Western Shelf Research Articles

Upwelling processes variability and water circulation along the northern margin of the Gulf of Cadiz

The present study investigates the contributions of upwelling mechanisms (coastal upwelling from boundary divergence and Ekman pumping from spatially variable wind) at the northern margin of the Gulf of Cadiz (NMGoC) based on high-resolution wind. The effects of the spatiotemporal upwelling mechanisms variability are then explored using current observations along with sea surface temperature and sea level anomaly. Upwelling favourable conditions occur throughout the year along the NMGoC, with the strongest intensity near Cape São Vicente due to a persistent positive wind stress curl. In winter, the surface water divergence is restricted to the coastal boundary due to strong coastal upwelling events that result in a cross-shore sea-level gradient close to the coast. Towards the summer, the gradient increases and extends further offshore (over the slope) at the western region due to the intensification of the Ekman pumping. Therefore, the seasonal and spatial variability of coastal upwelling and Ekman pumping contributes significantly to the offshore position of the geostrophic Gulf of Cadiz Current over the western shelf slope. Furthermore, a permanent sea level depression corresponds to the location of the strongest Ekman pumping, near Cape São Vicente. The dynamic adjustment of this feature may drive the cyclonic cell and alongshore poleward currents often observed in the area.

Suspended particulate matter in the Gulf of Oman: Spatial variations in concentration, bulk composition, and particulate metals controlled by physical and biogeochemical processes

The present work aims to assess the biogeochemical and physical sources of variation in the spatial distribution of suspended particulate matter (SPM), its major biotic and abiotic components, particulate metals, and the Redfield (N:P) stoichiometry of particles in a poorly understood basin of the Gulf of Oman. Particulate samples were collected in February 2022 from the Gulf of Oman aboard the R/V Persian Gulf Explorer, revealing surface SPM concentrations ranging from 140 to 1145 μg/l. The elemental composition of crustal-type elements in the surface offshore region confirmed the input of lithogenic components by aeolian dust from the surrounding deserts. The highest mid-depth SPM levels, with remarkable contribution from CaCO3, were observed at the western shelf edge at 100–300 m depth, supported by the Persian Gulf outflow. Conversely, mid-depth maxima with elevated concentrations of terrigenous elements were observed in the eastern edge, emanating from sediment resuspension and lateral transport under eddy-topography interaction. Organic matter is the most important phase, followed by biogenic silica from the basin-wide winter bloom of diatoms. While signs of CaCO3 dissolution are evident at depths >500 m, the oxidative precipitation of Mn (II) in the upper boundary of the oxygen minimum zone leads to the appearance of perceptible maxima in the vertical profile of particulate MnO2. Seasonal variations in the organic N:P ratio, from summer to winter, at the western station were linked to shifts in phytoplankton assemblage structure, transitioning from cyanobacteria dominance in summer to chain-forming diatoms in winter. The particulate pool of biologically important trace metals was dominated by a non-crustal fraction with enrichment factor followed a descending order: Cd > Mo > Pb > Zn > Ni in surface offshore samples. Metal/P ratios comparison with some previous data from the open ocean SPM and lab cultures of phytoplankton reveals that the Zn/P ratio is significantly exceeded in cultured communities, whereas the Cd/P ratio reflected the consistent demand in the Gulf of Oman compared to reported lab culture requirements.

