Composition of sulfides from the Sierra Leone Fracture Zone (Central Atlantic Ocean)

An analysis of data on the location of hydrothermal fields, seismicity, and satellite altimetry evidences that in mid-ocean ridges with low spreading rate hydrothermal fields tend to be grouped in areas with generally low seismic activity and at intersections of discontinuities and rift zones. Based on this assumption, the Sierra Leone Fracture Zone was studied in 2000 during Cruise 22 of R/V Akademik Nikolaj Strakhov. A study of gabbrodolerite and dolerite showed that sulfide ore minerals in them were formed both by hydrothermal and magmatic processes. An analysis of melt inclusions demonstrated that magmatic complexes formed from a high-temperature (1210-1255°C) low-potassium melt of the N-MORB type. Investigations of fluid inclusions revealed that gabbro and dolerite formed under influence of an active hydrothermal system at temperature 205-226°C. Thus, the Sierra Leone Fracture Zone is considered to be perspective for a discovery of a new hydrothermal field.

Geochemistry of basalts near the Tamayo Fracture Zone

Recent investigations of the southern Gulf of California (22°N) on Leg 65 of the Deep Sea Drilling Project (DSDP) allow important comparisons with drilled sections of ocean crust formed at different spreading rates. During Leg 65 the Glomar Challenger drilled seven basement holes at sites forming a transect across the ridge axis near the Tamayo Fracture Zone. An additional site was drilled on the fracture zone itself, where a small magnetic "diapir" was located. Together with the material from Site 474 (drilled during Leg 64) the cores recovered at these sites are representative of the upper basaltic and sedimentary crust formed since the initial opening of the Gulf. The pattern of magmatic accretion at the ridge axis is conditioned by the moderate to high rate of spreading (~6 cm/y.) and comparatively high sedimentation rates that now characterize the Gulf of California. In terms of spreading rate, this region is intermediate between the "superfast" East Pacific Rise axis to the south (up to 17 cm/y.) and the slow-spreading Mid-Atlantic Ridge (2-4 cm/y.) both of which have been extensively studied by dredging and drilling.

Depth profiles of pore-water and solid-phase constituents, respiration rates and grain size distribution in sediments of the Clarion-Clipperton Fracture Zone

Manganese nodules of the Clarion-Clipperton Fracture Zone (CCFZ) in the NE Pacific Ocean are highly enriched in Ni, Cu, Co, Mo and rare-earth elements, and thus may be the subject of future mining operations. Elucidating the depositional and biogeochemical processes that contribute to nodule formation, as well as the respective redox environment in both, water column and sediment, supports our ability to locate future nodule deposits and evaluates the potential ecological and environmental effects of future deep-sea mining. For these purposes we evaluated the local hydrodynamics and pore-water geochemistry with respect to the nodule coverage at four sites in the eastern CCFZ. Furthermore, we carried out selective leaching experiments at these sites in order to assess the potential mobility of Mn in the solid phase, and compared them with the spatial variations in sedimentation rates. We found that the oxygen penetration depth is 180 - 300 cm at all four sites, while reduction of Mn and NO3- is only significant below the oxygen penetration depth at sites with small or no nodules on the sediment surface. At the site without nodules, potential microbial respiration rates, determined by incubation experiments using 14C-labelled acetate, are slightly higher than at sites with nodules. Leaching experiments showed that surface sediments covered with big or medium-sized nodules are enriched in mobilizable Mn. Our deep oxygen measurements and pore-water data suggest that hydrogenetic and oxic-diagenetic processes control the present-day nodule growth at these sites, since free manganese from deeper sediments is unable to reach the sediment surface. We propose that the observed strong lateral contrasts in nodule size and abundance are sensitive to sedimentation rates, which in turn, are controlled by small-scale variations in seafloor topography and bottom-water current intensity.

Petrography and geochemical composition of the Atlantis II Fracture Zone

SeaBeam echo sounding, seismic reflection, magnetics, and gravity profiles were run along closely spaced tracks (5 km) parallel to the Atlantis II Fracture Zone on the Southwest Indian Ridge, giving 80% bathymetric coverage of a 30- * 170-nmi strip centered over the fracture zone. The southern and northern rift valleys of the ridge were clearly defined and offset north-south by 199 km. The rift valleys are typical of those found elsewhere on the Southwest Indian Ridge, with relief of more than 2200 m and widths from 22 to 38 km. The ridge-transform intersections are marked by deep nodal basins lying on the transform side of the neovolcanic zone that defines the present-day spreading axis. The walls of the transform generally are steep (25°-40°), although locally, they can be more subdued. The deepest point in the transform is 6480 m in the southern nodal basin, and the shallowest is an uplifted wave-cut terrace that exposes plutonic rocks from the deepest layer of the ocean crust at 700 m. The transform valley is bisected by a 1.5-km-high median tectonic ridge that extends from the northern ridge-transform intersection to the midpoint of the active transform. The seismic survey showed that the floor of the transform contains up to 0.5 km of sediment. Piston-coring at two locations on the transform floor recovered more than 1 m of sand and gravel, which appears to be turbidites shed from the walls of the fracture zone. Extensive dredging showed that more than two-thirds of the crust exposed in the transform valley and its walls were plutonic rocks, principally gabbros and residual mantle peridotites. In contrast, based on dredging and seafloor morphology, only relatively undisrupted pillow basalt flows have been exposed on crust of the same age spreading away from the transform. Magnetic anomalies are well defined out to 11 m.y. over the flanking transverse ridges and transform valley, even where layer 2 appears to be absent. The total opening rate is 1.6 cm/yr, but the arrangement of the anomalies indicates that the spreading for each ridge is asymmetric, with the ridge flanks facing the transform spreading at a rate of 1.0 cm/yr. Such an asymmetric spreading pattern requires that both the northern and southern ridges migrate away from each other at 0.2 cm/yr, thus lengthening the transform at 0.4 cm/yr for the last 11 m.y. To the north, the fracture zone valley is oriented differently from the present-day transform, indicating a paleospreading direction change at 17 m.y. from N10°E to due north-south. This change placed the transform into extension for the 11-m.y. period required for simple orthogonal ridge-transform geometry to be reestablished and produced a large transtensional basin within the transform valley. This basin was split by continued transform slip after 11 m.y., with the larger half moving to the north with the African Plate.

(Table 1) Average aluminium accumulation rate at 40 DSDP Sites for the time interval 0-10 Ma and relative rates for other time intervals

A six-fold increase in the rate of accumulation of Al in north and central Atlantic and Pacific Ocean sediments indicates vastly increased denudation of the continents during the past 15 Ma. The increase is more apparent in hemipelagic than pelagic sites, demonstrating widely distributed local controls. Similarities in the rate of increase in the Atlantic and Pacific show that tectonic elevation is not responsible for the difference in sedimentation rate. Also, similarities in the difference at sites of low and high latitude suggest that glaciation is not the most significant source. A lack of correspondence between sedimentation rates and Vail's sea-level curve similarly rule out that effect. The conclusion drawn here is that worldwide climatic deterioration during the late Tertiary is the explanation for the striking increase in detrital sedimentation in the World ocean.