This initial survey found that only one sample had anomalous levels of Ir (Table T1). The other 90 samples had Ir levels ranging from 9 to 37 pg/g, average = 17 pg/g. Such low concentrations are probably reasonable for rapidly accumulating terrigenous sediments. Sample 178-1096C-2H-4, 50-51 cm (172.13 mbsf), was found to contain 88 pg/g, an Ir concentration five times the background level. To test whether this might be a meteoritic signal, two additional samples were taken at 172.10 and 172.15 mbsf, closely bracketing the anomalous sample. These samples and another split of the sample from 172.13 mbsf were irradiated and analyzed in a manner similar to the original batch. Unfortunately, the split of the anomalous sample from 172.13 mbsf was spilled during chemical processing and the Ir value has not been replicated. The two adjacent samples were found to have normal Ir concentrations of ~20 pg/g.
As a final check of the sample with anomalous Ir at 172.13 mbsf, the remaining 3.2 g of material was wet sieved at 79 and 44 µm and these fractions were examined microscopically. A similar procedure was applied to 2.9 g of sediment from a background sample from 172.28 mbsf. In both samples, the >79-µm fraction contained only a few rock and mineral grains that were probably mostly quartz. No vesicular melt particles or impact spherules typical of the Eltanin impact debris (Kyte and Brownlee, 1985; Margolis et al., 1991) were identified. The >44-µm fractions contained abundant felsic mineral grains (probably quartz and feldspars) and pyrite. Both samples were quite similar, and neither contained identifiable meteoritic impact debris. The normal Ir concentration in samples adjacent to the one at 172.13 mbsf and the absence of particulate impact debris in that sample suggest that the Ir anomaly may have been spurious, likely caused by contamination or a stray cosmic spherule.