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From the BIOPEARL 1 and 2 Programs to the Amundsen and Scotia Seas a large series of Tanaidacea was collected using an epibenthic sled. We carried out a thorough morphological analysis of 169 individuals and provide a description of 14 pseudotanaid species distributed in three genera. Twelve species of the genus Pseudotanais are represented by three morphogroups: “affinis+longisetosus”, “denticulatus+abathagastor”, and “forcipatus”. We provide a redescription of Akanthinotanais gaussi, supplement the definitions of the genera Akanthinotanais and Beksitanais, and transfer Pseudotanais abyssi to the genus Beksitanais. Based on the literature and new data, we summarize knowledge of the Antarctic Pseudotanaidae and discuss this family’s spatial and bathymetric distribution. http://www.zoobank.org/urn:lsid:zoobank.org:pub:1A8F69F7-454D-4CB1-B745-3660AFD3BB82

Funding for this research was provided by the Spanish Ministry of Science and Innovation (grants CTM2014-60451-C2-1/2-P and CTM2017-89711-C2-1/2-P) co-funded by the European Union through FEDER funds. The research was conducted in collaboration with `The Drifters Research Group' of the Royal Holloway University of London (UK), and is also related to the projects CTM 2012-39599-C03, CGL2016-80445-R, CTM2016-75129-C3-1-R and CGL2015-74216-JIN. We thank the commander, officers and crew of the BIO HESPERIDES for their support in obtaining the data, sometimes under severe sea conditions. Jean Sanders reviewed the English style of the manuscript. Rob Larter (BAS, UK) and an anonymous reviewer provided numeroususeful remarks that improved an initial version of the manuscript. Seismic interpretations were made using the IHS Kingdom T software, thanks to the participation of the Instituto Andaluz de Ciencias de la Tierra in the IHS University Grant program., Multibeam bathymetric imagery and acoustic sub-bottom profiles are used to reveal distribution patterns of subsurface sedimentation in Dove Basin (Scotia Sea). The goals of the study are to determine the imprint of the inflow of deep Antarctic water masses from the Weddell Sea into the Scotia Sea, to establish the factors driving the styles of contourite deposition and to discern the relative contribution of alongslope versus downslope processes to the construction of the uppermost late Quaternary sedimentary record in the basin. The most significant morpho-sedimentary features in Dove Basin are linked to contouritic processes and to mass movements. Plastered drifts on the flanks of the basin constitute the most common contouritic deposits. Basement-controlled drifts on top of structural elevations are common along the central ridge, the central basin plain and scattered along the basin flanks. Sheeted drifts occur on top of adjacent banks or are restricted to the deep basin. In contrast, mounded drifts are poorly represented in Dove basin. A laterally extensive contouritic channel runs along the central ridge. Contouritic channels are also identified in the upper parts of the lateral banks and slopes. Numerous slide scars along the upper parts of the slopes evolve downslope into semitransparent lens-shaped bodies, with occasional development of across-slope channels. Semitransparent lenses occur intercalated within stratified deposits in the slopes of the basin, in the central ridge and in the deepest basin plain. The spatial arrangement of contouritic morphologies points to the influence of the water column structure and the basin physiography. In the eastern sub-basin, two different fractions (lower and upper) of Weddell Sea Deep Water (WSDW) leave an imprint on contourite deposits owing to the sloping interface between the two fractions. Contouritic influence is more subdued in the western sub-basin, and limited to the imprint of the lower WSDW. The upper parts of the surrounding banks are under the influence of deep-reaching Circumpolar waters (i.e., Lower Circumpolar Deep Water), which develops both depositional and erosional morphologies. The crosssection V-shaped morphology of the basin and the common occurrence of structural highs drive the predominance of plastered and basement-controlled drifts in the sediment record. The frequent alternation between contourites and downslope gravity-flow deposits is likely due to different processes associated with oversteepening in the basin, such as basement-controlled steep slopes, deformed drifts atop basement elevations, and the development of thick contouritic piles. Dove Basin is an example of a basin without mounded, plastered or mixed hybrid drifts in the transition between the lower slope and the deep basin, because the upper boundary of the deepest water mass —the Weddell Sea Deep Water— flows shallower along the middle slope. This fact underlines the relevance of the position and depth of water masses in shaping the morphology of the feet of slopes along continental margins., Spanish Ministry of Science and Innovation - European Union through FEDER funds CTM2014-60451-C2-1/2-P CTM2017-89711-C2-1/2-P CTM 2012-39599-C03 CGL2016-80445-R CTM2016-75129-C3-1-R CGL2015-74216-JIN

Author(s): Prend, Channing J; Gille, Sarah T; Talley, Lynne D; Mitchell, B Greg; Rosso, Isabella; Mazloff, Matthew R

The Scotia Sea is a productive pelagic ecosystem in the Southern Ocean, which is rapidly changing as a consequence of global warming. Species range shifts are particularly evident, as sub-Antarctic species expand their range from North to South, potentially rearranging the structure of this ecosystem. Thus, studies are needed to determine the current extent of variation in food web structure between these two biogeographic regions of the Scotia Sea and to investigate whether the observed patterns are consistent among depth zones. We compiled a database of 10,888 feeding interactions among 228 pelagic taxa, underpinned by surveys and dietary studies conducted in the Scotia Sea. Network analysis indicated that the Northern Scotia Sea (NSS), relative to the Southern Scotia Sea (SSS), is more complex: with higher species richness (more nodes) and trophic interactions (more links) is more connected overall (greater connectance and linkage density). Moreover, the NSS is characterised by more groups of strongly interacting organisms (greater node clustering) than the SSS, suggesting a higher trophic specialisation of Antarctic compared to sub-Antarctic species. Depth also played a key role in structuring these networks, with higher mean trophic position and more dietary generalism in the mesopelagic and bathypelagic zones relative to the epipelagic zones. This suggests that direct access to primary producers is a key factor influencing the trophic structure of these communities. Our results suggest that under current levels of warming the SSS ecosystem will likely become more connected and less modular, resembling the current structure of the NSS., SEA-DNA, SI

