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The abundance and composition of the coarse-sand fraction (250 μm-2 mm) of samples from Deep Sea Drilling Project Sites 579 and 580 were investigated to determine the chronology of ice rafting at these locations. The ice-rafting signal at these sites is reduced relative to the generalized ice-rafting chronology of the North Pacific (Kent et al., 1971) because of increased distance to the ice-rafting source areas. These data, however, do record the general increase in ice-rafting importance as sediment age decreases, especially between 0 and 1.0 m.y. ago. In addition, peaks in ice-rafting importance since 1.0 m.y. ago at Sites 579 and 580 are generally synchronous with periods of major ice-rafting influence throughout the North Pacific. The first occurrence of ice-rafted debris at the Matuyama/Gauss magnetic boundary supports a proposed age of approximately 2.47 m.y. for the onset of major Northern Hemisphere glaciation. The coarse-sand fraction at Sites 579 and 580 contains a significant component of non-ice-rafted material, particularly biogenic particles and volcanic pumice. Under these conditions, the standard assumption that the weight percent of the coarse-sand fraction reflects variations in ice-rafting importance may not be valid. For samples taken at some distance from the source of ice-rafted debris, both the abundance and the composition of the coarse-sand fraction must be considered to obtain a valid measure of the influence of sediment transport via ice rafting.

The Occurrence, Abundance, and Composition of Ice-Rafted Debris in Sediments from Deep Sea Drilling Project Sites 579 and 580, Northwest Pacific