Abunance of Iridium and other elements from ODP Hole 121-752B (Table 1)

Following neutron activation, a study was made with the Iridium Coincidence Spectrometer (ICS) of the abundance profile of iridium in the Cretaceous-Tertiary boundary region of cores from Hole 752B taken from the Indian Ocean west of Australia. The ICS was modified to measure 13 other elements simultaneously with the Ir determination, and data are included for Co, Cr, Cs, Fe, Hf, and Se. The K-T region was sampled continuously in 1 cm increments from 8 cm below to 22 cm above the boundary, and an additional 30 spot samples were taken in the region from 10 m below to 6.7 m above.A large Ir anomaly of 3800 parts per trillion (ppt) over a background of 22 ppt was found in interval 121-752B-11R-3, 91-93 cm. This interval overlaps the extinction level of globotruncanids determined by the Shipboard Scientific Party. After correcting for the estimated clay content, no Ir over the background was found in the region from 22 cm to 10 m below the main Ir peak. Iridium above background levels, however, was observed in all of the remaining samples up to 1 m above the main Ir peak. Between 2 and 5.7 m above the main Ir peak, the Ir level averaged 59 ppt and is attributed to basaltic volcanism. Another large Ir anomaly of 1500 ppt was found in a spot sample from interval 121-752B-11R-3, 13-14 cm, and its tails extend from at least 10 cm below to at least 20 cm above the anomaly. The stratigraphic extent of this second anomaly parallels that of reworked Cretaceous nannofossils determined by the Shipboard Scientific Party, and the anomaly is attributed to reworking of K-T boundary Ir from another location.