80 ± 3.09, and 18.81 ± 3.81 Tg C/yr, respectively. Under continuously increasing atmospheric CO2 concentration, the BS changed from a weak source to a weak sink, the YS experienced interannual fluctuations but did not show significant trend, while the ECS acted as a strong sink with CO2 absorption increased from ∼10 Tg C in 2003 to ∼19 Tg C in 2019. In total, CO2 uptake in the entire study area increased by 85 % in 17 years. For the first time, we present the most refined variation in the satellite-derived pCO2 and air-sea CO2 flux dataset. These complete ocean carbon sink statistics and new insights will benefit further research on carbon fixation and its potential capacity.

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.

Impact of Synoptic Wind Intensification and Relaxation on the Dynamics and Heat Budget of the South Senegalese Upwelling Sector

Abstract In addition to their well-known seasonal cycle, eastern boundary upwelling systems (EBUS) undergo modulation on shorter synoptic to intraseasonal time scales. Energetic intensifications and relaxations of upwelling-favorable winds with 5–10-day typical time scales can impact the EBUS dynamics and biogeochemical functioning. In this work the dynamical effects of wind-forced synoptic fluctuations on the South Senegalese Upwelling Sector (SSUS) are characterized. The region geomorphology is unique with its wide continental shelf and a major coastline discontinuity at its northern edge. The ocean response to synoptic events is explored using a modeling framework that involves applying idealized synoptic wind intensification or relaxation to a five-member climatological SSUS ensemble run. Model evaluation against sparse midshelf in situ observations indicates qualitative agreement in terms of synoptic variability of temperature, stratification, and ocean currents, despite a moderate but systematic bias in current intensity. Modeled synoptic wind and heat flux fluctuations produce clear modulations of all dynamical variables with robust SSUS-scale and mesoscale spatial patterns. A mixed layer heat budget analysis is performed over the continental shelf to uncover the dominant processes involved in SSUS synoptic variability. Modulations of horizontal advection and atmospheric forcing are the leading-order drivers of heat changes during either wind intensification or relaxation while vertical dynamics is of primary importance only in a very localized area. Also, modest asymmetries in the oceanic responses to upwelling intensification and relaxation are only identified for meridional velocities. This brings partial support to the hypothesis that synoptic variability has a modest net effect on the climatological state and functioning of upwelling systems dynamics.

Coastal sea levels and wind-waves in the Mediterranean Sea since 1950 from a high-resolution ocean reanalysis

In the Mediterranean Sea, coastal extreme sea levels are mainly caused by storm surges driven by atmospheric pressure and surface winds from extratropical cyclones. In addition, wind-waves generated by the same atmospheric perturbations may also contribute to coastal extremes through wave setup (temporal rise above the mean sea level due to dissipation and breaking of waves in shallow waters close to the shore). This study investigates the spatial and temporal variability of coastal extreme sea levels in the Mediterranean basin, using a new ocean hindcast generated with a coupled hydrodynamic-wave model that simulates storm surges and wind-waves. The numerical simulation covers the period 1950-2021 with high temporal sampling (1h) and at unprecedented spatial resolution for a basin scale analysis, that reaches 200 m along the coastlines. Coastal storm surges and wave heights are validated with available observations (tide gauges, waves buoys and satellites). Comparison against tide gauges shows an average RMSE of 7.5 cm (7.7 cm for extreme events) and mean linear correlation of 0.64 for the whole period. Similarly, comparison of simulated and observed significant wave height shows good agreement, with RMSE lower than 0.25 m and a coefficient correlation as high as 0.95. The results confirm that coastal extreme sea levels are more likely to be located in regions with wide continental shelves favouring the wind contribution to storm surges along with shallow waters that favour wave setup induced by depth-breaking. The contribution of waves to coastal extreme sea levels has been quantified, using the hindcast in combination with an uncoupled simulation and has been shown to be significant, with an assessed wave setup spatial footprint at regional scale and observed maximum sea levels increased by up to 120% in the presence of waves.

