Evidence for a recent increase in delivery of atmospheric ~(210)Pb to Oualidia lagoon, coastal Morocco

摘要

Two sediment cores were collected from the Oualidia lagoon, on the Atlantic coast of Morocco, and analyzed for Pb-210 and Cs-137 activity by gamma spectrometry. The Pb-210 profiles were characterized by high activity at specific depths in each core, which were attributed to substantial increases in atmospheric Pb-210 input to the sediment. A modified CRS model was applied to develop age-depth relations (chronologies) for the cores and calculate sediment accumulation rates, taking into account changing unsupported Pb-210 delivery and specifying the year when the increase began. Calculated Pb-210 inventories (activity/area) and fluxes (activity/area/time) depend strongly on sedimentation rates and were much higher than mean values in similar coastal systems worldwide. We attempted to use Cs-137 as a time marker to support the modified CRS chronologies for both cores. The Cs-137 profiles, however, were affected by post-depositional cesium migration in the sediment which made it difficult to identify the 1963 atmospheric bomb-testing peak, especially in the core with low sedimentation rate. We conclude that the high activities of Pb-210 detected at specific depths in the Oualidia lagoon sediment cores are a consequence of decay of radioactive Rn-222, which displayed periodic high concentrations in the overlying atmosphere.