Description: Submarine landslides cause slope instabilities and might damage sea-floor infrastructure or even generate tsunami waves. Therefore it is of major importance to understand all processes related to gravity-driven mass wasting and slope stability at continental margins. New swath bathymetry and parametric echo-sounding data from the Uruguay shelf and slope north-east of the Rio de la Plata were collected during METEOR Cruise M78/3. These data are investigated in order to understand mass wasting and sedimentary transport processes at the margin off Uruguay. Large amounts of silty suspension freight are delivered from the Rio de la Plata estuary. These sediments are potentially unstable, leading to numerous mass transport units in the working area. Prominent features identified in the study include slide deposits, channels and several prominent escarpment structures. The situation is complicated by intensive slope parallel sediment transport due to strong contour currents. Our data indicate interaction between gravity-driven downslope transport and alongslope sediment transport. Contourite deposits in the headwall areas of mass wasting events suggest widespread weak layers at those deposits. The up to 70m-high headwalls, are focusing contour currents resulting in small alongslope channels. Future work will concentrate on the reconstruction of the geological/sedimentological history of the study area in order assess slope stability and sediment transport in greater detail.

Description: New geophysical acquired during cruise M78 with RV “Meteor” in 2009 reveal a large-scale slope failure complex. Positioned between 1800 and 3300 m water depth, the slope failure affected an area of at least 1200 km〈sup〉2〈/sup〉. The failure is hosted in contouritic deposits. The morphology of the up 70 m high headwalls is underlain by a deeper reflector which we interpret as detachment. Listric faults positioned upslope these headwalls root in this detachment and are precursor of future failure at this location. The detachment correlates with a regional BSR mapped by Uruguayan colleagues. Cores recovered from 3 transects across the failure complex confirm that the acoustic transparent units are debrites. Sedimentological evidence in accordance with hydro-acoustic data indicate that debrites deposited downslope this failure complex are recent features on the slope.

Description: About 90% of the sediments generated by weathering and erosion on land get finally deposited at the ocean margins. The sediment distribution processes and landscape evolution on land are relatively well understood, but comparably little is known about the role and relative importance of marine sediment dynamics in controlling the architectural evolution of ocean margins. Important players include hemi-pelagic settling, down-slope and current-controlled along-slope sediment transport, depositional and post-depositional sedimentary processes (e.g. consolidation and diagenesis), as well as the destabilization of sediment bodies and their erosion. Submarine landslides in this context thus may represent an important sediment transport process, but also a major geo-hazard due to the increasing number of offshore constructions as well as their potential to instantaneously displace large water masses triggering waves in densely populated coastal areas. Here we present first results from a seagoing expedition that aimed at investigating the interaction processes of sediment redistribution, partitioning, deposition and diagenesis from the coast to the deep-sea along the western South-Atlantic passive continental margin. During RV Meteor Cruise M78/3 in May-July 2009 the shelf, slope and rise offshore Argentina and Uruguay have been investigated by means of hydroacoustic and seismic mapping as well as geological sampling with conventional coring tools as well as the new MARUM seafloor drill rig (MeBo) that revealed recovery of geological strata sampled from up to 50m below seafloor. The working area is characterized by a high amount of fluvial input by the Rio de la Plata river. The continental slope is relatively wide and shows average slope gradients between 1 and 2.5 but locally higher slope gradients may occur (〉5). The transition for the continental rise with low slope gradients is found in ~3000 m water depth. The working area is located in a highly dynamic oceanographic regime. Cold Antarctic water masses of the northward flowing Malvina current meet warm water masses of the southward flowing Brazil current in the working area. Various types of sediment instabilities have been imaged in geophysical and core data, documenting particularly the continental slope offshore Uruguay to be locus of frequent submarine landslides. Apart from individual landslides, however, gravitational downslope sediment transport along the continental slope is restricted to the prominent Mar del Plata Canyon and possibly to smaller canyons indentified in the bathymetric data. The location of the canyons might be controlled by tectonics. In contrast, many morphological features (e.g. progradational terraces and slope parallel scarps with scour-geometries) reveal that sediment transport is predominantly influenced/controlled by strong contour bottom currents. This suggests a significant impact of the western boundary currents on the overall architectural evolution of the margin. Future studies using the acquired geophysical, sedimentological, physical property and geochemical data will (i) quantify the relative contribution of gravitational down-slope vs. along-slope processes through time in shaping this ocean margin and how it relates to the global ocean circulation pattern and sea-level change through time, (ii) investigate depositional and post-depositional processes and how they control submarine slope stability and submarine landslide initiation and (iii) explore the interaction and relative contribution of the various processes in controlling margin evolution, sediment dynamics and geohazard off Uruguay and Northern Argentina.

Description: The continental margin off Uruguay and northern Argentina is characterized by high fluvial input by the de la Plata River and a complex oceanographic regime. Here we present first results from RV Meteor Cruise M78/3 of May–July 2009, which overall aimed at investigating sediment transport processes from the coast to the deep sea by means of hydroacoustic and seismic mapping, as well as coring using conventional tools and the new MARUM seafloor drill rig (MeBo). Various mechanisms of sediment instabilities were identified based on geophysical and core data, documenting particularly the continental slope offshore Uruguay to be locus of submarine landsliding. Individual landslides are relatively small with volumes 〈2km3. Gravitational downslope sediment transport also occurs through the prominent Mar del Plata Canyon and several smaller canyons. The canyons originate at a midslope position, and the absence of buried upslope continuations strongly suggests upslope erosion as main process for canyon evolution. Many other morphological features (e.g., slope-parallel scarps with scour geometries) and abundant contourites in a 35-m-long MeBo core reveal that sediment transport and erosion are controlled predominantly by strong contour currents. Despite numerous landslide events, their geohazard potential is considered to be relatively small, because of their small volumes and their occurrence at relatively deep water depths of more than 1,500 m.

