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Adélie penguins are considered indicators of Antarctic ecosystems. Their populations have declined by more than 50% in the West Antarctic Peninsula, an area strongly affected by global warming, and that concentrates most of Antarctic krill harvesting. The use of high-resolution data to identify foraging areas regularly used by krill predators could provide valuable information for current discussions on the development of small-scale management and conservation measures for this region. We used information on the foraging trips of 57 individuals breeding in King George Island, tracked over 2019/2020, 2020/2021 and 2021/2022 breeding seasons during the chick-rearing stage, to identify their key foraging areas. Using an accelerometry-based latent behavioral analysis approach, we identified an area within 10 km of the colony consistently used by over 60% of the population throughout and between seasons. We also observed that almost 20% of the population uses the area near a seamount located 35 km from the colony for foraging, mainly during the late guarding phase when chick energy demands are highest or the effects of prey depletion might become more evident. The distances and duration of trips and the area explored increased as the season progressed and varied between seasons, consistent with annual differences in krill availability observed in the region. Foraging dives comprise roughly 40% of the dives performed during foraging trips, irrespective of the stage of the chick-rearing period, or the season analyzed. Our results emphasize the need to understand how variability in environmental conditions, prey availability, and energetic demands affect how predators use space, and the role that bathymetric features might play in providing reliable foraging grounds, for penguins, in a rapidly changing region. [ABSTRACT FROM AUTHOR]

This study investigates the spatio-temporal variations of the chl-a blooms in the Bransfield Strait (BS) at a climatological scale (1998-2018). We propose that a suitable monitoring of these blooms can be achieved through remotely-sensed observations only if the Bransfield Strait is divided following the Peninsula Front, which ultimately influence the phytoplankton assemblage. Our analysis is based on characterizing climatological fields of Sea Surface Temperature (SST), air temperature, Sea-Ice Coverage (SIC), chl-a concentrations and wind stress, guided by synoptic novel and historical in situ observations which reveal two niches for phytoplankton assemblage: the Transitional Bellingshausen Water (TBW) and Transitional Weddell Water (TWW) pools. The TBW pool features stratified, less saline, warmer waters with shallow mixed layers, while the TWW pool features well-mixed, colder, and saltier waters. We identify that the 0.6°C isotherm corresponds to the climatological Peninsula Front location, effectively dividing the Bransfield Strait in two different scenarios. Furthermore, the 0.5 mg m-3 chl-a isoline aligns well with the 0.6°C isotherm, serving as a threshold for chl-a blooms of highest concentrations around the South Shetland Islands. These thresholds enable for the first time the monthly climatological description of the two blooms developing in BS at both sides of the Peninsula Front. We think this approach underscores the potential of combining SST and chl-a data to monitor the year-to-year interplay of the chl-a blooms occurring in the TBW and TWW pools contoured by the Peninsula Front. [ABSTRACT FROM AUTHOR]

Abstract—The paper discusses the spatial distribution of the main parameters of primary production in the Bransfield Strait in the austral summer. The integrated primary production in the strait varied from 435 to 741 mgC m–2 day–1. The share of primary production in the total production of phyto- and bacterioplankton in the upper 10 m layer was 82–91%. Potential photosynthetic capacity (Fv/Fm) was high within the euphotic layer (0.418–0.749) throughout the area. The production parameters in the two main water masses in the strait did not differ. Photosynthetic efficiency (the ratio of the assimilation number and the relative electron transport rate, AN/rETR) varied in different subregions of the study area by almost six times. [ABSTRACT FROM AUTHOR]

Abstract—The article studies the abiotic characteristics in waters of the Bransfield Strait in January 2022. The cross-section of January 2020 was repeated from the Antarctic Peninsula to the Southern Shetland Islands; new data were also obtained: two cross-sections in the eastern and western parts of the strait. The scheme of currents in the Strait reflects modern knowledge; absolute values have been measured up to 50 cm/s in the northeast direction and up to 35 cm/s in the southwest. In the studied area, modified waters of the Bellingshausen Sea are clearly distinguished (warm and least saline, with reduced values of total alkalinity and all nutrients); modified Weddell Sea water (colder and more saline, with reduced values of silicates and nitrates), as well as Circumpolar Deep Water in the 200–450 m layer (warm and salty, with low values of dissolved oxygen and pH and increased content of phosphates, silicates. and nitrates). In bottom waters of the strait, there were no serious changes in structure. The amplitude of the interannual variability of the abiotic characteristics of Bransfield Strait waters is poorly expressed, nevertheless it can serve as a climate change marker in the Southern Ocean. [ABSTRACT FROM AUTHOR]

