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

Abstract

Concentrations of meteoric 10Be 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 10Be 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 10Be/9Be ratios after selective chemical extraction of reactive authigenic phases. These phases mainly comprise Fe–Mn-(hydr)oxides that are the main carriers of 9Be and 10Be. We explore the distribution of 10Be and its stable 9Be 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 10Be concentrations show a strong decrease with increasing particle size, with up to 20 times more 10Be extracted from the finest analyzed (<30μm) grain size when compared to the coarsest (90–125μm) grain size. We attribute this decrease in 10Be concentrations mostly to subsequent dilution by quartz in coarse bedload. However, when normalized to stable 9Be concentrations that are also extracted from reactive phases, grain size effects are effectively removed and the resulting “reactive” 10Be/9Be ratio is uniform for all particle sizes. The reactive 10Be/9Be ratio thus offers important advantages over methods employing single meteoric 10Be concentrations, such that this ratio can now be developed as quantitative tracer for Earth surface processes.