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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 deep water circulation patterns in an area that makes a significant cold water contribution to the global conveyor belt system. As a long-term factor 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. [ABSTRACT FROM AUTHOR]

New swath bathymetry with multichannel and high resolution seismic profiles shows a variety of contourite drift, sediment wave morphologies, and seismic facies in the central Scotia Sea. The deposits are to be found at the confluence between the two most important bottom current flows in the southern ocean: the eastward flowing Antarctic Circumpolar Current (ACC) and the northward outflow of the Weddell Sea Deep Water (WSDW). The contourite drifts are wedge-like deposits up to 1 km thick, that exhibit aggradational reflectors along axis thinning towards the margins. The contourite drifts occur in areas of weaker flows along the margins of contourite channels and in areas protected by obstacles. The elongate-mounded drifts are best developed along the left-hand margins of channelized bottom current flows, due to the Coriolis force. A contourite fan has a main channel and two distributary channels that expand over a gentle seafloor. The proximal fan exhibits sediment waves with the distal fan incised by furrows. Sediment wave fields are well developed in areas of intensified bottom flows without channels, particularly at the confluence of the ACC and the WSDW. Small sediment waves occur where unidirectional bottom current flows predominate. Sediment waves may develop under the influence of internal waves produced by the interaction of the flows and sea-bottom relief. The stratigraphic sequence above the oceanic crust of Early to Middle Miocene age contains six seismic units separated by major reflectors. All the units were shaped under the influence of strong bottom current flows, although they exhibit distinct seismic facies changes that record the variations of the bottom current pathways over time. The age of the units was calculated based on the age of the oceanic crust and sedimentation rates of deep-sea deposits in the region. The oldest, Units VI–IV, are of Early to Middle Miocene age and developed under the influence of the ACC. They are characterized by a southward progradational pattern of the seismic units and sedimentation rates of 5–8 cm/ky. Unit III, with an estimated Middle Miocene age, evidences the first incursion of WSDW into the central Scotia Sea, when plate movement caused openings in the South Scotia Ridge and allowed the connection with the northern Weddell Sea through Jane Basin and gaps in the ridge. Unit II, estimated to be of Late Miocene to Early Pliocene age, extends over the area and is characterized by internal unconformities. A major unconformity at the base of Unit II records an important reorganization of bottom current flows that may predate the onset of grounded ice sheets on the Antarctic Peninsula shelf. Unit I, of Late Pliocene to Quaternary age, shows intensified bottom currents. The unconformity at the base of Unit I probably predates the onset of major Northern Hemisphere glaciations and the greater expansion of Antarctic ice sheets during the Late Pliocene. The extensive distribution of contourite deposits above the oceanic crust testifies to the long-term production of Antarctic Bottom Water. Cold, deep water was swept northward from the Weddell Gyre, interacting with the ACC, and possibly exerting profound influences on the global circulation system and the onset of major glaciations. [Copyright &y& Elsevier]

The Scotia Arc, situated between South America and Antarctica, is one of the Earth's most important ocean gateways and former land bridges. Understanding its structure and development is critical for the knowledge of tectonic, paleoenvironmental and biological processes in the southern oceans and Antarctica. It extends from the Drake Passage in the west, where the Shackleton Fracture Zone forms a prominent, but discontinuous, bathymetric ridge between the southern South American continent and the northern tip of the Antarctic Peninsula to the active intra-oceanic volcanic arc forming the South Sandwich Island in the east. The tectonic arc comprises the NSR to the north and to the south the South Scotia Ridge, both transcurrent plate margins that respectively include the South Georgia and South Orkney microcontinents. The Scotia and Sandwich tectonic plates form the major basin within these margins. As the basins opened, formation of first shallow sea ways and then deep ocean connections controlled the initiation and development of the Antarctic Circumpolar Current, which is widely thought to have been important in providing the climatic conditions for formation of the polar ice-sheets. The evolution of the Scotia Arc is therefore of global palaeoclimatic significance. The Scotia Arc has been the focus of increasing international research interest. Many recent studies have stressed the links and interactions between the solid Earth, oceanographic, paleoenvironmental and biological processes in the area. This special issue presents new works that summarize significant recent research results and synthesize the current state of knowledge for the Scotia Arc. [ABSTRACT FROM AUTHOR]

The Scan Basin is the easternmost of the southern Scotia Sea basins. It is located in a complex tectonic setting, north of the boundary between the Scotia and Antarctic plates. In addition, Scan Basin is directly to the north of Bruce Passage, which connects the Scotia and Weddell seas through the northward overflow of the Weddell Sea Deep Water (WSDW). Scan Basin has been described as an isolated small oceanic basin, where the fan-shaped abyssal plain is bounded by Bruce and Discovery banks. Sedimentary processes involved in the development of the Scan Basin sediment record during the late Pliocene-Quaternary are discussed here, through the examination of the distribution and morphological and acoustic characteristics of erosional, depositional, gravitational, fluid-escape, and volcanic features in correlation to the geological and oceanographic setting. The abyssal plain reflects the action of high-energy bottom-current circulation along its western side and lower-energy bottom currents along the eastern side. In contrast, Discovery Bank reflects a highly dynamic interplay between bottom-current activity, mass-wasting processes, and processes linked to the water mass interfaces. Variations in the abyssal plain stratigraphy are associated with events that may be related to climate changes since the late Miocene, involving increased bottom-current circulation and variations in the interaction between the WSDW and the Antarctic Circumpolar Current. The Scan Basin sector is one example of a contourite depositional system developed downstream of a deep gateway exit, and its major depositional characteristics allow its classification as a contourite fan. These new findings contribute to clarifying the development of contourite fans in regions of significant bottom-water interchange. [ABSTRACT FROM AUTHOR]

