Continental Shelf Research Articles

Asymmetry response of storm surges along the eastern coast of the Taiwan Strait

Spatiotemporal variation of storm surges in the Taiwan Strait (TWS) is studied using water level datasets from 14 tidal gauge stations located in the TWS from summer to fall of 2016. The effects of bathymetry on storm surges and tropical cyclone (TC)-induced continental shelf wave (CSW) are explored. By comparing water level response along the east coast of TWS, it is found that storm surges are asymmetric on the north and south sides of the Zhan-Yun Ridge (ZYR), regardless of the different categories and tracks of TCs passing by. Observations indicate that the ZYR could modulate the storm surges and the CSW propagation; ZYR can not only amplify the storm surges that generally peak around the ZYR, but also block the CSW propagation by dramatically dissipating its kinetic energy as revealed by the dispersion relation for the first mode CSW. Moreover, local wind work and the remote forcing, which are induced by TCs and cold fronts, respectively, can also modulate the synoptic variations of water level in the TWS.

Biogeochemical factors regulating photosynthetically dissolved organic carbon produced by phytoplankton in the Taiwan Strait

The distribution and mechanisms of photosynthetically dissolved organic carbon (PDOC) released by marine phytoplankton are frequently neglected and inadequately understood because most studies on carbon sequestration capacity have focused on photosynthetic particulate organic carbon. In this study, percentage extracellular release (PER) and its environmental influencing factors were investigated for 10 cruises in the Taiwan Strait during 2006–2023. The results indicated that the PER increased horizontally from the nearshore to the off-shelf and vertically from the surface to the bottom within the euphotic zone. PER tends to be low in eutrophic waters such as upwellings and estuaries and high in oligotrophic waters. The study revealed that the average contribution of PDOC to total primary productivity (TPP) in the Taiwan Strait could reach 18.2 ± 11.7%, which is similar to the previously estimated global oceanic values. PDOC production satisfied approximately 25% the carbon requirements of heterotropic bacteria (HB). A detailed analysis of the PER combined with model simulations proved that the distribution of the PER in the Taiwan Strait was caused by the joint contribution of irradiance, size-fractionated phytoplankton, and nutrient stoichiometry. Our results contradict the view that the PER is a constant factor that is unaffected by TPP. However, there was a significant negative correlation between the PER and TPP. The PDOC was always lower than the bacterial carbon demand for a broad range of bacterial growth efficiencies, suggesting a weak coupling between phytoplankton exudation and bacterial metabolism. This challenges the idea that there is a well-coupled relationship between bacteria and phytoplankton present on the continental shelf. These findings indicate significant discrepancies in PDOC mechanisms and the quantitative importance of nearshore eutrophic and off-shelf oligotrophic environments. Consequently, it is unwise to use uniform PERs without differentiation under trophic conditions when reevaluating and appraising marine carbon fixation.

Seafloor geomorphology of the Wrigley Gulf shelf, Amundsen Sea, West Antarctica, reveals two different phases of glaciation

AbstractKnowledge of the behaviour of marine‐based ice sheets during times of climatic warming, such as the last deglaciation, provides important information to understand how ice sheets respond to external forcing. We analysed swath bathymetric and acoustic sub‐bottom profiler data from Wrigley Gulf on the western Amundsen Sea shelf, West Antarctica, to identify glacial features and reconstruct past changes in the extent of the West Antarctic Ice Sheet (WAIS) and ice flow directions. Glacial bedforms mapped within a bathymetric cross‐shelf trough include features showing cross‐cutting and overprinting relationship and indicate changes in ice‐flow orientation. Here, we distinguish at least two phases of different ice‐flow patterns on the Wrigley Gulf shelf. During the earlier phase, seaward ice stream flow on the inner shelf was deflected towards the east due to the existence of an ice dome on the middle‐outer continental shelf. Retreat of grounded ice towards the centre of this dome is indicated by the asymmetric cross profile of recessional moraines mapped on the middle shelf. The later glaciation phase was characterized by fast, NNW‐directed ice flow across the shelf along a broad front and subsequent stepwise landward retreat, which is evident from the common occurrence and orientation of mega‐scale glaciation lineations and grounding zone wedges on the middle‐inner shelf. It is uncertain whether the two phases of glaciation recorded on the seafloor occurred during the last and penultimate glacial periods or at different times of the last glaciation. Reliable chronological constraints from sediment cores and additional geomorphological information are needed to understand the cause of the changes in WAIS dynamics reflected by the two ice‐flow phases.

