Forschungsschiff METEOR Reise Nr. 61 (2004) - Nordost-Atlantik

R/V METEOR Cruise No. 61 was divided into three different legs, which all focused on the NEAtlantic to the west of Ireland from the Porcupine Seabight towards the Rockall Bank. Legs 1 and 3 concentrated on geo-biological studies on the carbonate mounds in this region, which are covered by a unique cold water coral fauna. Leg 2 dealt with seismic investigations in order to investigate the extension processes that led to the development of the Porcupine rift basin. The foci of the individual legs were on the following themes. M61-1 was a multidisciplinary cruise addressing biological, paleo-geological and hydrographical scientific objectives in the carbonate mound provinces west of Ireland in the eastern Porcupine Seabight and on the Rockall Bank. The cruise started in Lisbon (Portugal) and ended in Cork (Ireland). M61-1 activities were embedded within the ESF-DFG MOUNDFORCE project of the EUROMARGINS Programme. Together with the succeeding M61-3 cruise, these Meteor activities document Germany´s strong scientific and logistic support for the success of this challenging programme. Investigations are also designed as a preparatory cruise for the EUproject HERMES (Hotspot Ecosystem Research on the Margins of European Seas; start April 2005). All institutions participating in M61-1 are partners in HERMES Work package 2 "Coral Reef and Carbonate Mound Systems". M 61-2 was directed at researching the earth's crust in the vicinity of the Porcupine rift basin. During this leg, seismic research has been undertaken in the Porcupine Basin west of Ireland, an area that represents a natural laboratory for the investigation of extensional processes. Firstly, both sides of a rift basin occurring in close proximity to each other could have been studied here, allowing questions about the symmetry of extension to be addressed by several east-west profiles parallel to the direction of extension. Secondly, the amount of extension increases from north to south, so a series of east-west cross sections on different latitudes has provided information on crustal structure during variable extension. The spatial changes between these sections also represent the temporal development of the rift through continued extension. In order to achieve these research goals, a series of east-west oriented wide angle reflection profiles in the Porcupine Basin has been acquired. These profiles aid in the explanation of extensional processes and their development through continued extension. They also address insufficiently explained questions about the initiation of large scale magmatism and intrusion, the onset of mantle serpentinisation and the development of detachment faults. M61-3 During this leg, the only recently discovered 'carbonate mounds' on the NWEuropean continental margin have been investigated, which represent unique geo- and ecosystems for European waters. The broad scientific interest that is directed at these mounds is reflected in three EU-projects, which until recently almost exclusively concentrated their efforts on the mounds, as well as the currently operating ESF-EUROMARGINS project MOUNDFORCE M 61-3 focused on the use of a 'Remotely Operated Vehicle' (ROV) for the investigation of the carbonate mounds. The primary tasks of Bremen's QUEST ROV were a detailed characterization of individual mound structures, selective sample collection and the retrieval of sensor systems placed at the seafloor one year before. These ROV tasks have been supplemented by hydro-acoustic measurements and conventional sediment sampling in order to work - in close collaboration with M61-1 - on the main research focuses of the MOUNDFORCE project: (a) analysis of the environmental factors that drive the development of the 'carbonate mounds', (b) surveying the benthic communities in dependence of changing environmental factors and (c) investigations to the stabilization and lithification of the mound sediments

Along- and across-axis variations in crustal thickness and structure at the Mid-Atlantic Ridge at 5°S obtained from wide-angle seismic tomography: implications for ridge segmentation

Two end‐member styles of crustal accretion are observed at two adjacent spreading segments at the Mid‐Atlantic Ridge at 5°S: focused accretion to the segment center with rapid crustal thinning toward the transform in the northern segment and crustal thickening toward the transform at an oceanic core complex in the southern segment. Our results were obtained by tomographic inversion of wide‐angle seismic reflection and refraction data collected along three intersecting profiles. The segment north of the 5°S fracture zone is characterized by a well‐developed median valley with a pronounced seafloor bulge in the segment center. A discrete portion of anomalously low velocities (−0.4 to −0.5 km/s relative to average off‐axis structure) at depths of ∼2.5 km beneath this bulge is possibly related to the presence of elevated temperatures and perhaps small portions of partial melt. This suggests that this segment is currently in a magmatically active period, which is confirmed by the observation of fresh lava flows and ongoing high‐temperature hydrothermal activity at the seafloor. Close to the current spreading axis, the crust thins rapidly from 8.5 km beneath the segment center to less than 3 km beneath the transform fault which indicates that melt supply here is strongly focused to the segment center. The reduction in crustal thickness is almost exclusively accommodated by the thinning of velocity portions indicative of seismic layer 3. The transform fault is characterized by more uniform velocity gradients throughout the entire crustal section and very low upper mantle velocities of 7.2–7.3 km/s indicating that serpentinization could be as much as 25% at 3.5 km depth. In contrast, ∼4.1 Ma old crust of the northern segment shows only minor thinning from the segment center toward the segment end. Here, the transform is characterized by a normal seismic layer 2/3 transition suggesting robust melt supply to the segment end at those times. In the adjacent southern segment, the crust thickens from ∼2.5 km beneath the flank of an oceanic core complex to ∼5.0 km at the segment boundary. The observed changes in crustal thickness show a significant temporal and lateral variability in melt supply and suggest a more complex crustal emplacement process than predicted by models of focused melt supply to the segment centers

Mid-depth internal wave energy off the Iberian Peninsula estimated from seismic reflection data

