Single beam survey data and links to sea-bottom video files off Helgoland

Kelp forests represent a major habitat type in coastal waters worldwide and their structure and distribution is predicted to change due to global warming. Despite their ecological and economical importance, there is still a lack of reliable spatial information on their abundance and distribution. In recent years, various hydroacoustic mapping techniques for sublittoral environments evolved. However, in turbid coastal waters, such as off the island of Helgoland (Germany, North Sea), the kelp vegetation is present in shallow water depths normally excluded from hydroacoustic surveys. In this study, single beam survey data consisting of the two seafloor parameters roughness and hardness were obtained with RoxAnn from water depth between 2 and 18 m. Our primary aim was to reliably detect the kelp forest habitat with different densities and distinguish it from other vegetated zones. Five habitat classes were identified using underwater-video and were applied for classification of acoustic signatures. Subsequently, spatial prediction maps were produced via two classification approaches: Linear discriminant analysis (LDA) and manual classification routine (MC). LDA was able to distinguish dense kelp forest from other habitats (i.e. mixed seaweed vegetation, sand, and barren bedrock), but no variances in kelp density. In contrast, MC also provided information on medium dense kelp distribution which is characterized by intermediate roughness and hardness values evoked by reduced kelp abundances. The prediction maps reach accordance levels of 62% (LDA) and 68% (MC). The presence of vegetation (kelp and mixed seaweed vegetation) was determined with higher prediction abilities of 75% (LDA) and 76% (MC). Since the different habitat classes reveal acoustic signatures that strongly overlap, the manual classification method was more appropriate for separating different kelp forest densities and low-lying vegetation. It became evident that the occurrence of kelp in this area is not simply linked to water depth. Moreover, this study shows that the two seafloor parameters collected with RoxAnn are suitable indicators for the discrimination of different densely vegetated seafloor habitats in shallow environments.

Compression and expansion index and void ratio of samples from Lomonosov Ridge and Yermak Plateau

With the coupled use of multibeam swath bathymetry, high-resolution subbottom profiling and sediment coring from icebreakers in the Arctic Ocean, there is a growing awareness of the prevalence of Quaternary ice-grounding events on many of the topographic highs found in present water depths of <1000 m. In some regions, such as the Lomonosov Ridge and Yermak Plateau, overconsolidated sediments sampled through either drilling or coring are found beneath seismically imaged unconformities of glacigenic origin. However, there exists no comprehensive analysis of the geotechnical properties of these sediments, or how their inferred stress state may be related to different glacigenic processes or types of ice-loading. Here we combine geophysical, stratigraphic and geotechnical measurements from the Lomonosov Ridge and Yermak Plateau and discuss the glacial geological implications of overconsolidated sediments. The degree of overconsolidation, determined from measurements of porosity and shear strength, is shown to result from consolidation and/or deformation below grounded ice and, with the exception of a single region on the Lomonosov Ridge, cannot be explained by erosion of overlying sediments. We demonstrate that the amount and depth of porosity loss associated with a middle Quaternary (~ 790-950 thousand years ago - ka) grounding on the Yermak Plateau is compatible with sediment consolidation under an ice sheet or ice rise. Conversely, geotechnical properties of sediments from beneath late Quaternary ice-groundings in both regions, independently dated to Marine Isotope Stage (MIS) 6, indicate a more transient event commensurate with a passing tabular iceberg calved from an ice shelf.

Fly-through movie for the reconstructed folds in the onset region of the Petermann Glacier, North Greenland

The increasing catalogue of high-quality ice-penetrating radar data provides a unique insight in the internal layering architecture of the Greenland ice sheet. The stratigraphy, an indicator of past deformation, highlights irregularities in ice flow and reveals large perturbations without obvious links to bedrock shape. In this work, to establish a new conceptual model for the formation process, we analysed the radar data at the onset of the Petermann Glacier, North Greenland, and created a three-dimensional model of several distinct stratigraphic layers. We demonstrate that the dominant structures are cylindrical folds sub-parallel to the ice flow. By numerical modelling, we show that these folds can be formed by lateral compression of mechanically anisotropic ice, while a general viscosity contrast between layers would not lead to folding for the same boundary conditions. We conclude that the folds primarily form by converging flow as the mechanically anisotropic ice is channelled towards the glacier.