Field Observations Of The Influence Of Infragravity Waves On Wave Overtopping At A Dike On A Shallow Foreshore

Infragravity (IG) waves are low-frequency waves (0.0033 Hz < f < 0.05 Hz) that exist: (i) being bound to the wave groups of high-frequency sea-swell (SS) waves (0.05 Hz < f < 1 Hz), (ii) generated by the moving breakpoint due to the groupiness of the SS waves (Bertin et al., 2018), and (iii) as free IG waves released from the SS wave groups and reflected from neighbouring coasts (Rijnsdorp et al., 2021). Offshore in deeper water (h/Hm0,o > 4, with h the water depth and Hm0,o the offshore significant wave height), IG waves become most apparent during storm conditions, but are still rather limited in wave height (order of a couple of cm). However, at the toe of coastal structures with a shallow foreshore depth (1 < ht/Hm0,o < 4, with ht the water depth at the toe), the energy of IG waves becomes more significant relative to the SS wave frequency band of the wave spectrum (Hofland et al., 2017), becomes equally important for very shallow foreshores (0.3 < ht/Hm0,o < 1), and even dominates SS wave energy for extremely shallow foreshore depths (ht/Hm0,o < 0.3). IG wave growth on mildly sloped shallow foreshores is a result of IG wave shoaling and SS wave energy transfer to IG waves via nonlinear wave-wave interactions (Bertin et al., 2018), and the relative growth of IG to SS wave energy is further aided by SS wave breaking. Coastal urban areas along the Belgian coast (and in other low-lying countries worldwide) rely on a hybrid beach-dike coastal defense system for protection against flooding. During design storm conditions, the nourished beach erodes to a very to extremely shallow foreshore in front of a low-crested impermeable sea dike. Wave overtopping at this type of structure has been shown to be affected by IG waves, based on physical (Altomare et al., 2016) and numerical modelling (Lashley et al., 2021) for shallow to extremely shallow foreshores. However, field observations of the influence of IG waves on wave overtopping have been scarce (van Gent and Giarusso, 2003) and the influence of free IG waves has not been investigated yet. Indeed, the amount of IG wave energy that is expected to arrive at the coastline, not only depends on IG waves naturally bound to the SS wave groups, but also on free IG waves generated offshore or locally reflected from the coast (edge waves). The Belgian continental shelf is characterized by the prevalence of offshore sandbanks (see Figure 1a), which might be an additional source of free IG waves, due to ss wave breaking on the shallow crest of those banks. These free IG waves are location specific and their existence can only be determined by field observations. Knowledge of the offshore boundary conditions with detailed IG wave information is indispensable for the correct prediction and modelling of surf zone hydrodynamics and beach morphodynamics (Fiedler et al., 2019), and the consequent wave overtopping on the dike. Simultaneous measurements of IG waves, offshore and on the beach-dike system during storm conditions together with wave overtopping has not been done before, as it is complex and needs a specific setup with high precision measurement equipment. In-situ measurements provide an invaluable dataset (i.e., no model nor scale effects) and are essential for validating safety assessment and design methodologies of coastal defense systems.

