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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.

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 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 (γn = 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 γn 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. Fil: Ruiz Barlett, Eduardo Mariano. Ministerio de Relaciones Exteriores, Comercio Interno y Culto. Dirección Nacional del Antártico. Instituto Antártico Argentino; Argentina Fil: Tosonotto, Gabriela V.. Ministerio de Relaciones Exteriores, Comercio Interno y Culto. Dirección Nacional del Antártico. Instituto Antártico Argentino; Argentina Fil: Piola, Alberto Ricardo. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina. Ministerio de Defensa. Armada Argentina. Servicio de Hidrografía Naval; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Sierra, Marta E.. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina. Ministerio de Relaciones Exteriores, Comercio Interno y Culto. Dirección Nacional del Antártico. Instituto Antártico Argentino; Argentina Fil: Mata, Mauricio M.. Universidade Federal Do Rio Grande; Brasil

Deception Island (South Shetland Islands) is one of the most active volcanoes in Antarctica. In the 1988–1989 austral summer, after the most recent eruptive process on the island (1967–1970), monitoring of volcanic activity through geophysical and geodetic techniques was resumed by Spanish and Argentinean scientists. In order to monitor the island's tectonic and volcanic behavior, a geodetic network was deployed. Currently, this network consists of 15 geodetic benchmarks located around Port Foster, Deception's inner bay open to the sea. Two additional geodetic benchmarks were installed outside Deception Island to be used as reference benchmarks for the differential positioning strategy. Since 1991–1992, geodetic ground-displacement velocities between the successive austral summer Antarctic campaigns have been computed and analyzed. The overall geodynamic behavior of Deception Island within the South Shetland Islands, Antarctic Peninsula and Bransfield Basin regional environment has been analyzed from geodetic ground-displacements. Results obtained demonstrate that Deception and Livingston island have a similar behavior derived from the Bransfield Basin extension and the Phoenix micro-plate subsidence processes. However, Deception Island is also highly influenced by its volcanic activity. Deception Island's volcanic behavior between 1991 and 2018 is shown by the velocity field, strain tensors and pressure source evolution obtained from the ground-displacements at the geodetic benchmarks. During this time period, it is possible to identify different inflation and deflation phases separated by transitional (or mixed) stages of extension without uplift and compression without subsidence. The most representative inflation and deflation periods were analyzed in detail, to show how they correlate with high and low seismic activity, respectively. The transitional or mixed stages, seem to be the precursors of the next inflation or deflation phase being the Bransfield basin rifting and NW-SE extension the potential related process. Finally, we have analyzed the processes that occurred prior to the volcanic unrests of 1999–2000 and 2012–2013. In both cases, an increase in detected seismic activity and/or soil and seawater temperature was observed and a mixed phase of extension without uplift seems to be precursory to the volcanic unrest. The correlation between the inflation processes, identified by ground-displacement of the network geodetic benchmarks, the increase in seismicity and the increment of soil and seawater temperature makes these transitional mixed phases potential precursors of Deception Island's volcanic unrest periods. © 2019 Elsevier B.V.

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.

The Bransfield Strait and the adjacent areas is a high-productivity region for polar biological resources in the Antarctic. Physical oceanographic processes produce strong ecosystem dynamics via mass transport and water exchange. However, few studies have focused on these processes from coast to shelf and across slopes by tidal dynamics. Therefore, a numerical model was implemented with a high resolution to simulate the influence of the tide on mass transport. The output of the model indicated that the tidal residual current produced by the diurnal tide is dominant and primarily distributed along with the shelf break and near the coast, and water stratification amplifies this residual current system. Water exchange processes were evaluated utilizing passive tracer dyes to demonstrate the influence of the tides on the channel and bay. The tracer residence time ranged from a few days to a week and was affected by the considered representative region's topography and area. Correspondingly, Lagrangian particles tracking method is also used to illustrate the mass transportation, which primarily move along the isobaths on the slope but spiral on the flat shelf. All experiments conducted herein indicate that tidal dynamics is indispensable in this region when studying cross-shelf mass transport.

