Beschreibung: Assessing frequency and extent of mass movement at continental margins is crucial to evaluate risks for offshore constructions and coastal areas. A multidisciplinary approach including geophysical, sedimentological, geotechnical, and geochemical methods was applied to investigate multistage mass transport deposits (MTDs) off Uruguay, on top of which no surficial hemipelagic drape was detected based on echosounder data. Nonsteady state pore water conditions are evidenced by a distinct gradient change in the sulfate (SO42−) profile at 2.8 m depth. A sharp sedimentological contact at 2.43 m coincides with an abrupt downward increase in shear strength from ∼10 to 〉20 kPa. This boundary is interpreted as a paleosurface (and top of an older MTD) that has recently been covered by a sediment package during a younger landslide event. This youngest MTD supposedly originated from an upslope position and carried its initial pore water signature downward. The kink in the SO42− profile ∼35 cm below the sedimentological and geotechnical contact indicates that bioirrigation affected the paleosurface before deposition of the youngest MTD. Based on modeling of the diffusive re-equilibration of SO42− the age of the most recent MTD is estimated to be 〈30 years. The mass movement was possibly related to an earthquake in 1988 (∼70 km southwest of the core location). Probabilistic slope stability back analysis of general landslide structures in the study area reveals that slope failure initiation requires additional ground accelerations. Therefore, we consider the earthquake as a reasonable trigger if additional weakening processes (e.g., erosion by previous retrogressive failure events or excess pore pressures) preconditioned the slope for failure. Our study reveals the necessity of multidisciplinary approaches to accurately recognize and date recent slope failures in complex settings such as the investigated area.

Beschreibung: The area around the Antarctic Peninsula is projected to undergo rapid climatic changes affecting seasonal sea ice cover, water column stratification, terrestrial meltwater run-off, and related nutrient input and thus the conditions for primary production and organic carbon export. The impact of such environmental changes on benthic microbial communities is poorly understood. In this study, we investigated the impact of different sea ice cover and redox conditions on microbial community compositions from 7 different stations (330–450 m water depth) along a 400-mile transect from the eastern shelf of the Antarctic Peninsula to the west of the South Orkney Islands. Two deep stations (3000 m depth) were sampled for comparison. Samples were collected from 6 different intervals down to a depth of 16 cm. The diversity and composition of microbial communities were determined by 16S ribosomal RNA (rRNA) gene sequencing. Redox conditions in sediments with long ice-free periods showed that iron and sulfate reduction are dominant anaerobic pathways for carbon mineralization. In contrast, sediments at a heavily ice-covered station were dominated by the aerobic pathway, which accounted for 〉94 % of the total carbon degradation. Our results reveal that the microbial community composition at the station under heavy ice-cover differs significantly from stations under low ice-cover and tends to cluster separately, suggesting that sea ice cover is the main driver for changes in microbial community composition in the shelf sediments. Further, the frequency of marginal sea ice conditions (here defined as 5-35% sea ice cover) is significantly different between stations (p 0.001) and can explain 5 to 13% of the variation between microbial communities. The bacterial communities at stations under low ice-cover co-varied significantly with TOC content and porewater concentrations of ammonia, dissolved iron, and sulfide. This was reflected in the microbial community composition, where stations with low ice-cover were dominated by Desulfuromonadia, a taxon including many iron and sulfate reducers. At the station with heavy ice-cover, this class showed very low abundances. Our findings demonstrate that the benthic microbial composition and mediated-processes at various sites around the eastern Antarctic Peninsula are regulated by sea ice cover.