15 pages, 5 figures, 2 tables.-- This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY), Physical and biogeochemical processes in the Southern Ocean are fundamental for modulating global climate. In this context, a process-based understanding of how Antarctic diatoms control primary production and carbon export, and hence global-ocean carbon sequestration, has been identified as a scientific priority. Here we use novel sediment trap observations in combination with a data-assimilative ocean biogeochemistry model (ECCO-Darwin) to understand how environmental conditions trigger diatom ecology in the iron-fertilized southern Scotia Sea. We unravel the role of diatoms assemblage in controlling the biogeochemistry of sinking material escaping from the euphotic zone, and discuss the link between changes in upper-ocean environmental conditions and the composition of settling material exported from the surface to 1,000 m depth from March 2012 to January 2013. The combined analysis of in situ observations and model simulation suggests that an anomalous sea-ice episode in early summer 2012–2013 favored (via restratification due to sea-ice melt) an early massive bloom of Corethron pennatum that rapidly sank to depth. This event drove high biogenic silicon to organic carbon export ratios, while modulating the carbon and nitrogen isotopic signals of sinking organic matter reaching the deep ocean. Our findings highlight the role of diatom ecology in modulating silicon vs. carbon sequestration efficiency, a critical factor for determining the stoichiometric relationship of limiting nutrients in the Southern Ocean, This work was funded by the Spanish Polar Program through the Spanish Research and Innovation (I+D+i) National Plan (grant numbers CTM2009-08287-E/ANT and CTM2011-14056- E/ANT), and supported by the Catalan Government Grups de Recerca Consolidats Grant (2017 SGR 315) and the Internal Research and Technology Development program of the Jet Propulsion Laboratory, California Institute of Technology. DZ was funded by a postdoctoral fellowship (Plan I2C) from Xunta de Galicia (Spain) and performed diatom analysis during her stay at Instituto Português do Mar e da Atmosfera (IPMA), Lisbon, Portugal. Diatom analysis was sponsored by Portuguese national funding through FCT-Fundação para a Ciência e a Tecnologia (UIDB/04326/2020 and DiatBIo PTDC/AAG-GLO/3737/2012 projects). GS was supported by the Deutsche Forschungsgemeinschaft (DFG) in the framework of the priority programme “Antarctic Research with comparative investigations in Arctic ice areas” SPP 1158 by Grant SITAnt (365778379)

International Ocean Discovery Program (IODP) Expedition 382 in the Scotia Sea’s Iceberg Alley recovered among the most continuous and highest resolution stratigraphic records in the Southern Ocean near Antarctica spanning the last 3.3 Myr. Sites drilled in Dove Basin (U1536/U1537) have well-resolved magnetostratigraphy and a strong imprint of orbital forcing in their lithostratigraphy. All magnetic reversals of the last 3.3 Myr are identified, providing a robust age model independent of orbital tuning. During the Pleistocene, alternation of terrigenous versus diatomaceous facies shows power in the eccentricity and obliquity frequencies comparable to the amplitude modulation of benthic δ18O records. This suggests that variations in Dove Basin lithostratigraphy during the Pleistocene reflect a similar history as globally integrated ice volume at these frequencies. However, power in the precession frequencies over the entire ∼3.3 Myr record does not match the amplitude modulation of benthic δ18O records, suggesting Dove Basin contains a unique record at these frequencies. Comparing the position of magnetic reversals relative to local facies changes in Dove Basin and the same magnetic reversals relative to benthic δ18O at North Atlantic IODP Site U1308, we demonstrate Dove Basin facies change at different times than benthic δ18O during intervals between ∼3 and 1 Ma. These differences are consistent with precession phase shifts and suggest climate signals with a Southern Hemisphere summer insolation phase were recorded around Antarctica. If Dove Basin lithology reflects local Antarctic ice volume changes, these signals could represent ice sheet precession-paced variations not captured in benthic δ18O during the 41-kyr world.

Highlights • Core-log-seismic correlation allows to assign ages to the Scotia Sea seismic record. • Major implications are derived on the relation between regional and global events. • The main stratigraphic events are much younger than previously proposed. • Three major phases for the regional oceanography are observed from late Miocene. • These phases appear to be closely linked to the Antarctic Ice Sheet dynamics. Scotia Sea and the Drake Passage is key towards understanding the development of modern oceanic circulation patterns and their implications for ice sheet growth and decay. The sedimentary record of the southern Scotia Sea basins documents the regional tectonic, oceanographic and climatic evolution since the Eocene. However, a lack of accurate age estimations has prevented the calibration of the reconstructed history. The upper sedimentary record of the Scotia Sea was scientifically drilled for the first time in 2019 during International Ocean Discovery Program (IODP) Expedition 382, recovering sediments down to ∼643 and 676 m below sea floor in the Dove and Pirie basins respectively. Here, we report newly acquired high resolution physical properties data and the first accurate age constraints for the seismic sequences of the upper sedimentary record of the Scotia Sea to the late Miocene. The drilled record contains four basin-wide reflectors – Reflector-c, -b, -a and -a' previously estimated to be ∼12.6 Ma, ∼6.4 Ma, ∼3.8 Ma and ∼2.6 Ma, respectively. By extrapolating our new Scotia Sea age model to previous morpho-structural and seismic-stratigraphic analyses of the wider region we found, however, that the four discontinuities drilled are much younger than previously thought. Reflector-c actually formed before 8.4 Ma, Reflector-b at ∼4.5/3.7 Ma, Reflector-a at ∼1.7 Ma, and Reflector-a' at ∼0.4 Ma. Our updated age model of these discontinuities has major implications for their correlation with regional tectonic, oceanographic and cryospheric events. According to our results, the outflow of Antarctic Bottom Water to northern latitudes controlled the Antarctic Circumpolar Current flow from late Miocene. Subsequent variability of the Antarctic ice sheets has influenced the oceanic circulation pattern linked to major global climatic changes during early Pliocene, Mid-Pleistocene and the Marine Isotope Stage 11.

Ona Basin, the westernmost oceanic basin in the southern Scotia Sea, is affected by the opposite flows of Circumpolar Deep Water (CDW) and Weddell Sea Deep Water (WSDW); thus , it represent s a key location for exploring seafloor morphologies influenced by bottom currents. The present study aims to capture the spatial arrangement of recent subsurface contourite features , assuming a latitudinal influence of water masses and the interactions between along - and downslope processes, in order to contribute to the knowledge of regional deepwater flow pathways and to the sedimentary model of small sediment -starved oceanic basins. To this end, the investigation combine s an interpretation of multibeam bathymetry and parametric echo sounder seismic data complemented with hydrological data. The distribution of morpho -sedimentary features in Ona Basin reveals two major domains. The southern margin of the basin can be regarded as a mixed/hybrid system containing abundant sediment drifts with channels and contourite moats and a lateral continuity interrupted by downslope morphologies. In contrast, the northern abyssal setting comprises relatively homogeneous large sheeted drifts with superimposed sediment waves, mounded drifts , and several scattered erosive features, likely reflecting the more distinct influence of deepwater contourite processes. Our work demonstrates that tectonic features in the southern basin control the interaction between deepwater along - and downslope processes, as the westward flow of the WSDW is deflected, channelized , and intensified along its westward route. In the northern region, the study indicates an overall clockwise rotation of the WSDW flow, with the spatial and vertical variability of CDW and WSDW affecting the distribution of bottom -current features around seamounts and/or structural highs. The results underscore the importance of sloping interphases in the water mass vertical structure, the degree of basin confinement, and the influence of local bathymetric elevations in sedimentation models of small sediment -starved oceanic basins., Sí

Large earthquakes in the vicinity of Antarctica have the potential to cause post-seismic deformation on the continent, affecting measurements of displacement and gravity field change from GRACE or those attempting to constrain models of glacial isostatic adjustment.In November 2013 a magnitude 7.7 strike-slip earthquake occurred in the Scotia Sea around 650 km from the northern tip of the Antarctic Peninsula. GPS coordinate time series from the Peninsula region show a change in rate after this event indicating a far-field post-seismic deformation signal is present. At these far-field locations, the effects of fault after-slip are likely negligible and hence we consider the deformation to be due to post-seismic viscoelastic deformation. Here we use a global spherical finite element model to investigate the extent of post-seismic viscoelastic deformation in the northern Antarctic Peninsula. We investigate possible 1D earth models that can fit the GPS data and consider the effect of including a simple 3D earth structure in the region. These results, combined with previous results showing East Antarctica is still deforming following 1998 Mw 8.2 intraplate earthquake, suggest that much of Antarctica is deforming due to recent post-seismic deformation.