Age, growth and feeding habits of early life stages ofChionodraco rastrospinosusDewitt & Hureau, the most abundant channichthyid in the larval fish assemblages of the Bransfield Strait, were studied by otolith microincrement counts and stomach content analyses. Individuals measuring 39–69 mm standard length were caught in the uppermost depth strata down to 300 m from Brabant to Joinville islands along the northern Antarctic Peninsula. The sample consisted of post-larvae and juveniles aged 105–211 days, with a mean growth rate of 0.25 mm day-1. Larval size at hatching was estimated to bec.17.2 mm. Hatching was spread over a relatively long period from August–November. Sagittal otoliths were characterized by a strong check located at 23–52 microincrements of distance from the core, tentatively associated with the onset of first exogenous feeding. The relatively long period during which larvae can rely on yolk reserves and the large size at hatching enable them to utilize a wide size range of prey, as well as cope with occasional food shortages. The stomach contents consisted exclusively of euphausiids (furcilia and adults) and larvae ofPleuragramma antarcticumBoulenger. Based on growth rate, the residence time in pelagic waters of juvenileC. rastrospinosuswas estimated to be about a year and a half.

Surface deformation parameters and its use in volcano monitoring have evolved from classical geodetic procedures up to those based on Global Navigation Satellite Systems (GNSS), in particular the most widely used and known Global Positioning System (GPS), profiting from the automated data processing, positioning precision and rates, as well as the large storage capacity and low power consumption of its equipments. These features have enabled the permanent GNSSGPS data acquisition to ensure the continuous monitoring of geodetic benchmarks for the evaluation of surface deformation in active tectonic or volcanic areas. In Deception Island (Antarctica), a normal vector analysis is being used to give surface deformation based on three permanently observed GNSSGPS benchmarks. Due to data availability, both in the past and for near real-time use, all benchmarks used are inside the monitored volcanic area, although the reference is away from thermal springs and/or fumaroles, unlike the other two. The time variation of slope distances to the reference benchmark and of the magnitude and inclination of the normal vector to the triangle defined by the reference benchmark and any other two, provides the spatial deformation in the volcanic area covered. The normal vector variation in magnitude gives information on compression or expansion, here called spatial dilatometer, while the changes in inclination gives information on relative uplift or subsidence, here called spatial inclinometer. In geodesy, the triangle is a basic geometric unit and the areal strain is commonly applied in tectonics and volcanism. The normal vector analysis conjugates both, benefiting from the method's precision, simplicity and possibility to model the surface using several triangles. The proposed method was applied to GNSSGPS data collected every austral summer between 20012002 and 20092010 in Deception Island. The results evidence that Deception Island acts as a strain marker in the Bransfield Basin volcano-tectonic setting. Spanish Ministry of Education and Science as part of the National Antarctic Program; Recognition and fast evaluation of volcanic activity on Deception Island (GEODESY) [ANT1999-1430-E/HESP]; Geodetic Studies on Deception Island: deformation models, geoid determination and Scientific Information System [REN2000-0551-C03-01/ANT]; Acquisition of scientific software for GPS data processing [REN2000-2690-E]; Geodetic Control of the volcanic activity of Deception Island [CGL2004-21547-E/ANT]; Update of the Spanish Cartography for Deception Island [CGL2004-20408-E/ANT]; Volcanotectonic activity on Deception Island: geodetic, geophysical investigations and Remote Sensing on Deception Island and its surroundings [CGLl2005-07589-c03-01/ANT]; Geodetic and Geothermal Researches, Time Serial Analysis and Volcanic Innovation in Antarctica (South Shetland Islands and Antarctic Peninsula (GEOTINANT) [CTM2009-07251/ANT] info:eu-repo/semantics/publishedVersion