Late Pleistocene to Holocene variations in marine productivity and terrestrial material delivery to the western South Atlantic

Despite the increased number of paleoceanographic studies in the SW Atlantic in recent years, the mechanisms controlling marine productivity and terrestrial material delivery to the South Brazil Bight remain unresolved. Because of its wide continental shelf and abrupt change in coastline orientation, this region is under the influence of several environmental forcings, causing the region to have large variability in primary production. This study investigated terrestrial organic matter (OM) sources and marine OM sources in the South Brazil Bight, as well as the main controls on marine productivity and terrestrial OM export. We analyzed OM geochemical (bulk and molecular) proxies in sediment samples from a core (NAP 63-1) retrieved from the SW Atlantic slope (24.8°S, 44.3°W, 840-m water depth). The organic proxies were classified into “terrestrial-source” and “marine-source” groups based on a cluster analysis. The two sources presented different stratigraphical profiles, indicating distinct mechanisms governing their delivery. Bulk proxies indicate the predominance of marine OM, although terrestrial input also affected the total OM deposition. The highest marine productivity, observed between 50 and 39 ka BP, was driven by the combined effects of the South Atlantic Central Water upwelling promoted by Brazil Current eddies and fluvial nutrient inputs from the adjacent coast. After the last deglaciation, decreased phytoplankton productivity and increased archaeal productivity suggest a stronger oligotrophic tropical water presence. The highest terrestrial OM accumulation occurred between 30 and 20 ka BP, with its temporal evolution controlled mainly by continental moisture evolution. Sea level fluctuations affected the distance between the coastline and the sampling site. In contrast, continental moisture affected the phytogeography, changing from lowlands covered by grasses and saltmarshes to a landscape dominated by mangroves and the Atlantic Forest. Our results suggest how the OM cycle in the South Brazil Bight may respond to warmer and dryer climate conditions.

Continental shelves as detrital mixers: U–Pb and Lu–Hf detrital zircon provenance of the Pleistocene–Holocene Bering Sea and its margins

AbstractContinental shelves serve as critical transfer zones in sediment routing systems, linking the terrestrial erosional and deep‐water depositional domains. The degree to which clastic sediment is mixed and homogenised during transfer across broad shelves has important implications for understanding deep sea detrital records. Wide continental shelves are thought to act as capacitors characterised by transient sediment storage during sea‐level rise and sediment remobilisation during sea‐level fall. This study attempts to test the hypothesis that sea‐level lowstand yields more efficient and direct sediment transfer from fluvial sources to deep sea sinks compared to highstand when sediment is sequestered and mixed on the shelf. This hypothesis is tested by evaluating U–Pb and Lu–Hf detrital zircon provenance trends along the vast Bering Sea shelf and deep‐marine Beringian continental margin. Presented here are 5884 U–Pb ages and 402 Lu–Hf analyses from 30 samples to characterise the provenance of modern to Pleistocene sediment across the Bering Sea region. Both forward and inverse numerical mixture modelling was used to estimate the abundance of distinct fluvial sources in shelfal and deep‐water deposits. These results demonstrate that sediment in the Bering Sea is derived from a mixture of regional fluvial sources, but that the Yukon River is the primary detrital source for sediment throughout the region. Although Yukon River signatures are abundant in all basin samples, the relative proportions of Yukon River versus other sources vary spatially across the shelf. A comparison of Holocene and surficial sediment with Pleistocene deposits shows that sediment across the shelf and in the deep sea remains well‐mixed between climate states. Thus, detrital provenance signatures in deep‐marine deposits outward of broad transfer zones are likely to represent mixtures of fluvial sources regardless of sea level.