Geohazard features of the Tyrrhenian Calabria

ABSTRACT This paper accompanies the Maps of Geohazard features of the Cilento and the Calabro-Tyrrhenian continental margin in the southern Tyrrhenian Sea (Italy). The main geohazard-related features were derived from extensive seafloor mapping through the collection of high-resolution multibeam data acquired during several oceanographic cruises. They encompass many fluids seepage features, fault scarps, landslides scars, gullies, channels, and canyons. Hazards related to coastal landslides and shelf-indenting canyons are very high in these sectors (especially in southern Calabria) due to active seismicity coupled with rapid uplift, high sedimentation rates and narrow or totally absent continental shelf, thus promoting a direct connection between steep slopes and coastal areas. In this setting, mass-wasting features can directly impact coastal or submarine infrastructures or indirectly create local tsunami waves, as observed in historical times. Moreover, this physiographic setting of the margin facilitates the transfer of marine litter toward deep-sea areas.

Eurasian Arctic Rifted Margin Composite Tectono-Sedimentary Element

Abstract The Eurasian Arctic Rifted Margin (EARM) spans a poorly studied transition region between the Eurasian continental shelf and deep-water Nansen Basin. It is a conjugate margin to the Lomonosov Ridge as both domains were united before opening of the oceanic Eurasian Basin. The EARM is considered as a Composite Tectono-Sedimentary Element (CTSE) consisting of two individual tectono-sedimentary elements (TSEs): (1) an undefined group of poorly known pre-rift Mesozoic and Paleozoic sedimentary successions, and (2) the Late Cretaceous-Paleocene syn-rift strata. The latter are bounded by ∼56 Ma breakup unconformity and overlain by the Eurasia Basin prograded margin TSE, which is not included in EARM. In the absence of drilled wells, we utilize multichannel seismic reflection (MCS) data for seismic stratigraphic correlations with nearby, better-explored areas. The EARM is subdivided into three along-strike segments (from west to east): Svalbard-Barents Sea, the North Kara Sea, and the Taimyr-Laptev Sea. Across-strike, the EARM is composed of a proximal zone of stretched crust characterized by normal faulting, and a relatively narrow distal transition zone that may contain unroofed serpentinised mantle. The lack of drilled wells makes it challenging to evaluate petroleum potential of the CTSE, although some indications of a working petroleum system presence exist.

Indigo-dyed cellulose fibers and synthetic polymers in surface-feeding seabird chick regurgitates from the Gulf of Alaska

We provide evidence of anthropogenic materials ingestion in seabirds from a remote oceanic area, using regurgitates obtained from black-legged kittiwake (Rissa tridactyla) chicks from Middleton Island (Gulf of Alaska, USA). By means of GPS tracking of breeding adults, we identified foraging grounds where anthropogenic materials were most likely ingested. They were mainly located within the continental shelf of the Gulf of Alaska and near the Alaskan coastline. Anthropogenic cellulose fibers showed a high prevalence (85 % occurrence), whereas synthetic polymers (in the micro- and mesoplastics dimensional range) were less frequent (20 %). Most fibers (60 %) were blue and we confirmed the presence of indigo-dyed cellulosic fibers, characteristic of denim fabrics. In terms of mass, contamination levels were 0.077 μg g−1 wet weight and 0.009 μg g−1 wet weight for anthropogenic microfibers and synthetic polymers, respectively. These results represent the only recent report of contamination by anthropogenic fibers in seabirds from the Gulf of Alaska.

Downscaling the ocean response to the Madden–Julian Oscillation in the Northwest Atlantic and adjacent shelf seas

AbstractSubseasonal-to-seasonal (S2S) prediction is a global effort to forecast the state of the atmosphere and ocean with lead times between two weeks and a season. This study explores the feasibility of S2S prediction of the ocean using a variety of tools including statistical analysis, a statistical-dynamical mixed layer model, and a regional, high-resolution ocean circulation model based on physical principles. Ocean predictability on S2S timescales is analyzed by compositing winter sea surface temperature (SST) anomalies in the North Atlantic with respect to the state of the Madden–Julian Oscillation (MJO). It is found that statistically significant, large-scale SST changes, particularly along the eastern seaboard of North America, can be related to the MJO. This signal is shown to be driven by anomalous air–sea heat fluxes caused by atmospheric perturbations in response to the MJO. The high-resolution model of the Gulf of Maine and Scotian Shelf is used to downscale the mean ocean response to the MJO. The model is able to capture the observed relationship between the MJO and SST in the northwest Atlantic. It is also shown that the anomalous atmospheric circulation in response to the MJO leads to anomalous upwelling on the Scotian Shelf. Overall, this study demonstrates that it is feasible, and of value, to use regional ocean models for S2S prediction.

