Mathematics – Logic
Scientific paper
Oct 1976
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1976e%26psl..32..277b&link_type=abstract
Earth and Planetary Science Letters, Volume 32, Issue 2, p. 277-296.
Mathematics
Logic
38
Scientific paper
The distribution of 210Po and 210Po in dissolved (<0.4 μm) and particulate (>0.4 μm) phases has been measured at ten stations in the tropical and eastern North Atlantic and at two stations in the Pacific. Both radionuclides occur principally in the dissolved phase. Unsupported 210Pb activities, maintained by flux from the atmosphere, are present in the surface mixed layer and penetrate into the thermocline to depths of about 500 m. Dissolved 210Po is ordinarily present in the mixed layer at less than equilibrium concentrations, suggesting rapid biological removal of this nuclide. Particulate matter is enriched in 210Po, with 210Po/210Pb activity ratios greater than 1.0, similar to those reported for phytoplankton. Box-model calculations yield a 2.5-year residence time for 210Pb and a 0.6-year residence time for 210Po in the mixed layer. These residence times are considerably longer than the time calculated for turnover of particles in the mixed layer (about 0.1 year). At depths of 100-300 m, 210Po maxima occur and unsupported 210Po is frequently present. Calculations indicate that at least 50% of the 210Po removed from the mixed layer is recycled within the thermocline. Similar calculations for 210Pb suggest much lower recycling efficiencies. Comparison of the 210Pb distribution with the reported distribution of 226Ra at nearby GEOSECS stations has confirmed the widespread existence of a 210Pb/226Ra disequilibrium in the deep sea. Vertical profiles of particulate 210Pb were used to test the hypothesis that 210Pb is removed from deep water by in-situ scavenging. With the exception of one profile taken near the Mid-Atlantic Ridge, significant vertical gradients in particulate 210Pb concentration were not observed, and it is necessary to invoke exceptionally high particle sinking velocities to account for the inferred 210Pb flux. It is proposed instead that an additional sink for 210Pb in the deep sea must be sought. Estimates of the dissolved 210Pb/226Ra activity ratio at depths greater than 1000 m range from 0.2 to 0.8 and reveal a systematic increase, in both vertical and horizontal directions, with increasing distance from the sea floor. This observation implies rapid scavenging of 210Pb at the sediment-water interface and is consistent with a horizontal eddy diffusivity of 3-6 × 107 cm2/sec. The more reactive element Po, on the other hand, shows evidence of rapid in-situ scavenging. In filtered seawater, 210Po is deficient, on the average, by ca. 10% relative to 210Pb; a corresponding enrichment is found in the particulate phase. Total inventories of 210Pb and 210Po over the entire water column, however, show no significant departure from secular equilibrium.
Bacon Michael P.
Brewer Peter G.
Spencer Derek W.
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