Wolfram H. Geissler

Applying distance sampling to fin whale calls recorded by single seismic instruments in the northeast Atlantic.

Automated methods were developed to detect fin whale calls recorded by an array of ocean bottom seismometers (OBSs) deployed off the Portuguese coast between 2007 and 2008. Using recordings collected on a single day in January 2008, a standard seismological method for estimating earthquake location from single instruments, the three-component analysis, was used to estimate the relative azimuth, incidence angle, and horizontal range between each OBS and detected calls. A validation study using airgun shots, performed prior to the call analysis, indicated that the accuracy of the three-component analysis was satisfactory for this preliminary study. Point transect sampling using cue counts, a form of distance sampling, was then used to estimate the average probability of detecting a call via the array during the chosen day. This is a key step to estimating density or abundance of animals using passive acoustic data. The average probability of detection was estimated to be 0.313 (standard error: 0.033). However, fin whale density could not be estimated due to a lack of an appropriate estimate of cue (i.e., vocalization) rate. This study demonstrates the potential for using a sparse array of widely spaced, independently operating acoustic sensors, such as OBSs, for estimating cetacean density.

The Yermak Plateau in the Arctic Ocean in the light of reflection seismic data-implication for its tectonic and sedimentary evolution

SUMMARY The separation of Northeast Greenland and Svalbard is the result of large-scale strike slip movements during Cenozoic times. Geological evidence for these movements can be found onshore both on North Greenland and Svalbard. However, the role of the submarine Yermak Plateau (YP) in this process is unclear. The compilation of available multichannel reflection and wide-angle seismic data give new insights into the sedimentary and crustal structure and evolution of the plateau. The flat surface of the present-day plateau is a quite young feature. Up to 2 km of Cenozoic sediments cover a rough basement, which show similarities to the rough topography and strike of geological structures of Spitsbergen Island. In some basins more than 4 km of sedimentary rocks could be mapped. The most pronounced structure is the Sverdrup Bank, which appears to be part of a larger crustal block. P-wave velocities of about 4.5 km s–1 derived from sonobuoy data indicate that its uppermost part is most probably composed of sedimentary or volcanic rocks. We have made a correlation of previously defined seismic units across the YP to outline the history of sediment deposition in the area. The existing graben structures on the plateau might have provided early shallow pathways for water exchange between the Arctic and the Atlantic Oceans. A chaotic sedimentary apron east of the Sverdrup Bank and bright reflections near the Mosby Seamount interpreted as magmatic sills suggest tectonic and magmatic events during the Miocene.

Crustal thickness and Vp/Vs ratio in NW Namibia from receiver functions: Evidence for magmatic underplating due to mantle plume‐crust interaction

A seismological network was operated at the junction of the aseismic Walvis Ridge with the northwestern Namibian coast. We mapped crustal thickness and bulk Vp/Vs ratio by the H-k analysis of receiver functions. In the Damara Belt, the crustal thickness is ~35 km with a Vp/Vs ratio of <1.75. The crust is ~30 km thick at the coast in the Kaoko Belt. Strong variations in crustal thickness and Vp/Vs ratios are found at the landfall of the Walvis Ridge. Here and at ~150 km northeast of the coast, the crustal thickness increases dramatically reaching 44 km and the Vp/Vs ratios are extremely high (~1.89). These anomalies are interpreted as magmatic underplating produced by the mantle plume during the breakup of Gondwana. The area affected by the plume is smaller than 300 km in diameter, possibly ruling out the existence of a large plume head under the continent during the breakup.

Gas hydrate distribution and hydrocarbon maturation north of the Knipovich Ridge, western Svalbard margin

A bottom-simulating reflector (BSR) occurs west of Svalbard in water depths exceeding 600 m, indicating that gas hydrate occurrence in marine sediments is more widespread in this region than anywhere else on the eastern North Atlantic margin. Regional BSR mapping shows the presence of hydrate and free gas in several areas, with the largest area located north of the Knipovich Ridge, a slow-spreading ridge segment of the Mid Atlantic Ridge system. Here, heat flow is high (up to 330 mW m-2), increasing towards the ridge axis. The coinciding maxima in across-margin BSR width and heat flow suggest that the Knipovich Ridge influenced methane generation in this area. This is supported by recent finds of thermogenic methane at cold seeps north of the ridge termination. To evaluate the source rock potential on the western Svalbard margin, we applied 1D petroleum system modeling at three sites. The modeling shows that temperature and burial conditions near the ridge were sufficient to produce hydrocarbons. The bulk petroleum mass produced since the Eocene is at least 5 kt and could be as high as ~0.2 Mt. Most likely, source rocks are Miocene organic-rich sediments and a potential Eocene source rock that may exist in the area if early rifting created sufficiently deep depocenters. Thermogenic methane production could thus explain the more widespread presence of gas hydrates north of the Knipovich Ridge. The presence of microbial methane on the upper continental slope and shelf indicates that the origin of methane on the Svalbard margin varies spatially.

