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Sediment gravity flows are ubiquitous agents of transport, erosion, and deposition across Earth’s surface, including terrestrial debris flows, snow avalanches, and submarine turbidity currents. Sediment gravity flows typically erode material along their path (bulking), which can dramatically increase their size, speed, and run-out distance. Hence, flow bulking is a first-order control on flow evolution and underpins predictive modeling approaches and geohazard assessments. Quantifying bulking in submarine systems is problematic because of their large-scale and inaccessible nature, complex stratigraphy, and poorly understood source areas. Here, we map the deposits and erosive destruction of a giant submarine gravity flow from source to sink. The small initial failure (~1.5 cubic kilometers) entrained over 100 times its starting volume, catastrophically evolving into a giant flow with a total volume of ~162 cubic kilometers and a run-out distance of ~2000 kilometers. Entrainment of mud was the critical fuel, which promoted run-away flow growth and extreme levels of erosion. An initially small underwater avalanche entrained so much sediment along its path downslope that it grew over 100 times in size.

Bottom contact trawling from commercial fishing activity can have profound impacts on the sea floor, as trawling gear can both resuspend the surface sediments and shift sediment to the sides of the gear, forming furrows and mounds. This disturbance can thus have profound impacts on the benthic biogeochemistry, as these surface sediments generally contain the most labile organic matter, and the porewaters can be elevated in dissolved redox-sensitive metals (Fe and Mn). Disturbance can thus mix these Fe- and Mn- rich porewaters with oxygenated bottom waters, which can reoxidize and form particles, potentially making their distribution more heterogeneous and acting as a substrate for sorption processes. As these particulate iron oxy(hydr)oxides and manganese oxides can be reduced by the sulfide produced by microbial sulfate reduction, the distribution of these phases has profound implications for the habitability of surface sediments by modifying sulfide concentrations and related toxicity for higher life. Here, we report on a research endeavor in Fehmarn Belt, an extensively fished region in the Southern Baltic Sea, Germany. Inside of this area, we collected sediment cores from a variety of sites ranging from undisturbed (due to a nearby shipwreck and boulders) to heavily trawled. From these cores we analyzed a suite of porewater parameters (including: dissolved sulfide, Fe, Mn, SO4, nutrients, and 13C-DIC), solid phase parameters (including: Hg, TIC, CNS, reactive Fe, and reactive Mn), as well as rates of sulfate reduction (SRR). Due to the addition of an ultra-short baseline acoustic positioning system (USBL) on our multicorer (MUC), we are able to relate these parameters not just to coarse estimates of areal trawling density, but also obtain a fine (about 1 m) estimate of the MUC location in relation to specific trawl marks. Thus, we are well equipped to broaden our understanding of the impact of bottom contact trawling on benthic biogeochemical element cycling.

Backscatter mosaics based on a multi-frequency multibeam echosounder survey in the continental shelf setting of the North Sea were compared. The uncalibrated backscatter data were recorded with frequencies of 200, 400 and 600 kHz. The results showed that the seafloor appears mostly featureless in acoustic backscatter mosaics derived from 600 kHz data. The same area surveyed with 200 kHz reveals numerous backscatter anomalies with diameters of 10–70 m deviating between −2 dB and +4 dB from the background sediment. Backscatter anomalies were further subdivided based on their frequency-specific texture and were attributed to bioturbation within the sediment and the presence of polychaetes on the seafloor. While low frequencies show the highest overall contrast between different seafloor types, a consideration of all frequencies permits an improved interpretation of subtle seafloor features.

The physical properties of biologically impacted seafloor are poorly known, including the effects of biology on local roughness. We present an in-situ laser line scanning method to determine the impact of macrofauna on seafloor microtopography and roughness. The local microtopography was determined with mm-level accuracy for the sand mason Lanice conchilega offshore of the island of Sylt in the German Bight (south-eastern North Sea). Ground truthing was done using underwater video data. Two stations were populated by tubeworm colonies of different population densities, and one station showed 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, eventually independent of the tubeworm density. This effect was not detected by commonly used 1D roughness profiles but required consideration of the complete spectrum. The microroughness of different blue mussel and oyster shells were determined in the laboratory. The potential impact of roughness on high-frequency acoustic scatter – widely used for marine remote sensing and strongly influenced by seafloor roughness – is discussed based on an extensive multi-frequency multibeam echo sounder dataset recorded in shallow water (< 3m) offshore Sylt Island. The results indicate that new indicator variables for benthic organisms may be developed based on the microroughness of benthic organisms.

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 m2 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.

Agadir Canyon is one of the largest submarine canyons in the World, supplying giant submarine sediment gravity flows to the Agadir Basin and the wider Moroccan Turbidite System. While the Moroccan Turbidite System is extremely well investigated, almost no data from the source region, i.e. the Agadir Canyon, are available. Understanding why some submarine landslides remain as coherent blocks of sediment throughout their passage downslope, while others mix and disintegrate almost immediately after initial failure, is a major scientific challenge, which was addressed in the Agadir Canyon source region during Cruise MSM32. We collected ~ 1500 km of seismic 2D-lines in combination with a dense net of hydroacoustic data. About 1000 km2 of sea floor were imaged during three deployments of TOBI (deep-towed sidescan sonar operated by the National Oceanography Centre Southampton). A total of 186 m of gravity cores and several giant box cores were recovered at more than 50 stations. CTD casts were collected at nine stations including one 13 hour Yo-yo CTD. The new data show that Agadir canyon is the source area of the world's largest submarine sediment flow, which occurred about 60,000 years ago. Up to 160 km3 of sediment was transported to the deep ocean in a single catastrophic event. For the first time, sediment flows of this scale have been tracked along their entire flow pathway. A major landslide area was identified south of Agadir Canyon. Landslide material enters Agadir canyon in about 2500 m water depth; the material is transported as debrite for at least another 200 km down the canyon. Initial data suggest that the last major slide from this source entered Agadir canyon at least 130,000 years ago. Living deep-water corals were recovered from a large mound field north of Agadir canyon. To our knowledge, these are the first living cold water corals recovered off the coast of Morocco (except for the Gulf of Cadiz). They represent an important link between the known cold