Description: Large amounts of the greenhouse gas methane are released from the seabed to the water column where it may be consumed by aerobic methanotrophic bacteria. This microbial filter is consequently the last marine sink for methane before its liberation to the atmosphere. The size and activity of methanotrophic communities, which determine the capacity of the water column methane filter, are thought to be mainly controlled by nutrient and redox dynamics, but little is known about the effects of ocean currents. Here, we report measurements of methanotrophic activity and biomass (CARD-FISH) at methane seeps west of Svalbard, and related them to physical water mass properties (CTD) and modelled current dynamics. We show that cold bottom water containing a large number of aerobic methanotrophs was rapidly displaced by warmer water with a considerably smaller methanotrophic community. This water mass exchange, caused by short-term variations of the West Spitsbergen Current, constitutes a rapid oceanographic switch severely reducing methanotrophic activity in the water column. Strong and fluctuating currents are widespread oceanographic features common at many methane seep systems and are thus likely to globally affect methane oxidation in the ocean water column.

Description: The interplay between sediment deposition patterns, organic matter type and the quantity and quality of reactive mineral phases determines the accumulation, speciation and isotope composition of pore water and solid phase sulfur constituents in marine sediments. Here, we present the sulfur geochemistry of siliciclastic sediments from two sites along the Argentine continental slope--a system characterized by dynamic deposition and reworking, which result in non-steady state conditions. The two investigated sites have different depositional histories but have in common that reactive iron phases are abundant and that organic matter is refractory--conditions that result in low organoclastic sulfate reduction rates. Deposition of reworked, isotopically light pyrite and sulfurized organic matter appear to be important contributors to the sulfur inventory, with only minor addition of pyrite from organoclastic sulfate reduction above the sulfate-methane transition (SMT). Pore-water sulfide is limited to a narrow zone at the SMT. The core of that zone is dominated by pyrite accumulation. Iron monosulfide and elemental sulfur accumulate above and below this zone. Iron monosulfide precipitation is driven by the reaction of low amounts of hydrogen sulfide with ferrous iron and is in competition with the oxidation of sulfide by iron (oxyhydr)oxides to form elemental sulfur. The intervals marked by precipitation of intermediate sulfur phases at the margin of the zone with free sulfide are bordered by two distinct peaks in total organic sulfur. Organic matter sulfurization appears to precede pyrite formation in the iron-dominated margins of the sulfide zone, potentially linked to the presence of polysulfides formed by reaction between dissolved sulfide and elemental sulfur. Thus, SMTs can be hotspots for organic matter sulfurization in sulfide-limited, reactive iron-rich marine sedimentary systems. Furthermore, existence of elemental sulfur and iron monosulfide phases meters below the SMT demonstrates that in sulfide-limited systems metastable sulfur constituents are not readily converted to pyrite but can be buried to deeper sediment depths. Our data show that in non-steady state systems, redox zones do not occur in sequence but can reappear or proceed in inverse sequence throughout the sediment column, causing similar mineral alteration processes to occur at the same time at different sediment depths.

Description: Methane hydrate is an ice-like substance that is stable at high-pressure and low temperature in continental margin sediments. Since the discovery of a large number of gas flares at the landward termination of the gas hydrate stability zone off Svalbard, there has been concern that warming bottom waters have started to dissociate large amounts of gas hydrate and that the resulting methane release may possibly accelerate global warming. Here, we can corroborate that hydrates play a role in the observed seepage of gas, but we present evidence that seepage off Svalbard has been ongoing for at least three thousand years and that seasonal fluctuations of 1-2°C in the bottom-water temperature cause periodic gas hydrate formation and dissociation, which focus seepage at the observed sites.

Description: Acoustic direct line-of-sight distance measurements offshore Etna volcano by means of a network of five transponders. The data include travel times between all transponders, pressure, temperature (inside pressure tube), sound velocity, and inclination.

