Assessing the utility of 210 Pb geochronology for estimating sediment accumulation rates on river floodplains in Fiji James P. Terry, 1 Rajeev Lal 2 and Sitaram Garimella 3 1 Department of Geography, National University of Singapore, Singapore 2 Department of Nuclear Physics, Research School of Physics and Engineering, The Australian National University, Canberra, Australia 3 (formerly) Physics Division, School of Engineering and Physics, University of the South Pacific, Suva, Fiji Correspondence: James P. Terry (email: geojpt@nus.edu.sg) 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 accre- tion 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. Keywords: floodplain sedimentation, gamma ray spectroscopy, 210 Pb, sugarcane farming, Fiji Introduction and aims On Vanua Levu in northern Fiji, sugarcane farming has been the economic mainstay for over a century. The drainage basins of the Labasa River and neighbouring Qawa, Wailevu and Bucaisau rivers are the principal areas for this intensive agriculture on the north (leeward) side of the island. Over recent decades there has been growing concern that the expansion of cane farming out of flat valley bottoms, a response to favourable sugar prices in the 1950s–1960s, is causing accelerated soil erosion on marginal sloping land (e.g. Clarke & Morrison, 1987; Drysdale, 1994) and associated siltation of the Labasa River. With the farming of slopes up to 20° now commonplace, the hilly land of the lower Labasa River basin is therefore a principle source of sediment supply through runoff and erosion during tropical rainstorms. In consequence, the adjacent floodplains of the Labasa River are experiencing net sediment accumulation. The main process of floodplain accretion is repeated overbank flooding (Blake & Ollier, 1971; Gregory & Walling, 1973). Annual flood events each lay down a new layer of fine deposits on top of older alluvium. This leads to floodplain growth, as long as the area is not experiencing rapid subsidence. Within this context, this study attempts to measure the recent historical rate of net sediment accretion on the Labasa floodplain using measurements of the environmental doi:10.1111/j.1467-9493.2011.00414.x Singapore Journal of Tropical Geography 32 (2011) 102–114 © 2011 The Authors Singapore Journal of Tropical Geography © 2011 Department of Geography, National University of Singapore and Blackwell Publishing Asia Pty Ltd