Evidence for a recent increase in delivery of atmospheric 210 Pb 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.