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

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. (orig./WB)