Synoptic Flow Variability in a River-Influenced Inner Shelf off Central Chile

The response of inner shelf circulation and bottom temperature variability to synoptic wind forcing and freshwater outflow is evaluated in an area with a wide continental shelf off central Chile. This forced circulation, with a strong seasonal evolution from upwelling- to downwelling-favorable conditions, is a key process modulating the exchange of water properties in a coastal zone characterized by multiple river outflows and high biological productivity. Ocean currents on the inner shelf (34 m depth) off the Itata River mouth were obtained and analyzed for a nine-month record (December 2008 to September 2009). The synoptic wind and current variability was defined between 2 and 16 days (0.02 to 0.0026 cph). The subinertial coastal circulation within the inner shelf off Itata River was dominated by the along-shelf flow, while cross-shelf flows driven by along-shelf winds were substantially reduced. The alongshore synoptic currents showed two distinct modal structures: (i) A mean two-layer flow field during upwelling-favorable wind stress with northward (southward) flow at the surface (bottom), and (ii) a mean southward flow through the entire water column during downwelling-favorable wind stress periods, which were intensified during peak river discharge events in winter. Calculations of the wind index clarified the relative importance of wind versus buoyancy forcing on the alongshore flow. The wind forcing dominated during summer when the river discharge was minimum, whereas the relative importance of the freshwater outflow from the Itata River became dominant in winter (May-August) when the buoyancy forcing, in conjunction with southward wind events, modulated strong barotropic southward flows. The change in the long-term regime of coastal winds and river discharges, with a dominance of upwelling winds and reduced river outflows in the last decade, is discussed in the context of observations of thinner river plumes under frequent upwelling conditions off central Chile.

Dissolved Organic Carbon Source Attribution in the Changjiang Outflow Region of the East China Sea.

The variable optical properties of chromophoric dissolved organic matter (CDOM) under the complicated dynamic marine environment make it difficult to establish a robust inversion algorithm for quantifying the dissolved organic carbon (DOC). To better understand the main factors affecting the relationship between the DOC and the CDOM when the Changjiang diluted water (CDW) interacts with the marine currents on the wide continental shelf, we measured the DOC concentration, the absorption, and the fluorescence spectra of the CDOM along the main axis and the northern boundary of the CDW. The sources of DOC and their impacts on the relationship between the optical properties of the DOC and CDOM are discussed. We reached the following conclusions: There are strong positive correlations between the absorptive and fluorescent properties of the DOC and the CDOM as a whole. The dilution of the terrestrial DOC carried by the CDW through mixing with saline sea water is the dominant mechanism controlling the characteristics of the optical properties of the CDOM. CDOM optical properties can be adopted to establish inversion models in retrieving DOC in Changjiang River Estuary. It is concluded that the introduction of extra DOC from different sources is the main factor causing the regional optical complexity leading to the bias of DOC estimation rather than removal mechanism. As whole, the input of polluted water from Huangpujiang River with abnormally high and Fs(355) will induce the overestimation of DOC. In the main axis of CDW, the impact from autochthonous DOC input to the correlation between DOC and CDOM can be neglected in comparison with conservative dilution procedure. The relationship between the DOC and the CDOM on the northern boundary of the CDW is more complicated, which can be attributed to the continuous input of terrestrial material from the Old Huanghe Delta by the Subei Coastal Current, the input of materials from the Yellow sea by the Yellow Sea Warm Western Coastal Current, and the input of materials from the Changjiang Basin by the CDW. The results of this study suggest that long-term observations of the regional variations in the DOM inputs from multiple sources in the interior of the CDW are essential, which is conducive to assess the degree of impact to the DOC estimation through the CDOM in the East China Sea.

