ALBERT

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

Description: There is significant commercial and research interest in the application of sea cucumbers as nutrient recyclers and processors of particulate waste in polyculture or integrated multitrophic aquaculture (IMTA) systems. The following article reviews examples of existing IMTA systems operating with sea cucumbers, and details the role and effect of several sea cucumber species in experimental and pilot IMTA systems worldwide. Historical observations and quantification of impacts of sea cucumber deposit-feeding and locomotion are examined, as is the development and testing of concepts for the application of sea cucumbers in sediment remediation and site recovery. The extension of applied IMTA systems is reported, from basic piloting through to economically viable farming systems operating at commercial scales. The near-global recognition of the ecological and economic value of deposit-feeding sea cucumbers in IMTA applications within existing and developing aquaculture industries is discussed. Predictions and recommendations are offered for optimal development of sea cucumber IMTA globally. Future directions within the industry are indicated, and key areas of ecological, biological and commercial concern are highlighted to be kept in mind and addressed in a precautionary manner as the industry develops.

Description: Northwestern Namibia, at the landfall of the Walvis Ridge, was affected by the Tristan da Cunha mantle plume during continental rupture between Africa and South America, as evidenced by the presence of the Etendeka continental flood basalts. Here we use data from a passive-source seismological network to investigate the upper mantle structure and to elucidate the Cretaceous mantle plume-lithosphere interaction. Receiver functions reveal an interface associated with a negative velocity contrast within the lithosphere at an average depth of 80 km. We interpret this interface as the relic of the lithosphereasthenosphere boundary (LAB) formed during the Mesozoic by interaction of the Tristan da Cunha plume head with the pre-existing lithosphere. The velocity contrast might be explained by stagnated and ‘‘frozen’’ melts beneath an intensively depleted and dehydrated peridotitic mantle. The present-day LAB is poorly visible with converted waves, indicating a gradual impedance contrast. Beneath much of the study area, converted phases of the 410 and 660 km mantle transition zone discontinuities arrive 1.5 s earlier than in the landward plume-unaffected continental interior, suggesting high velocities in the upper mantle caused by a thick lithosphere. This indicates that after lithospheric thinning during continental breakup, the lithosphere has increased in thickness during the last 132 Myr. Thermal cooling of the continental lithosphere alone cannot produce the lithospheric thickness required here. We propose that the remnant plume material, which has a higher seismic velocity than the ambient mantle due to melt depletion and dehydration, significantly contributed to the thickening of the mantle lithosphere.

Description: The Himalaya and the Tibetan Plateau are uplifted by the ongoing northward underthrusting of the Indian continental lithosphere below Tibet resulting in lithospheric stacking. The layered structure of the Tibetan upper mantle is imaged by seismic methods, most detailed with the receiver function method. Tibet is considered as a place where the development of a future craton is currently under way. Here we study the upper mantle from Germany to northern Sweden with seismic S receiver functions and compare the structure below Scandinavia with that below Tibet. Below Proterozoic Scandinavia, we found two low velocity zones on top of each other, separated by a high velocity zone. The top of the upper low velocity zone at about 100km depth extends from Germany to Archaean northern Sweden. It agrees with the lithosphere-asthenosphere boundary (LAB) below Germany and Denmark. Below Sweden it is known as the 8°discontinuity, or as a mid-lithospheric discontinuity (MLD), similar to observations in North America. Seismic tomography places the LAB near 200km in Scandinavia, which is close to the top of our deeper low velocity zone. We also observed the bottom of the asthenosphere (the Lehmann discontinuity) deepening from 180km in Germany to 260km below Sweden. Remnants of old subduction in the upper about 100km below Scandinavia and Finland are known from controlled source seismic experiments and local earthquake studies. Recent tomographic studies indicate delamination of the lithosphere below southern Scandinavia and northern Germany. We are suggesting that the large scale layered structure in the Scandinavian upper mantle may be caused by processes similar to the ongoing lithospheric stacking in Tibet.

Description: Passive continental margins offer the unique opportunity to study the processes involved in continental extension and break up and the role of hot-spot related magmatism. Several geophysical experiments (Seismics, Magnetotellurics and Seismology) were therefore carried out 2011 and 2012 in northern Namibia to image crust and upper mantle at the landfall of the Walvis Ridge. The aim of these studies is to shed light on the present-day structure of this area to understand the dynamics of the breakup of Pangaea and the processes involved. First results of the seismic part of the project show anomalous velocity structures and reflective properties in the mid- and lower crust. The lower crust (onshore) is characterized by an unusual high velocity body which might be associated with magmatic processes of the plume continent interaction. The distribution of the upper mantle wave propagation velocities shows a rather complex, very inhomogeneous pattern. Onshore magnetotelluric (MT) data were acquired at 167 sites in a ~140 km wide and ~260 km long corridor from the Atlantic Ocean through the Kaoko Mobile Belt onto the Congo Craton. The data are generally of excellent quality. A first inspection of magnetotelluric and vertical magnetic transfer functions indicates significant three-dimensional (3-D) structures in the crust and upper mantle, particularly in the Western Kaoko Zone in the vicinity of prominent shear zones. The seismological team operates a passive-source seismic experiment for two years onshore/offshore NW Namibia. The seismic network consists of 28 land-based and 12 ocean-bottom stations covering an area of 400km x 800km. Different seismic methods, such as body wave and surface wave tomography, receiver function, shear wave splitting, etc, will be used to image the seismic anomalies in the upper mantle and to map the thickness of the crust and mantle lithosphere in this ocean-continental transition area. The aim is to find the mantle deformation styles related to the plume-lithosphere interaction along the Walvis Ridge.

Description: The Walvis Ridge is the connecting link between the Tristan da Cunha hotspot and the Etendeka continental flood basalt province. This is an ideal location to get a better understanding for the plume-lithosphere interaction and the breakup of the continents. In the midst of the controversial topic of the likes of plume theory and various deep-Earth processes it is essential to have a clear image of this region to put forward any kind of geodynamical model. Within the frame of the WALPASS project twelve broadband ocean-bottom seismometer (BBOBS) had been deployed off the north-west coast of Namibia. as well as 28 land stations in northwestern Namibia. The seismological net covers an area of approximately 400 x 800 km. Data from eleven months (BBOBS) and up to two years (land stations) are available. We are presenting some preliminary work done on the BBOBS data set. Due to problems with internal power of the instruments, the internal clock drift could be measured at only two stations. We test ambient noise correlation to estimate the clock drift for the remaining 10 seafloor instruments. Furthermore, we will outline following steps of analyses of ambient noise and surface waves.