Variability of the Bottom Boundary Layer Induced by the Dynamics of the Cross‐Isobath Transport Over a Variable Shelf

AbstractThe bottom boundary layer (BBL) dynamics play an important role in regulating the energy, momentum balance, and circulation in the shallow shelf areas. Unlike previous studies that disconnected BBL with background variable shelf circulation, we investigate the dynamic connection between the wind‐driven shelf circulation and BBL dynamics, and show the spatial characteristics of BBL dynamics in response to three‐dimensional (3D) heterogeneous transport over the highly variable shelf topography in the Northern South China Sea. Our process‐oriented modeling study demonstrates that the mixing dynamics and upslope buoyancy transport over varying shelf topography alter the spatial variability of BBL dynamics. Driven by southwesterly upwelling‐favorable winds, the along‐shelf current generated a frictional upslope Ekman transport. The along‐isobath pressure gradient force () formed by the flow‐topography interactions over the meandering shelf induces the geostrophic cross‐isobath transport. The downwave (upwave) enhances (offset) the frictional upslope transport over the east (west) of the shelf that has a concaving (uniform) bottom topography. Over the eastern shelf with concave isobaths, the intensified and upslope cross‐isobath dense water transport strengthen stratification and weaken the effect of bottom stress‐induced mixing, limiting the development of the BBL. The antithesis occurs over the western shelf, where a small bottom stress controls the BBL. River discharge and the tidal current modulate the alongshore current, upslope transport, bottom stress intensity, and BBL development. We model the trajectory of seabed particles as they respond to the BBL dynamic regimes, and find that high (low) concentration, short (long) suspension time, and strong (weak) shoreward transport occurover the eastern (western) shelf, respectively.

Haline Tolerance of Stomatopod (Crustacea) Larvae and Pelagic Dispersal of Two Common Species in the Western Atlantic

AbstractPelagic larval stages play a critical role in the dynamics of marine populations since they are the main way of dispersal and habitat colonization. Here, we examined the larval dispersal pathways of two common stomatopod species in the western Atlantic: Squilla empusa and Lysiosquilla scabricauda. To complement this goal, we also analyzed the haline tolerance of the stomatopod larvae collected in an estuary from the southern Gulf of Mexico. Larval dispersal was simulated using a Lagrangian particle‐tracking module coupled to the Global HYCOM model and consisted of releasing 100 passive particles from each starting site. Results indicated a high level of larval retention in the west Florida shelf and over the narrow western shelf of the Gulf of Mexico. In the South Atlantic Bight, Central America, and northern South America the larval transport was almost unidirectional following the pattern of currents. Generally, connections were among nearby sites, but long‐distance transport can also occur when larvae are trapped by great high‐speed currents. Retention of larvae and connection with neighboring sites were due to local atmospheric and hydrological conditions. During fieldwork, we found two kinds of larvae: antizoea and alima. Morphological characteristics of the antizoea correspond to the superfamily Lysiosquilloidea, and those of the alima, with the superfamily Squilloidea. The antizoea larvae were found in salinity values as low as 21.9 psu, while the alima were at 23.2 psu. Salinity tolerances and dispersal potential of larvae indicate a high level of colonization of new habitats and a broad intrusion into the estuaries.

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.

Sediment provenance of the East Siberian Arctic Shelf and evidence of Holocene climate-driven fluvial events in the Indigirka River based on detrital mineral analysis

Analysis of sediment provenance in the East Siberian Arctic Shelf is crucial for the understanding of source to sink pathways and their link to climatic and environmental changes. In this paper, we conducted detrital mineral analysis on surface sediments and one sedimentary core (LV77–41-1) from the eastern Laptev Sea and the western shelves of the East Siberian Sea. Based on the results of light and heavy mineral analysis in surface sediments, two distinct zones were identified in the eastern Laptev Sea: an eastern zone is characterized by sources from the Lena River and a northern zone is characterized by sources that transition from riverine dominance to more sea-ice transported sources in an offshore direction. In the western part of the East Siberian Sea, detrital mineral assemblages in the outer Indigirka River mouth region are influenced by polygenic sources comprising sediments mainly derived from the river itself, and some material supplied by the Siberian Coastal Current, and the erosion of coastal permafrost and nearby islands. In the outer shelf and basin region, detrital mineral assemblages contain signatures of apatite, garnet, and hypersthene, indicating various material sources are represented, including the Lena River, Taimyr Peninsula, and Kamchatka Peninsula. Integrating this understanding of the sources and transport pathways of surface sediments, detrital mineral analysis of core LV77–41-1 indicate that coarse-grained Holocene sediments were mainly transported by the Indigirka River, with subordinate sources from coastal permafrost erosion, materials carried by Siberian Coastal Current, and island erosion. Since 7 ka BP, four distinct fluvial events have occurred, with the most prominent fluvial event occurring between 4.6 and 4.3 ka when the depocenter shifted significantly toward the sea. The fluvial events occurred following the onset warm periods and the high frequency of volcanic glass observed in layers marking these fluvial events serves as evidence of the influx of glacial meltwater into the Indigirka River basin during these warmer episodes.

