Intercomparison of ²¹⁰Pb measurements at GEOSECS station 500 in the northeast Pacific

Abstract

During reoccupation of the GEOSECS-I test station in May, 1979, more than eighty 30-liter Niskin samples were collected in profile, many as replicates, for ²¹⁰Pb intercomparison measurements by the WHOI, SIO and Yale groups. In addition to the inter-laboratory comparisons, the SIO group also carried out extensive experiments to test the effect of sample scavenging method. Pb equilibration time (storage effect), and filtration process on the measured ²¹⁰Pb results. The intercomparison measurements indicate that there is a general agreement between the various sets of data. The sample set which allows a direct comparison at the same depth was available in most cases only between two of the three groups. The direct paired comparison shows that (1) the WHOI data are systematically 3% lower than the SIO data; (2) there are no systematic differences observed between the SIO and Yale data although the scatter is rather large; (3) the Yale data are systematically higher than the WHOI data by about 8%. The SIO experiments show that (1) the two scavenging methods employed (Fe(OH)₃ and Co-APDC co-precipitation) yield identical ²¹⁰Pb results; (2) variation of Pb carrier equilibration time or of storage time has no discernible effect; (3) the filtration apparatus and procedure employed at this station do not result in ²¹⁰Pb loss or contamination. The ²¹⁰Pb profile structure and absolute concentration measured earlier at the same location (GOGO-II test station and GEOSECS station 347) agree with those of station 500 within 10%. The present profile shows a minimum ²¹⁰Pb concentration around 500 m depth, marking the penetration depth of the flux of excess ²¹⁰Pb from the atmosphere. There is a mild mid-depth maximum around 2500-3000 m. The ²¹⁰Pb/²²⁶Ra activity ratio decreases monotonically from about 1 at the ²¹⁰Pb minimum to about 0.5 near the bottom. The particulate ²¹⁰Pb profile shows a systematic increase from the subsurface water to the bottom water by a factor of 5. This feature has been observed in many GEOSECS particulate ²¹⁰Pb profiles.