ALBERT

Description: The Red Sea is a textbook case of a modern-day oceanic rift basin forming as a result of continental break-up. A characteristic feature of this setting in the central and northern Red Sea are more than two dozens of isolated bathymetric depressions, "deeps", filled with brines derived from leaching of the early-breakup Miocene salt deposits that lie beneath the entire Red Sea. The Atlantis II Deep is the largest basin of this type in the axial rift zone of the Red Sea. A topographic depression enclosing a volume of an estimated 15 km3 at water depths from 1900 to 2200 m, the Atlantis II Deep contains layered fluids with temperatures of up to 66°C and salinities of up to 27%. Beneath the brines, up to 30 m of fine-grained metalliferous sediments have been accumulating for the past 23 000 years (Anschutz, 1995). Unlike those in modern hydrothermal systems at mid-ocean ridges, where most of the metals are expelled to the open ocean in buoyant hydrothermal plumes, metals in the Atlantis II Deep are trapped and precipitated beneath a 200-m-thick brine layer. These sediments show extremely high concentrations of zinc, copper, silver, and gold (90 Mt of dry salt-free sediment at Zn〉2%, Cu〉0.5%, Ag〉39 g/t., Au〉0.5 g/t.). While the mode of metal deposition found in the Atlantis II Deep is not known from anywhere else on the seafloor today, it has yet been widely suggested as an analog for many ancient sediment-hosted ore deposits (Laurila et al. 2015). From 1975 to 1981 the German company PREUSSAG was contracted by the Saudi-Sudanese Commission for the Exploitation of the Red Sea Resources to explore the Atlantis II Deep. The project was aimed at assessing the overall technical viability of recovering and processing metalliferous muds on board of a mining vessel. The program encompassed several sampling cruises, a pre-pilot mining test (PPMT), environmental surveys, a study of pre-mining environmental conditions, as well as an economical evaluation. However, despite a highly successful PPMT, economic interest in the project waned due to declining commodity prices in the early 1980s. A substantial data set was derived from this campaign, including several kilometers of well preserved sediment cores and 〉20 000 pages of analog information. Utilizing this information, several GIS-based data mining efforts were carried out in recent years, driven by renewed interest in developing this unique resource. Our presentation will discuss results from these efforts and focus on the technical, economic and environmental boundary conditions of ocean mining of metalliferous sediments. Recently acquired geophysical data in the central Red Sea provide an unprecedented level of ground truth to historic data on Atlantis II.

Description: Verfahren zur Analyse der chemischen Zusammensetzung eines Gewässerbodens umfassend folgende Schritte a. Bereitstellen einer Vorrichtung umfassend eine Eindringsonde umfassend ein Messfenster mit einer Scheibe und Mittel zur Durchführung eines Röntgenfluoreszenz- Verfahrens, welche über die Verbindungsvorrichtung mit einem Wasserfahrzeug verbunden ist, wobei die Eindringsonde in einer Wassertiefe bis zu 6000 m vorliegt, b. Initialisierung eines schwerkraftbedingten Falls der Vorrichtung, c. zumindest teilweises Eindringen der Eindringsonde in den Gewässerboden, in eine Tiefe von bis zu 18 m, bevorzugt in eine Tiefe von bis zu 12 m, d. Herausziehen der der Eindringsonde aus dem Gewässerboden, mit einer bevorzugten Geschwindigkeit von 0,1 m/s, e. Durchführung eines Röntgenfluoreszenz-Verfahrens während des gesamten Herausziehens, wobei in der Eindringsonde erzeugte Röntgenstrahlung durch das Messfenster abgestrahlt wird und im Gewässerboden generierte Fluoreszenzstrahlung durch das Messfenster einfällt und detektiert wird, g. Verbleiben der Vorrichtung in einem gewässerbodennahen Arbeitsbereich von bevorzugt 10 m über dem Gewässerboden.