Deception Island shows the most recent exposed active volcanism in the northern boundary of the Bransfield Trough. The succession of the volcanic sequence in the island is broadly divided into pre- and post-caldera collapse units although a well-constrained chronological identification of the well-defined successive volcanic episodes is still needed. A new paleomagnetic investigation was carried out on 157 samples grouped in 20 sites from the volcanic deposits of Deception Island (South Shetlands archipelago, Antarctic Peninsula region) distributed in: (1) volcanic breccia (3 sites) and lavas (2 sites) prior to the caldera collapse; (2) lavas emplaced after the caldera collapse (10 sites); and (3) dikes cutting pre- and the lowermost post-caldera collapse units (5 sites). The information revealed by paleomagnetism provides new data about the evolution of the multi-episodic volcanic edifice of this Quaternary volcano, suggesting that the present-day position of the volcanic materials is close to their original emplacement position. The new data have been combined with previous paleomagnetic results in order to tentatively propose an age when comparing the paleomagnetic data with a global geomagnetic model. Despite the uncertainties in the use of averaged paleomagnetic data per volcanic units, the new data in combination with tephra occurrences noted elsewhere in the region suggest that the pre-caldera units (F1 and F2) erupted before 12,000 year BC, the caldera collapse took place at about 8300 year BC, and post-caldera units S1 and S2 are younger than 2000 year BC., Departamento de Geología y Geoquímica, Universidad Autónoma de Madrid, España, Instituto Geológico y Minero de España, España, Departamento de Geodinámica, Universidad de Granada, España, Instituto Andaluz de Ciencias de la Tierra, Consejo Superior de Investigaciones Científicas, España, Instituto Andaluz de Ciencias de la Tierra, Universidad de Granada, España, Unidad de Zaragoza, Instituto Geológico y Minero de España, España, Área de Geodinámica Interna, Universidad de Zaragoza, España, ESGEMAR, España

[EN] Geochemical analysis, micropalaeontological analysis and radiometric dating techniques were performed on four gravity cores, G-1, G-2, A-3 and A-6, recovered during the BIO Hesperides expeditions GEBRA-93 and FRUELA-96 from the Bransfield Strait (Antarctica). Moreover, in order to improve the sedimentation rate control we tentatively relate abundance variations in the sea-ice taxa group (SITG) to air temperature estimations based on Deuterium contents in Vostok ice-core. The results of diatom analyses were related to the sequence of neoglacial events that have occurred over the last three millennia. For these periods, a restricted communication between the Weddell, Bransfield and Bellingshausen seas has been proposed. The abundance patterns of diatom valves, resting spores (RS) of the diatom Chaetoceros and opal content agree with the high productivity values previously reported for the area. The significant reduction Chaetoceros RS towards the present is interpreted as a reduction in surface productivity. Trend differences between Chaetoceros RS and TOC contents are explained in terms of organic matter preservation. Diatom communities from the Bransfield Strait did not play an important role in the global CO2 cycle during cold periods. Bio- and geochemical changes have overprinted high frequency cyclicity at about 200-300 yr, which might be related to the 200-yr solar cycle., [ES] Se han realizado análisis geoquímicos y micropaleontológicos, así como técnicas de datación radiométricas sobre cuatro testigos de gravedad (G-1, G2, A-3 y A-6) recuperados durante las campañas del BIO Hespérides Gebra-93 y Fruela-96 en el Estrecho de Bransfield (Antártida). Se han correlacionado las estimaciones de temperatura del aire, derivadas del contenido en Deuterio en el testigo de hielo Vostok, con la abundancia del grupo de taxones de mar helado (SITG). Los resultados obtenidos del análisis de diatomeas han sido relacionados con la secuencia de eventos neoglaciales que tuvieron lugar durante los últimos 3000 años. Se propone una restricción en la comunicación entre el Mar de Weddell, Estrecho de Bransfield y Mar de Bellingshausen durante los episodios neoglaciales. Los patrones de abundancia de valvas de diatomeas, esporas (RS) de Chaetoceros y el contenido en ópalo biogénico indican valores altos en productividad, previamente descritos para el área. La progresiva reducción en Chaetoceros (RS) hacia techo se ha interpretado como reducción en la productividad superficial. Las diferencias entre la abundancia de Chaetoceros (RS) y el contenido en carbono orgánico (TOC) puede deberse a diferencias en la preservación de la materia orgánica. Por otro lado, los datos analizados indican que las comunidades de diatomeas del estrecho de Bransfield no jugaron un papel determinante en el ciclo global del CO2 durante los episodios fríos. Los cambios bio-geoquímicos están marcados por una ciclicidad de alta frecuencia de 200-300 años que podría estar relacionada con el ciclo solar de 200 años, Funding for this work was generously provided by the Spanish Ministry of Science and Technology for the Ramón y Cajal Research contract 13749957M, and the projects REN2003-09622 and CGL2004-23341-E

