Sand, Gravel, Cobbles, and Boulders: Detrital Thermochronology Shows that One Size Does Not Tell All.

Sierra Nevada (Calif. & Nev.); California

Detrital thermochronology has been used to measure sediment source elevations, and thus to quantify spatial variations in sediment production and erosion in steep mountain catchments. Samples commonly include a small fraction of the sediment sizes present on mountain streambeds, which according to previous modeling may not adequately represent sediment production where hillslope sediment sizes vary or where sediment breaks down during transport. Here we explore what can be learned from multiple sizes by quantifying source elevation distributions for 12 sediment size classes collected from Inyo Creek in the eastern Sierra Nevada, California. To interpret these data, we use a new analytical framework that identifies both the elevations where sediment sources deviate from catchment hypsometry and the likelihood that observed cumulative deviations could occur by chance. We find that sediment in four gravel and cobble size classes originates preferentially from higher elevations, either because erosion rates are faster or because these sizes are disproportionately represented in the sediment from high elevations. Conversely, boulders in the stream originate mostly from low elevations near the sample point, possibly reflecting the breakdown of boulders from high elevations during transport. While source elevations of finer sediment sizes are statistically indistinguishable from hypsometry, we show that these sizes are unlikely to be consistent with uniform sediment production because they cannot be considered in isolation from the coarser sizes. Our source elevation distributions from sand, gravel, cobbles, and boulders show that no one size can tell the rich story of sediment production and evolution, and highlight opportunities for future work. Plain Language Summary: Geochemical fingerprints in stream sediment can be used to trace individual clasts back to their points of origin and thus to understand how sediment is generated by weathering and erosion on catchment slopes. We apply the tracing method to all the sizes present in a steep mountain stream, ranging from sand to boulders, expanding on previous studies that have studied a narrower range of sizes. We also introduce a new analytical framework for evaluating whether the observations are statistically meaningful and to pinpoint the parts of the landscape with excesses and deficits in the amount of sediment being produced. We find that sediment origins differ across sizes: cobbles in the stream originate mostly from high elevations, boulders originate from low elevations near the sample point, and finer sizes originate from all over the landscape. Comparing these observations from the stream to direct measurements of sediment size on hillslopes provides strong evidence for sediment breakdown during transport. Our results represent an unusually large data set for quantifying the origins of all sizes of sediment across a catchment and highlight opportunities for future research on how sediment evolves on its journey from slopes to streams. Key Points: We present 699 new apatite (U‐Th)/He ages from 12 size classes from Inyo Creek, California, and a new analytical framework to interpret themCobbles and gravel originate from high elevations, boulders originate from low elevations and finer sizes originate from all elevationsNo one size adequately represents spatial patterns of sediment production across the landscape [ABSTRACT FROM AUTHOR]

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