Geochronology of Recent Sediments from the Upper Bonny Estuary (Niger Delta) Using Naturally Occurring Radionuclides

Abstract

The research evaluated temporal trends of naturally occurring radionuclides and their vertical distributions in sediment cores collected from three sites of diverse sedimentation regimes in Bonny Estuary, and developed and implemented sediment dating with unsupported lead-210. Radium (Ra) isotopes exhibited a very similar distribution throughout all sampled cores in each month, with 228Ra, while 226Ra displayed some variability with generally lower specific activities in dry months than in wet months. The values found in the study were all less than unity ranging from 0.3 to 0.8, which suggested sediment accretion and that the study area corresponded to zones with different sedimentation regimes. Temporal distribution of total 210Pb in the sampled cores determined via its daughter 210Po by alpha spectrometry assumed that the secular equilibrium between both radionuclides was achieved. However, the activities in Station 3 were found to be lower than those in Station 1, with Station 2 having the lowest activity registered in the samples collected in December month. The specific activities of 228Ac (228Ra), 212Pb (228Th) and 40K against depth in the sampled cores were observed to be almost equal to one another at each stratigraphic interval. The average sedimentation rate throughout the core obtained from Constant Rate of Supply model (0.068 ± 0.015g.cm-2.y-1) was nearly the same as that obtained from the Constant Initial Concentration model (0.065 ± 0.004). However, the estimated ages were quite variable with depths of various stratigraphic layers. The age of the sediment core was dated approximately 80 years, which was qualitatively validated using 137CS whose activity was definitely undetected in the basal part of the core. The sedimentation rates calculated suggested low sedimentation characterized by low energy environment dominated by weak tidal currents.