Shelf Sections Research Articles

Local hydrography and interactions with benthic megafauna explain fish assemblage structure on the shallow inner shelf of Paraná, southern Brazil

An important objective of fish ecology is to understand pure and shared effects of abiotic, biotic, and spatial predictors of fish assemblages’ structure on continental shelves. This study aims to assess the structure of the bottom-associated fish assemblage on the shallow inner continental shelf of Paraná, southern Brazil, in response to physicochemical, biological, and spatial factors. We hypothesize that this structure is associated with the benthic megafauna, particularly with the distribution of the Brazilian seabob shrimp Xiphopenaeus kroyeri, an abundant fishing resource on the shallow coast of Paraná. We used uni and multivariate analyses to examine the distribution of abundance, richness, and composition of ichthyofauna in the region. Using variance partitioning analysis, we determined the pure and shared effect of spatial (latitude and longitude), physicochemical, and spatiotemporal distribution patterns of crustaceans, echinoderms, and mollusks as potential predictors of the structure of the fish assemblage. The data were obtained monthly from a one-year bottom trawl survey, comprising five transects and four depths (six, nine, 12 and 15 m) in a 60 km long section of the inner continental shelf, under the influence of estuaries. We captured 57578 demersal fish, comprising 37 families and 88 species. The average abundance of fish per haul was 387 individuals at a depth of 6 m, decreasing to 114 individuals per haul at 15 m. Average richness also decreased along the bathymetric profile, from 15 to 10 species per haul. Fish abundance and richness responded positively to a seasonal estuarine discharge pattern that was stronger in summer and autumn. High biomass of juvenile seabob shrimp in shallower regions also explained the higher abundances and richness. The results also suggest the existence of two fish assemblages in the region, associated with the distribution of the megafauna and segregated by depth, water surface transparency, and sediment particle size composition. Despite significant seasonal variations, controlled by precipitation and bottom water temperature, the structure of fish assemblages responded to the pure and shared effects of the three predictive components in all seasons. In particular, local coastal hydrography and the prey-predator relationship between fish and seabob shrimp were ecologically relevant processes for structuring fish assemblages at the inner continental shelf of Paraná. This study highlights the complexity of ecological interactions within demersal fish assemblages and the importance of considering biotic and abiotic factors in understanding fish distribution patterns at the spatial scale of inner continental shelves that are influenced by estuaries.

Submarine morphology of Pantelleria volcano: The interplay between volcanic and erosive-depositional processes modulated by sea-level fluctuations.

High-resolution multibeam data integrated with seismic reflection profiles are used to identify and characterize the main primary volcanic and erosive-depositional features along the submarine part (about the 80%) of the active Pantelleria volcano located in the Sicily Channel. Volcanic features include lava flows, cones and elongated ridges. Lava flows are mainly recognized over the insular shelf, while volcanic cones and ridges are mostly concentrated along the steep submarine flanks, especially along the wider SE and NW ones. A strong volcano-tectonic interaction is envisaged for their formation, as indicated by their preferential elongation or alignment along the (main) SE-NW and (secondary) SW-NE directions that have controlled the evolution of the whole volcanic edifice. Erosive-depositional features mainly include small-scale landslide scars and narrow gullies affecting the edge of the insular shelf and overlying submarine depositional terraces. Gullies sometimes merge downslope in larger channels, whose formation is primarily controlled by the distribution of volcanic features and/or shelf sectors characterized by different age or lithologies. Based on the marked morphological differences between the different flanks of the Pantelleria volcano, we infer an overall migration of the volcanic activity from SE to NW over time. This migration is apparently in contrast with the presence of a much wider but shallower NW insular shelf with respect to the SE one. This anomaly can be explained through a two-stage model, with the formation, in the NW sector, of a polygenic shelf rejuvenated by volcanic progradation during the last eustatic hemicycle. The different depths of the insular shelf edge around the island also provide insights on vertical deformations that affected the Pantelleria volcano during the Late-Quaternary.

Seasonal variation of Marine Heat Wave in the East China Sea

The satellite-based daily sea surface temperature was used to study the Marine Heat Wave (MHW) in the East China Sea. The seasonal variation of MHW is significant. In winter, the 200 m isobath on the shelf is identified as a section with high-frequency MHW. The duration of MHW is relatively long on the slope and deep basin, and mean intensity of MHW is obviously high in the central Taiwan Strait. In summer, the frequency of MHW is relatively high near the coast of the China Mainland. Meanwhile, the shelf section of the 200 m isobath is also high in frequency. Otherwise, the duration of MHW on the shelf is short. In the northern shelf, the mean intensity of MHW is obviously higher. The total days and maximum intensity of MHW are also discussed, aiming at giving near-fully description of MHW in the East China Sea.

