The dependence of meteoric 10Be concentrations on particle size in Amazon River bed sediment and the extraction of reactive 10Be/9Be ratios

Concentrations of meteoric Be-10 extracted from detrital Amazon River sediment are strongly particle size-dependent. Such grain size dependency presents a formidable obstacle to routine applications of meteoric cosmogenic Be-10 to basin-wide erosion studies. In this study, we explore means to eliminate these grain size effects in bedload from rivers of the Amazon basin by measuring Be-10/Be-9 ratios after selective chemical extraction of reactive authigenic phases. These phases mainly comprise Fe-Mn-(hydr)oxides that are the main carriers of Be-9 and Be-10. We explore the distribution of Be-10 and its stable Be-9 counterpart between these phases and the partitioning of Be into remaining silicate fractions. This extraction procedure was carried out on bedload samples comprising the three main tectonic units of the Amazon basin, namely the Andes, the Guyana and Brazilian Shields, and the central Amazonian lowlands. For all samples, extracted Be-10 concentrations show a strong decrease with increasing particle size, with up to 20 times more Be-10 extracted from the finest analyzed (<30 mu m) grain size when compared to the coarsest (90-125 mu m) grain size. We attribute this decrease in Be-10 concentrations mostly to subsequent dilution by quartz in coarse bedload. However, when normalized to stable Be-9 concentrations that are also extracted from reactive phases, grain size effects are effectively removed and the resulting "reactive" Be-10/Be-9 ratio is uniform for all particle sizes. The reactive Be-10/Be-9 ratio thus offers important advantages over methods employing single meteoric Be-10 concentrations, such that this ratio can now be developed as quantitative tracer for Earth surface processes. (C) 2012 Elsevier B.V. All rights reserved.