BEYOND EROSION RATES: SEDIMENT GENERATION AND TRANSPORT DYNAMICS REVEALED BY IN SITU COSMOGENIC 26AL/10BE CONCENTRATION RATIOS

Quartz from 1000s of river sand samples has been analyzed since the 1990s under the assumption that the concentration of in situ produced cosmogenic nuclides is directly related to the rate at which the source drainage basin erodes. Recently published ²⁶Al/¹⁰Be ratio data from eastern Australia rivers suggest that the history of river sand there includes significant burial prior to and/or during transport through river networks. This finding (lower than expected ²⁶Al/¹⁰Be ratios) has implications both for understanding the behavior of sediment as it moves downslope and then downstream, and for the veracity of cosmogenic nuclides as erosion rate monitors.

To learn more about sediment behavior in drainage basins, we compiled a global data set of published ²⁶Al and ¹⁰Be concentration measurements made in quartz purified from river sediment (n = 645). Our data set includes 117 new measurements of ²⁶Al in samples with previously-published ¹⁰Be data. We then performed morphometric and climatological analyses of each drainage basin.

Nearly two-thirds (n = 367) of measured ²⁶Al/¹⁰Be ratios are lower than expected under a simple exposure/erosion scenario. We find that basin mean slope and basin area exhibit statistically-significant correlations with ²⁶Al/¹⁰Be ratios. Smaller, steeper basins are more likely to shed sediment with ²⁶Al/¹⁰Be ratios indicative of constant near-surface exposure. Larger, low-slope basins are more likely to have sediment with ²⁶Al/¹⁰Be ratios indicative of burial during hillslope and/or river transport. Basins of all sizes from tropical regions tend to have lower-than-expected ²⁶Al/¹⁰Be ratios.

Measuring ²⁶Al along with ¹⁰Be indicates that sediment found in many river channels has a history that includes either burial during or after near-surface exposure or non-random quartz distribution. Burial revealed by lowered ²⁶Al/¹⁰Be ratios can be caused by bioturbation-induced vertical mixing on hillslopes, and/ or extended sediment storage in terraces during transport down river networks. ²⁶Al/¹⁰Be ratios below those indicative of steady erosion, such as we found in the majority of basins for which both nuclides have been measured in sediment, imply loss of nuclides to radio-decay or overestimation of integrated nuclide production rates, and thus overestimation of long-term erosion rates.