Transport and dispersion of PFOA and PFOS in the Black Sea

A 3-D transport and dispersion model was applied to study the recent past and future dynamics of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) concentrations in the Black Sea for the 2016–2030 period. The modelled surface concentrations show a distinct seasonal behaviour, shaped by winter to spring convective mixing. A significant increasing long-term trend in PFOS concentrations is established, with concentrations in water layers 200 m below the surface increasing at 4–8% per year. Driving mechanisms for PFOA and PFOS transport and accumulation in the subsurface and deeper layers are the cooling of the surface water in winter and the transport of water masses from the North Western Shelf (NWS) of the Black Sea. A simulated 50% phase-out of PFOA and PFOS from 2020 to 2030 shows a 21% reduction in PFOA, while PFOS continues to increase.

СТРУКТУРНО-ПЛОТНОСТНАЯ МОДЕЛЬ ЗЕМНОЙ КОРЫ ЗАПАДНОГО ШЕЛЬФА О-ВА САХАЛИН И ЕЕ ГЕОЛОГИЧЕСКАЯ ИНТЕРПРЕТАЦИЯ

Structural and density modeling of the Earth's crust carried out using a deep seismic sounding profile previously tested along the western shelf of Sakhalin Island offered a clearer view of the layered block structure of the main tectonic faults and their newtwork within the crust of the region, where a large number of strong crustal earthquakes are concentrated. The density structures of the resulting model were compared with geological data on the adjacent territory of Sakhalin Island. Our structural and density model made it possible to separate volcanic blocks and blocks of sialic crust and to trace the submerged prolongation of the largest geological complexes of the western edge of Sakhalin Island on the shelf. There are signs of spatial correlation between seismic events and some tectonic faults.

Population Status of the Ctenophores Mnemiopsis Leidyi Agassiz, 1865 and Beroe Ovata Bruguiere, 1789 in Waters of the North and Middle Caspian Western Shelf in August–September, 2022

Population Status of the Ctenophores Mnemiopsis Leidyi Agassiz, 1865 and Beroe Ovata Bruguiere, 1789 in Waters of the North and Middle Caspian Western Shelf in August–September, 2022

Contributors to tidal duration asymmetry with varied coastline configurations on western shelf of Yellow Sea

Coastal management in China is confronted with an urgent choice between natural restoration and maintenance of existing seawalls and reclaimed land for economic development. A key criterion for making this decision is the resilience to coastal flooding, which depends on the ability to predict tidal level. Tidal duration asymmetry (TDA) is a key parameter in determination of the arrival and duration of flood tides. This study selected the western inner shelf of the Yellow Sea (WYS) as the study area and investigated the responses of TDA to different shoreline configurations and relative sea level rise. The responses of TDA to shoreline reconstruction yielded spatial variability locally and remotely. In the nearshore area, the responses of TDA to the complex ocean environment mainly originated from the combined functions of reflection, bottom friction, and advection, which controlled the energy transfer from M2 or S2 constituents to their overtides or compound tides. The sensitivity of TDA to coastline typologies was not limited to coastal waters but could stretch over the entire inner shelf. The vulnerability of tidal responses was due to the displacement of the M2 amphidrome of the Kelvin wave on the WYS, which in turn changed tidal energy fluxes over the regime. The relative sea level rise could intensify the feedback of TDA to seawalls and land reclamation.