We report on three decades of repeat surveys of beached marine debris at two locations in the Scotia Sea, in the Southwest Atlantic sector of the Southern Ocean. Between October 1989 and March 2019 10,112 items of beached debris were recovered from Main Bay, Bird Island, South Georgia in the northern Scotia Sea. The total mass of items (data from 1996 onwards) was 101 kg. Plastic was the most commonly recovered item (97.5% by number; 89% by mass) with the remainder made up of fabric, glass, metal, paper and rubber. Mean mass per item was 0.01 kg and the rate of accumulation was 100 items km−1 month−1. Analyses showed an increase in the number of debris items recovered (5.7 per year) but a decline in mean mass per item, suggesting a trend towards more, smaller items of debris at Bird Island. At Signy Island, South Orkney Islands, located in the southern Scotia Sea and within the Antarctic Treaty area, debris items were collected from three beaches, during the austral summer only, between 1991 and 2019. In total 1304 items with a mass of 268 kg were recovered. Plastic items contributed 84% by number and 80% by mass, with the remainder made up of metal (6% by number; 14% by mass), rubber (4% by number; 3% by mass), fabric, glass and paper (

Long-range atmospheric transport is one of the most important ways in which persistent organic pollutants can be transported from their source to remote and pristine regions. Here, we report the results of the first Argentinian measurements of organochlorine pesticides in the Antarctic region. During a 9665-km track onboard OV ARA Puerto Deseado, within the framework of Argentinian Antarctic Expeditions, air samples were taken using high-volume samplers and analyzed using GC-μECD. HCB, HCHs, and endosulfans were the major organic pollutants found, and a north-south gradient in their concentrations was evident by comparing data from the Argentinian offshore zone to the South Scotia Sea. Fil: Rimondino, Guido Noé. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; Argentina Fil: Pepino, Ana Julieta. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; Argentina Fil: Manetti, Martin Diego. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; Argentina Fil: Olcese, Luis Eduardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; Argentina Fil: Argüello, Gustavo Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; Argentina

The scientific basis for the research program of the dynamics and ecosystem of the Drake Passage region and the Scotia Sea is given. Measurements will be carried out in an interdisciplinary expedition of the Russian Academy of Sciences with the participation of 9 institutes of the Russian Academy of Sciences on the R/V "Akademik Mstislav Keldysh". The basis of the research will be the study of the functioning of the Antarctic marine ecosystem and measurement in the Antarctic Circumpolar Current.

The variability of sedimentation patterns and processes driven by late Quaternary glacial-interglacial paleoclimatic and paleoceanographic changes are investigated in Ona Basin, southwestern Scotia Sea. The interest of this area lies in the fact that the nearby Antarctic Peninsula has recorded extreme climatic variability, and the Drake Passage-Scotia Sea oceanic domain is influenced by two major Southern Ocean water masses, the eastward-flowing Antarctic Circumpolar Current (ACC) and the westward-flowing Weddell Sea Deep Water (WSDW). These goals are achieved through the examination of a grid of very high-resolution sub-bottom profiles and two gravity cores collected in Ona Basin. Multi-proxy data derived from the gravity cores include C-derived ages, descriptions of sedimentary units and diatom assemblages, and continuous logging of physical properties and micro-XRF core scanning. The sub-surface seismic stratigraphy is composed of four seismic units (U4 to U1) with a dominant sub-parallel configuration, with local occurrence of wavy facies and intercalations of transparent seismic facies. Additionally, four sedimentary units were recognized through sediment core analysis from bottom to top: Unit IV is composed of slightly bioturbated diatom-rich mud and silty mud with sparse ice-rafted debris (IRDs); Unit III is composed of gravelly silty to sandy mud with large amounts of IRDs; Unit II mostly contains bioturbated diatomaceous mud; and Unit I is composed of diatom-rich silty to sandy mud. The highest diatom abundances are found in Unit II, whereas highly variable abundances are found in Unit IV. The most common diatoms are Fragilariopsis kerguelensis and Chaetoceros subg. Hyalochaete. Overall, these characteristics document a change in the depositional style from terrigenous during the Last Glacial Maximum (LGM) to hemipelagic sedimentation during the deglaciation. The high-resolution seismic stratigraphy analysis reveals significant fluctuations in the regional bottom-current patterns during the late Quaternary (i.e., after 0.4 Ma) glacial-interglacial cycles. An overall strengthening of the westward-flowing WSDW is postulated in relation to latitudinal displacements of the interphase between the deeper ACC and the WSDW, together with enhanced interactions between along- and downslope processes. In addition, sedimentological, geochemical, and micropaleontological analyses revealed two distinctive phases during the late Pleistocene, in terms of paleoenvironments and paleoceanographic conditions. During the LGM, extensive sea-ice coverage limited biogenic productivity in the ocean. Increased terrigenous input was largely supplied by the westward-flowing WSDW, under a reduced ACC influence due to the northward location of fronts. During deglaciation, the sediment record indicates reduced sea-ice cover and increased open-ocean conditions and surface water productivity, as well as a long-term intensification of the WSDW flow. We postulate bottom-current strengthening was driven by an increased Weddell Sea water export and the southward migration of fronts as a consequence of major retreat of sea ice, enhancing the ACC influence in the southern Ona Basin, and thus, affecting the sloping interphase between the deeper ACC and the WSDW., Funding for this research was provided by the Spanish Ministry of Science and Innovation (Grant CTM2017-89711-C2-1/2-P) co-financed by the European Regional Development Fund (FEDER). We thank the Commander, officers, crew, and scientific staff of the BIO HESPERIDES for their support in obtaining the data, sometimes under severe sea conditions. We also acknowledge the help of Dr. Rocío Márquez Crespo (Scientific Instrumentation Center, University of Granada) for her assistance using the FESEM. We would also thank Prof. David M. Harwood (University of Nebraska, USA) for helping with diatom identification, and Dr. Ignacio López-Cilla and Luis Galán (Geological Survey of Spain - IGME, Spain) for the analytical support. Prof. Rubens Figueira and Dr. Paulo Ferreira (Oceanographic Institute, University of São Paulo, Brazil) provided useful remarks for age assignments of sediment cores. Seismic interpretations were made using Kingdom Suite™ software, thanks to the participation of the Instituto Andaluz de Ciencias de la Tierra in the IHS University Grant program. We are grateful to the Editor-in-Chief Michele Rebesco, Guest Editor Uisdean Nicholson and to three anonymous reviewers for their valuable suggestions that greatly improved the manuscript.

