Abstract

The 87Sr/86Sr ratios and concentrations of rubidium and strontium of <100 mesh noncarbonate fractions of sediment taken from the tops of piston cores vary systematically throughout the Ross Sea. The respective ranges of variation are: 87Sr/86Sr: 0.710 to 0.726; Sr = 91 to 238 ppm, Rb = 56 to 158 ppm. These data are interpreted in terms of two-component mixing of weathering products of old sialic rocks (high 87Sr/86Sr ratio, low Sr concentration) and of young volcanic rock of basaltic composition (low 87Sr/86Sr ratio, high Sr concentration). The relevant mixing equation was derived from the data by fitting a straight line to data points in coordinates of 87Sr/86Sr and 1/Sr. The strontium concentrations of the two components were calculated from the mixing equation for assumed values of the 87Sr/86Sr ratios. For the basaltic component (87Sr/86Sr = 0.704 ± 0.001): Sr = 567 ± 143 ppm, for the sialic component (87Sr/86Sr = 0.729 ± 0.001): Sr = 82 ± 3 ppm. The sialic component consists primarily of clay minerals, but the basaltic component is represented by less-weathered detrital particles. The concentration of volcanic detritus ranges from nearly zero just north of the Ross Ice Shelf to values in excess of 30 percent in the vicinity of Ross Island and off the coast of northern Victoria Land. Quartz and feldspar correlate positively and are both concentrated in sediment collected along the edge of the continental shelf and along the coast of Victoria Land. Illite and kaolinite/chlorite are most abundant in the immediate vicinity of the Ross Ice Shelf. The mineral compositions of the sediment may have been modified by strong bottom currents which apparently limit contemporary deposition of sediment to sheltered depressions. A plot of rubidium versus strontium concentrations of the noncarbonate sediment shows a wide range of scatter along the Rb-axis which is attributed to uptake of rubidium from sea water but may also indicate inhomogeneity of the rubidium concentrations of the sialic component. The 87Sr/86Sr and 87Rb/86Sr ratios are positively correlated, but this relationship is the result of mixing and does not reflect the age of the provenance nor the time of deposition of the sediment.

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