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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 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]

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]

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]

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]

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]

Phytoplankton assemblages around the South Shetland Islands (SSI) were closely related to mesoscale physical features, based on high spatial resolution sampling performed during the summer of 2010. Sampling was done in 8 transects with stations 9 km apart. Phytoplankton groups were described using flow cytometry, FlowCAM and HPLC/CHEMTAX pigment analysis. Nanophytoplankton (2-20 μm) was predominant throughout the study area, which was dominated by small diatoms. They were distributed along the stratified waters of the SSI shelf and in the centre of the Bransfield Strait where an anticyclonic eddy was detected, established between two frontal structures [Bransfield Front and Peninsula Front (PF)]. The highest concentrations correlated with mid-to-high temperatures (1.07 °C) and mid-salinities (34.03) corresponding with Transitional Bellinghausen Water stations. Haptophytes distribution co-varied with small diatoms but also appeared in those vertical mixed stations with Transitional Zonal Water with Weddell Sea influence. A shift from smaller to larger diatoms was detected at the ice edge in the Antarctic Sound. Cryptophytes were restricted to stratified stations of the SSI shelf and those associated with the PF, while small prasinophytes were the only group occupying deeper and colder waters of the Drake Passage, beneath the Antarctic Surface Water, north of a narrow frontal region described here for the first time (Shetland Front). Phytoplankton assemblages around the SSI were strongly connected with the Bransfield Current System, supporting a clockwise circulation around the archipelago. The Bransfield Current System components are permanent structures during the austral summer suggesting that the distribution of phytoplankton, which responds to these structures, must also be a quasi-permanent feature. [ABSTRACT FROM AUTHOR]

The Bransfield Strait is a tectonically active back-arc rift basin located between the South Shetland archipelago (SSI) and the Antarctic Peninsula (AP). We use teleseismic P-wave receiver functions from twelve seismic stations in order to improve the current understanding of the crustal and upper mantle structure of the region. We calculate the thickness and average Vp / Vs ratio of the crust, the thickness of the Mantle Transition Zone (MTZ) and their spatial variability along the study area. Our analysis reveals a homogeneous crustal thickness in the AP, averaging 33.5 ± 1.2 km. The V P / V S ratio increases from east to west, with an average of 1.78 ± 0.04 typical of continental crust. In the SSI results are highly heterogeneous. The minimum crustal thickness is observed in Deception Island (~15 km), near the rift axis, while the crust thickens towards the edges of the basin to ~30 km. High crustal V P / V S ratios (>1.95) in the archipelago correlate with a shallow Moho and may be linked to the presence of melts in the crust, in accordance with the active volcanism in the region. We detected an underplating layer and the Moho of the Phoenix slab beneath the SSI. There is evidence of a low velocity zone (LVZ) in the mantle wedge under the SSI and also under the AP, suggesting there is widespread partial melting of the upper mantle. We infer that the Phoenix slab dips steeply, as we observe a thickened MTZ under the AP. • The South Shetland Islands feature large heterogeneity regarding crustal thickness and Vp/Vs. • An underplating layer is found below the South Shetland Islands. • Low velocity bodies in the upper mantle may indicate widespread partial melting. • The Phoenix slab affects the Mantle Transition Zone below the Antarctic Peninsula. [ABSTRACT FROM AUTHOR]

The Bransfield Basin is a back-arc basin located in Western Antarctica between the South Shetland Islands and Antarctic Peninsula. Although the subduction of the Phoenix plate under the South Shetland block has ceased, extension continues through a combination of slab rollback and transtensional motions between the Scotia and Antarctic plates. This process has created a continental rift in the basin, interleaved with volcanic islands and seamounts, which may be near the transition from rifting to seafloor spreading. In the framework of the BRAVOSEIS project (2017–2020), we deployed a dense amphibious seismic network in the Bransfield Strait comprising 15 land stations and 24 ocean-bottom seismometers, as well as a network of 6 moored hydrophones; and acquired marine geophysics data including multibeam bathymetry, sub-bottom profiler, gravity & magnetics, multi-channel seismics, and seismic refraction data. The experiment has collected a unique, high quality, and multifaceted geophysical data set in the Central Bransfield Basin, with a special focus on Orca and Humpback seamounts. Preliminary results confirm that the Bransfield region has slab-related intermediate depth seismicity, with earthquake characteristics suggesting distributed extension across the rift. Gravity and magnetic highs delineate a segmented rift with along-axis variations that are consistent with increased accumulated strain to the northeast. Orca volcano shows evidences of an active caldera and magma accumulation at shallow depths, while Humpback volcano has evolved past the caldera stage and is currently dominated by rifting structures. These differences suggest that volcanic evolution is influenced by the position along the rift. Although a lot of analysis remains, these results provide useful constraints on the structure and dynamics of the Bransfield rift and associated volcanoes. [ABSTRACT FROM AUTHOR]