Examining the effect of the UK oil and gas windfall tax on the economics of new fields in the UKCS province

Recent events in the global political economy have led to record high levels of oil and gas prices. In the UK, this has meant (1) a cost-of-living crisis; and (2) a significant increase in profits earned by companies in the upstream UK Continental Shelf (UKCS) sector. To help fund a cost-of-living support scheme for UK households, the UK Government recently introduced a windfall tax on upstream UKCS operations. There have been concerns however that the new tax would downgrade the value of UKCS assets hence diminishing the global competitiveness of the province. In this paper, I develop an optimisation model that fully incorporates all instruments of the new UK windfall taxation regime. The model is then applied to twelve wholly new UKCS fields currently under review by the UK Government for development approval. I show that whilst the new UK windfall tax reduces field economic value for all fields, the extent of the reduction is unlikely to cause premature abandonment of the fields. Consequently, the tax would not significantly injure the global competitiveness of the province. For the UK Government, the windfall tax would raise about $ 1.97 billion in tax revenues from the new fields in question. Finally, I derive the critical oil price levels that should trigger an accelerated phase out of the windfall tax in order to maintain the global competitiveness of the UKCS province should future oil prices slump.

Вопросы научного изучения арктических территорий в трудах отечественных и зарубежных исследователей (XIX-начало XX веков)

The Arctic region attracts attention of different states as a potential source of geopolitical influence. The struggle for the resources of the Extreme North is not over - the states are trying to prove their rights to various sections of the Arctic shelf. Therefore, it is important to turn to the experience of previous generations, especially to the development of science in the region according to the changing geopolitical situation, to identify what difficulties the researchers of the past periods faced and how cooperation of states progressed not to harm the fragile environment of the Arctic along the search for new resources. The paper compares the ideas of the Russian and western researchers on the development of science in the Arctic in the XIX and early XX centuries. The study analyzes the sources and literature of the period under study. The paper examines such aspects as the prevailing geopolitical thoughts in the study period - what influenced the appeal specifically to marine resources, the development of Arctic resources - what resources scientists paid attention to, research potential - what tasks were set when studying the region and what difficulties science faced. Common are ideas on the ocean resources development, search for sea passages and the need of natural resources exploitation in the region. Different is realization of thoughts about cooperation and, in general, about development of the Arctic resources.

ГИДРОЛОГО-ГИДРОХИМИЧЕСКАЯ СТРУКТУРА ВОД В ЗОНЕ АПВЕЛЛИНГА У ЮГО-ЗАПАДНОГО ПОБЕРЕЖЬЯ О. САХАЛИН

Based on the spatial distributions of sea surface temperature according to satellite observations and observational data on standard oceanographic sections of the southwestern shelf of the Sakhalin Island in summer (August) and autumn (November) shows the existence of seasonal upwelling, which is formed under the influence of northerly and northwesterly winds, typical of the winter monsoon. In this area, it manifests to a lesser extent than in the northern part of the Tatar Strait (Shevchenko et al., 2011) due to the features of the bottom topography (short deep shelf) and the large thickness of the upper heated layer, but quite clearly. At the same time, deep water rises, rich in biogenic elements and mineral phosphorus, which is important for the development of young growth of kelp. In the warm period of the year, under the conditions of the prevailing southerly wind, upwelling is observed in this area sporadically (4 ̶ 5 times per season) in the form of sharp, up to 12 ̶ 15 °C, water temperature drops in the coastal area lasting from 3 to 12 days. Most often they occur during the passage of cyclones over the southern part of Sakhalin or Hokkaido Island, when they enter the southern part of the Sea of Okhotsk or the South Kuril Islands and formed east and northeast blowing winds over the study area.