Tracing Atlantic water transit times in the Arctic Ocean: Coupling reprocessing-derived 236U and colored dissolved organic matter to distinguish different pathways

The Fram Strait is a key region for investigating the exchange of Atlantic water with the Arctic Ocean. Uranium-236 (236U) from the two European nuclear reprocessing plants (NRPs) at La Hague and Sellafield provides a unique fingerprint in Atlantic water which can be used for studying its circulation patterns in the Arctic Ocean. And NRPs-derived 236U (236UNRPs) can be identified by its 233U/236U signature. In this study, we use colored dissolved organic matter (CDOM) absorption to constrain the selection of three Atlantic branch waters that carried different inputs of 236UNRPs in Fram Strait. This can potentially provide better estimates of transit times of Atlantic waters in the Arctic Ocean. High CDOM levels (a350≥0.35 m−1) in Fram Strait reflect the passage of Atlantic water transported to the Arctic by the Norwegian Coastal Current (NCC) and its extension and subsequently along the Siberian continental slope and shelf where the Ob, Yenisei and Lena rivers supply terrestrial organic matter with significantly high CDOM content. Conversely, low CDOM water represents Atlantic water that has remained off the shelf. Based on CDOM absorption, potential temperature (Θ), potential density (σΘ) and 236U concentration, the path of a given body of Atlantic water could be inferred and an appropriate NRPs input function constrained so that transit times could be estimated. Our results indicate that Arctic High CDOM Water (a350≥0.35 m−1) sourced from the NCC and Barents Sea Branch Water (BSBW) in the Barents Sea Opening has an average of 7–27 yrs transit time in the upper ∼200 m of the western shelf of the Fram Strait. Atlantic Low CDOM Water (a350<0.35 m−1, Θ>2°C) sourced from the Fram Strait Branch Water (FSBW) has a short pathway from the eastern Fram Strait. Arctic Low CDOM / High 236U Water (a350<0.35 m−1, Θ≤2°C, σΘ≤27.97 or 236U concentration ≥15 × 106 atom/L) sourced from the BSBW and the FSBW has an average of 22–28 yrs transit time. These findings demonstrate how combining measurements of CDOM with 236UNRPs can improve the robustness in estimation of transit times of different Atlantic water pathways in the Arctic Ocean. There are limited ways to empirically derive estimates and the values provided offer unique data for comparison with estimates from regional circulation models.

Assessing ecological connectivity of blacktip sharks (Carcharhinus limbatus) in the Gulf of Mexico using stable isotope analysis of multiple tissues

Effective management of blacktip sharks relies on a comprehensive understanding of population distribution and stock structure. Previous stock separations and the implementation of regional quotas have proven successful in maintaining the sustainable harvest of blacktip sharks in both the Western North Atlantic and United States Gulf of Mexico (US GoM), in commercial and recreational fisheries. Within the US GoM, finer scale biological separation between blacktips in western and eastern regions has been supported through tagging, genetic, and diet data. This study provides additional evidence of unique ecological isotopic niche areas and δ15N and δ13C values for blacktip sharks from the western, central, and eastern US GoM across muscle and vertebral tissues that reflect different isotope turnover rates. Blacktip sharks from western and central regions exhibited isotope values and niches that were significantly smaller compared to eastern sharks, with enriched δ13C and depleted δ15N west of 88ºN. Ontogenetic shifts were noted for most regions and there was no crossover or overlap in average isotope values from early to recent life between regions. These spatiotemporal patterns suggest that in the year following birth and prior to the time of capture, blacktip sharks on the central and western shelves have separated ecologically from blacktips on the eastern US GoM Florida shelf.

