Radium-226 measurements from three Geosecs intercalibration stations

This paper presents the results of²²⁶Ra intercalibration measurements made by three groups of investigators (LDGO, USC and SIO) on the seawater samples collected in profile at three Geosecs intercalibration stations. A common radium standard prepared from an NBS bulk standard for the Geosecs program has been adopted by all groups. At Geosecs-I station in the Northeast Pacific, the new²²⁶Ra results obtained from reoccupation of the station show that the agreement of the three groups has been significantly improved over the initial comparison made in 1970.At Geosecs-II in the Northwest Atlantic, the initial comparison of the²²⁶Ra profiles showed that the USC data were systematically higher than the SIO data by 1 radium unit of 10^?14 g/kg. This corresponds to a relative difference of 20% due to the very low radium content of Atlantic waters. The new results obtained from reoccupation of the station show that both the USC and SIO data are consistent with their previous data. Thus, a systematic difference of 1 radium unit still exists. However, the new LDGO profile falls in between those of USC and SIO, lying closer to the SIO profile.At Geosecs-III station in the Southwest Pacific, the²²⁶Ra measurements show that the LDGO data are systematically higher than the SIO data by about 10% above 3.4 km depth. Below this depth, both sets of data agree and show a sharp decrease in radium concentration. This radium discontinuity corresponds to the benthic front which is a density discontinuity separating the Deep and Bottom Water in the South Pacific.