The spatial distribution of the biomass of macro- and mesozoopankton species, as well as the size and sex structure of the species Euphausia superba (krill), Salpa thompsoni, and Ihlea racovitzai in the Bransfield Strait during the Antarctic summer of 2020 were studied. Krill is at the heart of the food chain of the Antarctic ecosystem because numerous populations of seabirds, seals, and whales feed on them. In recent decades, the Antarctic ecosystem has been under significant changes caused by global warming, but the nature and extent of this impact on krill stocks, especially its juveniles in the Bransfield Strait, remains insufficiently studied. At the same time, this particular region is a highly productive zone of the Antarctic ecosystem and one of the areas where concentrations of Antarctic krill accumulate. The aim of this research was to study the spatial variability of the structure, numbers, and biomass of zooplankton in the Bransfield Strait during the Antarctic summer of 2020. Zooplankton samples in the Bransfield Strait were collected with a Bongo net in January 2020. The samples were processed on the research vessel in the intervals between sampling according to the standard procedure. At all stations studied, the main zooplankton biomass consisted of salps (S. thompsoni and I. racovitzai). According to the coenotic significance index, salps were of primary importance in the zooplanktocenosis, in which E. superba occupied third place with a biomass ranging from 6.3 to 96.3 mg/m3; krill was encountered only in the northwestern deep-water part of the Bransfield Strait. This is due to food competition between salps (S. thompsoni, I. racovitzai) and krill. The presence of krill in the deep-water part of the Bransfield Strait is explained by its introduction from the Bellingshausen Sea due to the prevalence of western winds. The biomass of other zooplankton species in all areas of the strait was very small vesus that of salps and krill and was uniformly distributed. When comparing the ratio of the biomass of the above species obtained during the expedition with the data of the 1990s and 2000s, there is a clear trend of an increase in salps and a decrease in E. superba, probably associated with the general trend of an increase in water temperature in Antarctic water. Thus, the case study of the Bransfield Strait clearly shows a negative correlation between the biomass of salps and krill. [ABSTRACT FROM AUTHOR]

Internal waves based on measurements on cruise 87 of the R/V Akademik Mstislav Keldysh in the Bransfield Strait near Half Moon Island and calculations using a numerical model are analyzed. The measurements were carried out on January 25, 2022 over 4 h using a line of temperature and pressure sensors along with CTD probe measurements. Temperature fluctuations based on the data from these sensors showed that the amplitude of internal waves was close to 5 m, sometimes reaching 15 m. Irregular semidiurnal tides predominate in the study area based on the calculation results using the TPXO9 global tidal model. Numerical calculations of the internal wave parameters show that the baroclinic tide generated on a steep slope breaks up into higher frequency waves. [ABSTRACT FROM AUTHOR]

The Southern Ocean has undergone significant climate-related changes in recent decades. As a result, pelagic communities inhabiting these waters, particularly mesozooplankton, have adapted to new conditions. The present study considers the patterns of horizontal and vertical (up to 1000 m) distribution, the composition, abundance, and biomass of mesozooplankton, and the relationships of these parameters to the extreme environmental conditions off the western Antarctic Peninsula throughout the record-warm austral summer season of 2022. Sampling was conducted using the opening/closing Multinet system (0.25 m2 aperture) equipped with five 150-μm mesh nets and a WP-2 net. The mesozooplankton was represented by the three most abundant groups: eggs and larvae of euphausiids such as Euphausia superba, small copepods such as Oithona similis, and large calanoid copepods such as Calanoides acutus, Calanus propinquus, Metridia gerlachei, and Rhincalanus gigas. The composition and quantitative distribution of the mesozooplankton significantly varied: the copepods were abundant in the west, off the Antarctic Peninsula, while eggs and larvae of euphausiids were abundant in the east, off the South Orkney Islands. Most mesozooplankton occurred in the upper 200 m layer, and each taxon showed characteristic depth preference: small copepods, euphausiids larvae, and cirripeds cypris larvae were abundant in the epipelagic layer, while large calanoid copepods, euphausiids eggs, amphipods, pelagic polychaetes, and ostracods were found mostly in the mesopelagic layer. The composition and quantitative distribution of mesozooplankton had clear relationships with environmental factors, particularly with a combination of variables such as water salinity, temperature, and chlorophyll a concentration. [ABSTRACT FROM AUTHOR]

The carbon dioxide concentration in the atmosphere has progressively risen since pre-industrial times. About one-third of the anthropogenically generated CO2 is absorbed by the waters of the World Ocean, whereas the waters of the Southern Ocean take up about 40% of this CO2. The concentrations of oxygen and carbon dioxide dissolved in seawater are sensitive to climate changes, transferring anthropogenic pressures with consequences for the biogeochemical cycles in the World Ocean. The Southern Ocean is a key region for the exchange of oxygen and carbon between the surface water and the atmosphere and for their transfer with cold water masses to the deep layers of the Ocean. In this paper, we discuss the dynamics of the carbon dioxide partial pressure (pCO2) and dissolved oxygen (O2) in the surface waters of the Atlantic Southern Ocean based on data collected during the 87th cruise of the R/V "Academik Mstislav Keldysh". The study area includes the Bransfield Strait, Antarctic Sound, the Powell Basin, the Weddell, and Scotia Seas. We have analyzed the spatial distribution of pCO2 and oxygen for the areas of transformation of water masses and changes in biogeochemical processes. In the zone of Scotia and Weddell Seas, we have observed an increase in pCO2 and a decrease in oxygen concentrations at the transect from the Weddell Sea at 56° W to the Powell Basin. From the Antarctic Sound to the Bransfield Strait, a decrease in oxygen saturation and an increase in pCO2 has been traced. The surface waters of the Bransfield Strait have revealed the greatest variability of hydrochemical characteristics due to a complex structure of currents and intrusions of different water masses. In general, this area has been characterized by the maximum pCO2, while the surface waters are undersaturated with oxygen. The variability of the AOU/ΔpCO2 (w-a) ratio has revealed a pCO2 oversaturation and an O2 undersaturation in the waters of the Bransfield Strait. It is evidence of active organic carbon decomposition as the major controlling process. Yet, photosynthesis is the major biogeochemical process in the studied areas of the Weddell and Scotia seas, and their waters have been undersaturated with pCO2 and oversaturated with O2. As it comes from the analysis of the distribution and correlation coefficients of AOU and the sea-air gradient of pCO2 with other physical and biogeochemical properties, the predominance of the biotic processes to the dynamics of O2 and pCO2 in the surface water layer has been demonstrated for the studied areas. Yet, there is evidence of additional sources of CO2 not associated with the production and destruction processes of organic matter. [ABSTRACT FROM AUTHOR]

