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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

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.

During evolution of the South Sandwich subduction zone, which has consumed South American plate oceanic lithosphere, somehow continental crust of both the South American and Antarctic plates have become incorporated into its upper plate. Continental fragments of both plates are currently separated by small oceanic basins in the upper plate above the South Sandwich subduction zone, in the Scotia Sea region, but how fragments of both continents became incorporated in the same upper plate remains enigmatic. Here we present an updated kinematic reconstruction of the Scotia Sea region using the latest published marine magnetic anomaly constraints, and place this in a South America-Africa-Antarctica plate circuit in which we take intracontinental deformation into account. We show that a change in fracture zone orientation in the Weddell Sea requires that previously inferred initiation of subduction of South American oceanic crust of the northern Weddell below the eastern margin of South Orkney Islands continental crust, then still attached to the Antarctic Peninsula, already occurred around 80 Ma. We propose that subsequently, between ~71-50 Ma, the trench propagated northwards into South America by delamination of South American lithosphere: this resulted in the transfer of delaminated South American continental crust to the overriding plate of the South Sandwich subduction zone. We show continental delamination may have been facilitated by absolute southward motion of South America that was resisted by South Sandwich slab dragging. Pre-drift extension preceding the oceanic Scotia Sea basins led around 50 Ma to opening of the Drake Passage, preconditioning the southern ocean for the Antarctic Circumpolar Current. This 50 Ma extension was concurrent with a strong change in absolute plate motion of the South American Plate that changed from S to WNW, leading to upper plate retreat relative to the more or less mantle stationary South Sandwich Trench that did not partake in the absolute plate motion change. While subduction continued, this mantle-stationary trench setting lasted until ~30 Ma, after which rollback started to contribute to back-arc extension. We find that roll-back and upper plate retreat have contributed more or less equally to the total amount of ~2000 km of extension accommodated in the Scotia Sea basins. We highlight that viewing tectonic motions in a context of absolute plate motion is key for identifying slab motion (e.g. rollback, trench-parallel slab dragging) and consequently mantle-forcing of geological processes.

The Scotia Sea is one of the tectonically most complex and least understood back-arc basins on Earth, which partly results from its remote location making the acquisition of data challenging. Here, we provide a review of current, publicly available geophysical and geological data in the Scotia Sea realm, including magnetic, Bouguer gravity anomaly, high-resolution bathymetric, heat flow and reflection seismic data and rock type data from cored and dredged samples. With this inter-disciplinary data-set, we performed an offshore geological mapping exercise that allowed us to identify lithologies in the predominantly submerged Scotia Sea domain. Cross-sections combining crustal structure and mantle tomography enabled us to address some of the still persisting geological challenges in this tectonically complex area. The data-review revealed that basalt is the dominant lithology in the Scotia Sea area, occupying most of the West and East Scotia Sea (WSS and ESS). Andesitic and more felsic lithologies are identified in the Central Scotia Sea (CSS) and the northern East Scotia Sea (ESS). Mesozoic/Palaeozoic metamorphic/sedimentary lithologies are reported from the highs along the North and South Scotia Ridges (NSR and SSR). These highs originate from a land bridge that, until the late-Mesozoic, connected the South American and Antarctic continents. Scarcely available and contradicting data prevent the age determination of several structural units surrounding and located on the Scotia plate, but our mapping exercise allowed us to confirm the presence of the early Oligocene - late Miocene Ancestral South Sandwich Arc (ASSA) in the east of the CSS, setting the minimum age of the older segment of the CSS crust to Eocene-earliest Oligocene. Three cross-sections cross-cutting the Scotia Sea reveal two high velocity zones, indicating cold mantle material. One is situated below the structural highs along the SSR, which we interpreted as remnant slab material of the ASSA and another below South Georgia and the CSS, which we interpreted as the front of the slab that currently subducts at the South Sandwich Trench (SSaT). The neutral velocity discontinuity below the WSS implies mantle conditions of a recently extinct spreading centre. The upper mantle low velocity anomaly below the CSS is interpreted as warmer toroidal flows around the slab edges of the subducting plate at the SSaT. We have demonstrated that the geological and geophysical data publicly available today allows us to create offshore geological maps in remote, inaccessible offshore domains. This is a less time-consuming, economically advantageous exercise, which re-uses existing geological and geophysical data for a new purpose. It is the most data-inclusive study there is today of the Scotia Sea region and serves as a guideline for future expeditions targeting the CSS and the structural features along the NSR to identify their age and origin. A georeferenced version of the map is provided in the supplementary material.