Description: The relationship of sea-level changes and short-term climatic changes with turbidite deposition is poorly documented, although the mechanisms of gravity-driven sediment transport in submarine canyons during sea-level changes have been reported from many regions. This study focuses on the activity of the Dakar Canyon off southern Senegal in response to major glacial/interglacial sea-level shifts and variability in the NW-African continental climate. The sedimentary record from the canyon allows us to determine the timing of turbidite events and, on the basis of XRF-scanning element data, we have identified the climate signal at a sub-millennial time scale from the surrounding hemipelagic sediments. Over the late Quaternary the highest frequency in turbidite activity in the Dakar Canyon is confined to major climatic terminations when remobilisation of sediments from the shelf was triggered by the eustatic sea-level rise. However, episodic turbidite events coincide with the timing of Heinrich events in the North Atlantic. During these times continental climate has changed rapidly, with evidence for higher dust supply over NW Africa which has fed turbidity currents. Increased aridity and enhanced wind strength in the southern Saharan-Sahelian zone may have provided a source for this dust.

Description: Mass wasting processes are a common phenomenon along the continental margin of NW-Africa. Located on the high-upwelling regime off the Mauritanian coastline, the Mauritania Slide Complex (MSC) is one of the largest events known on the Atlantic margin with an affected area of ~30000 km**2. Understanding previous failure events as well as its current hazard potential are crucial for risk assessment with respect to offshore installations and tsunamis. We present the results of geotechnical measurements and strain analyses on sediment cores taken from both the stable and the failed part of the MSC and compare them to previously published geophysical and sedimentological data. The material originates from water depths of 1500-3000 m and consists of detached slide deposits separated by undisturbed hemipelagic sediments. While the hemipelagites are characterized by normal consolidation with a downward increase in bulk density and shear strength (from 1.68 to 1.8 g/cm**3, 2-10 kPa), the slid deposits of the uppermost debris flow event preserve constant bulk density values (1.75 and 1.8 g/cm**3) with incisions marking different flow events. These slid sediments comprise three different matrix types, with normal consolidation at the base (OCR = 1.04), strong overconsolidation (OCR = 3.96) in the middle and normal consolidation to slight overconsolidation at the top (OCR = 0.91-1.28). However, the hemipelagic sediments underlying the debris flow units, which have been 14C dated at 〈24 ka BP, show strong to slight underconsolidation (OCR = 0.65-0.79) with low friction coefficients of µ = 0.18. Fabric analyses show deformation intensities R 〉= 4 (ratio Sigma1/Sigma3) in several of the remobilized sediments. Such high deformation is also attested by observed disintegrated clasts from the underlying unit in the youngest debrites (14C-age of 10.5-10.9 ka BP). These clasts show strong consolidation and intense deformation, implying a pre-slide origin and amalgamation into the mass transport deposits. While previous studies propose an emplacement by retrogressive failure for thick slide deposits separated by undisturbed units, our new data on geotechnical properties, strain and age infer at least two different source areas with a sequential failure mechanism as the origin for the different mass wasting events.

Description: On November 18, 1929, a M7.2 earthquake occurred beneath the Laurentian Channel off the coast of Newfoundland. Nearly simultaneously, 12 undersea trans-Atlantic communication cables were severed and within two hours, a devastating tsunami struck the south coast of Newfoundland, claiming 28 lives. Only in 1952, it was understood that a slump-generated turbidity current caused the sequential severance of the cables and likely generated the tsunami. The 1929 Grand Banks events were pivotal, as they led to the first unequivocal recognition of a turbidity current and landslide-triggered tsunami. The landslide site was visited numerous times as underwater survey technologies evolved. No major head scarp related to the event is recognized. The landslide appears to have affected shallow sediments (top 5-100 m) and was laterally extensive. In order to test the hypothesis that a distributed, laterally extensive, shallow submarine mass failure event caused the tsunami, we collected ~ 1500 km of seismic lines in combination with a dense net of hydroacoustic data. A total of ~130 m of gravity cores were recovered at 30 stations. Giant box cores were taken at 15 stations. Three CPT (free-fall cone penetrating testing) transects were collected across landslide scarps. The data in the failure area show abundant small scarps and several young landslide deposits. The existing bathymetric data were slightly expanded to the shelf break but no obvious major scarp was discovered. The combined interpretation of existing and new data will allow estimating the volume of the failed material, which is an important input parameter for tsunami modelling. Another important aspect will be to assess the activity of listric faults in the failure area with special emphasis on their role for the failure dynamics and the triggering of the tsunami. The deposits of the related turbidity current were investigated in a complex channel area downslope of the failure area. Several coring transects will allow to reconstruct the flow lines of the 1929 turbidity current from bypass- dominated to depositional areas. Very coarse gravel was sampled up to 150 m above the canyon thalweg. First estimates suggest high concentrations of sediments in the flow, which was able to run out over 1000s of kilometers. The collected raw data of the surface and shallow subsurface is attached here. A detailed description of the dataset und utilised methods and the overview of track lines can be found in the official cruise report (Krastel et al. 2016, doi:10.2312/cr_msm47).