Paleotectonic and paleogeographic criteria for forecasting hydrocarbon accumulations in the water area of the Sakhalin shelf

Background. The article considers paleotectonic and paleogeographic criteria for forecasting hydrocarbon accumulations and determining the sites of priority exploration on the Sakhalin shelf.Aim. To determine the priority objects of geological exploration on the Sakhalin shelf based on paleotectonic and paleogeographic criteria.Materials and methods. Generalization and statistical processing of available data, fieldwork materials, reference literature, and scientific publications. Modeling of the tectonic evolution of the Cenozoic sedimentary basin in the studied section of the Sakhalin shelf and its individual sections. The formation and evolution of hydrocarbon systems in the waters of the Sakhalin shelf occurred in the process of geodynamic evolution of the West Okhotsk megabasin covering modern Sakhalin and the adjacent water area. The PetroMod (Schlumberger) and Qgis software packages were used.Results. The developed paleotectonic and paleogeographic criteria are suitable for both determining the sites of priority exploration and developing appropriate recommendations. Schemes with the allocation of areas that meet the developed criteria were constructed. For all three license areas, the stage of prospecting and evaluation of the oil and gas deposits is required, along with the completion of the exploration and production stage at already discovered fields. Within the license areas, the most promising structures were identified and boreholes were outlined.

Geochronological constraints on Cryogenian ice ages: Zircon U Pb ages from a shelf section in South China

Geochronological constraints on Cryogenian ice ages: Zircon U Pb ages from a shelf section in South China

Santos Basin continental shelf morphology, sedimentology, and slope sediment distribution

The ocean bottom morphology, depth, sediment grain size, sediment sorting, carbonate content, and water depth are used to compose seafloor maps as support for biological studies. Sediments are reworked by waves and currents and accumulate according to the hydrodynamic level on the continental shelf. In contrast, sediments are accumulated by pelagic settling, mass wasting, turbidity flows, and boundary currents on the slope. Sea level oscillations during the Quaternary also played an important role in modifying the shelf seabed's morphology and sedimentary mosaic composition. The work addresses these topics in the Santos Basin based on extensive bathymetric data, shallow seismic records, and stratified bottom sediment samples in water depths ranging from 25 to 2,400m, as part of The Santos Basin Regional Environmental Characterization Project (PCR-BS). The main objective is to provide broad background information on the sedimentology and morphology of Santos Basin, focusing on the continental shelf. The morphology, sediment supply, shelf orientation, and ocean climate imprint the Santos Basin's characteristics. The Northern shelf sector is the most dynamic because of the narrow shelf, steep gradient, and East-West shoreline orientation, providing conditions for storm waves to approach and mobilize sediment in the ocean bottom shallow than 50 m water depth. The Southern sector of the shelf is much broader than the Northern sector; it displays a gentler gradient, and its orientation is Northeast-Southwest; therefore, less subject to storm waves than the Northern sector.

Quantitative Evaluation of CO2 Storage Potential in the Offshore Atlantic Lower Cretaceous Strata, Southeastern United States

The geological storage of CO2 in the Earth’s subsurface has the potential to significantly offset greenhouse gas emissions for safe, economical, and acceptable public use. Due to legal advantages and vast resource capacity, offshore CO2 storage provides an attractive alternative to onshore options. Although offshore Lower Cretaceous reservoirs have a vast expected storage capacity, there is a limited quantitative assessment of the offshore storage resource in the southeastern United States. This work is part of the Southeast Offshore Storage Resource Assessment (SOSRA) project, which presents a high-quality potential geological repository for CO2 in the Mid- and South Atlantic Planning Areas. This is the first comprehensive investigation and quantitative assessment of CO2 storage potential for the Lower Cretaceous section of the outer continental shelf that includes the Southeast Georgia Embayment and most of the Blake Plateau. An interpretation of 200,000 km of legacy industrial 2D seismic reflection profiles and geophysical well logs (i.e., TRANSCO 1005-1-1, COST GE-1, and EXXON 564-1) were utilized to create structure and thickness maps for the potential reservoirs and seals. We identified and assessed three target reservoirs isolated by seals based on their effective porosity values. The CO2 storage capacity of these reservoirs was theoretically calculated using the DOE-NETL equation for saline formations. The prospective storage resources are estimated between 450 and 4700 Mt of CO2, with an offshore geological efficiency factor of dolomite between 2% and 3.6% at the formation scale.

