Peter Feldens

Sedimentary deposits left by the 2004 Indian Ocean tsunami on the inner continental shelf offshore of Khao Lak, Andaman Sea (Thailand)

Tsunami waves leave sedimentary signatures both onshore and offshore, although the latter are hardly known. The objective of the present study is to provide new evidence for the 2004 Indian Ocean tsunami deposits left on the inner continental shelf of the Andaman Sea (Thailand) and to identify diagnostic sedimentological and geochemical properties of these deposits. Based on extensive seafloor mapping, three sediment cores were selected for study and were analysed for their sedimentary structures, grain size composition, chemical elemental composition, physical properties and 210Pb activity. Sediment cores retrieved from shallow water (9–15 m) within 7.5 km off the shore revealed distinct event layers, which were interpreted as being tsunami deposits. These 20–25 cm thick deposits were already covered with post-tsunami marine sediments. They were composed of several units, marine sand layers alternating with poorly sorted mud with terrigenous and anthropogenic components, representing different hydrodynamic conditions (probably during run-up and backwash phase). These sedimentological observations were supported by geochemical and physical data and were confirmed using 210Pb dating. A sediment core taken from a depth of 57 m at a distance of 25 km offshore did not reveal clear event deposits. Comparisons with available data from offshore tsunami deposits showed that there is no single set of signatures that could be applied to identify this kind of deposits.

Sediment distribution on the inner continental shelf off Khao Lak (Thailand) after the 2004 Indian Ocean tsunami

The coastline of Khao Lak (Thailand) was heavily damaged by the 2004 Indian Ocean tsunami. Onshore tsunami deposits and satellite images, which show large amounts of sediment transported offshore, indicate that the seafloor was impacted by tsunami run-up and backwash. In this study, high-resolution maps of sediment distribution patterns and the geological development of the seafloor are presented. These maps are based on multibeam, side-scan sonar and seismic profiling surveys offshore Khao Lak. Paleoreefs, with associated boulder fields and sandy sediment dominate the inner continental shelf. Patches of fine-grained (silt to fine sand) sediments exist in water depths of less than 15 m. The sediment distribution pattern is stable between 2008 and 2010, apart from small shifts regarding the boundaries of the fine-grained sediment patches. In one sediment core and several grab samples an event layer was documented, situated below a cover of modern sediments which is only a few cm thick. The event-layer is traced down to 18 m water depth. It consists mostly of sand and contains compounds of terrigenous origin. It is interpreted as a 2004 Indian Ocean tsunami deposit. However, over large areas of the study-site, the impact of the tsunami is hardly identifiable by seafloor morphology or sediment distribution.

Application of 2D Fourier filtering for elimination of stripe noise in side-scan sonar mosaics

Different types of along-track stripe noise are common artifacts in side-scan sonar mosaics, hampering both visual image quality and automatic seabed classification attempts. In this study, a 2D Fourier domain slope filter is applied to remove stripes from side-scan sonar mosaics. This technique has the advantage of using only two simple tuning parameters, namely, the slope angle of the stripe noise, and a filtering width. Furthermore, the image processing tool presented here can be applied to non-optimally processed side-scan mosaics without the need of reprocessing the original data. The performance of the filter technique is demonstrated by two datasets in which stripe noise, including nadir stripes, has been successfully removed. To one of the filtered dataset a simple unsupervised seabed classification was then applied, leading to a much improved result with only little misclassification due to residual stripe noise.

Effects of using inclined parametric echosounding on sub-bottom acoustic imaging and advances in buried object detection

This study reports an adaptation of a parametric echosounder system using 15 kHz as secondary frequency to investigate the angular response of sub-bottom backscatter strength of layered mud, providing a new method for enhanced acoustic detection of buried targets. Adaptions to achieve both vertical (0°) and non-vertical inclination (1–15°, 30°, 45° and 60°) comprise mechanical tilting of the acoustic transducer and electronic beam steering. Data were acquired at 18 m water depth at a study site characterized by a flat, muddy seafloor where a 0.1 m diameter power cable lies 1–2 m below the seafloor. Surveying the cable with vertical incidence revealed that the buried cable can hardly be discriminated against the backscatter strength of the layered mud. However, the backscatter strength of layered mud decreases strongly at >3±0.5° incidence and the layered mud echo pattern vanishes beyond 5°. As a consequence, the backscatter pattern of the buried cable is very pronounced in acoustic images gathered at 15°, 30°, 45° and 60° incidence. The size of the cable echo pattern increases linearly with incidence. These effects are attributed to reflection loss from layered mud at larger incidence and to the scattering of the 0.1 m diameter buried cable. Data analyses support the visual impression of superior detection of the cable with an up to 2.6-fold increase of the signal-to-noise ratio at 40° incidence compared to the vertical incidence case.

