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