The Bransfield Strait is a climate change hotspot at the tip of the northern Antarctic Peninsula (NAP). The region is marked by a mixture of relatively warm waters from the Bellingshausen Sea with cold shelf waters from the Weddell Sea. Additionally, its deep central basin (>800 m) preserves seawater properties from the north‐western Weddell Sea continental shelf. This study assessed long‐term changes in carbonate chemistry in the Bransfield Strait and found that the hydrographic setting (i.e., a mixture between modified‐Circumpolar Deep Water with Dense Shelf Water [DSW]) drives temporal variability of carbonate parameters. The western basin has experienced decreases in pH (seawater scale) over the last three decades (1996–2019), varying from −0.003 to −0.017 pH units yr−1, while Ωar decreased from −0.01 to −0.07 yr−1 throughout the water column. The central basin was characterized by a high contribution of DSW with high carbon dioxide (CO2) content and the decomposition of organic matter produced and transported into its deep layer. With lower variability for all carbonate system variables, the eastern basin was likely regulated by internal mixing. Overall, the entire strait is almost reaching a CO2‐saturated condition, highlighting how sensitive subpolar regions are to the effects of human‐induced climate change. Plain Language Summary: Although the entire world is experiencing the impacts of climate change, they may be occurring more rapidly in some regions, such as the northern Antarctic Peninsula. At the northern tip of this area is the Bransfield Strait, which can act as a sentinel for identifying what is happening around the peninsula during modern climate change. Areas of the strait that are connected to the open ocean showed a more rapid increase in acidification over time than areas mainly influenced by coastal zones. This occurs because the contribution of open‐ocean water masses supplies a signature of the decomposition of organic particles from the ocean around Antarctica. On the other hand, coastal zones are more influenced by atmospheric carbon dioxide from human activities. The steeper acidification trends in the Bransfield Strait draw our attention to the effects of climate change on ocean acidification and its biological and chemical impacts on the ocean. Moreover, there are many areas, such as the Bransfield Strait, for which there are few studies about these effects, which delays the identification of severe impacts even after changes have already been experienced, as was found in our assessment. Key Points: The western basin experiences steeper pH decreases than the surrounding areas at a rate of −0.017 pHsws units yr−1 due to Circumpolar Deep Water intrusionsDense Shelf Water inflow into the deep layer of the central basin promoted a CT increase of about 50 μmol kg−1 in the 2010s relative to the 2000sInternal mixing has likely reduced spatiotemporal variability of carbonate chemistry in the eastern basin since the 1990s [ABSTRACT FROM AUTHOR]

Three-dimensional structure of currents in the Bransfield Strait (BS) was measured in 2017 over 50 transects across the strait using a Shipborne Acoustic Current Doppler Profiler (SADCP) onboard the R/V Akademik Sergey Vavilov and Akademik Ioffe. These data were used to determine local features of the three-dimensional velocity field of the currents. The sections crossed all three basins of the BS and the main flows in the strait including the Bransfield Current and the southwestward flow of Weddell Sea waters along the Antarctic Peninsula. Two vessels are equipped with different SADCP systems with frequencies of 76.8 kHz and 38.4 kHz, which allows us to compare the data from different SADCPs. The observations reveal that very dense waters from the Weddell Sea propagate in deep layers over the shelf break; this flow is usually not observed at the sea surface. Vertical structure of the Bransfield Current significantly differs from the flow of the Weddell Sea waters and its velocity maximum is located in the upper ocean layer. We also used two gridded altimetry products of 0.25° and 0.1° resolution for investigations of circulation in the BS. We analyzed the efficiency of satellite altimetry in reproducing currents in conditions of a narrow strait and evaluated its accuracy using field velocity observations. We found that satellite altimetry can be used for identification of the mean ocean circulation patterns within the BS. However, daily altimetry data contain significant discrepancies with the in situ data and require direct measurements for studies of the BS circulation.

