Experimental study on the use of granulometric speciation for the radiometric dating of recent sediments

Theoretical and experimental studies have shown that activity concentrations of fallout radionuclides (such as ¹³⁷Cs and excess ²¹⁰Pb) decrease with particles size in aqueous suspensions. This paper is aimed at reviewing the theoretical fundamentals for granulometric speciation of radionuclides, and at exploring its practical use in the analytical context of gamma spectrometry for the radiometric dating of recent sediments, with view to: i) improving the detection of ¹³⁷Cs (since its use as independent chronostratigraphic mark is challenging in the southern hemisphere because its low fallout rate), ii) supporting refined CIC models and normalization techniques in ²¹⁰Pb-based radiogeochronologies. The work uses surface sediments sampled from the Tinto Estuary (SW Spain), affected by mining and phosphate-fertilizer industries, and from the Ankobra Estuary (Ghana), affected by intensive artisanal gold-mining. Granulometric classes have been separated by a sieving column with decreasing mesh sizes and the obtained cumulative percentage of mass mathematically described by a Rosin-Rammler particle-size distribution. The target radionuclides for gamma spectrometry were ²¹⁰Pb, ²²⁶Ra and¹³⁷Cs, complemented with ⁴⁰K, ²³⁴Th and ²²⁸Ra. Results revealed that, far from ideal experiments, under actual environmental conditions the increase in activity concentrations with decreasing particle sizes is too moderate, and in general they are affected by larger counting uncertainties due to the small available amount of mass. Indeed, there was no correlation between grain-size and ¹³⁷Cs concentrations (p = 0.25), and similarly for excess ²¹⁰Pb (p = 0.53). No effect of the organic matter content was observed in ¹³⁷Cs (p = 0.58) and excess ²¹⁰Pb (p = 0.85) concentrations. Present results pose some concerns to the general use of granulometric speciation in the context of gamma spectrometry for supporting the radiometric dating of recent sediments. A detailed discussion on the use of normalization methods is also presented.