13 pages, 10 figures, 5 tables, [EN] During January 1994, the photosynthetic response (P–E curves) of phytoplankton in the eastern part of the Bransfield Strait (Antarctica) was studied in relation to the mesoscale hydrographic structures in the area. The most important hydrographic features found in the study area were: (i) the Bransfield Strait front which separates surface Bellingshausen waters from surface Weddell Sea water in the northern part of the Strait; (ii) further to the North, the Weddell–Scotia Confluence north of Elephant Island; (iii) to the South, the ice-edge and the associated lenses of melting waters at the south-eastern part of the sampling area. These three structures were associated with zones of shallower mixing depth (1 mg Chl m−3). The photosynthetic response of the phytoplankton was also affected by the hydrographic structures. The phytoplankton of the well-mixed Weddell waters showed a lower light saturation parameter (Ek100 μmol m−2 s−1). There were no differences in the light saturation parameters of phytoplankton samples from the different water bodies found in the region as demonstrated by a t-test for paired comparisons (0.88>P>0.46). The average Ek was 87±26 μmol m−2 s−1 and not significantly different (t-test NS, P=0.83) from the mean irradiance in the upper mixed layer (Zuml) without the Zuml≥150 m stations. The slope of the PmB vs. α relationship was 59±5 μmol m−2 s−1 (r2=0.66, P1.5 g C m−2 d−1 between King George Island and Elephant Island, [FR] Pendant la campagne en Antarctique du navire BIO Hesperides, effectuée en janvier 1994, dans la partie est du détroit de Bransfield (Antarctique), la réponse photosynthétique du phytoplancton (courbes P–E) a été etudiée en relation avec les structures hydrographiques à mésoéchelle correspondantes. Pendant cette période, trois structures hydrographiques principales ont été déterminées: 1) Le front séparant les eaux de Bellingshausen de celles de la mer de Weddell entre 62°30′S–56°30′W et 62°S–53°W, en direction SE–NE; 2) La Confluence Weddell–Scotia au nord de la région; 3) La zone marginale de la glace, avec des nappes superficielles d'eaux de fonte, à la partie sud-est de la zone de travail. Ces structures étaient associées à des zones de mélange superficiel de faible épaisseur (1 mg Chl m−3). La réponse photosynthétique du phytoplancton était également affectée par les conditions hydrographiques. Dans les eaux de la Mer de Weddell profondément mélangées, le phytoplancton présentait un paramètre de saturation de lumière plus bas (Ek100 μmol m−2 s−1). Il n'y avait pas de différences significatives entre les valeurs des paramètres de saturation de lumière des échantillons de phytoplancton provenant de différentes masses d'eau, comme on l'a demontré par la comparaison t-test des paires d'échantillons (0.88>P>0.46). Le EK moyen était 87±26 μmol m−2 s−1 et il n'avait pas de différences significatives (t-test NS, P=0.83) avec l'irradiance moyenne de la couche de mélange supérieure (Zuml), sans les stations avec Zuml≥150 m. La pente de la relation PmB vs. α (r2=0.66, P1.5 g C m−2 j−1 entre les Îles du Roi George et l'Île de l'Éléphant

Rogers, Alex D. ... et. al.-- 17 pages, 6 figures, 2 tables, supporting information in https://doi.org/10.1371/journal.pbio.1001234, Since the first discovery of deep-sea hydrothermal vents along the Galápagos Rift in 1977, numerous vent sites and endemic faunal assemblages have been found along mid-ocean ridges and back-arc basins at low to mid latitudes. These discoveries have suggested the existence of separate biogeographic provinces in the Atlantic and the North West Pacific, the existence of a province including the South West Pacific and Indian Ocean, and a separation of the North East Pacific, North East Pacific Rise, and South East Pacific Rise. The Southern Ocean is known to be a region of high deep-sea species diversity and centre of origin for the global deep-sea fauna. It has also been proposed as a gateway connecting hydrothermal vents in different oceans but is little explored because of extreme conditions. Since 2009 we have explored two segments of the East Scotia Ridge (ESR) in the Southern Ocean using a remotely operated vehicle. In each segment we located deep-sea hydrothermal vents hosting high-temperature black smokers up to 382.8°C and diffuse venting. The chemosynthetic ecosystems hosted by these vents are dominated by a new yeti crab (Kiwa n. sp.), stalked barnacles, limpets, peltospiroid gastropods, anemones, and a predatory sea star. Taxa abundant in vent ecosystems in other oceans, including polychaete worms (Siboglinidae), bathymodiolid mussels, and alvinocaridid shrimps, are absent from the ESR vents. These groups, except the Siboglinidae, possess planktotrophic larvae, rare in Antarctic marine invertebrates, suggesting that the environmental conditions of the Southern Ocean may act as a dispersal filter for vent taxa. Evidence from the distinctive fauna, the unique community structure, and multivariate analyses suggest that the Antarctic vent ecosystems represent a new vent biogeographic province. However, multivariate analyses of species present at the ESR and at other deep-sea hydrothermal vents globally indicate that vent biogeography is more complex than previously recognised. © 2012 Rogers et al., The ChEsSo research programme was funded by a NERC Consortium Grant (NE/DO1249X/1) and supported by the Census of Marine Life and the Sloan Foundation, and the Total Foundation for Biodiversity (Abyss 2100)(SVTH) all of which are gratefully acknowledged. We also acknowledge NSF grant ANT-0739675 (CG and TS), NERC PhD studentships NE/D01429X/1(LH, LM, CNR), NE/H524922/1(JH) and NE/F010664/1 (WDKR), a Cusanuswerk doctoral fellowship, and a Lesley & Charles Hilton-Brown Scholarship, University of St. Andrews (PHBS)