Seismic volcanostratigraphy of the NE Greenland continental margin

Abstract The Early Eocene continental break-up between the NE Greenland and the mid-Norwegian–SW Barents Sea margins was associated with voluminous magmatism and led to the emplacement of massive volcanic complexes including wedges of seawards-dipping reflections (SDR). We study the distribution of these break-up-related volcanic rocks along the NE Greenland margin by revisiting existing seismic reflection data and comparing our observations to better-studied segments of the conjugate margin. Seismic facies types match between the conjugate margins and show strong lateral variations. Seaward-dipping wedges are mapped offshore East Greenland, the conjugate to the Vøring continental margin. The geophysical signature of the SDRs becomes less visible towards the north, as it does along the conjugate Lofoten–Vesterålen margin. We suggest that the Traill Ø volcanic ridge is a result of plume–ridge interactions formed between approximately 54 and 47 Ma. North of the East Greenland Ridge, strong basement reflections conjugate to the Vestbakken Volcanic Province are interpreted as lava flows or ‘spurious’ SDRs. We discuss our findings in conjunction with results from seismic wide-angle experiments, gravity and magnetic data. We focus on the spatial and temporal relationships of the break-up volcanic rocks, and their structural setting in a late rift and initial oceanic drift stage. Supplementary material: The figures show the original seismic data used as the base for the interpretations shown in this paper. The seismic profiles are marked on Figure 1 (in the paper) as numbers 1 to 10 and are available at https://doi.org/10.6084/m9.figshare.c.3593780

Seamounts and oceanic igneous features in the NE Atlantic: A link between plate motions and mantle dynamics

Abstract A new regional compilation of seamount-like oceanic igneous features (SOIFs) in the NE Atlantic points to three distinct oceanic areas of abundant seamount clusters. Seamounts on oceanic crust dated 54–50 Ma are formed on smooth oceanic basement, which resulted from high spreading rates and magmatic productivity enhanced by higher than usual mantle plume activity. Late Eocene–Early Miocene SOIF clusters are located close to newly formed tectonic features on rough oceanic crust in the Irminger, Iceland and Norway basins, reflecting an unstable tectonic regime prone to local readjustments of mid-ocean ridge and fracture zone segments accompanied by extra igneous activity. A SOIF population observed on Mid-Miocene–Present rough oceanic basement in the Greenland and Lofoten basins, and on conjugate Kolbeinsey Ridge flanks, coincides with an increase in spreading rate and magmatic productivity. We suggest that both tectonic/kinematic and magmatic triggers produced Mid-Miocene–Present SOIFs, but the Early Miocene westwards ridge relocation may have played a role in delaying SOIF formation south of the Jan Mayen Fracture Zone. We conclude that Iceland plume episodic activity combined with regional changes in relative plate motion led to local mid-ocean ridge readjustments, which enhanced the likelihood of seamount formation. Supplementary material: Figures detailing NE Atlantic seamounts and SOIF distribution, and the location of earthquake epicentres are available at https://doi.org/10.6084/m9.figshare.c.3459729

Submarine slope failures due to pipe structure formation

There is a strong spatial correlation between submarine slope failures and the occurrence of gas hydrates. This has been attributed to the dynamic nature of gas hydrate systems and the potential reduction of slope stability due to bottom water warming or sea level drop. However, 30 years of research into this process found no solid supporting evidence. Here we present new reflection seismic data from the Arctic Ocean and numerical modelling results supporting a different link between hydrates and slope stability. Hydrates reduce sediment permeability and cause build-up of overpressure at the base of the gas hydrate stability zone. Resulting hydro-fracturing forms pipe structures as pathways for overpressured fluids to migrate upward. Where these pipe structures reach shallow permeable beds, this overpressure transfers laterally and destabilises the slope. This process reconciles the spatial correlation of submarine landslides and gas hydrate, and it is independent of environmental change and water depth.There is a strong correlation between submarine slope failures and the occurrence of gas hydrates. Here, the authors use a combination of seismic data and numerical modelling to show that overpressure at the gas hydrate stability zone leads to potential destabilization of the slope and submarine landslides.