Description: 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.

Description: In this study we aim on a reconstruction of mechanisms and kinematics of slope-failure and mass-movement processes along the northeastern slope of Crete in the Hellenic forearc, eastern Mediterranean. Here, subsidence of the forearc basin and the uplift of the island of Crete cause ongoing steepening of the slope in-between. The high level of neotectonic activity in this region is expected to exert a key role in slope-failure development. Newly acquired reflection seismic data from the upper slope region reveal an intact sediment cover while the lower slope is devoid of both intact strata and mass-transport deposits (MTDs). In a mid-slope position, however, we found evidence for a ∼ 4-km³-sized landslide complex that comprises several MTDs from translational transport of coherent sediment bodies over short distances. Morphometric analysis of these MTDs and their source scars indicates that this part of the northeast Cretan slope can be characterized as a cohesive slope. Furthermore, we reconstruct retrogressive development for this complex and determine a critical slope angle for both pre-conditioning of failure and subsequent landslide deposition near source scars. Consequently, data imply that the investigated shallower slope is stable due to low angles in the order of 3°, whereas 5°-inclined mid-slope portions favour both slope destabilization and landslide deposition. The failed mid-slope parts are dominated by sediment truncations from faults almost correlating with the orientation of head- and sidewalls of scars. We suggest that cohesive landslides and MTDs are generated and preserved, respectively, in such critical slope regions. If once generated, cohesive landslides reach the lower slope further downslope that exceeds the threshold gradient for MTD deposition (∼ 5°), they are transported all the way down to the foot of the slope and disintegrate to mass flows. From these observations we suggest that the mass-wasting history of the investigated Cretan slope area over a longer period of time is characterized by repeated sediment erosion and transport into the deeper Cretan Sea basin. The relocation of the critical slope portion in upslope direction and therefore recurrence of mass-wasting events is thereby likely controlled by the progressive steepening of the slope. This mechanism and restriction of sediment failure to narrow, critically-inclined and relocating slope portions likely explains how such an active margin setting can exhibit only scarce findings of MTDs on the slope despite an expected, extensive and widespread mass wasting.

Description: The Nankai Trough off southwest Japan has an [~]1300 yr historical record of great earthquakes, including the most recent, the A.D. 1944 Tonankai (M = 8.2) earthquake. Evaluation of the activity of an individual submarine fault is difficult when only onland observations are available. Submarine core records can pinpoint individual fault activity. Here we present Integrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Zone Experiment data from the shallow portion. IODP Expedition 316 drilled and cored several holes in the shallow portion of the offshore Tonankai earthquake area, including sites in the hanging wall of a margin-dominating splay fault that has previously been interpreted to have ruptured coseismically during megathrust earthquakes. X-ray computed tomography scanning revealed that the uppermost core at one site contains repeated occurrences of mud breccia. Radioisotope dating of the uppermost mud breccia indicates a deposition time consistent with the 1944 Tonankai earthquake, suggesting that the mud-breccia layers result from episodic brecciation caused by seismic shaking. Mud brecciation provides a potential new tool to reconstruct ancient earthquake history in subduction zones.

Description: Seepage of methane-dominated hydrocarbons is heterogeneous in space and time, and trigger mechanisms of episodic seep events are not well constrained. It is generally found that free hydrocarbon gas entering the local gas hydrate stability field in marine sediments is sequestered in gas hydrates. In this manner, gas hydrates can act as a buffer for carbon transport from the sediment into the ocean. However, the efficiency of gas hydrate-bearing sediments for retaining hydrocarbons may be corrupted: Hypothesized mechanisms include critical gas/fluid pressures beneath gas hydrate-bearing sediments, implying that these are susceptible to mechanical failure and subsequent gas release. Although gas hydrates often occur in seismically active regions, e.g., subduction zones, the role of earthquakes as potential triggers of hydrocarbon transport through gas hydrate-bearing sediments has hardly been explored. Based on a recent publication (Fischer et al., 2013), we present geochemical and transport/reaction-modelling data suggesting a substantial increase in upward gas flux and hydrocarbon emission into the water column following a major earthquake that occurred near the study sites in 1945. Calculating the formation time of authigenic barite enrichments identified in two sediment cores obtained from an anticlinal structure called “Nascent Ridge”, we find they formed 38-91 years before sampling, which corresponds well to the time elapsed since the earthquake (62 years). Furthermore, applying a numerical model, we show that the local sulfate/methane transition zone shifted upward by several meters due to the increased methane flux and simulated sulfate profiles very closely match measured ones in a comparable time frame of 50-70 years. We thus propose a causal relation between the earthquake and the amplified gas flux and present reflection seismic data supporting our hypothesis that co-seismic ground shaking induced mechanical fracturing of gas hydrate-bearing sediments creating pathways for free gas to migrate from a shallow reservoir within the gas hydrate stability zone into the water column. Our results imply that free hydrocarbon gas trapped beneath a local gas hydrate seal was mobilized through earthquake-induced mechanical failure and in that way circumvented carbon sequestration within the sediment. These findings lead to conclude that hydrocarbon seepage triggered by earthquakes can play a role for carbon budgets at other seismically active continental margins. The newly identified process presented in our study is conceivable to help interpret data from similar sites.