Leg 4 of POLARSTERN cruise 29 was dedicated to the eastern Scotia Sea which is part of the antarctic South Atlantic Ocean. Main goal were investigations on fluid and gas circulation of the Sandwich Microplate. Recent investigations have proved hydrothermal and volcanic activities at the backarc spreading center. In contrast active fluid seepage in the forecarc between the island arc and deep sea trench was completely unknown and therefore a target of investigation on this expedition. Besides this, marine geoscientific work south of Falkland and around South Georgia was carried out and provided new insights to shallow gas seepage on the shelf of South Georgia.

The ATLAS HYDROSWEEP DS3 MBES was used for all multibeam survey. It is a nominal 15kHzs deep-water system, with water-column imaging capability, and 2 x 2 degree resolution, protected with a shatterproof window with respect to the icebreaker class of RV POLARSTERN.The system upgrade from generation DS2 to DS3 just happened on this cruise. The new functionality allowed for recording of up to 313 hard-beams and 920 soft-beams, depending on the water depth. The upgrade also contained a new module to investigate the water column online. This so called ATLAS Hydro-Viewer has been installed on a second acquisition computer and is receiving the signal via UDP from the processing unit. The tool is essential when looking for gas flares while mapping the sea floor. We note that despite some initial performance issues, gas flares were found on the first trials of the DS3 on Polarstern.Due to partial bad weather conditions the quality of the bathymetric data set is between fair and good. In addition, residual artefacts due to a rather callow bottom-detection-algorithm and sound velocity issues are limiting the quality and resolution of the grid. Responsible person during this cruise / PI: Paul Wintersteller (pwintersteller@marum.de)Postprocessing and creation of products were conducted by the Seafloor-Imaging group of MARUM, responsible/contact person: Paul Wintersteller (seafloor-imaging@marum.de). The open source software MB-system (Caress, D.W., and D.N. Chayes, MB-System Version 5, Open source software distributed from the MBARI and L-DEO web sites, 2000-2012.) was utilized for this purpose. A tide correction was applied, based on the Oregon State University (OSU) tidal prediction software (OTPS) that is retrievable through MB-System. Due to the fact that to few sound velocity profiles (SVP) were obtained during the cruise, residual artefacts are visible in higher resolutions.Some SVP issues were solved by applying SVP-models based on global databases (e.g. Levitus/WODC) and SVPs from former cruises. The changes in SVP were applied with the mbset function within mbsystem. Motion, lever-arm or offset corrections e.g. for roll, pitch or heading were not necessary since a calibration took place at the beginning of the cruise, guided by an ATLAS technician (Mr. Joern Ewert). Bathymetric data has been manually cleaned for existing artefacts with mbeditviz. NetCDF (GMT) grids of the product and the statistics were created using mbgrid. Quality assurance was furthermore proven by visual and statistical comparison of crossing survey-lines from different cruises throughout the Black Sea. No total propagated uncertainty (TPU) has been calculated to gather vertical or horizontal accuracy.The currently published bathymetric grid of the cruise has a resolution of 200m. A higher resolution is, at least partly, achievable and can be requested from the author.

Supplement to: Graham, Alastair G C; Kuhn, Gerhard; Meisel, Ove; Hillenbrand, Claus-Dieter; Hodgson, Dominic A; Ehrmann, Werner; Wacker, Lukas; Wintersteller, Paul; dos Santos Ferreira, Christian; Römer, Miriam; White, Duanne A; Bohrmann, Gerhard (2017): Major advance of South Georgia glaciers during the Antarctic Cold Reversal following extensive sub-Antarctic glaciation. Nature Communications, 8, 14798