333 resultados para Ford, George R.
em Publishing Network for Geoscientific
Resumo:
Organic-rich sediments (sapropels) deposited in the Mediterranean are presumed to have formed during periods of increased productivity, and/or deep water oxygen depletion, possibly including the development of sulfidic conditions (euxinia). Geochemical redox proxies (Re, Mo, Mo isotopes, V, Fe/Al, and multiple S isotopes) in 8 sapropels from the Pleistocene confirm water column euxinic conditions of varying intensity during sapropel deposition. These same proxies indicate an oxic origin for hemipelagic sediments deposited between sapropel-forming episodes. In one intensively sampled sapropel, deposited between 1.450 and 1.458 Ma, changing concentrations of organic carbon, Ba, Re, Mo, V, and Fe/Al track one another closely, reflecting coupling between water column euxinia and biological productivity. Multiple S isotope data from this sapropel suggest that the redox interface where oxidative sulfur cycling occurred was present in the sediments during hemipelagic sedimentation, but moved into the water column during sapropel deposition. Molybdenum isotopes of these 8 sapropels encompass a range of values (d98Mo = +0.2 to +1.7), but are all 98Mo-depleted relative to seawater (d98Mo = +2.3 per mil), suggesting that quantitative removal of Mo did not occur. This finding contrasts with modern Black Sea sediments. In general, Re/Mo ratios in sapropels are greater than in modern seawater, implying that the water column was not sufficiently sulfidic during sapropel-forming episodes to induce complete removal of both these elements. Surprisingly, the heaviest d98Mo values are found within hemipelagic sediments. Very few of the hemipelagic samples preserve the negative d98Mo values commonly associated with modern oxic marine sediments. Many of the hemipelagic samples also contained higher concentrations of Re and Mo than are common in oxic sediments. These features may be attributable to diffusion from the sapropels of a 98Mo-enriched component into the hemipelagic sediments.
Resumo:
New surveys were completed and data from the field sheets were kindly furnished by the U. S. Coast and Geodetic Survey to the Woods Hole Oceanographic Institution for use in dredging and coring operations. This field work, first reported in 1936, was continued from time to time until 1941 as new soundings became available. Rock dredging and coring has been carried out in every major canyon on the slope from Corsair Canyon at the tip of Georges Bank to Norfolk Canyon off the entrance to the Chesapeake. Numerous cores have also been taken from the areas in between; and while the whole slope from Georges to the Chesapeake has not been covered, it is believed that no significant areas have been missed. In the following report the tows and cores will be described by areas from Georges Bank southwards, as the same region was revisited in successive years. The various samples, however, will be referred to by number followed by the year in which they were taken. The material is in storage in the Woods Hole Oceanographic Institution and in the Museum of Comparative Zoology at Harvard University.
Resumo:
Chemical and mineralogical compositions of ferromanganese oxide coatings on rocks dredged from the New England Seamounts, the Sierra Leone Rise and the Mid-Atlantic Ridge near the Equator have been determined in an investigation of regional differences in Atlantic ferromanganese deposits. Most encrustations are clearly of hydrogenous origin, consisting mainly of todorokite and delta MnO2, but several recovered from the equatorial fracture zones may be hydrothermal accumulations. Differences in the chemistry of the water column and in growth rates of the ferromanganese coatings may be important in producing this regional contrast in composition. Fine-scale changes in element abundances within the encrustations indicate that the nature of the substrate has little influence on compositional variations.
