Spatial and temporal replicability of meteoric and in situ¹⁰Be concentrations in fluvial sediment

Abstract

Cosmogenic isotopes, short‐lived radionuclides, elemental concentrations and thermochronometric indicators are measured in river sand to quantify erosion rates and trace sediment sources, and/or infer erosional processes. Interpretations of detrital sediment analyses are often based on the rarely tested assumption of time‐invariant tracer concentration. A better understanding of when and where this assumption breaks down and what sampling strategies minimize temporal and small‐scale spatial variance will improve science done using detrital river sediment. Here, we present new and previously published spatial and temporal replicates measured for in situ and meteoric 10Be (10Bei and 10Bem, respectively). Our new data include 113 replicate pairs, taken from agricultural and/or tectonically active watersheds in China months to millennia apart and spatial replicates taken up to 2 km apart on the same day. The mean percentage difference is 10% (−122% to 150%) for both systems considered together; the mode is close to 0% for both systems; and 36% of pairs of samples replicate within our analytical accuracy at 2σ. We find that 10Bei replicates better than 10Bem (p < 0.01). 10Bei replicability is worse in steeper basins, suggesting that stochastic processes (i.e. landslides) affect reproducibility. 10Bem replicability is worse in larger basins, suggesting non‐conservative behavior of 10Bem as sediment moves downstream. Our results are consistent with the few previously published replicate studies. Considering all replicate data in a wide range of landscapes, in areas with deep erosional processes, replicability is poor; in other areas, replicability is good. This suggests that, in steep, tectonically active, and/or agricultural landscapes, individual detrital sediment measurements do not represent upstream rates as well as larger populations of samples. To ensure that measurements are representative of the upstream watershed, our data suggest that samples be amalgamated either over time or from several places close by in the same channel. Copyright © 2017 John Wiley & Sons, Ltd.

Cosmogenic 10Be in river sediment: where grain size matters and why

“…concentrations 10 Be concentrations in quartz grains depend on the 10 Be production rate and the exposure time of the grain to cosmic rays (Gosse and Phillips, 2001). Processes that preferentially transport sediment from locations with contrasting 10 Be production rates (i.e.…”

“…A certain sample location provides a spatially and temporally representative sediment sample when the sediment from different sources is sufficiently mixed (Binnie et al, 2006;Neilson et al, 2017;Niemi et al, 2005;Yanites et al, 2009). Because 10 Be production rates decrease exponentially with depth (Gosse and Phillips, 2001), hillslope sediment that is excavated over a larger depth interval by landslides will obtain a larger variation in 10 Be concentrations than sediment transported by diffusive processes near the surface (Aguilar et al, 2014;Belmont et al, 2007;Binnie et al, 2007;Brown et al, 1995;Puchol et al, 2014;Sosa Gonzalez et al, 2016a, b;Tofelde et al, 2018;West et al, 2014). In soil-mantled landscapes, bioturbation by burrowing animals and tree throw (Gabet et al, 2003) result in well-mixed surface layers with uniform 10 Be concentrations (Brown et al, 1995;Granger et al, 1996; (Fig.…”

Arrested development: Erosional equilibrium in the southern Sierra Nevada, California, maintained by feedbacks between channel incision and hillslope sediment production

“…6), even though the samples were collected and prepared by two different research teams at two different times and analyzed at two different AMS facilities. Replicate, independent erosion rate measurements such as these are valuable because they are rare (e.g., Balco et al, 2013;Sosa Gonzalez et al, 2017). The similarity in the inferred erosion rates instills confidence that the Dartmouth and Wyoming data sets can be combined without bias, despite any differences in sample collection, preparation, and analysis techniques.…”

Denudation systematics inferred from in situ cosmogenic 10Be concentrations in fine (50–100 µm) and medium (100–250 µm) sediments of the Var River basin, southern French Alps

“…Understanding landscape dynamics and denudation processes is crucial for human activities: for example, soil formation rates (Heimsath et al, 1997;Stockmann et al, 2014) control agricultural resources (Montgomery, 2007), whereas landslide occurrences (Hovius and Stark, 2007;Yu et al, 2019) are key to managing natural hazards (Kirschbaum et al, 2010). Denudation is the combined effect of physical erosion and chemical weathering processes, and it may vary with time.…”