The Quaternary sedimentary history and dynamics of the deep-water margin off the Hudson Strait ice stream

Sites of ice-stream discharge to the continental slope build trough-mouth fans from glacigenic debris flows (GDFs), subglacial meltwater flows, and suspended sediment. Thus, past ice configurations may be interpreted from the sedimentological record. This study links the records in cores, high-resolution seismic profiles, and multibeam swath bathymetry of Hatton Slope and Fan to the changing glaciological history in Hudson Strait, one of the largest ice streams of the late Pleistocene Laurentide Ice Sheet. 14C-AMS dates, X-ray fluorescence geochemical profiles, and oxygen isotopes in sixteen cores document sediment supply during the last glacial cycle, and earlier history is interpreted from seismic data. The Frobisher Bay slope sector has a long record of GDF deposits from severe glacial periods between MIS 6 and 22, with little impact of meltwater discharge. Mass-transport events and aggrading gravel-floored valleys dominate the northern Hatton slope sector. The central slope sector shows the greatest shelf break progradation by till deposition, with kilometre-spaced gullies heading at 700–730 mbsl from the last major grounding of the ice-stream on the upper slope during Heinrich event 3 (H3). The gullies pass downslope into ∼5 km wide valleys, which have been sites of sediment bypass since widespread meltwater flow in MIS 6 and show little aggradation since MIS 16. Intervening >50 m high ridges show alternating levee accretion and erosion by shallow mass-transport events. Downslope on Hatton Fan, GDFs are common and fill wide erosional channels that lead to the sandy braid plain of the central Labrador Sea. The channels of the southern sector and Saglek Slope have channeled turbidity currents down the NAMOC, and stratified contourite deposits accumulated on the fan. Over the MIS 1–3 period widely represented by cores, ice was grounded in Hatton Basin and on the upper slope to 700 m water depth only immediately prior to H3. Older and younger H events had a floating ice-shelf in Hatton Basin, and sedimentation was dominated by the retreat of that ice shelf to Resolution Island. Distal cores show no systematic difference between the H1–top H3 interval and the base H3–H5 interval. Given the supply of carbonate detritus of consistent composition back to at least H5, any MIS 3 deglaciation of Hudson Bay would require improbably high growth rates of ice sheets in the centre of the continent.

Morphological insights into the three-dimensional complexity of rhodolith beds

The ecological importance of rhodolith beds stems from their role as structurally complex three-dimensional habitat formed by free-living red calcareous algae. Their structural singularity is due to the great variety of complex and branching morphologies exhibited by rhodoliths that create interstitial spaces and increase their surface area. This increases the ecological niches for cryptofauna and provide refuge for a high number of organisms, which is why rhodolith beds are considered biodiversity hotspots. In this work, we studied a rhodolith bed located in the Menorca Channel, formed by several species of red calcareous algae that exhibit a great variety of morphologies and form an extensive and heterogeneous habitat. This study explored the morphological diversity of the rhodolith bed, comparing the ‘Core Habitat’ (within the center of the bed with the highest densities of rhodoliths) with the boundaries or ‘Adjacent Habitat’ where rhodolith density was lower. Our results show that all rhodolith growth forms (branched, pralines and boxwork) in the Core Habitat had higher interstitial space and were larger than the ones from adjacent zones. Moreover, we explored the three-dimensional techniques to study the morphological characteristics that have historically been studied in two dimensions. This study contributes to the knowledge of morphological diversity in well-preserved rhodolith beds from continental shelves in the western Mediterranean Sea and reinforces the use of three-dimensional measurements, specifically the interstitial space of branched rhodoliths, to provide more accurate data on habitat complexity.