An oceanographic survey around the South Shetland Islands (SSI) and the South Orkney Islands (SOI) was conducted during January 2007 and February 2008, respectively, as part of the United States Antarctic Marine Living Resources (AMLR) program ecosystems surveys. At 27 stations, concentrations of dissolved labile Fe (DFe) and total acid leachable (unfiltered, TaLFe) iron (Fe) were measured in the upper 200 m (including coastal and oceanic waters) to better resolve the factors limiting primary production in these regions. Northwest of the SSI, a region influenced by Drake Passage (DP) waters, mean DFe (∼0.26 nM) and TaLFe (∼1.02 nM) concentrations were the lowest, whereas intermediate concentrations for both DFe and TaLFe were measured in the Bransfield Strait (BS). Around Elephant Island (EI), over and off the continental shelf, Fe concentrations differed between the west and the east margins. DFe and TaLFe concentrations further support the argument that the effect of the Shackleton Transverse Ridge (STR) is a crucial structure affecting both the Fe and the chlorophyll distributions in this region. The waters around the SOI had DFe concentrations higher than those in the SSI, with the area north of the South Scotia Ridge (SSR) (60°S), having the highest DFe (0.54 nM) concentrations and the waters in Powell Basin (PB) having the lowest DFe (1.17 nM) and TaLFe (4.51 nM) and concentrations. These spatial patterns of Fe suggest that there are different Fe inputs from shelf waters near the Antarctic Slope Front (ASF). The overall TaLFe:DFe ratios, used as indicator for understanding the relative distance of Fe sources, were lower around the SOI compared to those in the SSI, suggesting that the Fe source for SOI waters was more distant. The spatial patterns between Fe and chlorophyll-a (Chl-a) concentrations in relation to the hydrography highlight the complexity and variability of the oceanographic processes in the region. These results improve the knowledge on the Fe sources and inputs in the less known SOI waters during the austral summer, and they further support the importance of advective processes from the Fe-rich waters that flow from the eastern margin of the Antarctic Peninsula (AP) into the Weddell-Scotia Confluence (WSC). Copyright © 2019 Sanchez, Reiss, Holm-Hansen, Hewes, Bizsel and Ardelan. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

Myctophids are the most abundant and diverse mesopelagic fishes in the Southern Ocean. They are a conduit of energy between primary consumers and higher marine predators, and between the surface layers and the mesopelagic depths. However, there remain major uncertainties about their ecology, particularly regarding their role in Southern Ocean food webs in waters south of the Antarctic Polar Front, which are often regarded as dominated by Antarctic krill. Limited data on the feeding ecology of myctophids has made it difficult to assess the importance of myctophids as consumers of krill and how they fit in the traditional view of a krill-dominated system (diatom-krill-higher predator). We provide a new assessment of the role of myctophids in Southern Ocean food webs using information from recent trophodynamic studies of myctophids conducted in the Scotia Sea, one of the most productive regions of the Southern Ocean and a region that sustains both major populations of higher predators (sea birds, seals, whales) and important commercial fisheries (krill, toothfish, and mackerel icefish). Collectively, these data show that myctophids have a central role in Southern Ocean food webs as both predators and prey. Large myctophid species are prevalent consumers of krill throughout their distributional range and in different seasons in the Scotia Sea. Moreover, best estimates of both myctophid and higher predator consumption of krill to date indicate that large myctophids are the greatest predators of krill in this region, consuming almost as much krill as do all other vertebrates. Nevertheless, there are several smaller myctophid species that do not eat krill, instead consuming copepods and other small euphausiids. Myctophids therefore link primary producers to higher predators through both krill-dependent and krill-independent trophic pathways, emphasizing their importance in regional food webs. Consequently, trophic pathways involving large myctophids are unlikely to be exempt from the direct consequences of projected redistribution and/or reduction in krill population biomass under scenarios of ocean-warming, whilst trophic pathways involving small myctophids may be more resilient to such effects and become increasingly important to higher trophic levels. Further studies are required to determine the extent to which myctophids can maintain food web stability and sustain higher predator populations during periods of prolonged reductions in krill abundance. Including knowledge of the role of myctophids in Southern Ocean food webs will be important for developing robust projections of the impacts of future change to inform decision making for conservation and management.

Zooplankton faecal pellets (FPs) are important conduits of carbon from the surface to the deep ocean, as shown by their presence in deep-sea sediment traps. Zooplankton themselves are thought to play an important role in the breakdown and reworking of FPs as they sink, whilst processes such as diel vertical migration (DVM) may enhance the supply of carbon to the mesopelagic. However, comparatively little is known about the processes or variability of FP sinking/ transport within the upper mesopelagic and how this relates to deeper ocean export. Profiles of FP type and size, and the contribution made by FPs to mesopelagic carbon flux to a depth of 400 m, were considered. Three contrasting locations in the Scotia Sea were compared, which together reflect the variability in physical regime and productivity encountered across the Southern Ocean. Comparing observed FPs with predictions from the mesozooplankton community, we show that, even at shallow depths, the smallest fraction of FP is under-represented, suggesting rapid remineralisation, incorporation into larger aggregates or reworking into larger FPs, and that the flux is dominated by FPs from larger zooplankton. In contrast to models where POC attenuation rates are set to increase with temperature, we find that FP carbon flux attenuates rapidly in low productivity, colder regions dominated by krill, while remineralisation is deeper in warmer areas where productivity is high and copepods dominate. This emphasises the strong modulation of the zooplankton community on the supply and transfer of FP carbon between the epi- and mesopelagic. Evidence was found to suggest that DVM enhances FP flux across the upper mesopelagic, producing a pulse of fresh, dense material that may support secondary production and heterotrophic respiration in the mesopelagic. This illustrates that variability in flux at short (daily) as well as longer (seasonal) timescales may have important implications for the supply of FP carbon to deeper waters.

