Discordant 26Al/10Be ratios as an indicator of bedrock plucking: case studies from northern Australia

Abstract

With the number of multi cosmogenic nuclide data from detrital samples increased, it becomes apparent that discordant ²⁶ Al/¹⁰ Be ratios are not exception but common. Traditionally, depressed ²⁶ Al/¹⁰ Be ratios, below the nominal production ratio of 6.8, have been interpreted as prior burial. However, in northern Australia, such scenario is highly unlikely as the region is subject to intensive annual floods and therefore river channels normally lack long-term sediment storage. Another possible interpretation for lower ²⁶ Al/¹⁰ Be ratios is non-steady state erosion, such as stochastic bedrock plucking. Such interpretation has an important implication to the sediment source and production mechanism, and therefore to the basin-wide erosion rate approach that assumes a steady-state erosion at sediment source. In this study, we measured ¹⁰ Be and ²⁶ Al concentrations from three different waterfall sites in northern Australia with contrasting lithological and physical characteristics, in an attempt to capture depleted ²⁶ Al/¹⁰ Be ratios. Our results indicate that ²⁶ Al/¹⁰ Be ratios from two sandstone-dominated sites show consistently lower values (4.3-6.1), consistent with non-steady erosion interpretation, whereas data from a quartzite-dominated site (5.9-6.9) are indistinguishable from steady-state interpretation. Detrital samples collected downstream at each site indicate the similar trend as respective bedrock sites, implying that sediments are largely derived from the waterfall bedrock surfaces. In this paper, we discuss the results in the context of lithological difference and physical erosion mechanism between the sites. © The Authors