Understanding subduction processes is critical in assessing the long-term evolution of the upper mantle and continental crust. We present new geochemical data on glassy submarine lavas from the Bransfield Strait, the Phoenix and West Scotia ridges, and previously unpublished data of marine sediments from atop the Phoenix Plate. We combined new and published geochemical data from across the northern sector of the Antarctic Peninsula to unravel both large-scale mantle composition and flow across the region. The geochemistry of Phoenix and West Scotia Ridge basalts supports the hypothesis that both ridges are underlain by Pacific MORB mantle, brought into the region through the eastward expansion of the Scotia Plate. Comparisons among lavas from the Phoenix/West Scotia ridges, the South Shetland Islands volcanic arc, and the Bransfield Strait/Prince Gustav Rift back-arc region reveal that the Pacific upper mantle flowed into the mantle wedge and was subsequently modified by subduction processes. The compositions of Bransfield Strait lavas range from strongly subduction-influenced near the strait's center to akin to Phoenix Ridge MORB toward the strait's edges. This suggests that the Phoenix MORB mantle has flowed around the slab edges into the mantle wedge, diluting the subduction signal and focusing the subduction-modified mantle towards the center of the strait. Monte Carlo simulations indicate that mixing of Phoenix MORB mantle with 0 to 3 % subduction component having a fluid/sediment ratio of ∼1 can explain the compositional range in the Bransfield Strait. Furthermore, our modeling suggests the presence of a more depleted Phoenix MORB mantle beneath the South Shetland Islands modified by the addition of ∼3 % subduction component with a fluid/sediment ratio ranging from ∼0.18 to 4. The increase in fluid/sediment ratios in the South Shetland Island lavas corresponds to a spatiotemporal progression of volcanism from the southwest (40–140 Ma) to the northeast (30–60 Ma). Finally, we have identified a set of lavas with unique trace element and isotope compositions typical of alkaline volcanism found across other parts of the Antarctic Peninsula. Surprisingly, we find occurrences of these lavas on both sides of the South Shetland Trench, suggesting the presence of a distinct enriched source in the upper mantle throughout the region. [ABSTRACT FROM AUTHOR]

Calcium carbonate hexahydrate (ikaite) is a rare mineral that forms as metastable species in the organic-carbon-rich sediments of the King George Basin, Bransfield Strait, Antarctica, as a consequence of early diagenetic decomposition of organic matter under cold water (−1.4 °C) and high pressure (200 bar) conditions. Large crystals grow in the sediment immediately below the diagenetic transition between microbial sulfate reduction and methanogenesis at ~320 cm below sea floor (bsf). This process is reflected in the dissolved sulfate, total carbon dioxide, and methane concentrations, as well as in the carbon, hydrogen, and oxygen isotope chemistries of the interstitial fluids and dissolved gases of the host sediment. The ikaite crystal faithfully records in its zonal structure the changing carbon isotope ratio of the total dissolved carbon dioxide pool as it gradually diminishes during methanogenesis (δ13Cikaite = −17.5 to −21.4‰). These changes in the crystal's host environment follow general Rayleigh carbon isotope fractionation. The oxygen isotopes of the ikaite carbonate (δ18Oikaite = 1.46 to 4.45‰) also show a strong zonal distribution, unrelated to temperature of formation, but perhaps controlled by the degree of recrystallization of ikaite to calcite. The crystal water of the ikaite is depleted 11‰ in 2H/1H (VSMOW) relative to the coexisting interstitial water, which is in excellent agreement with the isotope fractionation of other hydrated minerals. In addition to the in situ temperature and pressure, nucleation of the ikaite crystals in the Bransfield Basin sediments may be induced by the high alkalinity, high phosphate concentrations, and dissolved organic compounds. Intense microbial metabolism generates such compounds; of these, aspartic acid and glutamic acid may play an important role, as they do in biological and extracellular carbonate mineral precipitation. All indications are that low temperatures (such as of polar environments), high calcium carbonate supersaturation caused by interstitial methanogenesis, and a sufficiently large supply of dissolved phosphate and amino acids favor metastable ikaite formation. These conditions, modified by recrystallization, may be preserved in calcite glendonites, thinolites, and other calcitic pseudomorphs derived from ikaite and found throughout the ancient sedimentary record. [ABSTRACT FROM AUTHOR]

In January 2022, during scientific cruise 87 on the RV Academic Mstislav Keldysh in the Atlantic sector of the Southern Ocean, three hydrobiophysical cross‐sections were performed in the Bransfield Strait. Bioluminescent signals were measured in a layer of 0–200 m at each of the 24 stations located at three sites. For the first time, a new hydrobiological system 'Salpa MA +' was used, which made it possible to obtain novel data in the photic layer of the studied water area. Bioluminescence studies were accompanied by simultaneous measurements of background indicators: temperature, electrical conductivity, photosynthetically active radiation, and they were compared with the data from plankton samples processing. Bioluminescent potential was registered at almost all the stations. The maximum level of bioluminescence was registered in the area of the archipelago of the South Shetland Islands, where the maximum accumulation of Salpa thompsoni, Foxton 1861 was noted. The purpose of this work is to identify the main factors and patterns affecting the intensity of the bioluminescence field in the region under study. [ABSTRACT FROM AUTHOR]

The Bransfield Strait is a relatively deep and narrow channel between the South Shetland Islands and the Antarctic Peninsula contributing to the water transport between the Pacific and Atlantic sectors of the Southern Ocean. The strait can be divided into three deep separate basins, namely, the western, central, and eastern basins. The sources of deep waters in the three basins are different, leading to differences in thermohaline properties and water density between the basins. The difference in water density should in turn cause intense deep currents from one basin to another through narrow passages over the sills separating the basins. However, there are still no works dedicated to such possible overflows in the Bransfield Strait. In this study, we report our new CTD and LADCP measurements performed in 2022 over the watersheds between the basins. Quasisimultaneous observations of the main circulation patterns carried out at several sections allowed us to analyze the evolution of thermohaline and kinematic structures along the Bransfield Strait. Volume transports of waters in the strait were estimated on the basis of direct velocity observations. These new data also indicate the existence of intense and variable deep current between the central and eastern basins of the strait. The analysis of historical data shows that the mean flow is directed from the central to the eastern basin. In addition, LADCP data suggest the intensification of the flow in the narrow part of the sill between the basins, and the possible mixing of deep waters at this location. [ABSTRACT FROM AUTHOR]