The N–S trending Scan Basin is the easternmost deep basin north of the South Scotia Ridge, which is a geologically complex structural elevation that hosts the strike-slip boundary between the Scotia and Antarctic plates. We characterized the main morpho-structural features of the basin by analyzing the available multichannel seismic reflection profiles. The reconstruction of the seismo-stratigraphy reveals the growth patterns of the Scan Basin. Seismic data and gravity modeling support the interpretation that the basin is mainly floored by oceanic crust, however its northern and southern provinces exhibit different seismic attributes. Stratigraphic calibrations with adjacent regions together with the distribution of sedimentary units indicate that this basin was formed by rifting processes and subsequent spreading accretion from the Oligocene to the Miocene. This age attribution suggests that the Scan Basin might be one of the oldest oceanic basins of the southern Scotia Sea—possibly coeval with the Eocene–Oligocene opening of the Drake Passage. The basin is the most direct connection between the Weddell Sea and the Scotia Sea, whereas the stratigraphic features reveal the occurrence of major paleoceanographic changes. The initial phases of the evolution were influenced by mass-transport and turbidite processes of sediment supply from the nearby continental margins of the eastern tip of the Antarctic Peninsula. From the Middle Miocene to the Present-day, the eastward motion of the basin due to plate tectonic and the connection with the Weddell Sea through gateways enabled instauration of the overflow of Weddell Sea Deep Water (WSDW) into the Scan Basin. The WSDW forced the northward migration of the Circumpolar Deep Water (CDW) and became progressively dominant, controlling depositional patterns. The results that we report here should prove essential for understanding the formation of the Scotia Sea, the beginning of the Scotia Arc fragmentation, and the increasing role played by Weddell Sea/Scotia Sea water-mass exchange. [ABSTRACT FROM AUTHOR]

Based on an extensive dataset including swath bathymetry, chirp sub-bottom profiler (TOPAS) and multichannel seismic reflection profiles obtained during four cruises in the Scotia Sea aboard the R/V Hespérides , we report a variety of seismic and morphological structures related to focused fluid flow in the Scan Basin (southern Scotia Sea) and the central Scotia Sea (Antarctica). We show that both positive-relief (mounds) and negative-relief (craters and elongated depressions) seafloor morphologies are associated with deep seismic chimneys that link the deep source zone to the subsurface structures through a network of fractures that progressively breach sub-horizontal bands of anomalously high-amplitude reflections. Based on the recognition that these bands of reflections generally mimic the seafloor topography and locally cross-cut the stratigraphic seismic reflections, we recognize three different bottom simulating reflectors (BSRs). According to the theoretical model for hydrate and silica diagenesis stability conditions in the central and southern Scotia Sea and the calculations of temperature and seismic polarity for the three BSRs, we infer that BSR-2 and BSR-3 are reflections caused by the transformation between Opal-A/Opal-CT and Opal-CT/Quartz, respectively. We thus postulate that the successive diagenetic fronts were caused by significantly high geothermal gradients during the early-middle Miocene. In contrast, the low temperatures calculated for the depth of the BSR-1 event rule out its diagenetic origin but delineate the base of the gas hydrate stability zone (GHSZ). An evolutionary model is proposed to explain the plumbing system and chimney structures that help the focused flow of gas-rich fluids to migrate into the subsurface. Firstly, the formation of silica transformation zones may have acted as reservoir traps during Neogene times. Secondly, the progressive decrease of heat flow during the late Pliocene and Quaternary favored the development of the networks of polygonal faults forming collapses and downward tapering chimneys. Finally, seafloor mounds are formed as a result of the continuous injection of gas-enriched fluids through these networks of fractures; they are transformed into gas hydrates above the present base of the GHSZ and move upwards by buoyancy drive as they lose density and increase their volume. We present these structures as type cases that might represent highly concentrated hydrates around local seafloor fluid venting structures. Furthermore, they may be one of the most important conduits into the ocean–atmosphere system for deep methane in the Antarctic seafloor. The breach of BSRs influenced by global warming may induce the catastrophic release of greenhouse gases to the ocean–atmosphere system and, in turn, impact on the Earth's evolution. [ABSTRACT FROM AUTHOR]

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 represents 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 combines 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. • Ona Basin margin defined by first time as a mixed gravitational-drift system • The northern basin abyssal plain is mainly influenced by bottom flow processes. • Superimposed sediment waves confirm a clockwise rotation of the WSDW flow. • Drift-and-scour morphologies formed by deep-reaching CDW eddies • New insight for sedimentation models of small, sediment-starved oceanic basins [ABSTRACT FROM AUTHOR]