Energy levels of internal waves are estimated from seismic reflection data. Three legacy seismic sections from 1993 and 1997 obtained off the Iberian Peninsula have been analyzed for acoustic reflections within the water column. The reflections are aligned continuously for up to several kilometers over large parts of the sections and in the depth interval from 200 to 2000 m. Depth variations of these reflections are thought to be caused by the background internal wave field. From the variations we derive horizontal wave number spectra of normalized internal wave displacement. The general slope of the power density spectra is remarkably consistent for all sections and agrees well with model spectra for internal waves. Significant differences within the sections can be found when sufficiently large subsections are averaged. The spatial variation of the energy level indicates increasing internal wave activity with shallower water depths as well as near a subsurface eddy

A system for rating the stability and strength of medical evidence

BACKGROUND: Methods for describing one's confidence in the available evidence are useful for end-users of evidence reviews. Analysts inevitably make judgments about the quality, quantity consistency, robustness, and magnitude of effects observed in the studies identified. The subjectivity of these judgments in several areas underscores the need for transparency in judgments. DISCUSSION: This paper introduces a new system for rating medical evidence. The system requires explicit judgments and provides explicit rules for balancing these judgments. Unlike other systems for rating the strength of evidence, our system draws a distinction between two types of conclusions: quantitative and qualitative. A quantitative conclusion addresses the question, "How well does it work?", whereas a qualitative conclusion addresses the question, "Does it work?" In our system, quantitative conclusions are tied to stability ratings, and qualitative conclusions are tied to strength ratings. Our system emphasizes extensive a priori criteria for judgments to reduce the potential for bias. Further, the system makes explicit the impact of heterogeneity testing, meta-analysis, and sensitivity analyses on evidence ratings. This article provides details of our system, including graphical depictions of how the numerous judgments that an analyst makes can be combined. We also describe two worked examples of how the system can be applied to both interventional and diagnostic technologies. SUMMARY: Although explicit judgments and formal combination rules are two important steps on the path to a comprehensive system for rating medical evidence, many additional steps must also be taken. Foremost among these are the distinction between quantitative and qualitative conclusions, an extensive set of a priori criteria for making judgments, and the direct impact of analytic results on evidence ratings. These attributes form the basis for a logically consistent system that can improve the usefulness of evidence reviews

Movement along a low-angle normal fault: The S reflector west of Spain

[1] The existence of normal faults that moved at low angles (less than 20°) has long been debated. One possible low-angle fault is the S detachment at the west Galicia (Spain) margin and thought to occur at the top of serpentinized mantle. It is unlikely that S was a large submarine slide as it was probably active over several million years without the development of any compressional features such as toe thrusts, it appears to have rooted beneath the conjugate Flemish Cap margin, and it is similar to structures elsewhere that also appear to be rooted detachments. Here we analyze depth images to identify synrift sediment packages above S and use the geometry of these synrift packages to constrain the angle at which S both formed and remained active. We find that S must have remained active at angles below 15°, too low to be explained simply by the low friction coefficient of partially serpentinized peridotites. Instead, we suggest that transient high fluid pressures must have developed within the serpentinites and propose a model in which anastomosing fault strands are alternately active and sealed, enabling moderately high fluid pressures to develop

Seismicity trends and detachment fault structure at 13°N, Mid-Atlantic Ridge

At slow-spreading ridges, plate separation is commonly partly accommodated by slip on long-lived detachment faults, exposing upper mantle and lower crustal rocks on the seafloor. However, the mechanics of this process, the subsurface structure, and the interaction of these faults remain largely unknown. We report the results of a network of 56 ocean-bottom seismographs (OBSs), deployed in 2016 at the Mid-Atlantic Ridge near 13°N, that provided dense spatial coverage of two adjacent detachment faults and the intervening ridge axis. Although both detachments exhibited high levels of seismicity, they are separated by an ∼8-km-wide aseismic zone, indicating that they are mechanically decoupled. A linear band of seismic activity, possibly indicating magmatism, crosscuts the 13°30′N domed detachment surface, confirming previous evidence for fault abandonment. Farther south, where the 2016 OBS network spatially overlapped with a similar survey done in 2014, significant changes in the patterns of seismicity between these surveys are observed. These changes suggest that oceanic detachments undergo previously unobserved cycles of stress accumulation and release as plate spreading is accommodated.</p

Resolving the fine-scale velocity structure of continental hyperextension at the Deep Galicia Margin using full-waveform inversion

Continental hyperextension during magma-poor rifting at the Deep Galicia Margin is characterised by a complex pattern of faulting, thin continental fault blocks, and the serpentinisation, with local exhumation, of mantle peridotites along the S-reflector, interpreted as a detachment surface. In order to understand fully the evolution of these features, it is important to image seismically the structure and to model the velocity structure to the greatest resolution possible. Travel-time tomography models have revealed the long-wavelength velocity structure of this hyperextended domain, but are often insufficient to match accurately the short-wavelength structure observed in reflection seismic imaging. Here we demonstrate the application of two-dimensional (2D) time-domain acoustic full-waveform inversion to deep water seismic data collected at the Deep Galicia Margin, in order to attain a high resolution velocity model of continental hyperextension. We have used several quality assurance procedures to assess the velocity model, including comparison of the observed and modelled waveforms, checkerboard tests, testing of parameter and inversion strategy, and comparison with the migrated reflection image. Our final model exhibits an increase in the resolution of subsurface velocities, with particular improvement observed in the westernmost continental fault blocks, with a clear rotation of the velocity field to match steeply dipping reflectors. Across the S-reflector there is a sharpening in the velocity contrast, with lower velocities beneath S indicative of preferential mantle serpentinisation. This study supports the hypothesis that normal faulting acts to hydrate the upper mantle peridotite, observed as a systematic decrease in seismic velocities, consistent with increased serpentinisation. Our results confirm the feasibility of applying the full-waveform inversion method to sparse, deep water crustal datasets