Seasonal abundance, lipid storage, and energy density of Calanus finmarchicus and other copepod preyfields along the Northwest Atlantic continental shelf

Abstract Calanus finmarchicus is an abundant calanoid copepod in the New York Bight (NYB) that has energy-dense lipid stores. This study measured C. finmarchicus abundance, lipid storage and energy density and compared it to two other abundant calanoid species in the region. We compared the NYB C. finmarchicus preyfield characteristics with data from two other NW Atlantic coastal ecosystems, the Gulf of Maine (GoM) and Cape Cod Bay (CCB). Copepods from net tows were identified, enumerated, photographed and measured. These data were used to calculate animal biovolume and lipid content as well as the volumetric energy density of the copepod preyfield. C. finmarchicus were most abundant in spring and summer and the mean lipid storage of C. finmarchicus remained largely constant regardless of season and location within the NYB. Centropages typicus were abundant enough in the winter to be more energy-rich than C. finmarchicus. Temora longicornis, while also abundant, never surpassed the energy density of C. finmarchicus. C. finmarchicus in the NYB were comparably lipid-rich to those from the GoM and CCB. C. finmarchicus in the NYB can be a reliable energy source for local predators due to their high spring and summer abundances and year-round storage of high-energy wax esters.

Social structure and site fidelity of an oceanic delphinid: the rough‐toothed dolphin (Steno bredanensis) off the southwest Pacific coast of Mexico

AbstractRough‐toothed dolphins (Steno bredanensis) typically inhabit deep oceanic waters from tropical to temperate waters worldwide. However, their ecology and behavior remain poorly understood. We investigated site fidelity and social structure of rough‐toothed dolphins using photo‐identification data collected between 2014 and 2022 in the continental shelf waters of the southwestern coast of Mexico. A total of 133 groups of rough‐toothed dolphins were recorded. Mean group size was 4.9 individuals (SD = 4.5), and dolphins were encountered in waters of a mean depth of 110 m (SD = 188). Of 228 individuals identified, 55% were observed once, 39% were seen between two and four times, and 6% were seen five times or more. Site fidelity analysis suggests three levels of site fidelity: transients, occasional visitors, and regular visitors, which are likely to affect the social structure. Through network analysis, nonrandom social patterns were identified (S = 1.25 ± 0.26) and dolphins exhibited both preferences and avoidances of social partners. Cluster analysis suggested significant divisions within the population (Q = 0.43 ± 0.05). Despite limitations related to sample size and design, this preliminary study suggests that rough‐toothed dolphins are best characterized by a fission‐fusion society, which is similar to most coastal delphinid populations.

Bathymetric LiDAR and Semi‐Automated Feature Extraction Assist Underwater Archaeological Surveys

ABSTRACTCritical data concerning key developments in global human history now lie submerged on continental shelves where investigations confront significant challenges. Whereas underwater excavations and surveys are expensive and weather dependent and require specialized training and equipment, remote sensing methods can improve chances for success offshore. A refinement in one method, a semi‐automated analysis protocol that can help to identify Pleistocene and Holocene era archaeological deposits in bathymetric LiDAR datasets, is presented here. This method employs contour mapping to identify potential archaeological features in shallow water environments in Apalachee Bay, Florida. This method successfully re‐identified multiple previously recorded archaeological sites in the study region and detected at least four previously undocumented archaeological sites. These results suggest that this procedure can expand on methods to identify and record submerged archaeological deposits in sediment‐starved, shallow‐water environments.

Benthic foraminifera as depth estimators in the tropical carbonate shelf of northeastern Brazil

