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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.
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Resistant rock layers amplify cosmogenically‐determined erosion rates
Abstract Prior numerical modeling work has suggested that incision into sub‐horizontal layered stratigraphy with variable erodibility induces non‐uniform erosion rates even if base‐level fall is steady and sustained. Erosion rates of cliff bands formed in the stronger rocks in a stratigraphic sequence can greatly exceed the rate of base‐level fall. Where quartz in downstream sediment is sourced primarily from the stronger, cliff‐forming units, erosion rates estimated from concentrations of cosmogenic beryllium‐10 (10Be) in detrital sediment will reflect the locally high erosion rates in retreating cliff bands. We derive theoretical relationships for threshold hillslopes and channels described by the stream‐power incision model as a quantitative guide to the potential magnitude of this amplification of10Be‐derived erosion rates above the rate of base‐level fall. Our analyses predict that the degree of erosion rate amplification is a function of bedding dip and either the ratio of rock erodibility in alternating strong and weak layers in the channel network, or the ratio of cliff to intervening‐slope gradient on threshold hillslopes. We test our predictions in the cliff‐and‐bench landscape of the Grand Staircase in southern Utah, USA. We show that detrital cosmogenic erosion rates in this landscape are significantly higher (median 300 m/Ma) than the base‐level fall rate (~75 m/Ma) determined from the incision rate of a trunk stream into a ~0.6 Ma basalt flow emplaced along a 16 km reach of the channel. We infer a 3–6‐fold range in rock strength from near‐surface P‐wave velocity measurements. The approximately four‐fold difference between the median10Be‐derived erosion rate and the long‐term rate of base‐level fall is consistent with our model and the observation that the stronger, cliff‐forming lithologies in this landscape are the primary source of quartz in detrital sediments. © 2020 John Wiley & Sons, Ltd.
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- Award ID(s):
- 1735676
- PAR ID:
- 10130180
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Earth Surface Processes and Landforms
- Volume:
- 45
- Issue:
- 2
- ISSN:
- 0197-9337
- Page Range / eLocation ID:
- p. 312-330
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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