74 appears to be from the Bravo Crater area, the lagoonward distribution of this radionuclide may serve as a useful tracer. Cobalt-€0 is produced mainly by the reaction 360 (ny) ©°co. Cobalt-59 for this reaction may have been supplied from cobalt present in iron, which was used in large quantities for device and barge construction (Schell, 1975a; Adams et al., op. cit). Compared to the typical 23942405,8009 ratios of 5-10 found in lagoon sediments, lower ratios and high concentrations of 696, are present in both Bravo and Zuni Crater sediments. The absence of high concen- trations of 0c from crater sediments collected in Tewa Crater and at Station B-19 can be explained again by hypothesizing that surface sediments in this region are covered by (or diluted with) material transported to this region from the reef perimeter to the east. The relative depletion of 60 Co (ts, = 5.27 yrs.) from Station B-30 sediments can be explained by decay of the radio- nuclide between the Baker (1946) and post-Baker (1954-58) testing periods. 5.1-7 Distribution of the ordering sequence of radionuclide concentrations measured at each station. The distribution of the several radionuclides measured in surface sediments across the lagoon show only occasional patterns which are useful in tracing the specific origin of the radionuclides measured at each station. In an attempt to make more use of the concentration data, it was found that there exists patterns in the areal distribution of the ordering sequence of radionuclide concentrations. Except as noted in Fig. 18, the ordering sequence found can be separated into two types, where the concentrations of 23942405 24 ny and 155ey were always greater than the concentrations of 20755 13766 or 6009, 24) Am and 155 The first type is for the ordering of 23942405 Eu. The ordering sequence Pu >Eu >Am predominates across the lagoon, while the sequences Eu > Pu > Am, and Pu > Am > Eu were found in the