The structure of the Antarctic Slope Front (ASF) and the associated Antarctic Slope Current (ASC) on the Scotia Sea side of the Weddell-Scotia Confluence (WSC) is described using data from a hydrographic survey and three 1 year long moorings across the continental slope. The ASC in this region flows westward along isobaths with an annual mean speed of ∼0.2 m s−1, with time variability dominated by the K1 and O1 tidal diurnal constituents, a narrowband oscillation with ∼2-week period attributable to the spring/neap tidal cycle, and seasonal variability. Realistic and idealized high-resolution numerical simulations are used to determine the contribution of tides to the structure of the ASF and the speed of the ASC. Two simulations forced by realistic atmospheric forcing and boundary conditions integrated with and without tidal forcing show that tidal forcing is essential to reproduce the measured ASF/ASC cross-slope structure, the time variability at our moorings, and the reduced stratification within the WSC. Two idealized simulations run with tide-only forcing, one with a homogeneous ocean and the other with initial vertical stratification that is laterally homogeneous, show that tides can generate the ASC and ASF through volume flux convergence along the slope initiated by effects including the Lagrangian component of tidal rectification and mixing at the seabed and in the stratified ocean interior. Climate models that exclude the effects of tides will not correctly represent the ASF and ASC or their influence on the injection of intermediate and dense waters from the WSC to the deep ocean.

The \(M_w=7.6\) earthquake, known as Centenary Earthquake, occurred in the Scotia Sea region near the South Orkney Islands, Laurie Island, where is located the permanent Argentinean Antarctic Base Orcadas, here from 1997 operates a seismographic station ORCD, which has recorded several thousands of aftershocks, the most energetic ones recorded by all the instruments of the Antarctic Seismographic Argentinean-Italian Network (ASAIN). The aftershocks data available at ORCD station, till 60 days following the main shock were compiled. The plot of aftershocks rate with time was found to be oscillatory decay. Then, we inverted regional waveforms from ASAIN and International Federation of Digital Seismograph Networks (FSDN) stations to determine source parameters and source time functions for a set of aftershocks with magnitudes in the range 4.3–5.6 \(m_b\). For the regional inversion we applied a methodology for the determination of the seismic moment tensor by means of full waveform inversion. The results obtained reflect the normal character of the main system fault, characterizing the study area.

Researchers have launched a broad international survey of krill in the Southern Ocean to learn whether the fishing industry is leaving enough for this important crustacean9s natural predators, including whales and penguins. Six vessels will spend nearly 2 months mapping the abundance of krill in and around the Scotia Sea. Researchers will also test wave gliders and other tools for cheaper, more frequent surveys that could improve the management of the fishery. The survey itself won9t be able to reveal how the overall krill population in the Scotia Sea might have changed since the previous large survey, in 2000, but it could help confirm whether current fishing limits are precautionary enough.

A three-dimensional geological model was built to show and analyse a northern sector of the Scotia-Antarctica transform plate boundary. The South Scotia Ridge is a 400km long submerged continental structural high representing the eastern continuation of the Antarctic Peninsula. South Scotia Ridge runs approximately in the E-W direction, separating Scotia Sea Plate from Antarctica Plates. Structures, due to the transform plate margin, are considered to be concentrated inside this continental high. The three-dimensional model, built using seismic profiles and a digital elevation model, is a powerful tool to visualize and help to understand deep geological structures. Maps and profiles, on the contrary, only give a two-dimensional view, and do not show the structure of the continental-oceanic boundary at depth. The model shows that the deformation style of the continental-oceanic boundary, and of the oceanic crust nearby, is related to the left-lateral movement of the main transform fault system. Furthermore, it seems to be connected to the orientation and geometry of the South Scotia Ridge with respect to the homogeneous deformation regime, which affects the entire Scotia Plate. Moving from west to east, the NW-dipping main fault surface becomes almost vertical with a sinistral strike-slip movement in the central sector. To the east, a south-dipping plane decreases its inclination and changes orientation (from E-W to ESE-WNW): here shortening features are visible. The three-dimensional geological model presents a western and central province wherein the continental-oceanic boundary involves the fragmented continental blocks, the continental slope, the oceanic basement and sediments. The sedimentary cover and the oceanic basement are not deformed in the eastern province.

We introduce a new bathymetric compilation of the area around South Georgia in the Southern Ocean. Using a variety of data sources including multi and single-beam swath bathymetry we have constructed a gridded bathymetric dataset of the shelf and near-shelf sea-floor areas. The grid has been constructed using a layered hierarchy dependent upon accuracy of each dataset. The spikes and errors have been checked both manually and with a novel semi-automated process. We discuss the resulting bathymetry and the potential uses of the new dataset.

The northward outflow of cold, dense water from the Weddell Sea into the world ocean basins plays a key role in balancing the global heat budget. We estimate the geostrophic flow patterns in the northwestern Weddell Sea using box inverse methods applied to quasi-synoptic hydrographic data collected during the Brazilian DOVETAIL 2000 and 2001 austral summer cruises. The analysis is focused on the variations of the deep Weddell Sea outflow into the Scotia Sea within boxes that bound the main deep gaps over the South Scotia Ridge. To determine the geostrophic volume transports in each box, mass, salt, and heat are conserved within neutral density layers that are not in contact with the atmosphere. Implementing the inverse model and using property anomaly equations weighted by the flow estimate uncertainty our results are consistent with those reported in the literature. A bottom triangle extrapolation method is introduced, which improves the estimated property fluxes through hydrographic sections. In the austral summer of 2000 the transports of Weddell Sea Deep Water (WSDW) through the Philip Passage, Orkney Passage, and southwestern Bruce Passage are 0.01 7 0.01, 1.15 7 0.33, and 1.03 7 0.23 Sv (1 Sv ¼ 10 6 m 3 s 1 , 4 0 is northward), respectively. After extrapolation within bottom triangles these transports increase to 0.12 7 0.03, 3.48 7 1.81, and 1.20 7 2.16 Sv. Analysis of the hydrographic data reveal distinct oceanographic conditions over the Philip Passage region, with evidence of mesoscale meanders, warmer and saltier Warm Deep Water (WDW) and colder WSDW observed in 2001 than in 2000. Despite these differences the WSDW transport does not present a significant variation between 2000 and 2001. The WSDW transports through the Philip Passage in 2001 are 0.012 7 0.001 and 0.113 7 0.001 Sv after extrapolation within bottom triangles. The circulation derived from the inversion in the austral summer of 2001 suggests a sharp weakening of the barotropic cyclonic flow in the Powell Basin, which may be due to northerly and northeasterly winds associated with an atmospheric low-pressure center located west of the Antarctic Peninsula. We suggest that similar variations in atmospheric forcing may explain changes in the intensity of the cyclonic flow observed in the northwestern Weddell Sea and Powell Basin. Fil: Franco, Barbara Cristie. Universidade Federal do Rio Grande do Sul; Brasil Fil: Mata, Mauricio M.. Universidade Federal do Rio Grande do Sul; Brasil Fil: Piola, Alberto Ricardo. Universidad de Buenos Aires; Argentina. Ministerio de Defensa. Armada Argentina. Servicio de Hidrografía Naval; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Garcia, Carlos Alberto. Universidade Federal do Rio Grande do Sul; Brasil