In January 2022, several sections were made across the Bransfield Strait during an Antarctic expedition on the R/V Akademik Mstislav Keldysh. Calculations of geostrophic velocities and transports in the strait were performed based on the measured data on vertical salinity and temperature profiles. The geostrophic velocities and transports were compared with the data of direct instrumental measurements using a lowered acoustic current profiler. It was shown that it is quite justified to apply estimates of geostrophic velocities and transports to the Bransfield Current. The agreement between calculations and measurements for the water flow from the Weddell Sea is less satisfactory. [ABSTRACT FROM AUTHOR]

This study examines the adaptability of a Southern Ocean predator, which is dependent on Antarctic krill (Euphausia superba), to potential changes in food availability. Muscle fatty acids (FAs) of the spiny icefish Chaenodraco wilsoni collected from three areas in the Bransfield Strait (BS), northern Antarctic Peninsula during February–April 2016 give a good representation of their feeding variability. The compositions of 22:6n3 (DHA) and 20:5n3 (EPA) were both higher in the Transitional Zonal Water with Bellingshausen influence (TBW)‐controlled C. wilsoni than in the Transitional Zonal Water with Weddell Sea influence (TWW)‐controlled fish. This was positively correlated with photoadaptation and carbon sequestration in TBW‐controlled phytoplankton. Results for the FAs 16:1n7, 16:0, DHA and EPA indicate the presence of dinoflagellates in all three areas, suggesting that during late summer and early fall, there is a seasonal phytoplankton succession, where small phytoplankton become dominant, in the BS. In addition, the compositions of some long‐chain FAs (>20, such as 20:0, 20:1, 22:0 and 22:1n9) and ∑18 indicated that the food chain based on flagellates and copepods was more apparent in TWW‐controlled C. wilsoni, especially the effect of El Niño‐Southern Oscillation (ENSO) on the variation of prey communities in TWW‐controlled areas. FA markers such as SFA/(PUFA+MUFA), ∑15 + ∑17 and ARA were more pronounced in TWW‐controlled C. wilsoni, indicating a more strongly carnivorous and benthic food source. In the TBW–TWW confluence, the complex hydrological structure, including the presence of a large number of mesoscale eddies, allows rich nutrients and krill larvae to remain in it, providing a rich food source for the C. wilsoni. Overall, the FA data of this study show that the diet of C. wilsoni varies in different marine environments, aiding their survivability at the face of climate change. [ABSTRACT FROM AUTHOR]

Studies of the benthic fauna in the Atlantic sector of the Antarctic within the Project were carried out on the 87th cruise of the RV Akademik Mstislav Keldysh (January 19–February 14, 2022) in the Bransfield Strait, the Powell Basin of the Weddell Sea, and the Orkney and Laurie Troughs. At 11 stations in the depth range of 362–5490 m, eight Sigsbee trawls and three box corer were taken, and visual observations were made on two phototransects using the AUV MMT-3500. New data were obtained on the biodiversity and community structure of the benthic communities. [ABSTRACT FROM AUTHOR]

Methane concentrations were measured in the water layers on two transects crossing the central part of Bransfield Strait and along the Antarctic Sound. In the abyssal sea of the Bransfield Strait (1397 m), for the first time, an area with an abnormally high methane concentration (47.5 nmol/L) in the near bottom layer was found. An increased methane concentration (11.3–16.0 nmol/L) in the bottom layer of seawater was also recorded at a distance of 30–35 km from the central part of the methane anomaly. In the bottom layer of the southern part of the Antarctic Sound below the 400 m isobath, a zone with an increased methane concentration (6.7 nmol/L) was discovered for the first time, which exceeds the average methane concentration in the Weddell Sea. The set of studies carried out allows the conclusion that the source of methane found in the Bransfield Strait is endogenic, and the increase in the methane concentration in the Antarctic Sound is caused by the current. [ABSTRACT FROM AUTHOR]

The Orca Seamount is a submarine volcanic structure, located in a tectonic area of the Bransfield Strait, characterized by cortical extension and roll back-type subduction. Recent investigations have described the presence of hydrothermal activity and thermophilic microorganisms in this submarine volcano, raising questions regarding the role these microorganisms might play in the environment. The presence of hydrothermal activity interacting with cold Antarctic marine waters has probably exerted a great impact on the chemistry of the Orca Seamount area, providing different types of substrates capable to support complex microbial communities. In this work, we further study the Orca Seamount area with respect to the mineralogy present in this environment and the role microorganisms might play in the biogeochemical cycles. Here we show that the assemblage of minerals detected in the Orca Seamount area is like those commonly found in other hydrothermal environments, consistent with previous investigations reporting hydrothermal activity in this zone. Sulfur- and iron-bearing minerals in addition to inorganic soluble compounds are able to support chemosynthetic microbial communities inhabiting the Orca Seamount. The role of these microorganisms on the sulfur, iron, and carbon cycle is discussed and analyzed in the context of the mineralogy and conditions of the environment. [ABSTRACT FROM AUTHOR]