Contrasting sources and fates of sedimentary organic carbon in subtropical estuary-marginal sea systems

Sediments deposited in subtropical deltas and continental shelves are the “recorders” of sources, transport processes, and accumulation of organic carbon (OC) at the land-ocean interface, but their role in the global carbon cycle remains poorly constrained. In this study, we determined the abundance and carbon isotopic composition (δ13C and Δ14C) of bulk OC, as well as characteristics of specific biomarker compounds (plant wax n-alkanes, n-fatty acids, and phytoplankton sterols) in surface sediments from the Pearl River estuary (PRE) and adjacent continental shelf of the northern South China Sea (SCS) in order to examine the sources and fates of OC delivered to a large subtropical river-marginal sea system. We observed marked spatial variability in the sources, hydrodynamic processes and accumulation of sedimentary OC in PRE and northern SCS shelf, with the emergence of two distinct regimes reflected in the bulk OC isotopic characteristics of surface sediments. In Pearl River impacted regions, relatively fresh terrestrial OC (OCterr) was subject to substantial degradation and aging during transport within the estuary and from the estuary to the shelf. In contrast, sedimentary OC in the Taiwanese river impacted region comprised a mixture of fluvially-derived refractory OCterr from the Choshui River and modern marine OC. Burial efficiencies of OCterr are also spatially heterogeneous as a result of these different OC sources and hydrodynamic processes. The overall OCterr burial efficiencies are relatively low in the Pearl River influenced region, with an average value of 27 ± 10% in the PRE and an average value of 17 ± 6% in the western shelf, in contrast to the higher values in the region impacted by Taiwanese rivers (55% ± 39%).These findings suggest both sources and hydrodynamic processes during transport exert strong influence on the fate of OC, with important implications for the role of river-dominated marginal seas in the global carbon cycle.

Abundance, Biomass, and Production of Bacterioplankton at the End of the Growing Season in the Western Laptev Sea: Impact of Khatanga River Discharge (Arctic)

The structure and productivity of planktonic microbial communities in the ecosystems of the Siberian Arctic seas are significantly dependent on freshwater input. During the study, we determined the spatial distribution of the abundance, biomass, and production of heterotrophic bacterioplankton in the Western Laptev Sea on the transect from the Khatanga River estuary to the continental slope and assessed the impact of river freshwater discharge. The influence of fresh water on bacterioplankton was restricted mainly to Khatanga Bay (KHAB) and the transitional zone (TZ) and was poorly recognized in the Western shelf (WS) and continental slope (CS) areas. The total bacterial abundance decreased from KHAB to the CS. Particle-attached bacteria constituted on average 63.0% of the total abundance of bacterioplankton in KHAB and 1.0% at the CS. Average bacterial production in the water column was highest in KHAB (10.3 mg C m−3 d−1), decreasing towards the CS (0.7 mg C m−3 d−1). In KHAB and TZ, bacteria were the main component of the planktonic community (44−55%). These results show that at the end of the growing season, bacterial processes prevailed over autotrophic ones and contributed largely to the total biological carbon flux in the coastal ecosystem of the Western Laptev Sea.

Marine heatwaves in the Gulf of Mexico 1983‒2021: Statistics, recent intensifications, and threats on coral reefs

There is the current lack of comprehensive understanding of the hotspots, frequency, duration, spatiotemporal trends, and physical drivers of marine heatwaves (MHWs) within the Gulf of Mexico (GoM). Here, a series of high-resolution satellite and reanalysis products are used to examine their spatiotemporal characteristics, trends, and possible geophysical triggers of MHWs. Possible impacts of the MHW on coral reefs are also discussed. Results reveal an increasing trend in their frequency, duration, and intensities from 1983–2021, particularly after 2016. It identifies MHWs hotspots within the GoM, notably the northern and western shelves and the Loop Current. The study further documents an intense MHW event from late 2020 to early 2021 near the Yucatan Channel, south of 24°N, attributing its development to oceanic processes such as wind anomalies, anticyclonic eddies, and current-driven heat transport anomalies. The occurrence of this MHW event potentially increased thermal stress on the Campeche and Tuxtlas Reef Systems. This research illuminates the increasing trends and impacts of MHWs in the GoM, providing valuable insights for understanding and predicting the effects of climate change on marine ecosystems.

