Chert intervals in DSDP and ODP sites (Table 1)

Radiolarian cherts in the Tethyan realm of Jurassic age were recently interpreted as resulting from high biosiliceous productivity along upwelling zones in subequatorial paleolatitudes the locations of which were confirmed by revised paleomagnetic estimates. However, the widespread occurrence of cherts in the Eocene suggests that cherts may not always be reliable proxies of latitude and upwelling zones. In a new survey of the global spatio-temporal distribution of Cenozoic cherts in Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) sediment cores, we found that cherts occur most frequently in the Paleocene and early Eocene, with a peak in occurrences at ~50 Ma that is coincident with the time of highest bottom water temperatures of the early Eocene climatic optimum (EECO) when the global ocean was presumably characterized by reduced upwelling efficiency and biosiliceous productivity. Cherts occur less commonly during the subsequent Eocene global cooling trend. Primary paleoclimatic factors rather than secondary diagenetic processes seem therefore to control chert formation. This timing of peak Eocene chert occurrence, which is supported by detailed stratigraphic correlations, contradicts currently accepted models that involve an initial loading of large amounts of dissolved silica from enhanced weathering and/or volcanism in a supposedly sluggish ocean of the EECO, followed during the subsequent middle Eocene global cooling by more vigorous oceanic circulation and consequent upwelling that made this silica reservoir available for enhanced biosilicification, with the formation of chert as a result of biosilica transformation during diagenesis. Instead, we suggest that basin-basin fractionation by deep-sea circulation could have raised the concentration of EECO dissolved silica especially in the North Atlantic, where an alternative mode of silica burial involving widespread direct precipitation and/or absorption of silica by clay minerals could have been operative in order to maintain balance between silica input and output during the upwelling-deficient conditions of the EECO. Cherts may therefore not always be proxies of biosiliceous productivity associated with latitudinally focused upwelling zones.

Parameters of the near-bottom nepheloid layer off the northwestern Africa coast

Vertical profiles of light scattering at a right angle and turbidity profiles in seawater indicating suspended matter concentration in the near-bottom nepheloid layer (NNL) were measured simultaneously with temperature, salinity, and density profiles at the continental slope off the northwestern Africa. About 100 stations 5' apart in latitude and longitude were carried out over an ocean area of 6100 sq. km. Special features of the NNL variability in the area were analyzed. It was found that some structural parameters of the NNL (maximum transparency depth, that is the upper boundary of NNL; NNL thickness; maximum and total turbidity) correlate with ocean depth. On the average, thickness of the NNL in the area is 20-40% of the ocean depth. At most stations the NNL is fairly strong. In the shelf region NNL turbidity was influenced by the intensive near-shore upwelling. Formation of ''high-energy near-bottom layers'' in the shelf region resulted from passing of a mesoscale cyclonic eddy that caused redistribution of measured quantities within the entire water column.

Geochemistry of picrite basalts from the Liohi Volcano

Research of the ocean floor using the Mir submersibles carried out south of the Hawaiian Archipelago allowed to recover flows of recent picrite basalts. Lava vents are confined to a field of development of open fractures of a gjar type. Basalts represent initial lava flows in the structure of the Hawaiian volcanic archipelago. Considering contents of alkali and rare-earth elements in them, the picrite basalts of the bottom could be assigned to a series of island tholeiites. They are products of high level melting of asthenospheric matter at depth about 75-80 km as a result of decompression near a deep fracture that occurred in the lithosphere and asthenosphere. Similar picrite basalts were found in the base of the youngest volcano of the Hawaiian chain the Loihi Volcano. With respect to contents of alkali metals, these rocks are assigned to the subalkaline series of rocks formed during melting of garnet lherzolites. This could probably be explained by supply of melts from deeper levels of the asthenosphere after partial packing of an initial magma effluent fracture.

Mineral, chemical and isotopic compositions of hydrothermally altered sediments from the Middle Valley (Juan de Fuca Ridge), ODP Site 139-858

Li and Li isotopes have been measured in the clay fraction of sediments recovered from the Middle Valley hydrothermal site on the Juan De Fuca Ridge. The Li content of pure detrital clays is 51 ppm while hydrothermal clays and carbonates have lower Li (22+/-11 ppm). However, there is no clear relationship between the mineralogy of the hydrothermal alteration products and their Li content. The d7Li value of the detrital clays is +5.8?. Hydrothermal clays and carbonates have d7Li in the range of -3.9? to +7.8?; these values do not seem to be dependent on the temperature at which they formed. Modelling of the Li and Li isotope systematics indicates that the fluid from which the alteration products form is significantly enriched in Li (higher than 10000 µmol/kg) relative to pore fluids recovered from within the sediments (up to 589 µmol/kg; [Wheat, C.G., M.J. Mottl, 1994. Data report: trace metal composition of pore water from Sites 855 through 858, Middle valley, Juan De Fuca Ridge. In Mottl, M.J., Davis, E.E., Fisher, A.T., Slack, J.F. (Eds.), Proc. ODP, Sci. Res. 139: 749-755; doi:10.2973/odp.proc.sr.139.269.1994]), and that this Li is derived from sediment. Thus, the alteration products are not in equilibrium with their conjugate pore fluids; rather, the alteration minerals formed at lower water/sediment ratios. This suggests that fluid flow pathways at Middle Valley were more diffuse in the past than they are today.