Salt Flows in the Central Red Sea

The central Red Sea is a nascent oceanic basin. Miocene evaporites, kilometers in thickness, were deposited during its continental rifting phase and early seafloor spreading. With further seafloor spreading, increasing dissolution due to increasing hydrothermal circulation as well as normal fault movements removed lateral constraint of the evaporites at the walls of the axial rift valley. Because halite is a ductile material that forms a large part of the evaporite sequence, the evaporites started to move downslope, passively carrying their hemipelagic sediment cover. Today, flowlike features comprising Miocene evaporites are situated on the top of younger magnetic seafloor spreading anomalies. Six salt flows, most showing rounded fronts in plan view, with heights of several hundred meters and widths between 3 and 10 km, are identified by high-resolution bathymetry and DSDP core material around Thetis Deep and Atlantis II Deep, and between Atlantis II Deep and Port Sudan Deep. The relief of the underlying volcanic basement likely controls the positions of individual salt flow lobes. On the flow surfaces, along-slope and downslope ridge and trough morphologies parallel to the local seafloor gradient have developed, presumably due to extension of the hemiplegic sediment cover or strike-slip movement within the evaporites. A few places of irregular seafloor topography are observed close to the flow fronts, interpreted to be the result of dissolution of Miocene evaporites, which contributes to the formation of brines in several of the deeps. Based on the vertical relief of the flow lobes, deformation is taking place in the upper part of the evaporite sequence. Considering a salt flow at Atlantis II Deep in more detail, strain rates due to dislocation creep and pressure solution creep were estimated to be 10−14 1/s and 10−10 1/s, respectively, using given assumptions of grain size and deforming layer thickness. The latter strain rate, comparable to strain rates observed for onshore salt flows in Iran, results in flow speeds of several mm/year for the offshore salt flows in certain locations. Thus, salt flow movements can potentially keep up with Arabia–Nubia tectonic half-spreading rates reported for large parts of the Red Sea.

Highly dynamic biological seabed alterations revealed by side scan sonar tracking of Lanice conchilega beds offshore the island of Sylt (German Bight)

Hydroacoustic surveys are common tools for habitat investigation and monitoring that aid in the realisation of the aims of the EU Marine Directives. However, the creation of habitat maps is difficult, especially when benthic organisms densely populate the seafloor. This study assesses the sensitivity of entropy and homogeneity image texture parameters derived from backscatter strength data to benthic habitats dominated by the tubeworm Lanice conchilega. Side scan sonar backscatter surveys were carried out in 2010 and 2011 in the German Bight (southern North Sea) at two sites approx. 20 km offshore of the island of Sylt. Abiotic and biotic seabed facies, such as sorted bedforms, areas of fine to medium sand and L. conchilega beds with different tube densities, were identified and characterised based on manual expert analysis and image texture analysis. Ground truthing was performed by grab sampling and underwater video observations. Compared to the manual expert analysis, the k-means classification of image textures proves to be a semi-automated method to investigate small-scale differences in a biologically altered seabed from backscatter data. The texture parameters entropy and homogeneity appear linearly interrelated with tube density, the former positively and the latter negatively. Reinvestigation of one site after 1 year showed an extensive change in the distribution of the L. conchilega-altered seabed. Such marked annual fluctuations in L. conchilega tube cover demonstrate the need for dense time series and high spatial coverage to meaningfully monitor ecological patterns on the seafloor with acoustic backscatter methods in the study region and similar settings worldwide, particularly because the sand mason plays a pivotal role in promoting biodiversity. In this context, image texture analysis provides a cost-effective and reproducible method to track biologically altered seabeds from side scan sonar backscatter signatures.