Benthic foraminifera are valuable proxies for oceanographic and environmental research due to their high degree of species-specific habitat use. The present study describes the distribution of benthic foraminifera across the continental carbonate shelf of northeastern Brazil, and relates their distribution patterns to environmental parameters associated with the texture of the sediment and the depth of the water. Sediment samples were collected at depths between 0.5 and 57 m. In the laboratory, the samples were analyzed following the standard protocol established for Quaternary foraminifera research. Sediments dominated by carbonate sand, which were approximately symmetric in the first depth interval (0.5–20 m), and asymmetric in the second (20–40 m) and third (40–57 m) intervals. The foraminiferal assemblages composed of 96 species, although they were dominated by the five species – Archaias angulatus, Textularia agglutinans, Amphistegina lessonii, Quinqueloculina lamarckiana, and Peneroplis carinatus. Most tests were intact and the species composition varied across the continental shelf, with Pseudotriloculina laevigata and Globobulimina? sp. Only occurring from 0.5 to 20 m depth in shelf, while Pyrgo elongata, Quinqueloculina sp. A, Quinqueloculina sp. C, Fissurina sp. A, Fissurina sp. B, Elphidium morenoi, Glabratella brasilensis, and Siphogenerina sp. Were restricted from 20 to 40 m, and Sigmoilina sp., Fursenkoina sp., and Textularia sp. A was only recorded in depth >40 m. The diversity of the foraminiferal assemblages is favored by the attenuation of hydrodynamic conditions at deeper intervals (20–57 m). Therefore, it can be concluded that the benthic foraminiferal species respond to the depth and variation in sediment characteristics through changes in their population and assemblage structures.

A middle Miocene to Quaternary sedimentary and palaeoenvironmental record from the western continental shelf of South Africa

The sedimentary record of the western South African continental shelf is condensed compared to the continental slope and contains erosional unconformities, owing to periods of non-deposition, eustatic sea-level fluctuations, episodic uplift and intensified continental aridity. Despite this, the sedimentary record of the continental shelf provides important information on the depositional history and palaeoenvironmental evolution of the region. A core retrieved from the western shelf of South Africa was analysed for its sedimentary composition, lithological variation, foraminiferal content and its relation to the palaeoenvironment of the region. Four depositional facies were identified along the core, namely quartzitic sand, sandy mud, and glauco-phosphatic sand and a glauco-phosphatic gravel. The basal facies consisting of quartzitic sand is interpreted to have been deposited between 15.90 and 14.60 Ma, corresponding to the timing of the Mid-Miocene Climatic Optimum (MMCO). The highly quartzitic nature of the sediments indicate a high terrestrial influence from fluvial sources. The overlying sandy mud facies was deposited between 14.60 and 13.90 Ma based on planktic foraminiferal biostratigraphy. Foraminiferal analyses of these two facies that were deposited in the Langhian stage of the middle Miocene point to subtropical sea surface conditions and mesotrophic benthic environments. Sea level was noticeably higher during the MMCO and part of the cooling period following the MMCO. An erosional surface that spans 10.77 Myr, equal to the late Miocene (13.90 Ma) to early Pliocene (3.13 Ma), marks the boundary between the two Langhian facies and the overlying two Pleistocene facies, consisting of coarser grained glauco-phosphatic gravelly sand units. The Pleistocene environment on the shelf is interpreted to contrast with the Langhian environment, where cooler, shallower conditions and a more eutrophic benthic environment was prevalent, during a time that Benguela upwelling intensified with higher frequency and higher amplitude sea level fluctuations. Palaeobathymetric interpretations indicate that middle Miocene sea-level in the region were up to 77 m higher than present day and 101 m lower in the Pleistocene, in-line with previous global studies. Glauco-phosphatic content that increase up-core also marks the shallowing of the environment under high productivity conditions.

Lagrangian characterization of the southwestern Atlantic from a dense surface drifter deployment