Microbial community assembly and co-occurrence relationship in sediments of the river-dominated estuary and the adjacent shelf in the wet season

In the estuarine ecosystem, microbial community plays a vital role in controlling biogeochemical processes. However, there is currently limited comprehensive study on the deterministic and stochastic processes that drive the microbial community assembly in the estuaries and adjacent shelves. In this study, we systematically investigated the co-occurrence relationship and microbial community assembly in the sediments along a large river-dominated estuary to shelf in the northern South China Sea during the wet season. The sampling sites were divided into estuary, transection, and shelf sections based on their salinity values. The microbial co-occurrence networks, hierarchical partitioning-based canonical analysis, null model, neutral community model, and the Mantel test were used to investigate the community assembly. Results suggested that microbial community in the estuary section exhibited more interactions and a higher positive interaction ratio than those in the transition and shelf sections. Stochastic processes dominated community assembly in the study, with homogenizing dispersal contributing the most. The estuary exhibited a higher degree of heterogeneous selection than the transition and shelf sections, whereas homogeneous selection showed an opposite trend. Only the estuary section showed dispersal limitation and undominated processes. The river inflow and the resulting environmental heterogeneity were believed to be the key regulators of the community assembly in the studied area. Our study improved the understanding of how microbial community assembly in estuaries and adjacent shelves.

Bedform evolution and dynamics of a geostrophic current-swept shelf, northern KwaZulu-Natal, South Africa

When compared to tide-dominated shelves, shelves swept by geostrophic flows are relatively understudied, yet geostrophic currents have the potential to construct substantial current-generated shelf bedforms. This paper examines the evolution of a series of bedforms encountered along the narrow Agulhas Current-swept northern KwaZulu-Natal shelf. Their evolution is placed in the context of progressive current impingement and variation in flows associated with the postglacial transgression and culminating in the present-day highstand.Ultra-high resolution seismic reflection, multibeam, side-scan sonar and single beam data sets reveal several bedform scales and morphologies; wave ripples and shoreface-connected ridges are associated only with the inner shelf, and 2D and 3D very large dunes are more cosmopolitan and span the entire shelf sector. The inner shelf is marked by rock outcrop (aeolianite), surrounded by sandy sediment, grading seaward into sediment starved bedforms associated with bioclastic gravels. Where sufficient sediment exists in local depocenters, very large dunes form discontinuous fields along the outer shelf.The seismic stratigraphy of the outer shelf reflects the effects of postglacial flooding by rising sea levels and increasing current impingement by the Agulhas Current. The Holocene wave ravinement surface is overlain by flat-lying strata (early dune development and dune amalgamation with first current exposure), in turn covered by hummocky, sub-horizontal aggrading beds (amalgamation), overlain by inclined cross-bedded packages (lee faces of the bedforms formed during migration and full current interaction).Morphometric analyses show that for both the inner and outer shelf, no relationships exist between water depth, wavelength and spacing. Height to spacing (H/L) relationships are weak but nevertheless show a broadly positive trend. Bedform heights are lower on the inner shelf compared to the outer shelf, but bedform spacing is greater on the inner shelf, with a 40% overlap in H/L indices observed between the two areas. The departures in overlap can be linked to the competing offshore Agulhas Current and the inshore wave-dominated processes. Bedforms of the inner shelf plot below the global H/L mean and can be related to the more infrequent incursions of the Agulhas Current core. At the time of survey, the Agulhas Current was likely situated well offshore, resulting in reduced current activity in the survey area resulting in rounding of the dune crests, degradation of the dune crests and trough infilling. On the outer shelf, the H/L values plot above the mean global trend, suggesting vertical accretion due to the faster currents. Crest rounding and downlapping of the upstream lee faces onto the downstream stoss faces indicate dune degradation, which is related to a seaward location of the Agulhas Current at the time of survey.

Results of reinterpretation of marine geochemical survey on the Sakhalin shelf

Geochemical survey of hydrocarbons (HC) all over the world is a reliable tool of the complex of geological exploration, which allows to localize hydrocarbon saturation in structures exposed by seismic exploration, as well as to identify non-structural deposits. In 2011, a marine geochemical survey of the sorbed gases of bottom sediments was carried out on the shelf section of the northwest of Sakhalin Island. Based on the results of geochemical studies, 12 maps of the distribution of hydrocarbon and non-hydrocarbon gases in the work area and 7 maps of the distribution of metals in bottom sediments were constructed. Promising areas were distinguished by anomalies with the maximum content of parameters. The research area is characterized by a complex structure, located within the Baikal synclinal zone of the North Sakhalin oil and gas basin, which is part of the rift system of the Cenozoic sedimentary basins the Sea of Okhotsk. In 2019, the authors began to re-process and reinterpret the data in order to clarify the results. The work was based on modern theoretical foundations and methodological approaches of oil and gas prospecting geochemistry. The interpretation of the results was carried out on the basis of the model of interpretation of geochemical anomalies developed by the authors. Maps of anomalies were constructed according to 11 geochemical criteria and two geological and geochemical sections. The complex interpretation of geological and geochemical data was carried out taking into account the results of seismic exploration and drilling in a single project. According to the results of the complex interpretation, 6 promising sites were identified, which are ranked according to the degree of prospects.

