Global analysis of in situ cosmogenic 26Al and 10Be and inferred erosion rate ratios in modern 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 the quantitative determination of landscape mass loss rates (hereafter, erosion rates) at a basin scale. Paired measurements of in situ cosmogenic 26Al and 10Be in sediment are less common but offer insight into the integrated exposure and burial history of sediment moving down slopes and through drainage basins. Prolonged 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 at least in part shielded from cosmic rays. Here, we use a global compilation of 26Al and 10Be data generated from quartz-bearing fluvial sediment samples (nCombining double low line766, including 117 new measurements) to calculate the discordance between erosion rates derived from each nuclide. We find that over 30 % of samples (nCombining double low line234) exhibit discordance (>2σ analytical uncertainty) between erosion rates derived from 10Be and 26Al, indicating sediment histories that 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 weak correlation, allowing us to infer the likelihood of sediment burial during transport in different geomorphic settings. Paired 26Al and 10Be analyses in detrital fluvial samples 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 methodological assumptions are violated. Although previous studies have found 26Al/10Be erosion rate discordance to be common in the world's largest drainage basins, our analysis suggests that such discordance also occurs regularly in basins as small as 1000 km2, indicating that sediment storage mechanisms are more complex than suggested by simple floodplain area scaling laws. Moderately sized basins (1000-10 000 km2) with low average slopes in tectonically quiescent terrains appear conducive to extended sediment storage; thus, erosion rates from such basins are lower limits due to nuclide decay during storage. We find that 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 is thus more likely to provide reliable erosion rates.