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 approx. 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 approx. 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 SO4**2- the age of the most recent MTD is estimated to be <30 years. The mass movement was possibly related to an earthquake in 1988 (approx. 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.

Site GeoB 13804 at the continental slope off Uruguay was investigated with a focus on slope stability. Gravity core GeoB 13804-1, retrieved on 22 May 2009 during cruise M78/3 with the research vessel Meteor, penetrated a mass transport complex with a hummocky surface. Geotechnical parameters (undrained shear strength, porosity, bulk density), the total organic carbon (TOC) concentration, and pore water parameters were measured for the 608 cm long core. Additional lead-210 and pore water data were gained for the multicorer core GeoB 13804-2, retrieved on 24 May 2009. For a Limit Equilibrium slope stability analysis, additional geotechnical data of the gravity cores GeoB 13803-2 (321 cm), GeoB 13808-1 (467 cm), and GeoB 13854-1 (552 cm) were used. These cores were also gained during cruise M78/3.

Supplement to: Henkel, Susann; Strasser, Michael; Schwenk, Tilmann; Hanebuth, Till J J; Hüsener, Johannes; Arnold, Gail Lee; Winkelmann, Daniel; Formolo, Michael J; Tomasini, Juan; Krastel, Sebastian; Kasten, Sabine (2011): An interdisciplinary investigation of a recent submarine mass transport deposit at the continental margin off Uruguay. Geochemistry, Geophysics, Geosystems, 12, Q08009