The Scotia Sea in the South Atlantic holds a prominent position in geodynamics, because it has been proposed as a potential outlet of asthenosphere from under the shrinking Pacific into the mantle beneath the opening Atlantic. Shear wave splitting and geochemical studies have previously tested this hypothesis. Here, we take a different approach by calculating present-day dynamic topography of the region in search for a systematic trend in dynamic topography decreasing from west to east in response to a flow-related pressure gradient in the sublithospheric mantle. To this end, we reconstruct the kinematic history of the Scotia Sea, which is characterized by complex back-arc spreading processes active on a range of time scales. Our plate reconstructions allow us to derive an oceanic age-grid and to calculate the associated residual (dynamically maintained) topography of the Scotia Sea by comparing present-day isostatically corrected topography with that predicted from our reconstruction. The results provide no indication for a systematic trend in dynamic topography and we conclude that the material needed to supply the growing subatlantic mantle must be derived from elsewhere.

Limacina helicina antarctica is a common part of the Southern Ocean zooplankton community but little is known about its life cycle. Here we determine the population structure and standing stock biomass of this species in the Scotia Sea region and use this information to derive estimates for rates of growth, mortality and secondary productivity. Three non-overlapping cohorts were present in the size–frequency distribution, a G2 generation (0-year) with a modal peak at 0.3 mm shell diameter, a G1 generation (1-year) with a modal peak at 2.7 mm and a G generation (2-year) cohort with sizes between 4 and 10 mm. We surmise that at least some L. helicina ant. are capable of living 3 years or more, growing at an average rate of 0.01 mm d−1. Mortality rates were 0.01 d−1 or 3.83 year−1, with 2% of individuals surviving beyond 1 year of age and 0.05% beyond 2 years of age. Standing stock biomass was 178 mg DW m−2 or 32 mg C m−2, divided between 23 mg Corg m−2 and 9 mg Cinorg m−2. The inorganic fraction had a calcium carbonate composition equivalent to 72 mg CaCO3 m−2. Maximum daily productivity during the summer was 1.8 mg C m−2 d−1, made up of 1.3 Corg mg m−2 d−1 and 0.5 mg Cinorg m−2 d−1, and equivalent to 4.2 mg CaCO3 m−2 d−1. The mass specific growth rate (P:B d−1) was 0.06 within the summer period. Growth and production appeared to continue over the autumn and winter months at rates almost equivalent to those in summer. We propose that autumn peaks in sedimenting L. helicina ant., reported by other studies, do not appear to be the consequence of the termination of the life-cycle but are more likely to result from a combination of environmental and behavioural factors.

Mesopelagic fish are a key component of the pelagic ecosystem throughout the world’s oceans. Opening and closing nets were used to investigate patterns in the distribution and abundance of mesopelagic fish from the surface to 1000 m on a series of transects across the Scotia Sea from the ice-edge to the Antarctic Polar Front. A total of 141 non-target net hauls were undertaken during three cruises (Nov

The geology of South Georgia has long been interpreted as the product of processes acting at the Pacific margin of Gondwana in Jurassic and earlier times. This placement requires 1600 km of translation of South Georgia to its present location, for half of which there is no evidence. Assuming this means that 800 km of the translation cannot have occurred, it requires that those processes acted at a location much further east, within the interior of Gondwana. One such location could have been at the southern edge of the Deseado Terrane, which collided and sutured with Gondwana in Carboniferous times. Later, in this location, South Georgia would have experienced shearing and extension during the earliest stages of breakup of the supercontinent, eventually to form part of the ensuing ocean's northern margin. These, or possibly more recent tectonic events on the lengthening northern margin of the Scotia Sea, led to the northward obduction of ophiolitic rocks onto the island. A history like this reduces South Georgia to a peripheral role in the paleoceanographic development of Drake Passage, and strengthens the case for Eocene connection between the Pacific and Atlantic oceans.

Data on the surface distributions of chlorophyll, particulate inorganic carbon, biogenic silica and coccolithophores for a section in the western Scotia Sea (Drake Passage) made in December 2006 are presented. The relationships between PIC, total coccoliths and water-leaving radiance (as measured by MODIS) are largely consistent with published observations for warmer waters. However, for water temperatures

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.

