Cosmic Dust in Recent Deep-Sea Sediments

Abstract

By means of a simple density-measuring technique, black magnetic cosmic spherules, found in the deep-sea surface sediments, can be divided into two main groups. Further analysis shows the spherules of the first group (density about 3) to be a fine-grained magnesium-rich olivine; and those of the second group (density about 6) to be a partially oxidized nickel + iron alloy. These results strongly indicate a meteoritic origin, and, following Opik 1956, it is suggested that the spherules are derived from the Zodiacal Cloud. The expected amount of zodiacal material accreted to the earth is in fair agreement with direct observation, and it is tempting to regard the cloud as a swarm of small dust flakes produced by the collision of meteoritic (or asteroidal) bodies. Interaction with the upper atmosphere, at speeds near the earth’s escape velocity, would cause some of the larger flakes to melt, and in the case of olivine this would account for its blackened magnetic state. Only the magnetic fraction of the sediments have been examined; it is not known whether other extra-terrestrial material is present in the non-magnetic portion. Over 260 spherules were extracted from two Atlantic samples and one Pacific sample. A very simple method for comparing the recent rates of sedimentation for the various parts of the oceans is suggested. The size distributions of the spherules are discussed in terms of Whipple’s micro-meteorite theory. The minimum size of the iron spherules, occurring at about 15 μ diameter, is used to account for an unexpected depression or ‘forbidden region’ in the stony size distribution near 60 μ . Since the smallest stony spherules found were about 18 μ , it may be suspected that our observations were limited to sizes above about 15 μ . This may be true, as settling-out techniques had to be used in order to rid the sediment of fine magnetite grains, particularly troublesome in the Atlantic samples. However, on occasions the settling process was omitted for the Pacific sediment and careful attention paid to the smaller particles; no spherules below about 15 μ could be found. It is very important to obtain the correct lower limit of the size distributions, and the examination of magnetite-free sediments would be extremely helpful in any future work.