Abiotic characteristics of the waters in the Bransfield Strait were studied in a section from the South Shetland Islands to the Antarctic Peninsula. The Bransfield Current was recorded northeastward flowing along the archipelago with a speed of 20–47 cm/s. It is marked by the water temperature and salinity in the core (2.7°C and 34.18%). The depth of the lower boundary of the euphotic layer varies from 40 m near the South Shetland Islands to 80 m on the shelf of the Antarctic Peninsula. The oxygen content and parameters of the carbonate system indicate the predominance of production over destructive processes in the surface layer of the central part of the Strait during the period under study. A countercurrent from the Weddell Sea was recorded on the shelf of the Antarctic Peninsula, which is distinguished by the oxygen saturation (90%) and silica content (74 µM). Four water masses were identified in the region under studied: Antarctic Surface Water, Circumpolar Deep Water, modified water of the north-western Weddell Sea shelf, and Bottom Bransfield Water. [ABSTRACT FROM AUTHOR]

An expedition to study the ecosystems of the Atlantic sector of the Antarctic was carried out in cruise 87 of R/V Akademik Mstislav Keldysh between January 19 and February 14, 2022. The studies were carried out in the key regions for understanding modern processes in the ecosystem of the Southern Ocean and in the regions of interest for Russian fisheries, namely, the Powell Basin of the Weddell Sea, the Bransfield Strait, Antarctic Sound, and at the test site east of the South Orkney Islands. Materials characterizing the state of the environment, pelagic communities, including the population of the Antarctic krill and mesopelagic fish, benthic fauna, and the spatial and temporal variability of the most important components of ecosystems, were obtained. [ABSTRACT FROM AUTHOR]

Gravitational modeling along the DSS 1 profile was carried out, which made it possible to obtain the density structure of the lithosphere of Bransfield Strait between the Antarctic Peninsula and King George Island to a depth of 80 km, to clarify the geological structure of the upper part of the crust, obtained as a result of the interpretation of the seismic survey, and draw conclusions on the course of the evolution of the strait in the cross section of the profile. The identical structure of the bowels of King George Island and Trinity Peninsula, together with the inside of the Bransfield Platform, confirm the common origin of these formations. The crust under the Bransfield Trough has a distinct suboceanic type. The structure of the lithosphere of the Bransfield Strait allows us to distinguish three episodes of tectonic-magmatic activity, which manifested themselves in the introduction of the mantle substance into the crust followed by subsidence of the surface as a result of smelting crystallization. Asymmetric spreading can be explained by the fact that the crust growth to the southeast coincides with the rotational movement of the Antarctic Plate in the east direction, and the pressure from the Phoenix Plate prevents growth from the north-west side. The formation of the oceanic crust of the strait occurs in the areas of the crust that successively pass over the fixed mantle astenolite, i.e. the «hot spot» effect is evident.

The results of measurement of the thermohaline parameters of water and currents in the Bransfield Strait in Antarctica during the expedition of the Shirshov Institute of Oceanology, Russian Academy of Sciences, in 2020 are presented. A section of nine stations across the strait located in its central part and vertical distributions of temperature and salinity are analyzed. Intrusion of deep water from the southern branch of the Antarctic Circumpolar Current was found. Estimates of the current velocity and water transport are given. The total transport to the northeast was 5.03 Sv (1 Sv = 106 m3/s). [ABSTRACT FROM AUTHOR]

The Bransfield Strait connects the Bellingshausen and Weddell seas and is strongly affected by intense inflows of their water masses. In this work we analyzed Shipborne Acoustic Current Doppler Profiler (SADCP) measurements carried out in the Bransfield Strait from 2015 to 2022. The new dataset includes over 100 transects, crossing the main currents in the strait (namely, the Bransfield Current and the Transitional Weddell Water (TWW) flow). Based on these data, we studied spatial variability of water mass transports in the Bransfield Strait during austral summer season. As shown below in this research, the Bransfield Current was observed down to the 600 m depth. By considering deeper layers, we managed obtaining more precise assessments of mean water mass transports of the Bransfield Current. They were estimated at 1.4–2.1 Sv. We found that the transports of the Bransfield Current and TWW flow in the austral summer season (November–March) do not show significant variability. The water mass transport of the Bransfield Current and the TWW flow was stable within the central basin of the Bransfield Strait and differed in its western basin. We also found that in austral summer the heat transport into the Bransfield Strait varied between 0.2 and 0.4 Sv·°C and salinity transport was −0.07 to −0.06 Sv·psu, their sign reflecting a dominant northeastward flow of relatively warm and fresh water. In addition, we compared Lowered Acoustic Current Doppler Profiler (LADCP) and SADCP measurements and proved that the technical limitations of the SADCP do not significantly affect the estimates of the transports in this region. • New field measurements confirm cyclonic circulation in the Bransfield Strait. • Measured transport of the Bransfield Current is 1.4–2.1 Sv. • Measured transport of the current from the Weddell Sea is 0.8–1.1 Sv. • Shipboard and lowered ADCP data are in good agreement in the Bransfield Strait. [ABSTRACT FROM AUTHOR]

