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

Two sediment cores were collected from the Oualidia lagoon, on the Atlantic coast of Morocco, and analyzed for 210 Pb and 137 Cs activity by gamma spectrometry. The 210 Pb profiles were characterized by high activity at specific depths in each core, which were attributed to substantial increases in atmospheric 210 Pb 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 210 Pb delivery and specifying the year when the increase began. Calculated 210 Pb 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 137 Cs as a time marker to support the modified CRS chronologies for both cores. The 137 Cs 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 210 Pb detected at specific depths in the Oualidia lagoon sediment cores are a consequence of decay of radioactive 222 Rn, which displayed periodic high concentrations in the overlying atmosphere.