Fifty-four specimens of pycnogonids belonging to twenty-two species, eight genera and six families were collected with a rocky dredge during the cruise Gebrap-96 in the central basin of Bransfield Strait, from depths between 647 and 1592 m. The richest station in terms of abundance and biomass was DR6 (south of Livingston Island), which was also the shallowest one; at this relatively shallow depth food is more available than in deeper waters. The families Nymphonidae and Colossendeidae had the same number of specimens (21 specimens; 39% abundance each). The most abundant species were Nymphon villosum and N. proximum. Pallenopsis buphthalmus was collected for only the third time. The collections increased the geographical distribution of three species and the depth range of six species. The volcanic structures sampled were inactive during 1996, since none of the specimens showed signs of hydrothermal phenomena. This collection was typically representative of the west Antarctic benthic zone. En la expedición Gebrap-96 se capturaron 54 picnogónidos pertenecientes a 22 especies, 8 géneros y 6 familias. Se prospectaron fondos del Estrecho de Bransfield, entre 647 y 1592 m con una draga de roca. La estación con más riqueza biológica fue la DR6 (sur de Livingston), la menos profunda de entre las prospectadas; a dicha profundidad hay más disponibilidad alimentícia que en aguas inferiores. Las familias más abundantes (Colossendeidae y Nymphonidae) tienen el mismo número de individuos (21; 39% abundancia cada una). Las especies más abundantes fueron Nymphon proximum y N. villosum, siendo Pallenopsis buphthalmus tercera cita mundial. Tres especies aumentan su distribución geográfica y seis su batimetría. El análisis de la picnogonifauna permite afirmar que las formaciones volcánicas permanecían inactivas en el año del muestreo, siendo esta fauna la típica de la Antártida occidental.

Six events with magnitude between 3 and 5.6 have been analysed based on regional waveforms recorded by the temporal Seismic Experiment in Patagonia and Antarctica seismic broad-band network in the Bransfield Strait and the South Shetland Islands in the period 1997–1998. The source parameters have been retrieved using a robust methodology (INDirect PARametrization) to stabilize the inversion of a limited number of noisy records. This methodology is particularly important in oceanic environments, where the presence of seismic noise and the small number of stations makes it difficult to analyse small magnitude events. The source mechanisms obtained are quite variable but consistent with the active tectonic processes and the complicated structure of the South Shetland Island region.