Species richness of the shallow-water bivalves from the Scotia Arc islands was studied on the basis of new collections and by reviewing extant information. Seventy-three species are recognised from the entire area. South Georgia, the South Orkney Islands and the South Shetland Islands were similar in species richness to the Antarctic Weddell sector. New records for 51 bivalve species are provided and the presence of 18 undescribed species is reported. The faunistic similarity of the islands of the Scotia Arc to the Magellan region and the Antarctic Weddell sector is re-examined. These islands show a high similarity to the Antarctic Weddell sector (49 to 85%) and a low similarity to the Magellan region (12 to 32%). Evidence from bivalves clearly supports the placement of the Scotia Arc islands within the Antarctic region. La diversidad de bivalvos que habitan en las aguas someras del Arco de Scotia fue estudiada sobre la base de recientes colecciones desarrolladas en el área y por la revisión de la información preexistente. Un total de 73 especies fueron reconocidas para el área; el número de especies hallado en las islas Georgias del Sur (49), Orcadas del Sur (43) y Shetland del Sur (42) resultó similar al actualmente conocido para el cuadrante antártico de Weddell (41 especies). Se proporcionan nuevas citas para 51 especies de bivalvos y se informa sobre la existencia de 18 especies aún no descriptas. Las similitudes faunísticas de las islas del Arco de Scotia con la región magallánica y con el cuadrante antártico de Weddell fueron reanalizadas. Estas islas mostraron una elevada similitud con el cuadrante antártico de Weddell (49 a 85%, dependiendo del archipiélago) y una baja similitud respecto a la región magallánica (12 a 32%). Las evidencias procedentes de los bivalvos claramente soportan el emplazamiento de las islas del Arco de Scotia dentro de la región antártica.

Earthquake data from the Scotia Arc to early 2002 are reviewed in the light of satellite gravity and other data in order to derive a model for the motion of plates in the Scotia Sea region. Events with magnitude greater than or equal to5, which occurred on or near the boundaries of the Scotia and Sandwich plates, and for which Centroid Moment Tensor (CMT) solutions are available, are examined. The newer data fill some of the previous sampling gaps along the boundaries of the Scotia and Sandwich plates, and provide tighter constraints on relative motions. Variations in the width of the Brunhes anomaly on evenly spaced marine magnetic profiles over the East Scotia Ridge provide new estimates of Scotia-Sandwich plate spreading rates. Since there are no stable fracture zones in the east Scotia Sea, the mean azimuth of sea floor fabric mapped by sidescan is used to constrain the direction of spreading. 18 newrate estimates and four azimuths from the East Scotia Ridge are combined with 68 selected earthquake slip vectors from the boundaries of the Scotia Sea in a least-squares inversion for the best-fitting set of Euler poles and angular rotation rates describing the 'present-day' motions of the Scotia and Sandwich plates relative to South America and Antarctica. Our preferred model (TLP2003) gives poles that are similar to previous estimates, except for Scotia Plate motion with respect to South America, which is significantly different from earlier estimates; predicted rates of motion also differ slightly. Our results are much more robust than earlier work. We examine the implications of the model for motion and deformation along the various plate boundaries, with particular reference to the North and South Scotia Ridges, where rates are obtained by closure.

Marine free-living nematode communities were studied at similar depths (similar to 500 m) at two sides of the Antarctic Peninsula, characterised by different environmental and oceanographic conditions. At the Weddell Sea side, benthic communities are influenced by cold deep-water formation and seasonal sea-ice conditions, whereas the Drake Passage side experiences milder oceanic conditions and strong dynamics of the Antarctic Circumpolar Current. This resulted in different surface primary productivity, which contrasted with observed benthic pigment patterns and varied according to the area studied: chlorophyll a concentrations (as a proxy for primary production) were high in the Weddell Sea sediments, but low in the surface waters above; this pattern was reversed in the Drake Passage. Differences between areas were largely mirrored by the nematode communities: nematode densities peaked in Weddell stations and showed deeper vertical occurrence in the sediment, associated with deeper penetration of chlorophyll a and indicative of a strong benthopelagic coupling. Generic composition showed some similarities across both areas, though differences in the relative contribution of certain genera were noted, together with distinct community shifts with depth in the sediment at all locations.

The major structural and tectonic features of the Fuegian Andes provide an outstanding onshore geological framework that aids in the understanding of the tectonic evolution of the Scotia Arc, mainly known from offshore studies. The orogenic history of the Fuegian Andes (Late Cretaceous-Miocene) is thus compared and integrated with the tectonic history of the Scotia Sea. Late Cretaceous-Paleocene structures in the Fuegian Andes suggest a N-directed contraction consistent with an oroclinal bending of the southernmost South America-Antarctic Peninsula continental bridge. This N-directed contraction in the Fuegian Andes continued during the spreading of the West Scotia Ridge, between 40-50 and 10 Ma ago. The onset of major strike-slip faulting in Tierra del Fuego is here considered to be not older than the late Miocene, consistent with the recent history of the North Scotia Ridge; thus forming part of a tectonic regime superposed to the prior contraction in the Fuegian Andes. Fil: Torres Carbonell, Pablo Juan. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Austral de Investigaciones Científicas; Argentina Fil: Dimieri, Luis Vicente. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto Geológico del Sur; Argentina Fil: Olivero, Eduardo Bernardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Austral de Investigaciones Científicas; Argentina Fil: Bohoyo, Fernando. Instituto Geológico y Minero de España; España Fil: Galindo Zaldívar, Jesús. Universidad de Granada; España

Sedimentary processes in small, isolated oceanic basins that form adjacent to continental margins but detached from continents remain poorly understood. This work describes two such basins located in the southern Scotia Sea, the Protector and Pirie basins. We analysed multichannel seismic profiles to interpret morphostructural features and stratigraphy of these basins. Sedimentary stacking patterns and depocentre distribution illustrate basin development patterns. Basal units infill basement depressions formed by the submerged banks of thinned continental crust that abut the basin plains. These lower and middle deposits of the sedimentary record are interpreted as pre- and syn-rift deposits. The laterally extensive upper deposits are interpreted as post-rift deposits. These include five discrete units evident in seismic profiles. A prominent regional reflection referred to as Reflector-c, separates in these upper deposits two sets of seismic units that have recorded major shifts in the dominant sedimentary processes, stacking patterns and paleo-environmental conditions. The most important processes controlling deposition of the older units (those beneath Reflector-c), include down-slope gravity processes that infill depressions created by crustal thinning and seafloor spreading. These occurred under the coeval influence of Circumpolar Deep Water circulation. The major processes influencing younger units (those above Reflector-c) include bottom water circulation of the Circumpolar Deep Water and Weddell Sea Deep Water water masses,which coursed along bathymetric contours of the seafloor. The Reflector-c discontinuity developed concurrently with middle Miocene tectonic changes, which led to the opening and deepening of deep gateways in the South Scotia Ridge. These facilitated overflow of Weddell Sea Deep Water from the Weddell Gyre into the Scotia Sea. This overflow in turn forced the Circumpolar Deep Water northwards. Analysis of the Protector and Pirie basins shows that their tectonic evolution influenced regional deepwater circulation patterns in an area that makes a significant coldwater contribution to the global conveyor belt system. As a long-termfactor controlling basin evolution and sedimentary processes, tectonics events in this region therefore influenced the present day climate system. These results further clarify our understanding of deep, isolated oceanic basins in terms their sedimentologic, climatologic and oceanographic significance., SI

