Global analysis of in situ cosmogenic ²⁶Al/¹⁰Be ratios in fluvial sediments indicates widespread sediment storage and burial during transport

Abstract. Since the 1990s, analysis of cosmogenic nuclides, primarily ¹⁰Be, in quartz-bearing river sand, has allowed for quantitative determination of erosion rates at a basin scale. Paired measurements of in situ cosmogenic ²⁶Al and ¹⁰Be in sediment are less common but offers insight into the history of riverine sediment moving down slopes and through drainage basins. Prolonged sediment burial (>10⁵ years), a violation of assumptions underlying erosion rate calculations, is indicated by higher ²⁶Al-based than ¹⁰Be-based erosion rates due to preferential loss of shorter-lived ²⁶Al by decay when quartz is shielded from cosmic rays.

Here, we use a global compilation of ²⁶Al and ¹⁰Be data generated from quartz-bearing fluvial sediment samples (n = 624, including 121 new measurements) and calculate the discordance between erosion rates derived from each nuclide. We test for correlations between such discordance and topographic metrics for drainage basins, allowing us to infer the likelihood of sediment burial during transport in different geomorphic settings. We find that nearly half of samples (n = 276) exhibit discordance (> 1σ uncertainty) between erosion rates derived from ¹⁰Be and ²⁶Al, indicating sediment histories that must include extended burial during residence on hillslopes and/or in the fluvial system after or during initial near-surface exposure. Physical basin parameters such as basin area, slope, and tectonic activity exhibit significant correlation with erosion rate discordance whereas climatic parameters have little correlation.

Our analysis suggests that ²⁶Al/¹⁰Be erosion rate discordance occurs more regularly in basins larger than 1,000 km², particularly when such basins have low average slopes and are in tectonically quiescent terrains. Sediment sourced from smaller, steeper basins in tectonically active regions is more likely to have similar ¹⁰Be and ²⁶Al erosion rates indicative of limited storage and limited burial during residence in the hillslope and fluvial sediment system. The data and analysis we present demonstrate that paired ²⁶Al and ¹⁰Be analyses in detrital fluvial samples can provide a window into watershed processes, elucidating landscape behavior at different spatial scales and allowing a deeper understanding of both sediment routing systems and whether erosion rate assumptions are violated. Large lowland basins are more likely to transport detrital sediment that has experienced prolonged sediment storage and burial either on hillslopes and/or in fluvial networks; thus, erosion rates from such basins are lower limits due to nuclide decay during storage. Conversely, samples from smaller upland basins are more likely to provide reliable erosion rates.