A novel application of radionuclides for dating sediment cores from sandy, anthropogenically disturbed estuaries

Reliable sedimentation histories are difficult to obtain in sandy or anthropogenically impacted coastal systems with disturbed sediment profiles and low initial radionuclide activities. This study addresses the problem using radionuclides in sediment cores from Naples Bay estuary, Florida, USA. Non-steady sedimentation and nuclide scavenging processes are shown to limit application of traditional radiometric dating models in this system. Whole-core inventories of excess Pb activity (Pbxs) varied from 21 to 96 dpm cm 2 among sites, and initial sediment Pbxs activities were low, decreasing non-uniformlywith depth inmost cores. Activities of three radioisotopes used for sediment dating (Ra, Pb, and Cs) were compared with grain size and organic matter (OM) distributions to assess the factors that influence accumulation of radionuclides. Regression analysis indicated that radionuclide activities were more strongly correlated with OM content than with grain size parameters, and a novel OM-normalisation procedure was developed to correct for preferential nuclide associations. Normalised Pbxs profiles provide evidence for shifts in sedimentation rates and episodic erosion events in regions of the estuary where anthropogenic disturbance is known to have occurred. Our results emphasise the need to consider radionuclide scavenging by OM in sandy coastal sediments when establishing sedimentation histories. Additional keywords: Cs-137, estuaries, grain size effect, organic matter, Pb-210, preferential scavenging, radioisotopes, Ra-226.