A storm hazard matrix combining coastal flooding and beach erosion

Coastal storms cause widespread damage to property, infrastructure, economic activity, and the environment. Along open sandy coastlines, two of the primary coastal storm hazards are coastal flooding by elevated ocean water levels and beach erosion as the result of storm wave action. At continental margins characterized by a shallow, wide continental shelf, coastal storms are more commonly associated with amplified storm surge and the damaging impacts caused by flooding of low-lying land. In contrast, along margins where the continental shelf is narrow and deep, coastal storm impacts are more often characterized by beach erosion, due to the typically lower magnitude of storm surge but a higher proportion of deepwater wave energy reaching the shoreline. A new Storm Hazard Matrix is presented that integrates these two distinct but inherently linked open coast hazards. The approach is based on the combination of two hazard scales. The first is a ‘coastal flooding hazard scale’ that follows an established framework in which hazards are predominately driven by the vertical increase in ocean water levels during storms and any significant morphological changes caused by the storm are inferred. The second is a storm wave ‘beach erosion hazard scale’ where hazards are predominately determined by the horizontal recession of the sandy beach and dune without necessarily large increases in water levels. The resulting framework comprises a total of sixteen unique combinations of flooding/erosion storm hazard regimes, each potentially requiring different disaster risk reduction approaches. Real-world application of the Storm Hazard Matrix is explored at contrasting coastlines for two major storm events, encompassing an extratropical cyclone that impacted the coastline of southeast Australia in June 2016, and a large hurricane (Hurricane Ivan) that impacted the Gulf Coast of the United States in 2004. The new approach identifies and distinguishes between the severity of localized coastal flooding and/or coastal erosion, and also provides enhanced insight to the nature, magnitude and alongshore variation of coastal storm hazards along the impacted coastline. Within the context of disaster risk reduction, preparedness and operational early warning, implementation of the Storm Hazard Matrix has the potential to deliver robust evaluations of storm hazards spanning a wider variety of both wave-dominated and surge-dominated coasts.

Response of Total Suspended Sediment and Chlorophyll-a Concentration to Late Autumn Typhoon Events in the Northwestern South China Sea

Strong typhoon winds enhance turbulent mixing, which induces sediment to resuspend and to promote chlorophyll-a (Chl-a) blooms in the continental shelf areas. In this study, we find limited Chl-a responses to three late autumn typhoons (typhoon Nesat, Mujigae and Khanun) in the northwestern South China Sea (NWSCS) using satellite observations. In climatology, the Chl-a and total suspended sediment (TSS) concentrations are high all year round with higher value in autumn in the offshore area of the NWSCS. After the typhoon passage, the Chl-a concentration increases slightly (23%), while even TSS enhances by 280% on the wide continental shelf of the NWSCS. However, in the southern area, located approximately 100 km from the typhoon tracks, both TSS and Chl-a concentrations increase 160% and 150% after typhoon passage, respectively. In the deeper area, the increased TSS concentration is responsible for the considerable increase of the Chl-a. An empirical analysis is applied to the data, which reveals the TSS and Chl-a processes during typhoon events. The results of this study suggest a different mechanism for Chl-a concentration increase and thus contribute toward further evaluation of typhoon-induced biological responses.

Post-rift magmatism and hydrothermal activity in the central offshore Otway Basin and implications for igneous plumbing systems

Spatially and volumetrically extensive igneous rocks are exposed onshore in southeastern Australia and provide important information about the Cenozoic evolution of the region. However, the morphology and distribution of igneous rocks are still not well constrained in the ~70 km wide continental shelf of the central Otway Basin. Within this region, we present seismo-geomorphological characteristics of 30 volcanoes and 27 hydrothermal vents, as well as several lava flows and magmatic sills, based on 2D and 3D seismic reflection data. Hydrothermal vents formed mainly during the mid-Eocene, while igneous complexes developed between the mid-Eocene to late Miocene, both post-dating continental break-up in the basin. The distribution and morphology of igneous rocks and hydrothermal vents is influenced by rift-related faults, as most of these features are linked to the underlying faults by near-vertical zones of disruption and are elongated in an NW-SE direction that parallels the primary fault trend. Magma for volcanoes and heat sources for hydrothermal vents within the study area are likely supplied by deep-sourced dykes, as magmatic sills only have a scattered distribution in this region. This implies a dyke-dominated plumbing system for magmatism in the Otway Basin. In comparison, the nearby Bight Basin, which has a thin basement but a relatively thick sedimentary sequence, displays magma propogation more prone to forming igneous sills. There are significant lithological and tectonic similarities between the Otway and Bight basins; thus, we propose that a relatively thin sedimentary sequence over a thick basement in the Otway Basin produced sufficient magma pressure for a predominantly dyke-dictated igneous plumbing system. This work highlights the critical role of basin structures, such as the thickness of basement and overlying strata, in the control of igneous plumbing styles and the distribution of post-rift igneous complexes along magma-poor continental margins. Our work, therefore, aids the estimation of different magmatic components within the sedimentary basins and facilitates the understanding of their diverse impacts on the exploration of frontier basins. This result also provides some constraints on the prediction of possible future eruption centres within active volcanic fields.