Interannual variations of heat budget in the lower layer of the eastern Ross Sea shelf and the forcing mechanisms in the Southern Ocean State Estimate

AbstractThe temporal variation of heat budget in the lower layer of the eastern Ross Sea shelf (ERSS) is crucial for understanding the stability of ice shelves in the Ross Sea. In this study, a 6‐year (2005–2010) simulation from the Southern Ocean State Estimate (SOSE) is employed to analyse the interannual variations of heat budget in the lower layer of the ERSS and the controlling mechanisms. The results reveal that the annual change in the heat content of the study region is dominated by the horizontal heat advection term, and only in 2015 the vertical advection and diffusion terms also play an important role. The horizontal advection term is affected by the intrusion of warm circumpolar deep water (CDW) onto the shelf across the northern boundary of the ERSS, the transport of cold dense shelf water (DSW) from the western Ross Sea shelf, and the transport of CDW across the western boundary of the ERSS. The contributions of these physical processes to the change in annual heat content vary between years. The interannual variation of the CDW intrusion is associated with the strength of eddy activity over the slope, and the interannual variation of heat transports associated with DSW is affected by both the extent of DSW on the western shelf and the coastal circulations that affect the eastward spread of DSW.

Three decades of variability and warming of nearshore waters around Tasmania

The ocean region around Tasmania forms an interface between major ocean basins and represents a significant global ocean hotspot due to a rate of warming higher than the global average. Nearshore Tasmanian waters are of central importance for shallow-water ecosystems and a range of human activities, including recreation, aquaculture and fisheries which have become increasingly impacted by warming in recent decades. However, the fine-scale spatial and temporal variability of sea surface temperature (SST) in these waters is not well understood, limiting opportunities for effective local-scale mitigation and adaptation measures in the face of ongoing warming. We use a high-resolution (0.02 °x 0.02°, 1992–2020) satellite SST received at local stations (validated against in situ temperature) to explore variability over seasonal, interannual, decadal and multi-decadal timescales at spatial scales from the nearshore up to the entire study domain. The SST is used as a proxy for seasonal circulation by removal of the broad-scale radiative forcing (estimated by a region-wide latitudinal gradient of SST). The main circulation drivers of nearshore SST are the East Australian Current Extension (EACx) and Zeehan Current (ZC) acting on the east and west coasts with respective peaks in summer and winter. New results include the seasonality of three summer upwelling systems, the first direct evidence of EACx inflow into eastern Bass Strait, a time series of the endpoint of the ZC/EACx intersection, and a full year description of the ZC, from warm winter inflow transitioning into a summer cooler than ambient intrusion. We find that local SST variations are controlled by coastal geometry with intensified atmospheric forcing in enclosed waters. We have identified two modes of low frequency SST variability (including 1, 2 and 10-year periods) associated with the EACx (63% of the variance) and the ZC (12%). The EACx mode induces a rapid adjustment of the entire ocean region around Tasmania. Multi-decadal warming trends occur in all regions, modulated by coastal boundary flows, changes in upwelling mechanisms and local topography. Trends in the eastern shelf arise from intrusions of EACx waters, with local processes driving trends on the western shelf. Both east and west nearshore SST trends are around + 0.4°C decade−1 with the highest value of + 0.55°C decade−1 on the central west coast, possibly associated with outflow from Macquarie Harbour.