The long-term variability of chlorophyll a (Chl-a) was analyzed during a 12- year period (2002 - 2014) in the Bransfield Strait, Antarctica, to assess the interannual variability of the Chl-a and its main regulating environmental variables. We found that Chl-a was significantly correlated with the El Nino Southern Oscillation (ENSO) phase (r = -0.65), sea surface temperature (SST) (r = 0.70), and wind speed (r = 0.72), and changes in these factors preceded the Chl-a change by 0 - 4 months. High phytoplankton blooms (indicated by Chl-a levels) tended to coincide with higher SSTs, and lagged with wind speed variation during La Nina periods. This suggests that atmospheric teleconnections with ENSO-related processes, SST, and wind speed could have a direct impact on phytoplankton biomass in the Bransfield Strait in short-term (< 4 months) scales. Furthermore, our simple multivariable regression analysis showed the prominent possibility of usina the three variables to prediet Chl- a in the Bransfield Strait region. Overall, this study will help us to understand how atmospheric and oceanic fluctuations could affect phytoplankton dynamics in the Southern Ocean. [ABSTRACT FROM AUTHOR]

We present the first volatile contents (H 2 O, CO 2 , Cl, F, S) of young (< 6 Ma) submarine basaltic glasses from the Phoenix and West Scotia mid-ocean ridges and the Bransfield Strait back-arc of the South Shetland subduction zone in the Antarctic Peninsula. The volatile contents of the MORB glasses correspond well with those of published Pacific MORB and reflect covariations in source enrichment and extent of melting. Our results support the hypothesis that decreasing spreading rates at the Phoenix Ridge resulted in preferential melting of less abundant enriched MORB mantle, due to its greater fertility and higher volatile contents, relative to the more abundant depleted MORB mantle. The volatile contents of the Bransfield Strait back-arc glasses correlate with geochemical indicators of subduction processes and reveal an along-axis spatial distribution consistent with a toroidal inflow of sub-slab asthenosphere around the edges of the subducting plate into the mantle wedge. This inflow should be considered when assessing spatial and geochemical variability at subduction zones, particularly those with slab windows and tears. A small group of Bransfield Strait samples have volatile contents that do not correlate with geochemical signals of subduction influence. We speculate that these samples reflect flux melting of residual enriched mantle brought beneath the Bransfield Strait via corner flow following recent alkaline magmatism in the far eastern regions of the Antarctic Peninsula. Our new data on lavas from the W7 segment of the West Scotia Ridge reveal their source was significantly affected by subduction processes. Unexpectedly, these lavas have CO 2 -H 2 O pressures of vapor saturation that suggest they were collected in-situ and erupted relatively recently (∼6 Ma), at odds with previous interpretations of their origins. We suggest they originated from a subduction-modified mantle (lithosphere or asthenosphere) left behind by the eastward-migrating South Sandwich subduction zone sometime over the past ∼30 Myr. These lavas demonstrate the long-lasting effects of subduction processes on the upper mantle and their potential to influence melt compositions even in non-subduction environments today. We use the compositions of lavas from the Phoenix Ridge and Bransfield Strait to estimate mantle potential temperatures; our results agree well with global estimates for mid-ocean ridges and subduction zones, respectively. • Melts at dying ridges sample increasingly enriched mantle as spreading slows. • Subduction influence corresponds with volatile enrichment in most back-arc lavas. • Sub-slab asthenosphere can flow into the mantle wedge via slab windows and tears. • Subduction-modified mantle can remain ∼30 Myr after subduction has ceased/migrated. [ABSTRACT FROM AUTHOR]

Spiny icefish (Chaenodraco wilsoni), a common high-Antarctic ichthyofauna icefish, exhibits variable feeding habits in regions around the Southern Ocean. In the Bransfield Strait (BS), C. wilsoni has received considerable attention as it preys predominantly on krill. Antarctic krill (Euphausia superba), the keystone species in the Southern Ocean, has a versatile feeding strategy, particularly in a highly dynamic environment. The central basin of the BS is a highly dynamic environment, providing an ideal place to explore the application of biochemical tracers in a specialist predator to reveal the diet of its prey. In this study, fatty acid and stable isotopic data demonstrated that the trophic niche of C. wilsoni expanded from late austral summer to early fall, which was primarily reflected in variability in the food availability of krill, i.e., herbivorous features were weakened, benthic-feeding features were strengthened, and omnivorous features showed no significant difference. A highly dynamic hydrography, coupled with an unusual diatom bloom in the BS during 2016, may have resulted in krill's prolonged filtering on diatoms in the water column, which was further reflected in the difference in biochemical tracers of C. wilsoni from late austral summer to early fall. The study demonstrated that biochemical tracers can be used to reflect the variability in the diet of prey, particularly in a highly dynamic environment. • Fall diet of Chaenodraco wilsoni in a highly dynamic environment was examined. • Biochemical tracers of C. wilsoni can reflect changes in diet of Euphausia superba. • Study fills an important data gap on trophic dynamics of C. wilsoni and E. superba. [ABSTRACT FROM AUTHOR]

The Polish Geophysical Expedition to West Antarctica in 1979–1980 was carried out by the Institute of Geophysics, Polish Academy of Sciences. Beside deep seismic soundings, 12 multi-channel seismic profiles, with a total length of ca 1000 km have been recorded north and east of the South Shetland Islands and in the Bransfield Strait, but they have never before been completely interpreted and published. All profiles have been processed with modern processing flow including time migration. Profiles crossing the South Shetland Trench revealed distinct reflector inside continental slope, which has been interpreted as border between buried accretionary prism and overlying slope sediments of glacial-marine origin. Profiles in the Bransfield Strait show traces of the Last Glacial Maximum (LGM) in the form of glacial foreground valleys, with some of them used as weak spots for young age volcanic intrusions. This paper is the first comprehensive geological interpretation of collected dataset and differences between results from other expeditions are discussed.