Coastal erosion and structural damage due to four consecutive-year major hurricanes: Beach projects afford resilience and coastal protection

This paper presents a forensic analysis of the coastal erosion and structural damage associated with four consecutive-year, Atlantic-basin major hurricanes. The investigation focuses on Hurricanes Matthew (2016), Irma (2017), Michael (2018) and Dorian (2019) and their impact on the sandy shorelines and coastal construction of Florida, located in the southeastern United States. The study identifies the degree of resilience and coastal protection of different coastal construction methods and beach management programs under varied conditions associated with major hurricane impacts. The study results demonstrate a coherent relationship between observed peak storm tide and surveyed measures of coastal erosion and structural damage. The storm tide generated by the natural forcing of tropical-cyclone winds is variably influenced by the wide continental shelf of Florida's gulf coast and the meso-tidal regime of Florida's east coast. As our data show, beyond the obvious metric of storm strength, as expressed in terms of maximum wind speed, coastal impacts by hurricanes are additionally (or more predominantly) driven by storm size, as expressed in terms of wind radii for tropical storm- and hurricane-force winds. Of socioeconomic relevance, the results demonstrate that beach management projects of sand placement provide dune and berm protection as well as mitigation against damage to coastal property and infrastructure. The findings of this study are directly relevant to other coastal regions with recreational beach, dense populations and vulnerable infrastructure, subject to erosional and destructive events caused by tropical cyclones.

Cosmo-SkyMed and TerraSAR-X datasets for geomorphological mapping in the eastern of Marajó Island, Amazon coast

The Amazon coast is marked by the high discharge of sediments and freshwater, macrotidal influence, a wide continental shelf, extensive flood plains and lowered plateaus which make it unique as a delta and estuary landscape. Further, the tropical climate imposes heavy rains and incessant cloudiness that render microwave systems more suitable for cartography. This study proposed to recognize and map the Amazon coastal environments through the X-band Synthetic Aperture Radar, provided by Cosmo-SkyMed (CSK) and TerraSAR-X (TSX) systems. The SAR datasets consisted of interferometric and stereo pairs, restricted to single-revisit and obtained with small interval (1–11 days), under steeper (θ<35°) and shallow (θ≥35°) incidence angles, and during the rainy and dry seasons. From the 4 acquisitions of X-band SAR data, attributes such as the backscattering coefficient, coefficient of variation, texture, coherence, and Digital Surface Model (DSM) were derived, adding each variable in 5 scenarios. These combinations resulted in 20 models, which were submitted individually to the machine learning (ML) classification approach by Random Forest (RF). The backscattering and altimetry described the coastal environments which shared ambiguity and high dispersion, with the lowest separability for vegetated environments such as Mangrove, Vegetated Coastal Plateau and Vegetated Fluvial Marine Terrace. The coherence provided by interferometry was weak (<0.44), even during the dry season, in the other hand, the cross-correlation was strong (>0.60), during the rainy and dry season showing more suitability for radargrammetry on the Amazon coast. The RF models resulted in Kappa coefficient between 0.39 to 0.70, indicating that the use of X-band SAR images at an incidence angle greater than 44° and obtained in the dry season is more appropriated for the morphological mapping. The RF models given by TSX achieved the higher mapping accuracies per scenario of SAR products, in order of 0.48 to 0.63. Despite this, the best classification was carried out by 19 RF model with 0.70, provided by CSK in shallow incidence composed by intensity, texture, coherence and stereo DSM. The CSK and TSX data allowed to map the Amazon coast precisely, based on X-band at single polarization, high spatial resolution and revisit, which has demonstrated the support for detailed cartography scale (1:50,000) and frequent updating (monthly up to yearly).