This paper summarizes the results of a taxonomic study of the isopods collected during the Antártida 1986-11 expedition. A total of 123 specimens were studied, of which 99 were in the adult or juvenile phases and 24 were in the postmanca phase. The specimens were attributed to 11 species, 9 genera, 5 families and 4 suborders. The most abundant species were Glyptonotus antarcticus and Ceratoserolis trilobitoides. Several records are reported for the first time from the archipelagos of the Scotia Arc. En este trabajo se dan a conocer los resultados del estudio de la colección de isópodos recolectados durante la campaña “Antártida 1986-11”. Se han estudiado un total de 123 ejemplares, de los cuales 99 se encuentran en estado adulto o juvenil y 24 en estado postmanca. Los ejemplares se reparten en 11 especies, pertenecientes a 9 géneros, 5 familias y 4 subórdenes. Las especies más abundantes son Glyptonotus antarcticus y Ceratoserolis trilobitoides. Se dan diversas primeras citas para los diferentes archipiélagos del Arco de Escocia.

A well-marked volcanic ash layer was found at 145.9 m depth in a 154.3 m ice core recovered in 1981 on James Ross Island, Antarctic Peninsula. Ash composition analysis indicates that we are dealing with an eruption of the Deception Island volcano located some 200 km northwestward from James Ross Island. Regional lake sediments seem also to have recorded the eruption. Its date (some 350 years ago) has been determined precisely by comparison with a well-dated South Pole Station ice core. The strength and regional to global impact of the event are discussed. It is concluded it was most probably the largest eruption of this volcano over the studied time period. This exceptional layer was used to adjust the dating of the core.

Resume Differentes formes de Nothofagidites des types fusca et brassii ont ete identifiees dans l'ile du Roi Georges (iles Shetland du Sud) dans des sediments paleogenes. Elles sont associees a une microflore riche en fougeres, Podocarpaceae, Araucariaceae, Proteaceae, et Gunneraceae. L'etude comparative entre les pollens actuels et fossiles (microscope optique et a balayage) a permis d'etablir des affinites botaniques avec quelques especes sudamericaines. Cependant les Nothofagidites de l'ile Roi Georges ont l'exine pourvue d'epines tres fortes, semblables a celles de quelques especes du groupe brassii caracterisant le climat chaud et humide de Nouvelle-Guinee et de Nouvelle-Caledonie. Ces donnees completees par l'observation des caracteres anatomiques des bois fossiles etudies dans les memes sites, permettent de penser qu'au Paleogene, les Nothofagus etaient des arbres de climat chaud et humide a alternances de saison seche et de saison de pluie. Ces especes croissaient avec des plantes vivant actuellement en climat subtropical; a partir des glaciations et des evenements volcaniques et tectoniques de la fin du Paleogene, plusieurs especes ont migre et au moins un groupe parmi les Nothofagus s'est adapte aux changements climatiques et se trouve de nos jours dans les forets temperees froides de l'hemisphere sud, en particulier dans l'extreme Sud du Chili. Ainsi la seule presence de Nothofagus dans les sediments du Tertiaire ne doit pas etre utilisee comme critere d'existence d'un climat tempere froid ainsi que cela est souvent propose.

Limnological studies were carried out in the Potter Peninsula, “25 de Mayo Island” (South Shetland Islands ‐ Antarctica) on the chemistry of lentic environments. The relevant chemical characteristics of twenty‐two bodies of water in Potter Creek are described. The marine contribution index (I.A.M.) and Gibbs’ diagrams show that only 50% of dissolved load in these bodies of water can be ascribed to marine contribution. The analysis of factors allows one to explain the first factor as associated to these waters’ chemical characteristics, while the second one is linked to endoproduction. The order obtained by means of discriminant analysis may be explained as resulting from two processes: succession and microsuccession.

Chromophoric dissolved organic matter (CDOM) plays a key role regulating light attenuation in the ocean. This optically reactive pool of organic matter is driven by several physical and biological processes such as photobleaching, photohumification, and biogeneration, that act as primary sinks and sources of CDOM. In this study, we described the geographical and vertical distribution of CDOM in the Antarctic Peninsula area (Southern Ocean), and assessed its potential driving factors, with special emphasis on CDOM photoreactivity. CDOM values were between the detection limit and 2.17 m- 1 at 325 nm and between the detection limit and 0.76 m- 1 at 443 nm (average a325 = 0.36 ± 0.02 m- 1, average a443 = 0.11 ± 0.01 m- 1), with the highest values inside Deception Island in 2004, and the lowest in the Eastern Bransfield Strait. In Bellingshausen Sea, CDOM was higher below the mixed layer suggesting a significant role of photobleaching. By contrast in the Weddell Sea maximum values were found within the mixed layer. In the Weddell Sea, a positive correlation between CDOM and both chlorophyll a and bacterial production and a negative correlation with salinity suggest a biological source of CDOM likely associated to ice melting. Salinity was also negatively related to the spectral slopes from 275 to 295 nm, considered a good proxy for DOM molecular weight. The experimental results demonstrate the photoreactive nature of CDOM, with half lives from 2.1 to 5.1 days due to photobleaching in the upper layer and duplication times from 4.9 to 15.7 days due to photohumification, that highlight the highly dynamic nature of CDOM in this area. © 2009 Elsevier B.V. All rights reserved., This work was funded by the Spanish Ministry of Science and Technology (ICEPOS, REN2002-04165-CO3-02 to CD and DISPAR, CGL2005-00076 to IR).

During the EPOS leg 2 cruise (European Polarstern Study, November 1988–January 1989), the production rate of biogenic silica in the euphotic zone was measured by the 30Si method at stations in the Scotia and Weddell Seas. The highest integrated production rates were observed in the Scotia Sea (range: 11.2–20.6 mmol Si m−2 day−1), the marginal ice zone of the Weddell Sea exhibiting somewhat lower values (range: 6.0–20.0 mmol Si m−2day−1). Our results demonstrate that as far as biogenic silica production is concerned the marginal ice zone of the Weddell Sea is considerably less productive than that of the Ross Sea. Our results also indicate that the water of the Antarctic Circumpolar Current (ACC) could be more productive in late spring and early summer than at the beginning of spring. Possible reasons for the differences among the three subsystems (Ross Sea, Weddell Sea and ACC) are discussed.