The Salinity Budget of the Ross Sea Continental Shelf, Antarctica

AbstractToward understanding the trend and variability in the bottom water salinity in the Ross Sea, this study focuses on the seasonality of the salinity budget over the Ross Sea continental shelf, using a new coupled regional ocean‐sea ice‐ice shelf model. Owing to the sea ice production, the upstream advection, and the glacial melting, the Ross Sea continental shelf is characterized by the relatively saltier water on the western shelf than on the eastern shelf, with the saltiest water in the Terra Nova Bay Polynya. During the early freezing season (March–April), there is remarkable sea ice production over the broad continental shelf, which makes a significant contribution to the production of High Salinity Shelf Water (HSSW). The brine rejection in the polynyas leads to the salinification and deepening of the mixed layer, yet the upstream advection acts to decrease the salinity below the mixed layer until the deep convection is fully developed throughout the water column. The seasonal cycle of the salinity budget in the Ross Sea continental shelf is largely determined by the formation/melting of sea ice, the intrusion of modified Circumpolar Deep Water, and the outflow of HSSW, while the contribution of the Ross Ice Shelf melting is relatively small.

DESI Observations of the Andromeda Galaxy: Revealing the Immigration History of Our Nearest Neighbor

We present Dark Energy Spectroscopic Instrument (DESI) observations of the inner halo of M31, which reveal the kinematics of a recent merger—a galactic immigration event—in exquisite detail. Of the 11,416 sources studied in 3.75 hr of on-sky exposure time, 7438 are M31 sources with well-measured radial velocities. The observations reveal intricate coherent kinematic structure in the positions and velocities of individual stars: streams, wedges, and chevrons. While hints of coherent structures have been previously detected in M31, this is the first time they have been seen with such detail and clarity in a galaxy beyond the Milky Way. We find clear kinematic evidence for shell structures in the Giant Stellar Stream, the Northeast Shelf, and Western Shelf regions. The kinematics are remarkably similar to the predictions of dynamical models constructed to explain the spatial morphology of the inner halo. The results are consistent with the interpretation that much of the substructure in the inner halo of M31 is produced by a single galactic immigration event 1–2 Gyr ago. Significant numbers of metal-rich stars ([Fe/H] > − 0.5) are present in all of the detected substructures, suggesting that the immigrating galaxy had an extended star formation history. We also investigate the ability of the shells and Giant Stellar Stream to constrain the gravitational potential of M31, and estimate the mass within a projected radius of 125 kpc to be . The results herald a new era in our ability to study stars on a galactic scale and the immigration histories of galaxies.

Glacial sediments on the outer Chukchi Shelf and Chukchi Borderland in seismic reflection data

The up to 900 km broad shelves off East Siberia and northwest off Alaska, including the Chukchi Shelf and Borderland, are characterized by shallow water in the periphery of the Arctic Ocean, north of the Bering Strait. Seafloor investigations revealed the widespread presence of glacial bedforms, implying the former existence of grounded ice in this region. We discuss the erosion and deposition around and beneath ice sheets/shelves using a regional grid of 2D seismic reflection data, acquired in 2011 from R/V Marcus G. Langseth across the outer ~ 75 km of the Chukchi Shelf and the adjacent Chukchi Borderland. A high amplitude glacial base (GB) reflection extends over large parts of the shelf, separating glacial from preglacial strata. We define eleven seismic reflection characters, that we use to infer distinct depositional environments of glacial sediments. Thick well stratified sediments overlying the GB reflection in the south may have been impacted by fewer advance-retreat cycles than those near the northeastern and western shelf breaks. Here, the GB reflection pinches out at the seafloor next to reworked and eroded areas. Numerous meltwater channels, some up to several kilometers wide, together with grounding zone wedges and recessional moraines are hints for ice sheets in the Chukchi Region. These ice sheets built up a huge grounding zone wedge of 48 km × 75 km on the Chukchi Rise. More grounding zone wedges on the western sides of bathymetric highs of the Chukchi Borderland along with mega scale glacial lineations indicate later ice shelf advances from east during the late Quaternary. However, in the absence of deep sediment cores, the timing or origin of the ice grounding events cannot be fully reconstructed.