The Southwestern Atlantic (SWA) is characterized by its large Eddy Kinetic Energy as the result of the confluence of two major western boundary currents, the northward flowing Malvinas Current (MC) and the southward flowing Brazil Current. The SWA study was addressed in the literature based on altimetry data, in situ measurements, regional models and ocean reanalysis. The present study constitutes the first effort to sample a portion of the SWA, with a dense drifter array (N = 62) deployment. The drifters, drogued at 15 m depths, were deployed across the MC and the Argentine Continental Shelf along two zonal transects located at 47°S and 47.25°S, between the 8th and the September 9, 2021. Drifters were set to deliver their position every 10 and 60 min, providing accurate Lagrangian trajectories that provide information on a large range of space and time scales of the surface currents. Three regions are clearly identified based on the analysis of the speed of the drifters, of their trajectories and of the spectral density of their velocities: the continental shelf, the slope and the open ocean. The comparison of the trajectories of the drifters with satellite altimetry images shows that, in general, drifters follow mesoscale features that are detectable in satellite altimetry maps. The analysis of the drifter trajectories also allowed us the study of submesoscale features of the flow (1–10 km) that are not observable in satellite altimetry data. Comparison with cloud-free, high-resolution color images, shows that drifter trajectories organized by the mesoscale flow might also locally follow sub-mesoscale features. In frontal regions it was found that drifter velocities double satellite altimetry geostrophic velocities, which suggests that the dynamics at those regions is largely dominated by ageostrophic components. The ageostrophic Ekman component might explain the direction of the drifters when strong winds from a given direction prevail for several days and the drifters are not in a region with large sea surface height (SSH) gradients. The joint analysis of drifters’ trajectory and SSH clearly depicts that mesoscale features on the open ocean region control the cross-shelf exchanges between the MC and open ocean regions as well as the strength and width of the MC. Finally, the spatial density distribution of the drifters during the first hours after deployment and within a small eddy also allowed us to characterize the flow in terms of its divergence, vorticity and strain, indicating that the MC is geostrophic and has a jet-like behavior while the eddy is largely ageostrophic and has a dominant vorticity component over strain. We conclude observing that the analysis of a dense array of drifters provides valuable information of the flow that cannot be attained solely based on satellite data.

The Shelf‐To‐Basin Transport of Iron From the Northern U.S. West Coast to the Pacific Ocean

AbstractRelease of iron (Fe) from continental shelves is a major source of this limiting nutrient for phytoplankton in the open ocean, including productive Eastern Boundary Upwelling Systems. The mechanisms governing the transport and fate of Fe along continental margins remain poorly understood, reflecting interaction of physical and biogeochemical processes that are crudely represented by global ocean biogeochemical models. Here, we use a submesoscale‐permitting physical‐biogeochemical model to investigate processes governing the delivery of shelf‐derived Fe to the open ocean along the northern U.S. West Coast. We find that a significant fraction (∼20%) of the Fe released by sediments on the shelf is transported offshore, fertilizing the broader Northeast Pacific Ocean. This transport is governed by two main pathways that reflect interaction between the wind‐driven ocean circulation and Fe release by low‐oxygen sediments: the first in the surface boundary layer during upwelling events; the second in the bottom boundary layer, associated with pervasive interactions of the poleward California Undercurrent with bottom topography. In the water column interior, transient and standing eddies strengthen offshore transport, counteracting the onshore pull of the mean upwelling circulation. Several hot‐spots of intense Fe delivery to the open ocean are maintained by standing meanders in the mean current and enhanced by transient eddies and seasonal oxygen depletion. Our results highlight the importance of fine‐scale dynamics for the transport of Fe and shelf‐derived elements from continental margins to the open ocean, and the need to improve representation of these processes in biogeochemical models used for climate studies.

Drivers of organic carbon distribution and accumulation in the northern Barents Sea