Expelled subsalt fluids form a pockmark field in the eastern Red Sea

This study aimed to constrain the source area of fluids responsible for the formation of a pockmark field in the eastern Red Sea. The newly discovered field extends over an area of at least 1,000 km2 at a water depth of ~400 m. The pockmarks have modal diameters of 140–150 m and are either randomly distributed on the seafloor or aligned within valleys approximately 25 m deep and several kilometres in length. Seismic data show that chimneys and/or regions of acoustic turbidity prevail beneath the pockmark field down to the top of Miocene evaporites, which are widespread in the Red Sea. Four gravity cores were taken from the pockmark field. For most of the cores, geochemical analyses show that porewater has a higher Cl concentration than the local seawater and increased Cl/Br ratios, which indicate an origin from evaporites. The adsorbed hydrocarbons are of thermal origin, with C1/(C2+C3) ratios between 4 and 23 and stable carbon isotope data for methane varying from δ13C of –34 to –36.4‰ with respect to Vienna Pee Dee Belemnite. On the basis of the calculated maturity of the source rock of 1.2–1.4 Ro, local thermal gradients and sedimentation rates, its deeper depth boundary is approximated at 2,000 to 2,200 m. The results indicate that the adsorbed hydrocarbons sampled at the seafloor had to pass through an evaporite sequence of potentially several hundred metres to a few km in thickness. The most likely explanation for the increased permeability of the evaporite sequence is brittle deformation triggered by extensive local tectonic movements and supported by high fluid overpressure within the evaporite sequence.

Reconstructing the sediment concentration of a giant submarine gravity flow

Submarine gravity flows are responsible for the largest sediment accumulations on the planet, but are notoriously difficult to measure in action. Giant flows transport 100s of km3 of sediment with run-out distances over 2000 km. Sediment concentration is a first order control on flow dynamics and deposit character. It has never been measured directly nor convincingly estimated in large submarine flows. Here we reconstruct the sediment concentration of a historic giant submarine flow, the 1929 “Grand Banks” event, using two independent approaches, each validated by estimates of flow speed from cable breaks. The calculated average bulk sediment concentration of the flow was 2.7–5.4% by volume. This is orders of magnitude higher than directly-measured smaller-volume flows in river deltas and submarine canyons. The new concentration estimate provides a test case for scaled experiments and numerical simulations, and a major step towards a quantitative understanding of these prodigious flows.Giant submarine gravity flows are a key mechanism in global sediment transport, yet their properties remain enigmatic. Here, the authors reconstruct the properties of a historic giant submarine gravity flow from deposits across the seafloor.

Sub-marine Continuation of Peat Deposits From a Coastal Peatland in the Southern Baltic Sea and its Holocene Development

Coastal low-lying areas along the southern Baltic Sea provide good conditions for coastal peatland formation. During the Holocene, the transgression of the Littorina Sea has caused coastal flooding, submergence and erosion of ancient coastlines and former terrestrial material. The present Heiligensee & Hutelmoor peatland (located near Rostock in Northern Germany) was found to continue more than 90 m in front of the coastline based on on- and offshore sediment cores and geo-acoustic surveys. The seaward areal extent of the peatland is estimated with 0.16-0.2km2. The offshore limit of the former peatland roughly coincides with the offshore limit of a dynamic coast-parallel longshore bar, with peat deposits eroded seawards. While additional organic-rich layers were found further offshore below a small sand ridge system, no connection to the former peatlands can be established based on 14C age and C/N ratios. The preserved submerged peat deposits with organic carbon contents of 37 % in front of the coastal peatland Heiligensee & Hutelmoor was radiocarbon-dated to 6725 +/- 87 and 7024 +/-73 cal yr BP, respectively, indicating an earlier onset of the peatland as presently published. The formation time of the peat layers gives information about the local sea level rise. The local sea level curve derived from our 14C-dated organic-rich layers is in general agreement to nearby sea level reconstructions (North Rugen and Fischland, Northern Germany), with differences explained by local isostatic movements.

Flow Behaviour of a Giant Landslide and Debris Flow Entering Agadir Canyon, NW Africa

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. New acoustic and sedimentological data of the Agadir Canyon area were collected during RV Maria S. Merian Cruise 32 in autumn 2013. The data show a prominent headwall area around 200 km south of the head of Agadir Canyon. The failure occurred along a pronounced weak layer in a sediment wave field. The slab-type failure rapidly disintegrated and transformed into a debris flow, which entered Agadir Canyon at 2500 m water depth. Interestingly, the debris flow did not disintegrate into a turbidity current when it entered the canyon despite a significant increase in slope angle. Instead, the material was transported as debrite for at least another 200 km down the canyon. It is unlikely that this giant debris flow significantly contributed to the deposits in the wider Moroccan Turbidite System.