Seismic structure of the lithosphere beneath NW Namibia:Impact of the Tristan da Cunha mantle plume

Northwestern Namibia, at the landfall of the Walvis Ridge, was affected by the Tristan da Cunha mantle plume during continental rupture between Africa and South America, as evidenced by the presence of the Etendeka continental flood basalts. Here we use data from a passive-source seismological network to investigate the upper mantle structure and to elucidate the Cretaceous mantle plume-lithosphere interaction. Receiver functions reveal an interface associated with a negative velocity contrast within the lithosphere at an average depth of 80 km. We interpret this interface as the relic of the lithosphereasthenosphere boundary (LAB) formed during the Mesozoic by interaction of the Tristan da Cunha plume head with the pre-existing lithosphere. The velocity contrast might be explained by stagnated and ‘‘frozen’’ melts beneath an intensively depleted and dehydrated peridotitic mantle. The present-day LAB is poorly visible with converted waves, indicating a gradual impedance contrast. Beneath much of the study area, converted phases of the 410 and 660 km mantle transition zone discontinuities arrive 1.5 s earlier than in the landward plume-unaffected continental interior, suggesting high velocities in the upper mantle caused by a thick lithosphere. This indicates that after lithospheric thinning during continental breakup, the lithosphere has increased in thickness during the last 132 Myr. Thermal cooling of the continental lithosphere alone cannot produce the lithospheric thickness required here. We propose that the remnant plume material, which has a higher seismic velocity than the ambient mantle due to melt depletion and dehydration, significantly contributed to the thickening of the mantle lithosphere.

Post-Megaslide Slope Stability North of Svalbard, Arctic Ocean

In the light of a warming globe, increasing coastal population and human offshore activities, slope stability issues steadily gain significance. The Arctic Ocean is predicted to exhibit most drastic changes. Following the enormous Hinlopen/ Yermak Megaslide north of Svalbard 30,000 years ago, the adjacent slopes developed several failure types as a consequence of the partial removal of the Hinlopen trough mouth fan. The local slope to the east is structured by several detachment surfaces that facilitate large scale creeping. This soft sediment deformation includes turbulent structures like folds on a meter-scale. The creeping sediments partly cover the eastern main slide debris of the megaslide within Sophia Basin. The timing of this gravity-driven mass transport can roughly be assessed by the time interval that occurred between the megaslide and today. These features mark the slope as unstable.

Hot Upper Mantle Beneath the Tristan da Cunha Hotspot From Probabilistic Rayleigh‐Wave Inversion and Petrological Modeling

Understanding the enigmatic intraplate volcanism in the Tristan da Cunha region requires knowledge of the temperature of the lithosphere and asthenosphere beneath it. We measured phasevelocity curves of Rayleigh waves using cross-correlation of teleseismic seismograms from an array of ocean-bottom seismometers around Tristan, constrained a region-average, shear-velocity structure, and inferred the temperature of the lithosphere and asthenosphere beneath the hotspot. The ocean-bottom data set presented some challenges, which required data-processing and measurement approaches different from those tuned for land-based arrays of stations. Having derived a robust, phase-velocity curve for the Tristan area, we inverted it for a shear wave velocity profile using a probabilistic (Markov chain Monte Carlo) approach. The model shows a pronounced low-velocity anomaly from 70 to at least 120 km depth. VS in the low velocity zone is 4.1–4.2 km/s, not as low as reported for Hawaii (�4.0 km/s), which probably indicates a less pronounced thermal anomaly and, possibly, less partial melting. Petrological modeling shows that the seismic and bathymetry data are consistent with a moderately hot mantle (mantle potential temperature of 1,410–1,4308C, an excess of about 50–1208C compared to the global average) and a melt fraction smaller than 1%. Both purely seismic inversions and petrological modeling indicate a lithospheric thickness of 65–70 km, consistent with recent estimates from receiver functions. The presence of warmer-than-average asthenosphere beneath Tristan is consistent with a hot upwelling (plume) from the deep mantle. However, the excess temperature we determine is smaller than that reported for some other major hotspots, in particular Hawaii.