Figure 10. Magnetic anomaly distribution in the westernmost spreading corridors of the Scotia Sea, along magnetic profiles IT-41, IT-153, IT-244, and Shack71 (see Figure 1 for location), compared to a model (top of the figure) obtained assuming symmetrical spreading rates on both ridge flanks. Along line IT-153, the bathymetric profile is also displayed. The COB is the continent-ocean boundary determined from seismic data, reported here as line drawing along lines IT-244 and IT-41. TdF, Tierra del Fuego; SSR, South Scotia Ridge. See text for further details. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, B07199, doi:10.1029/2006JB004569, 2006

21 páginas, The photosynthesis-irradiance relationships (P-I curves) of natural plankton samples were studied in the Weddell Sea ice-edge zone, between Elephant Island and South Orkney Islands, during the austral summer of 1988–89. Three water bodies were distinguished in the region: Bellingshausen Sea waters modified after flowing through Drake Passage and Bransfield Strait, Weddell Sea waters and Weddell Sea waters modified by melting. The stations situated in modified Bellingshausen waters showed a net phytoplankton composition which was different from that of the other two water bodies. Weddell Sea waters and Weddell Sea waters modified by melting of sea ice had the same net phytoplankton composition. In the area of modified Weddell Sea waters, there was an accumulation of phytoplankton in the upper 40 m (>4 mg Chl m−1). pB, and α were independent of depth, but varied between stations. The P-I parameters exhibited an east-west gradient with the maximum values in the Elephant Island region, where large diatoms were absent. The light-saturation parameter Ik averaged 35 ± 13 μmol m−2 s−1. The mean irradiance of the upper mixed layer was 34±13 μmol m−2 s−1, thus virtually equal to Ik. These results are discussed in the context of critical depth (Zc, net compensation irradiance (In) and depth of the upper mixed layer (Zuml). We suggest that the phytoplankton in the region is adapted to maximize its carbon uptake and growth rate at the mean irradiance of the upper mixed layer (Iuml) in well-mixed zones. We also suggest that the light-saturation parameter (Ik) could be the irradiance at which photosynthesis approximately compensates the total losses of the community (In).

Three new species of Psolus Oken from Antarctica are described: Psolus byrdae sp. nov. from the Marie Byrd Seamount; Psolus carolineae sp. nov. from South Georgia; Psolus lockhartae sp. nov. from the South Orkney Islands.

A central tenet of Antarctic ecology suggests that increases in Chinstrap Penguin (Pygoscelis antarctica) populations during the last four decades resulted from an increase in prey availability brought on by the decrease in baleen whale stocks. We question this tenet and present evidence to support the hypothesis that these increases are due to a gradual decrease in the frequency of cold years with extensive winter sea ice cover resulting from environmental warming. Supporting data were derived from one of the first, major multidisciplinary winter expedition to the Scotia and Weddell seas; recent satellite images of ocean ice cover; and the analysis of long-term surface temperature records and penguin demography. Our observations indicate there is a need to pay close attention to environmental data in the management of Southern Ocean resources given the complexity of relating biological changes to ecological perturbations.

The nitrogenous nutrition of the phytoplankton in the Scotia Sea was investigated with 15 N tracer techniques during the austral summer of 1979. On a regional scale, ambient NH 4 + concentrations in the upper hundred meters were variable (0.1 to 2.5 μg-at. 1 −1 ) and one or more orders of magnitude lower than ambient NO 3 − concentrations (17 to 31 μg-at. 1 −1 ). Despite the abundance of NO 3 − , late summer phytoplankton showed a consistent preference for NH 4 + utilization relative to NO 3 − . This was determined by use of a relative preference index and the patterns are similar to those of other marine, estuarine, and fresh waters to which it has been applied. The ratio of NO 3 − utilization to total nitrogen utilization indicated that when ambient NH 4 + concentrations were > 1.0 μg-at. 1 −1 , NO 3 − accounted for s40% of the total nitrogen utilized. Thus, even when NO 3 − is present in quantities that approach the upper limits for near-surface open-ocean waters, the processes that recycle NH 4 + locally can make a major contribution to the nutrition of the phytoplankton. Data for NO 3 − uptake from the 0.1% light level at several of the southernmost stations appeared anomalous. The observed values were higher than would have been expected solely from phytoplakton uptake.

Here we synthesise available data and present 22 new chemical analyses of dredged volcanic rocks, recovered by the USNS Eltanin, from a wide variety of morphotectonic settings in the southern oceans. Generally, these samples have undergone the variable affects and styles of postmagmatic alteration. The products of alkali volcanism were recovered from scattered localities in the back-arc environment of the Scotia Sea. Volcanic rocks from the Pacific-Antarctic and Chile Rises mainly consist of quartz and olivine normative ocean-floor basalts similar in composition to those from the East Pacific Rise and Galapagos Spreading Centre. To the north of the Eltanin Fracture Zone, a wider compositional diversity seems to occur, including FeTi-basalts and trachyandesite, than that observed from within and to the south of this fracture zone and on the Chile Rise. Reported values for the ratios K/Rb Rb/Sr La/Yb and Sr87/Sr86 for fresh material, gen'era"lly hint at a derivation of these basalts from a typicall...

Marine magnetic anomalies are used to deduce the history of the opening of Drake Passage, the deep-water channel between South America and West Antarctica. Coherent spreading started in Drake Passage at anomaly 8 time (about 29 Ma), after an earlier, chaotic episode. In the early stages of opening, overlap of two shallow ridges at the Shackleton Fracture Zone confined the submarine connection between Pacific and Atlantic to shelf depths, but the cross-section grew rapidly after the ridge ends cleared at about 23.5 Ma. Reflection profiles from Drake Passage are used to support the existence of these ridges at that time. By comparison with other regions around Antarctica it is shown that the final barrier in an otherwise-complete deep circumpolar path lay at the Shackleton Fracture Zone; subsequently, the gap of smallest cross-section lay either always at the Shackleton Fracture Zone or possibly, for part of the time, in the developing East Scotia Sea. It is concluded that the Antarctic Circumpolar Current started at 23.5 ± 2.5 Ma, a time indistinguishable from the Oligocene—Miocene boundary.

Thirty-three collections of Gymnoscopelus nicholsi were made between 1976 and 1981 in the South Atlantic using both krill and bottom trawls. In the Antarctic waters the main part of the stock consisted of adult fish 3–7 years old, characterized by a benthopelagic mode of life. Fish of year-class I were observed only in the vicinity of South Georgia, South Sandwich Islands and the Scotia Sea; year-class II individuals were absent from Antarctic samples. In the notal zone of the western South Atlantic, fish aged 1–5 years were found. The existence of only one population in the region is a possible explanation of these findings: this population is divided into a pelagic stock of juveniles and sub-adults which occur in the offshore waters of the western South Atlantic and benthopelagic stocks of adults distributed over the slope regions off Argentina, South Georgia and Antarctic archipelagos. The computed values of K (von Bertalanffy growth) were 0.48, 0.41 and estimated total annual instantaneous mortality rates (Z) were 0.45, 0.67, and 0.70 for South Georgia, South Shetlands and notal zone of western South Atlantic stocks, respectively.