Seabed pockmarks, traditionally attributed to fluid venting from subsurface sediments, are widespread oceanic features. This study investigates an alternative hypothesis for the formation of these features, particularly focusing on a large number of shallow depressions in the North Sea. Previous research in the German Bight identified a pockmark field north of Heligoland, with thousands of depressions attributed to episodic methane degassing and storm sediment infilling. However, the methane discharge estimations associated with this interpretation seem unlikely given new data. This research employs high-resolution multibeam echosounder data, offering unprecedented centimeter-scale resolution, to re-examine the morphology of these seabed features. The study area, located within the Sylt Outer Reef (SOR) of the German Bight, is characterized by a postglacial sedimentary setting consisting of fine to medium-grained sands with low mud content and strong tidal currents. The integration of hydroacoustic data with information from behavioral biology, physical oceanography, remote sensing, and habitat mapping is used to propose a novel formation mechanism for the observed seabed depressions.

The literature review establishes the existing understanding of seabed pockmarks and alternative formation hypotheses. Geologic pockmarks are traditionally linked to fluid (gas or liquid) venting, creating cone-shaped depressions. Numerous studies have documented pockmarks globally in various marine and lacustrine settings. However, some research has proposed non-fluid-related mechanisms, including erosion and scouring around obstacles and benthic feeding activities of marine mammals. There is controversy regarding the role of fish in pockmark formation. The North Sea, a prolific sedimentary basin, is known for its pockmark fields, mostly interpreted as fluid seepage features. A prior study in the German Bight noted a large pockmark field and attributed its formation to episodic methane events and storm infilling. This study contrasts this interpretation, proposing a biologically driven mechanism. The review also covers existing knowledge of harbor porpoise foraging behaviors, specifically their benthic feeding preferences and the limited observational evidence of their interaction with the seafloor.

The study employed broadband multibeam echosounder (MBES) systems with centimeter-scale resolution during multiple research cruises. Three different MBES systems were used, with varying frequencies (70-100 kHz and 400 kHz) to achieve high-resolution bathymetric data. Modern inertial navigation systems were integrated to improve the accuracy of the data. The acquired data were processed to generate high-resolution bathymetric maps and point clouds. An algorithm was developed for automated mapping of seabed depressions, identifying polygons around regions with significant height changes. The algorithm's outputs underwent visual inspection to remove erroneous features. Sidescan sonar data and sub-bottom profiling were also acquired to assess the subsurface geology and to search for any signs of gas presence. Sediment samples were collected using a grab sampler to analyze grain size distribution. Video footage was obtained using an underwater camera system to examine the seafloor. Finally, existing harbor porpoise density models were utilized to estimate the abundance of these animals in the study area. The analysis considered various factors including water depth, proximity to sandeel habitats, sea surface temperature, and the position of tidal mixing fronts.

High-resolution bathymetric data revealed over 40,000 shallow depressions (pits and pit-scours), with a mean depth of 0.11 m and a mean surface area of 297 m². These pits exhibited irregular, non-circular shapes, unlike typical pockmark morphologies. Detailed analysis of point cloud data showed furrows, concentric highs, and mounds, indicating a complex formation process. Time-lapse bathymetric data revealed that initially small feeding pits served as nuclei for scouring, leading to the formation of larger pit-scours through merging. The study found no evidence of acoustic blanking, bright spots, or flares indicating shallow gas or gas ebullition. The sediment analysis revealed fine to medium-grained sands with low mud content. The pits were exclusively located in fine-grained sand, not in areas with coarser sediments. There was no correlation between pit occurrence and the thickness of Holocene sediments. The absence of clear signs of fluid seepage, combined with high-resolution images of the depressions, contradicted the gas-venting hypothesis. The distribution of pits strongly coincided with sandeel habitats and high harbor porpoise density areas, especially during spring. This suggests a strong correlation between porpoise foraging activity and pit formation.

The findings strongly suggest that the observed seabed depressions are primarily caused by the foraging activities of harbor porpoises. The porpoises' bottom-grubbing behavior, inferred from captive observations and the morphology of the pits, dislodges sandeels, initiating pit formation. Strong tidal currents and storms further shape and modify these pits, resulting in the observed merging and scouring. The absence of evidence for fluid seepage and the high concentration of pits within sandeel habitats reinforce this conclusion. The study's findings have important implications for understanding the interaction between megafauna and the geological processes shaping the seafloor. The significance of macro-bioturbation in influencing sediment transport, nutrient cycling, and ecosystem dynamics deserves further investigation. The ability to identify feeding grounds using high-resolution sonar data provides crucial information for marine environmental protection and sustainable fisheries.

This study provides compelling evidence that millions of seabed depressions in the North Sea, previously misidentified as pockmarks, are likely created by harbor porpoises during benthic foraging. This demonstrates the significant role of megafauna in shaping seafloor morphology and influencing marine ecosystem dynamics. The high-resolution data analysis and the integration of biological and oceanographic factors offered a novel understanding of seabed formation processes. Future research should focus on direct observation of porpoise foraging behavior, further investigation of the role of other megafauna in seabed modification, and the development of broader models to assess macro-bioturbation globally.

The study primarily relies on indirect evidence for the porpoise foraging hypothesis. Direct observation of the porpoises creating the pits is lacking, although captive observations support the bottom-grubbing behavior. The study is limited to a specific region in the North Sea. Further research is needed to confirm the generalizability of these findings to other areas and other megafauna species. The precise mechanisms of pit formation and the relative contributions of porpoise activity versus tidal currents and storms still need further investigation.