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

La distinction entre differents domaines de l'Ocean Austral, basee sur l'etude des rapports elementaires nitrate: silicate, introduite par Kamykowski et Zentara (Kamykowski, D., Zentara, S.J., 1985. Nitrate and silicic acid in the world ocean: patterns and processes. Mar. Ecol. Prog. Ser. 26, 47-59; et Kamykowski, D., Zentara, S.J., 1989. Circumpolar plant nutrient covariation in the Southern Ocean: patterns and processes. Mar. Ecol. Prog. Ser. 58, 101-111), se revele en bonne conformite avec nos calculs concernant les rapports d'assimilation de ces nutriments. Les eaux oligotrophes antarctiques presentent en general une consommation prononcee en silicates par rapport aux nitrates ce qui induit un potentiel pour un exces relatif en nitrates. Le regime d'assimilation de l'azote dans ces zones est caracterise par de faibles taux d'absorption des nitrates: les valeurs maximales des vitesses absolues et specifiques ne depassant guere 0.15 μM par jour et 0.005 par heure. Les valeurs du rapport f varient entre 0.39 et 0.86. Cette situation est fortement differente de celle observee dans les zones fertiles en bordure des glaces. Au debut du printemps, ces dernieres sont caracterisees par une augmentation significative, quoique de courte duree, des taux d'absorption des nitrates. Les vitesses absolues maximales sont de l'ordre de 2.18 μM par jour et les vitesses specifiques correspondantes de 0.063 par heure. Pendant cette periode, en plus de conditions physiques optimales pour le developpement du bloom, nous observons une convergence entre l'accumulation d'ammonium et l'accroissement des taux d'absorption des nitrates. Puisque la production d'ammonium dans les eaux de surface reflete l'activite heterotrophique. il est possible que des composes liberes par la degradation de la matiere organique stimulent l'utilisation des nitrates par le phytoplancton. La consommation prononcee en nitrates, qui en resulte, conduit d'autre part a un exces relatif en silicates. Au cours de l'evolution saisonniere, les taux d'absorption de nitrate diminuent toutefois progressivement en relation avec une augmentation de la disponibilite en ammonium (jusqu'a 8% du pool d'azote inorganique). Apres une courte periode d'utilisation intensive, les taux d'absorption de nitrates tendent alors vers des valeurs comparables a celles observees dans les zones oligotrophes.

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.

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.

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.

Pb, Nd and Sr isotope compositions of oceanic basalts have been used to identify recycled components of continent derivation in the mantle. The isotopic compositions of Sr, Nd and Pb, together with U, Pb, Sm, Nd, Rb, and Sr abundances have been determined for back-arc basalt glasses from the Scotia Sea and Parece Vela and West Philippine Basins, in addition to basalts from South Sandwich Islands, Ascension, St. Helena and Tristan da Cunha. Comparisons made between the isotopic compositions of South Sandwich Islands basalts and Atlantic MORB glasses permit the identification of recycled components of continent derivation in the source of the island arc basalts. Recycled Sr of continent derivation is also recognisable in back-arc basalt glasses from the Scotia Sea and Parece Vela and West Philippine Basins. However, contemporary reinjection of material with the isotopic structures similar to those identified as a component of island arc and back-arc regions cannot be the sole or dominant influence on the fine structure observed in MORB glasses from the Atlantic Ocean, nor the isotopic compositions of Tristan da Cunha, St. Helena and Ascension basalts. Recycled materials are likely to have been responsible for the generation of these heterogeneities only if they have been stored in the mantle for periods of time exceeding 109 years.

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.

During the 1981 multidisciplinary study of the eastern Scotia Sea, samples were analyzed for silicate, nitrate, and tritium. Most silicate sections showed a pronounced increase in concentrations from north to south in the surface waters. A frontal-type structure in the temperature and salinity fields in the section just north of the South Orkneys, where water flowing through the Drake Passage meets that from the Weddell Sea, is mirrored in the silicate section. Further downstream the silicate sections show no distinct trends in the deep waters, but the distributions are characteristic of meso-scale eddies. No significant horizontal gradients are evident in the nitrate sections, but a nitrate maximum is present in all sections, generally between 200 and 300 m. Tritium profiles are presented for eight stations on two of the sections. Surface tritium concentrations are low (0.3 to 0.4 TU) with the highest concentrations in the north. There was substatial tritium at two stations on the most western section, indicating up to 50% of the deep water may have come from the surface since the early 1960s. On the section to the east, tritium concentrations dropped rapidly below the temperature minimum, and there was essentially no tritium in deep waters. At most, 10% of the deep waters could have come from the surface since the advent of nuclear testing.

Satellite-fixed magnetic and bathymetric data confirm the presence of a spreading centre in the east Scotia Sea behind the South Sandwich island arc and trench. Spreading is slightly asymmetric, favouring the eastern, island arc side, and the total spreading rate has increased from 5 to 6 cm/yr 4 Ma ago, to 7 to 9 cm/yr at present. The close similarity between this feature and the signs of secondary extension found in the marginal basins of the Western Pacific suggests that the usual Vine-Matthews anomalies may be found there also.