Rapid and sustained environmental responses to global warming: the Paleocene–Eocene Thermal Maximum in the eastern North Sea

Abstract. The Paleocene–Eocene Thermal Maximum (PETM; ∼ 55.9 Ma) was a period of rapid and sustained global warming associated with significant carbon emissions. It coincided with the North Atlantic opening and emplacement of the North Atlantic Igneous Province (NAIP), suggesting a possible causal relationship. Only a very limited number of PETM studies exist from the North Sea, despite its ideal position for tracking the impact of both changing climate and NAIP activity. Here we present sedimentological, mineralogical, and geochemical proxy data from Denmark in the eastern North Sea, exploring the environmental response to the PETM. An increase in the chemical index of alteration and a kaolinite content up to 50 % of the clay fraction indicate an influx of terrestrial input shortly after the PETM onset and during the recovery, likely due to an intensified hydrological cycle. The volcanically derived zeolite and smectite minerals comprise up to 36 % and 90 % of the bulk and clay mineralogy respectively, highlighting the NAIP's importance as a sediment source for the North Sea and in increasing the rate of silicate weathering during the PETM. X-Ray fluorescence element core scans also reveal possible hitherto unknown NAIP ash deposition both prior to and during the PETM. Geochemical proxies show that an anoxic to sulfidic environment persisted during the PETM, particularly in the upper half of the PETM body with high concentrations of molybdenum (MoEF > 30), uranium (UEF up to 5), sulfur (∼ 4 wt %), and pyrite (∼ 7 % of bulk). At the same time, export productivity and organic-matter burial reached its maximum intensity. These new records reveal that negative feedback mechanisms including silicate weathering and organic carbon sequestration rapidly began to counteract the carbon cycle perturbations and temperature increase and remained active throughout the PETM. This study highlights the importance of shelf sections in tracking the environmental response to the PETM climatic changes and as carbon sinks driving the PETM recovery.

Sedimentology of a coastal shelf sector characterised by multiple bedload boundaries: Plettenberg Bay, inner Agulhas Bank, South Africa

The study deals with a large sand body (spit-bar) attached to the eastern tip of the Robberg Peninsula, Plettenberg Bay, South Africa. To date, the bar has prograded about 8 km beyond the tip of the peninsula. The bar top is predominantly composed of medium sand, the upper slope of fine sand, and the lower slope of fine muddy sand. Stratigraphically, the sedimentology thus documents an upward coarsening, calcareous quartz-arenitic depositional sequence. The spit-bar as a whole forms the eastern end of a sediment compartment that is clearly distinguishable from neighbouring compartments on the basis of its geomorphology, the textural characteristics of the sediment, and the distribution of sediment thicknesses. Aeolian overpass across the peninsula appears to have formed a fan-like sand deposit in its rear, which is perched upon the upper shoreface of the bay as suggested by the bathymetry to the north of the peninsula. It forms an integral part of the sediment body defining the spit-bar. The estimated volume of sand stored in the spit-bar amounts to 5.815 km3, of which 0.22 km3 is contributed by the aeolian overpass sand. The sediment sources of the spit-bar are located up to 100 km to the west, where a number of small rivers supply limited amounts of sediment to the sea and numerous coastal aeolianite ridges in the Wilderness embayment have been subject to erosion after becoming drowned in the course of the postglacial sea-level rise since about 12 ky BP. By contrast, the sediment volume in the adjacent compartment B to the north (Plettenberg Bay), which has been supplied by local rivers, amounts to only 0.127 km3. In a geological context, large sand bodies such as the Robberg spit-bar are excellent exploration models for hydrocarbons (oil and gas).

