Sediment geochronology and spatio-temporal and vertical distributions of radionuclides in the Upper Bonny Estuary (South Nigeria)

Surface deposits and sediment cores were collected from the Upper Bonny Estuary, located in Southwest Nigeria, and analyzed to determine spatio-temporal and vertical distributions of radio-nuclide activities expressed in Bq·kg^?1 dry weight. The results of activities of naturally occurring radionuclides of ²²⁶Ra (15 ± 2–34 ± 3 Bq·kg^?1), ²²⁸Ra (32 ± 5–48 ± 6 Bq·kg^?1), ⁴⁰K (264 ± 29–462 ± 36 Bq·kg^?1) were found to be all within the range of typical values reported for coastal regions. Ratios of ²²⁶Ra to ²²⁸Ra suggested accretion for all samples with low sediment accumulation registered during rainy months. In addition, vertical distributions at the three sampling sites were also studied with the initial aim of establishing chronologies from the decay of excess ²¹⁰Pb. In two cores, excess ²¹⁰Pb, estimated by subtracting ²²⁶Ra from total ²¹⁰Pb on a layer-by-layer basis, exhibit relatively constant activity with discrete minima and maxima. Therefore, these cores were excluded from radiometric dating. Only the third core could be dated by the constant rate of supply model, and ¹³⁷Cs was utilized to validate the ²¹⁰Pb chronology.