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Analysis has been made of tephra in deep-sea sedimentary cores downwind from Pleistocene and late Pliocene eruptions on the Balleny Islands in the southwest Pacific. The volcanic glass is finely dispersed and megascopically indistinguishable in the cores examined. Despite modification of the vertical distribution
by several causes, volcanic maxima can be correlated between cores separated by up to 2,000 n. mi. Paleomagnetic, micropaleontologic, and sedimentologic methods have been used.
The glass shards are mostly transparent, bubble-walled or platy in shape, and are silt size or smaller. The grain-size distribution of glass strongly parallels that of other sedimentary constituents wherever submarine transport or prolonged bioturbation has occurred, as determined by X-ray radiography. Concentration of glass also varies systematically with minor sedimentation-rate fluctuations, as independently shown by variations in microscopic manganese nodules and ice-rafted debris concentrations. Utilization of fine volcanic glass by siliceous plankton has resulted in productivity and diversity changes, and, perhaps, a corresponding modification of the glass distribution. High-power transmission and scanning electron microscope methods show that the relative amounts of glass finer than 10 microns decreased rapidly during volcanic episodes.
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