Field works on cruise 79 of the R/V Akademik Mstislav Keldysh aimed at investigating the ecosystems in the Atlantic sector of Antarctica were conducted from January 16 to March 1, 2020. Hydrophysical and biological studies were carried out in the Drake Passage and the Bransfield and Antarctic Sound straits. The regions of Russian fishery interest were covered. Data were collected that characterize the environmental state; plankton communities, including krill, ichthyoplankton, macronecton; bottom fauna; and spatiotemporal variations in the most important ecosystem components. [ABSTRACT FROM AUTHOR]

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Specimens of Pycogonida belonging to the genus Ammothea collected during the Polarstern ANT-XXIX/3 cruise (January 22-March 18, 2013), to Bransfield Strait and Eastern Weddell Sea, were studied. Eight species are recognized in this collection: Ammothea bicorniculata, A. calmani, A. carolinensis, A. clausi, A. minor, A. nadiae n. sp., A. spinosa, and A. striata. Morphological variability is discussed. The bathymetric range of A. bicorniculata and A. minor is enlarged and A. bicorniculata is reported for only the second time. Ammothea nadiae n. sp. is described and compared with its closest congeners. An updated key to species is presented. [ABSTRACT FROM AUTHOR]

The deep basins of the Bransfield Strait (BS) are ventilated by Weddell Sea (WS) waters from different origins. Depending on the source and density, these water masses follow different routes across the complex topography near the tip of the Antarctic Peninsula and thus into the Bransfield Strait abyss. Using a global setup of the Finite Element Sea-ice Ocean Model (FESOM) we show that the WS waters found at the western WS continental shelf break have a higher influence on the short period variability of BS bottom waters than the waters present over the continental shelf. Adding passive tracers to the glacial melt water (GMW) from two different origins, Larsen Ice Shelf (LIS) and Filchner-Ronne Ice Shelf (FRIS), we show that the GMW from FRIS has a larger influence on BS bottom waters than the GMW from LIS. FRIS GMW has a higher concentration in the BS eastern basin, while LIS GMW is more abundant in the BS central basin. This duality mainly leads to the difference between BS central and eastern basins seen on the observations. This is a novel result and we believe is a significant contribution to the understanding of the BS-WS circulation and interactions. [ABSTRACT FROM AUTHOR]

The Western Basin (WB) of the Bransfield Strait has a complex water mass structure derived from a variety of sources, and is located next to the Antarctic region most affected by climate change. The WB is one of the three Bransfield Strait deep basins and the one most directly influenced by the waters from the Bellingshausen Sea and Drake Passage. The objective of this paper is to study the variability of the inflow of water masses to the WB and to detect the possible impact of regional climate change. We focus on the modified Circumpolar Deep Water (mCDW) that enters the WB through the northwestern passages. To achieve this goal, this study uses historical hydrographic data collected between 1960 and 2014. Warming (0.0161 ± 0.0079 °C yr −1 ) and lightening (−0.0023 ± 0.0010 kg m −3 yr −1 ) of the mCDW core (γ n = 27.85–28.27 kg m −3 ) are observed in the WB between 1960 and 2013. The waters within the 27.85–28 kg m −3 γ n range presented warming and salinization. Waters deeper than 500 m showed cooling and freshening between the 1960s and 1990s; however, warming and lightening are observed if 1981–2013 is considered. These trends appear to be associated with the positive SAM trend observed since the mid-20th century. Statistically significant negative correlations between θ, S and γⁿ of the mCDW core with ENSO were found between 1980 and 2014. During La Niña phases, the mCDW core extended along the southern slope of the South Shetland Islands while during El Niño phases, it was present in patches or was totally absent. Since the 2000s, the mCDW core extends southward within the WB, possibly associated to positive SAM phases. Statistically significant negative correlations between θ and S of the WB deeper waters with ENSO and SAM were found. These waters were in general more saline and denser during negative ENSO and SAM phases. Positive SAM phases with La Niña events suggest the increase (decrease) of mCDW (Weddell Sea shelf waters) contribution to the deep WB during the 2000s. Thus, high interannual variability in the thermohaline properties within the WB seem to be linked to the effect of these modes of climate variability on the source water masses. [ABSTRACT FROM AUTHOR]

Antarctic krill (Euphausia superba), a typical aggregation species, is the keystone species in the Antarctic ecosystem and supports an important fishery in the Southern Ocean. The unique topographic conditions of this region provide an important habitat for krill, and it has therefore become a critical krill fishing ground. To investigate the factors affecting krill hotspot patterns in the central basin of the Bransfield Strait in autumn 2021, an area with a high level of krill biomass and a persistent fishing ground, we used Getis-Ord G i * approach to perform a hotspot analysis of the spatial and temporal variability of krill distribution. From this analysis we examined the relationship between krill distribution in hotspot and coldspot areas and environment variables such as geospatial features and eddy kinetic energy. We found that environmental variables generally showed opposing trends for krill distribution at hot- and coldspot areas. Moreover, geospatial features, including the distance to 200m and 1000m isobaths and depth, may be the dominant factor determining krill habitat characteristics, and moderate levels of eddy kinetic energy were conducive to krill aggregation. [ABSTRACT FROM AUTHOR]