Description: Structural, physical property, and magnetic data from Ocean Drilling Program Site 808 in the Nankai Trough, Japan, indicate that both sediment loading and plate convergence have driven dewatering and consolidation in this region. Evidence for this interpretation is provided by a migrated seismic reflection profile, computed tomography of core-scale structures, magnetic susceptibility and P-wave velocity data, and experimental studies of Site 808 samples. These data also show that the strain (including volume change) resulting from plate convergence is partitioned into both penetrative deformation structures as well as more discrete, core-scale structures with finite displacements. These core-scale structures range from relatively subtle, kink-like deflections of the primary phyllosilicate fabric to sharp discontinuities with probable displacements much greater than the dimensions of the core barrel. Although all of the structures acted at least in part as dewatering conduits, evidence of concentrated fluid flow in this region of the prism is limited to a narrow interval almost 150 m above the décollement (located at between 946 and 965 mbsf). This interval correlates with the middle of a hemipelagic sequence above the décollement that appears to have thinned, apparently through dewatering, relative to a more seaward section. Thinning and dewatering appear to have been induced by deposition of a more clastic sedimentary sequence (the outer marginal trench-wedge sediments) that grades upward into a coarse-grained trench-fill sequence. Importantly, the hemipelagic sequence below the décollement appears to have thinned very little, suggesting that these sediments are underconsolidated and overpressured. This interpretation is consistent with porosity measurements from below the décollement. Microscopic and submicroscopic studies of sediments from within the décollement record a cyclic deformation sequence of displacement-brecciation-porosity collapse and compaction that may also reflect deformation of an overpressured sequence. Finally, the structural, physical property, and magnetic data also yield kinematic and geometric results consistent with the present convergent vector between the Philippine Sea Plate and Eurasia. These data indicate a shortening direction that trends between 308° and 315°, consistent with plate convergent vectors that trend between 310° and 314°.

Description: Shipboard laboratory index property data, shore-based consolidation tests, and in-situ stress and pore-pressure measurements are used in this study to constrain the stress conditions at ODP Site 808, Nankai Trough. Results of these tests are presented along with additional intepretations of porosity rebound and permeability. The sediment at Site 808 is highly affected by excess fluid pressures throughout the sediment column. Excess fluid pressure is severe below the major fault boundary, the decollement. The in-situ measurement of lateral stresses, which are shallow in the sediment section, confirms that the principal stress direction is rotated from a "normal" basin-type condition where the principal stress direction is vertical.

Description: Sediments undergoing accretion in trench-forearc systems are subjected to conditions of large lateral thrusting. This stress regime controls the mechanism of faulting as well as the yield and strength properties of the sediment. Understanding them is therefore crucial for the construction of quantitative models of sediment dynamics in convergent margin settings. For this purpose triaxial and oedometer tests were performed on six whole-round core samples recovered from Site 808 from depths between 173 and 705 mbsf. Samples from five depth intervals were subjected to a triaxial test program that was primarily designed to define yield and strength behavior. Test specimens were cut parallel and normal to the core axis. Additional five oedometer tests with similarly prepared specimens were performed on samples from four depth intervals to evaluate the directional state and degree of sediment compaction. Test results show that the degree of sediment compaction is higher than expected from overburden. This overcompaction increases with depth. A well-developed mechanical anisotropy is evident in all samples tested, regardless of their depth and lithology. Values of yield limit, stiffness, and shear strength are up to 40% higher in the horizontal direction compared to the vertical direction. In addition the test data demonstrate that the axis of the volumetric yield loci have rotated into extensional stress field. This verifies that the mechanical state of sediment in the accretionary wedge is controlled by in-situ stress conditions of extensional nature. The coefficients of lateral stress inferred suggest that the extensional stress regime becomes increasingly effective with depth.

Description: Complete penetration of frontal thrust and décollement of the Nankai Trough accretionary complex in Site 808 during ODP Leg 131 provided a wealth of structural observations and physical property data. In this paper possible mechanisms are discussed that could be responsible for the development of irregular downhole trends in acoustic anisotropy observed in Site 808. After various steps of data reduction and screening, a paleomagnetic reorientation procedure is applied to a selected group of physical property data sets. This facilitates the integration of the observed changes in physical properties with the geotectonic framework at the deformation front of the Nankai Trough accretionary complex. The paleomagnetic database was employed in the reconstruction of directional properties of acoustic velocities of the Lower Shikoku Basin sedimentary sequence, which is divided by a sharply defined décollement into an accreting and a subducting portion. P-wave velocity anisotropies derived from paleomagnetically oriented samples in the upper part of this 420-m-thick hemipelagic sequence show maximum values in the direction parallel and normal to the inferred vector of plate convergence (31O°-315°). No preferred orientation of P-wave velocity anisotropy is found in the subducting part of the sequence. The preferred direction of maximum anisotropy parallel to the convergence vector is also in accordance with the true direction of the observed macro- to mesoscale structural features in Site 808. Microfractures and microcracks forming as stress relief and tensile fractures in cores of semilithified sediment normal and perpendicular to the maximum horizontal stress are discussed as control mechanisms for the development of the observed anisotropy pattern.