. The Earth beneath the sea : History . No of satellite impacts of micrometeorites per m^sec 10' 10" 5 10 20 40 10- Fig. 12. Size distribution of cosmic spherules from pelagic sediments, and from micro- meteorites in oviter space. Shaded histogram = iron spherules; Ime-boiuided histogram = silicate spherules from pelagic sediments (data from Hunter and Parkin, 1960); filled circles = impact observations from satellites 1958 Alpha and 1959 Eta (computed from data in LaGow and Alexander, 1960, assuming an average particle density of ). frequency below 25 y. and a comparatively large numb
. The Earth beneath the sea : History . No of satellite impacts of micrometeorites per m^sec 10' 10" 5 10 20 40 10- Fig. 12. Size distribution of cosmic spherules from pelagic sediments, and from micro- meteorites in oviter space. Shaded histogram = iron spherules; Ime-boiuided histogram = silicate spherules from pelagic sediments (data from Hunter and Parkin, 1960); filled circles = impact observations from satellites 1958 Alpha and 1959 Eta (computed from data in LaGow and Alexander, 1960, assuming an average particle density of ). frequency below 25 y. and a comparatively large number of big (90 to 300 [i) silicate chondrules. This corroborates the idea that the cosmic material found on the ocean floor represents debris of meteorites, as suggested by Fredriksson (1959), rather than the original cosmic dust. Large numbers of magnetic spherules of unknown origin and composition, ranging in size from less than a micron to several microns, are frequently observed in sediments (see, for example, Crozier, 1960). Analyses of such bodies indicate a heterogeneous origin. Many of the magnetic spherules previously assumed to have been derived from outer space have been shown by Fredriks- son (1961) to consist of volcanic glass with inclusions of magnetite and metallic iron. Others, in size-range of appear to consist of goethite and might have been accreted by marine bacteria. It therefore appears necessary to define individual cosmic spherules not only on the basis of shape and mag- netic properties but also on chemical composition. Considering the difficulties in 23—s. Ill
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Keywords: ., bookcentury1900, bookcollectionbiodivers, booksubjectoceanbottom
