Trade-winds during the glacial cycles

Two deep-sea cores (one north, the other south of the Cape Verde Islands) have been analysed for (i) size distribution of the quartz mineral grains, (ii) mineralogy and colour of carbonate-free clay, (iii) frequency of the Globorotalia menardii foraminifera, (iv) the total carbonate, and (v) the 18O/16O deviations. The size analyses establish a general law . The plot of the logarithm of the cumulative mass percentage for particles equal to or greater than diameter D against D2 is linear above a critical particle size, which in this case is about 7 μm. The linear form can be explained by a simple winnowing theory which expresses the slope of the line in terms involving the speed of bulk flow of the trade winds, the distance along a trajectory between the coast and the core site, and a characteristic height describing the vertical distribution of dust in the air. Variations of the slope of the size distributions down the core relate to trade wind conditions during the glacial cycles. Together with the mineralogical and biological data it seems that the trades have been more ‘vigorous’ during the glacial stages.