Sedimentary properties and accumulation rates on the continental shelf and in the deep sea reflect temporal oceanographic, biological and chemical processes occurring in the water column and the sediment surface. We used the radionuclides 210Pb, 226Ra, and 137Cs activities to estimate sedimentation rates during the last century at nine stations in the northern Barents Sea region. Elemental (C, N) and stable isotopic composition (δ13C, δ15N) were also analysed from the nine stations sampled in August 2018, and, for five other stations sampled in August and December 2019, and in March and May 2021. Sediment accumulation rates varied between 130 and 1 410 g m−2 y−1. The < 63 μm normalized total organic carbon (TOC63) and the total nitrogen from the sediment surface varied between 0.90–2.56 % and 0.13–0.33 %, respectively. Ice-free shelf stations had higher TOC63 and possibly fresher organic matter (high δ13C, low δ15N) than ice-covered more northern stations. The opposite trend was observed for total inorganic carbon. We found that these trends in biogeochemical parameters were spatially structured by the winter sea ice concentration and biological production differences, and exhibited a south-north separation of the Polar Front region. The low and stable organic carbon accumulation rate (1.7–13.4 g Corg/m−2(−|-) y−1; ARtoc) is a function of slow sedimentation rates, and high degradation and residence time in the water column and at the sediment–water interface. Overall, the ARtoc has been stable for the past 100 years, with a slight increase from the early 1970s to the present at the shelf and slope stations. Our results highlight that spatial scales of variability of the studied sedimentary parameters are linked to spatial patterns of important environmental variables (e.g., chlorophyll-a, sea ice concentration) in the region. In contrast, no seasonal differences were observed in the sediment parameters of revisited stations, and the dated sediment geochemical profiles did not exhibit substantial longer-term variation. This means that climate-induced changes in variables that modify the sedimentary geochemistry of the environment may affect benthic community activity and structure before leaving a record in ARtoc.

The role of ice-sheet dynamics in the Miocene-Pliocene depositional systems of the Ross Sea, Antarctica

The dynamics of the Antarctic ice sheet (AIS) have profound implications for the evolution of the Antarctic continental margin, global ocean currents, and climate changes. Marine sediments from the Antarctic continental margins recorded the history of the advance and retreat of the AIS. In this study, we present an integrated analysis of the sedimentary characteristics of the Early Miocene to Late Miocene of the Ross Sea by using 2D seismic reflection data and cores from International Ocean Discovery Program (IODP) Expedition 374 Sites U1521, U1522, and U1524. The characteristics of the seismic facies and sediments show that the expansion and grounding of the ice sheets across the continental shelf significantly influenced the depositional processes, leading to increased glacial erosion, the formation of prograding wedges, mass transport deposits (MTDs), and mixed (turbidite-contourite) depositional systems since the Early Miocene. The seaward migration of the continental shelf edge and the steepening of the continental slope were mainly influenced by the glaciation of extreme expansion of the AIS. Marked increases in the prograding wedge and MTD are associated with extensive sediment reworking and deposition at the grounding zones and along the continental slope as a result of ice sheet expansion. The development of the mixed (turbidite-contourite) depositional systems was the result of extensive sediment reworking and deposition by gravity flows and bottom currents on the continental slope and the rise. This research provides new insights into the complexity of the ice sheet and the sensibility of marginal sediments in responding to the past climate changes in Antarctica.

A distinctive rare earth element signature for pyrite oxidation and glacial weathering

The application of rare earth elements (REE) and neodymium (Nd) isotopes to authigenic Fe-(oxyhydr)oxide phases leached from marine sediments can be used for reconstructing past ocean circulation and chemical weathering patterns on nearby continental landmasses. To explore the behaviour of REE during chemical weathering in glacial environments, we analyzed the authigenic fraction of glacimarine sediments from the continental shelf off northern Svalbard (Arctic Ocean), where the evolution of ice sheet dynamics since the last deglaciation is well constrained. Our results show that leached authigenic fractions in Svalbard sediments exhibit shale-normalized REE patterns characterized by anomalously high mid-REE enrichment relative to both light- and heavy-REE, as expressed by concavity index values (CI > 2.5) that significantly depart from typical REE signatures for leached fractions in marine and river sediments worldwide. Using a compilation of literature data, we provide compelling evidence that the occurrence of pronounced mid-REE enrichment in authigenic fractions of Svalbard marine sediments links to terrestrial oxidation of pyrite following glacial weathering. While future investigation will be required to further understand the detailed mechanism accounting for the observed REE decoupling and its link to pyrite oxidation, we propose that preferential dissolution of MREE-enriched rock-bearing minerals such as apatite following glacial erosion, sulfide weathering and subsequent release of sulfuric acid could release a distinctive REE signature in glacial surface environments. Our findings suggest that the use of authigenic REE in the sedimentary record could provide a means for tracing glacial weathering and associated biogeochemical sulfur and iron cycling across geological times.