LAUSR

Abstract Detrital thermochronology has revolutionized the study of sediment provenance at orogen scales and spatial patterns in erosion at catchment scales. A strength of the method is that a handful of stream sand can be used to represent processes over landscape scales. However, it relies on both the widely recognized assumption that sand is supplied from every part of the catchment, and the implicit assumption that the fraction of eroded material that is sand-sized does not change across the landscape. These assumptions may be violated when the catchment contains spatial variations in the initial sizes of sediment produced by hillslope weathering or when abrasion during transport causes size reduction of sediment sourced from distal parts of the catchment. In either case, a detrital sample spanning a narrow range of sediment sizes (e.g., sand) may fail to represent the catchment as a whole, leading to bias in thermochronology of erosional and tectonic processes. We used forward modeling to quantify biases that can arise due to plausible abrasion rates and spatial variations in initial sediment size. Our results reveal significant reductions in the chance of detecting age populations originating from the highest, most distal parts of the landscape, leading to potentially erroneous interpretations in provenance studies. In tracer thermochronology, the biases distort detrital age distributions, leading to potentially profound misinterpretation of spatial patterns in erosion rates. Our analysis shows that the sediment size and abrasion biases increase with catchment area and relief but can be significant in small catchments ( 10 km 2 ) with moderate relief (>0.5 km). We show that the biases can be mitigated by analyzing a sufficient number of grains in multiple sediment size classes.