Quaternary evolution of the southeastern Korean continental shelf, East Sea: Paleo-incised valley and channel systems

About 1000 km of high-resolution seismic reflection profiles were used to study the morphology, distribution, nature, and sequence stratigraphy of incised valleys and valley fills within the Quaternary (<ca. 2.0 Ma) sedimentary section of the southeastern Korean continental shelf, East Sea. Seven sequences, S1 to S7 from bottom to top, were analyzed on the modern continental shelf. These sequences comprise a ~300 m thick sedimentary section containing, at minimum, one cycle of sea-level lowstand, transgression, and high stand deposits. The lowstand sections are composed mainly of laterally and vertically accreting fluvial channel deposits (i.e., point bars, channel lags) formed on an alluvial plain during a lowstand in sea level. The transgressive sections consist of confined, low-to moderate-amplitude continuous reflections interpreted as estuarine facies. The high stand sections, in contrast, are comprised of laterally extensive low amplitude reflections interpreted as open marine facies. Each of these sequences rests unconformably on its’ predecessor, particularly at the mid-southeastern edge of the shelf where angular, toplap, and onlapping relationships can be discerned.The sequence boundaries (SB1 to SB7) bounding these sequences are produced by fluvial incision (i.e., valleys and channels) during sea-level fall. SB4, SB5, and SB7 form the most prominent unconformities on the shelf and are incised by extensive paleo-incised valleys and channels. These valleys and distributary channels display simple U- or V-shaped cross sections or complex morphologies; they are ~1–8 km wide and ~10–60 m deep. Incised-valley fills reach a maximum thickness of over 80 m. The largest incised valley fills (~8 km wide, ~60 m thick) are found near the shelf break in the SE, indicating relative sea-level fall on this location (ca. 150 m lower than the present sea level). In the SE, the incised valleys are comprised dominantly of stacked lowstand successions (point bars and channel lags) and a lesser degree of transgressive successions (estuarine mud and levees) that are capped by subsequent high stand deposits (marine mud). The youngest channel above the valley was inferred to have been formed during the Last Glacial Maximum (LGM) on the shelf edge trajectory about 140 m lower than the present. These large incised valleys are thought to have developed through a complex history of repeated avulsion and reoccupation during the lowstand sea level. The most important factors regulating the valley morphology are the duration of sea-level fall, lithologic variations, climate (i.e., fluvial discharge), and the river gradient relative to the shelf gradient. The ancient drainage patterns are more-or-less northeast-southwest orientated, consistent with the trend of the present-day submerged fluvial channel. Superimposed upon this general pattern are the effects of tectonic activity (uplift, folds, and faults), which appear to have modified the courses of the rivers. Our results, therefore, provide a good example of how external factors (i.e., climate, tectonics, sediment supply, and paleogeomorphology) can determine the location and geometry of the valleys and the architecture of the valley-filling deposits of continental shelves.

Planktic foraminiferal response to an early Paleocene transient warming event and biostratigraphic implications

The Latest Danian Event (LDE, ~ 62.2 Ma) is characterized by global changes in the carbon cycle as indicated by two negative δ13C excursions of up to ~ 1‰. These δ13C shifts are accompanied by a 2–3 °C warming of both surface and deep waters based on benthic and planktic foraminiferal δ18O measurements, and the LDE has, thus, been considered as a so-called hyperthermal event. The event lasted for 200 kyr and has been identified in various ocean basins and shallow marine settings. Here, we present a compilation of data from three deep-sea cores covering the Pacific, North and South Atlantic oceans as well as a southern Tethyan shelf section to document the response of planktic foraminifera assemblages to ocean warming. In all studied successions, we observe the disappearance of the planktic foraminifer genus Praemurica on a global scale that took place close to the onset of the LDE. Moreover, on the long run, praemuricids were contemporaneously replaced by morozovellids. Both the decline of Praemurica and a temperature increase started between 200 and 260 kyr before the LDE onset and were punctuated by the LDE itself. In this paper, we propose two mechanisms that have controlled the environmental changes associated with this event, (1) increased activity of the North Atlantic Igneous Province acting on long time-scales, and (2) changes of orbital parameters resulting in insolation changes on shorter time-scales.In contrast to a proposed muted benthic foraminiferal response, the planktic community has been substantially impacted by the LDE as indicated by changes in planktic foraminifera faunas and calcareous nannofossils. Finally, our quantitative and conventional approaches identifying stratigraphically important planktic foraminifera datum levels justify a revision of the upper Danian to lower Selandian planktic foraminifera biozonation.