13 pages, 2 figures, 3 tables, As part of the FRUELA experiment, 11 sediment cores were recovered from the Bellingshausen Sea, the western Bransfield and Gerlache Straits (Antarctic Peninsula) to determine the spatial distribution of organic carbon (OC), biogenic silica (BSi), and nitrogen (N) in the sediment under regions of known irregular primary production rates (PPRs). OC, BSi, and N contents ranged from 0.3% to 1.2%, from 5.7% to 20.4%, and from 0.03% to 0.17%, respectively. Apparent mean sediment accumulation rates (SARs) varied between 0.25 and 3.11 mm year-1 (220 and 1750 g m-2 year-1). Central Gerlache and western Bransfield Straits (the Orleans Canyon) had the highest biogenic constituents contents and sediment accumulation rates, whereas the northern coast off Snow Island (Bellingshausen Sea) showed the lowest values. Central Gerlache Strait, with the highest primary production rates in the study area, was the only region where the OC and BSi contents in the sediment corresponded to the photosynthetic activity in the euphotic zone above it. At the extremes of Gerlache Strait as in western Bransfield Strait, sediment focusing by currents enhanced by depressions in the sea floor contributed to develop regions with high biogenic constituent concentrations, whereas at the Bellingshausen Sea and western Gerlache Strait, winnowing by currents may drift away the biogenic material exported from the euphotic zone. Apparently, bottom relief and currents rather than solely depth determined the modern distribution and accumulation rates of biogenic particulate matter in the study area. Paleoceanographic interpretations of the sedimentary record from Antarctic marginal seas, such as Gerlache and Bransfield Straits, should consider that focusing due to lateral transport and sea floor morphology and the preferential degradation of OC relative to BSi may increase to an important degree the biogenic contents in the sediment, especially the opal. OC and BSi accumulation in Antarctic marginal seas could be as important as the broad continental shelves despite large spatial differences. © 2004 Elsevier B.V. All rights reserved, The Comisión Interministerial de Ciencia y Tecnologı́a of Spain supported this research under contracts ANT94-1010, MAR96-1781-CO2-01, and ANT96-1346-E. This work was also benefited by the CONACYT (México) with the fellowship 92766

International audience; During the expeditions ANT-XV/2 with R/V Polarstern in 1997/98 and NBP 99-04 with R/V IB N.B. Palmer in 1999, the first samples of hydrothermally influenced sediments of Bransfield Strait were obtained at Hook Ridge, a volcanic edifice in the Central Basin of the Strait. The vent sites are characterized by white siliceous crusts on top of the sediment layer and temperatures measured immediately on deck are up to 48.5°C. The shallow depth of these vent sites (1050 m) particularly controls the chemistry of the pore fluids that are enriched in silica and sulfide and show low pH values. Chloride is depleted up to 20% and the calculated hydrothermal endmember concentration is in the range of 1–84 mM. Since other mechanisms for Cl depletion can be ruled out clearly, the composition of this fluid is attributed to phase separation. While the Cl-depleted fluid is emanating at Hook Ridge, a Cl-enriched fluid can be identified in the adjacent King George Basin. Using a p,x diagram the two corresponding endmember concentrations reveal that the phase separation takes place at subcritical conditions (total depth: ∼2500 m), probably along the whole volcanic edifice.

International audience; We carried out a search for hydrothermal vents in the Central Basin of Bransfield Strait, Antarctica. The ZAPS (zero angle photon spectrometer) chemical sensor and instrument package (Oregon State University), OFOS (ocean-floor observation system) camera sled and TVG (TV-grab) (GEOMAR) were used to explore the water column and underlying seafloor. These operations were supplemented with a series of dredges. Hydrothermal plumes over Hook Ridge at the eastern end of the basin are confined to the E ridge crest and SE flank. The plumes are complex and sometimes contain two turbidity maxima one widespread feature centered at 1150 m and a smaller, more localized but broad maximum at 600–800 m. We traced the source of the shallower plume to a sunken crater near the ridge crest using sensors on the ZAPS instrument package. Subsequently two TV-grabs from the crater brought back hot, soupy sediment (42–49°C) overlain by hard, siliceous crusts and underlain by a thick layer of volcanic ash. We also recovered chimney fragments whose texture and mineralogy indicate venting temperatures in excess of 250°C. Native sulfur and Fe-sulfides occur in fractures and porous layers in sediment from throughout the area. Pore water data from the crater site are consistent with venting into a thin sediment layer and indicate phase separation of fluids beneath Hook Ridge. The source of the deeper plumes at Hook Ridge has yet to be located. We also explored a series of three parallel volcanic ridges west of Hook Ridge called Three Sisters. We detected water column anomalies indicative of venting with the ZAPS package and recovered hydrothermal barites and sulfides from Middle Sister. We spent considerable time photographing Middle Sister and Hook Ridge but did not identify classic vent fauna at either location. We either missed small areas with our photography or typical MOR vent fauna are absent at these sites.