Description: This study presents a new in situ method to explore the impact of macrofauna on seafloor microtopography and corresponding microroughness based on underwater laser line scanning. The local microtopography was determined with mm-level accuracy at three stations colonised by the tubeworm Lanice conchilega offshore of the island of Sylt in the German Bight (south-eastern North Sea), covering approximately 0.5 m**2 each. Ground truthing was done using underwater video data. Two stations were populated by tubeworm colonies of different population densities, and one station had a hydrodynamically rippled seafloor. Tubeworms caused an increased skewness of the microtopography height distribution and an increased root mean square roughness at short spatial wavelengths compared with hydrodynamic bedforms. Spectral analysis of the 2D Fourier transformed microtopography showed that the roughness magnitude increased at spatial wavelengths between 0.020 and 0.003 m independently of the tubeworm density. This effect was not detected by commonly used 1D roughness profiles but required consideration of the complete spectrum. Overall, the results reveal that new indicator variables for benthic organisms may be developed based on microtopographic data. An example demonstrates the use of local slope and skewness to detect tubeworms in the measured digital elevation model.

Description: Continental shelves represent areas of highest economical and ecological importance. Nevertheless, these sedimentary systems remain poorly understood due to a complex interplay of various factors and processes which results in highly individual construction schemes. Previous studies of sedimentary shelf systems have mainly focused on a limited number of cores, retrieved from Holocene fine-grained depocentres. As such, the relation between shelf architecture and sedimentary history remains largely obscure. Here, we present new data from the NW Iberian shelf comprising shallow-seismic profiles, a large number of sediment cores, and an extended set of radiocarbon dates to reveal the Late Quaternary evolution of a low-accumulation shelf system in detail. On the NW Iberian shelf, three main seismic units are identified. These overly a prominent erosional unconformity on top of the basement. The lowermost Unit 1 is composed of maximal 75-m thick, Late Tertiary to Pleistocene deposits. The youngest sediments of this unit are related to the last glacial sea-level fall. Unit 2 was controlled by the deglacial sea-level rise and shows a maximum thickness of 15 m. Finally, Unit 3 comprises deposits related to the late stage of sea-level rise and the modern sea-level highstand with a thickness of 4 m in mid-shelf position. Two pronounced seismic reflectors separate these main units from each other. Their origin is related to (1) exposure and ravinement processes during lower sea level, and (2) to reworking and re-deposition of coarse sediments during subsequent sea-level rise. According to the sediment core ground-truthing, sediments of the Late Tertiary to Pleistocene unit predominantly display homogenous fine sands with exceptional occurrences of palaeosols that indicate an ancient exposure surface. Fine sands which were deposited in the run of the last sea-level rise show a time-transgressive retrogradational development. The seismic reflectors, bounding the individual units, appear in the cores as 0.1 to 1-m thick deposits consisting either of shell gravels or siliceous coarse sands with gravels. The modern sea-level highstand stage is characterised by zonal deposition of mud forming a mud belt in mid-shelf position, and sediment starvation on outer shelf zones. Radiocarbon ages indicate that this mud belt was the main depocentre for river-supplied fine material on the NW Iberian shelf at least over the past 5.32 ka BP. The initial onset of this depocentre is proposed to be related to a shift in the balance between rate of sea-level rise and amount of terrigenous sediment supply. Various other stratigraphical shelf reconstructions reveal analogies in architecture which indicate that timing and shaping of the individual units on low-accumulation shelves is fundamentally controlled by eustatic sea-level changes. Other factors of local importance such as differential elevation of the basement and the presence of morphological barriers formed by rocky outcrops on the seafloor have additionally modifying influence on the sedimentary processes.