Assessing the utility of 210 Pb geochronology for estimating sediment accumulation rates on river floodplains in Fiji

Low‐energy gamma ray spectroscopy has been employed to estimate floodplain sedimentation rates using measurements of 210 Pb in floodplain alluvium. The utility of the technique is assessed through the analysis of excess (unsupported) 210 Pb profiles in three sediment cores taken from the floodplain of the Labasa River on Vanua Levu in northern Fiji. A low‐energy germanium spectrometer (LEGe) was used for the nondestructive determination of excess 210 Pb in a region cultivated intensively with sugarcane. Measured average historical ( c. 25 years) vertical accretion rates are between 2.2 and 4.4 cm yr −1 . The findings are broadly comparable with published sedimentation rates from analyses of radionuclide profiles elsewhere in the tropical South Pacific Islands, but the rates are higher than those measured previously at the same Labasa River sites using 137 Cs profiles. Accelerated soil erosion owing to cane burning and land tillage seems to be largely responsible for sediment production, although flood‐related effects such as channel accretion by coarse bedload and the emplacement of large organic debris also influence floodplain sedimentation. However, application of the 210 Pb technique in Fiji (and perhaps neighbouring island countries) is found to have serious drawbacks compared to the more robust 137 Cs method, owing principally to the low 210 Pb concentrations in the sandy alluvial sediment tested.