Об одной задаче оптимизации конструкций с учетом требований устойчивости, прочности, при ограничениях первой частоты собственных колебаний

В одной из предыдущих работ авторов данной статьи рассматривалась задача оценки близости решения к проекту минимальной материалоёмкости при оптимизации ширины полок стержней двутаврового поперечного сечения при ограничениях по устойчивости или величины первой частоты собственных колебаний с учётом требований прочности в случае непрерывного изменения варьируемых параметров по длине стержня. При этом известно, что в строительной практике в основном стержни проектируются с кусочно-по­стоянным изменением параметров сечения. В другой статье авторов был сформулирован критерий оценки оптимальных решений при формирования кусочно-постоянных участков полки стержней двутаврового сечения при ограничениях по устойчивости или величине первой частоты собственных колебаний, но без учёта требований прочности. В данной статье рассматривается задача оценки оптимальных решений при формировании кусочно-постоянных участков ширины полок стержней двутаврового сечения при ограничениях по устойчивости или величины первой частоты собственных колебаний, но с учётом требований прочности и некоторых конструктивных ограничений.

Late Artinskian Nautiloids of the Shakh-Tau Reef (Bashkortostan)

A nautiloid assemblage is described for the first time from the Upper Artinskian of the Shakh-Tau shikhan, including 14 species of 12 genera of coiled and uncoiled taxa. Most species identified in this assemblage were originally described by V.N. Shimansky (Shimansky, 1954; Ruzhencev and Shimansky, 1954) from several Sakmarian and Artinskian open shelf sections of the South Urals. Previously, only two species of Pseudorthocerida were reported from Shakh-Tau, while no coiled taxa had been recorded from the region of the Sterlitamak shikhans. In its taxonomic diversity, the Shakh-Tau assemblage is comparable to the synchronous Zhil-Tau (=Gil-Tau) fauna of the South Urals.

Environmental impacts on walleye pollock (Gadus chalcogrammus) distribution across the Bering Sea shelf

Adult and juvenile (age-1) walleye pollock (Gadus chalcogrammus) were sampled by the US NOAA Alaska Fisheries Science Center summer bottom trawl survey in 2010, 2017, 2018, and 2019 in the northeastern and southeastern Bering Sea, with profiles of temperature collected concurrently. Similarly, the Russian Research Institute of Fisheries and Oceanography, Pacific branch, collected adult and juvenile pollock and temperature profiles on summer bottom trawl surveys in the northwestern Bering Sea. Results from these surveys show that adult pollock abundance in recent years (2017, 2018, 2019) has increased in northern regions of the Bering Sea shelf in both the US and Russian sectors. Lower abundances, compared to historic means, were observed in southern regions of the shelf, suggesting the pollock moved directionally from the south to the north. We relate changes in pollock distribution in recent intermediate (2017) and warm, low-ice years (2018–2019) to a prior cold, high-ice year (2010) and describe how these observations relate to our longer time series. We link temperature data from bottom trawl surveys (US and Russian), sea-ice indices (retreat timing and extent), as well as model-based estimates of ocean circulation to changes in pollock distribution and examine potential environmental factors driving the observed changes. Changes in sea-ice and bottom temperature (e.g., reductions in ice extent and shrinking of the cold pool), and changes in circulation (stronger northward currents over the northeastern shelf in warmer years, particularly in 2018) led to changes in distributions of adult and age-1 pollock. Adult pollock were concentrated north of St. Lawrence Island and had larger longitudinal distributions in warm years, 2017–2019; whereas they had a more southerly and narrow distribution over the outer shelf in the cold year, 2010. Age-1 pollock had higher densities over the inner eastern shelf in 2017–2019 compared to 2010. Northward flow around St. Lawrence Island (particularly in the spring) alternated between stronger flow on the west side of the island in 2010 and 2017 and stronger flow on the east side of the island in 2018 and 2019; variations in flow may have impacted the location of prey and movement of feeding pollock to the Chukchi Sea. Size structure comparisons between NW, NE and SE sections of the Bering Sea shelf suggest that movement of fish between US and Russian waters may have been highest in 2019, one of the two warmest years, and lowest in 2010, the coldest year. Spatial comparisons of distributions and size structure across the Bering Sea help provide a comprehensive view of factors affecting the movement of this highly important commercial fish species.