Are CRN-derived denudation rates representative of contemporary natural sediment fluxes?

Disentangling human signals from geomorphic, tectonic, and climatic drivers of contemporary sediment fluxes is key to understanding the magnitude of human impacts on river basins and the landscape. Denudation rates derived from cosmogenic radionuclides (CRN) can provide a useful baseline of ‘natural’ sediment fluxes, especially in regions where little to no undisturbed catchments remain or where contemporary monitoring networks are lacking. However, their integration time can span very long timescales (up to 100kyr), which may limit their suitability for establishing contemporary natural rates of sediment export. Here, we investigate whether (and where) CRN-derived denudation rates are representative of current climatic, geomorphic, and tectonic conditions and can provide relevant contemporary baseline fluxes. We do so by first compiling hundreds of contemporary sediment yield (SY) observations from ‘quasi-natural’ catchments worldwide. We define quasi-natural catchments as ones with little to no expected disturbance to their sediment transport regime due to anthropogenic changes to the landscape or river system (e.g., land cover/land use, dams and reservoirs, mining). In addition to clear indicators of human disturbance such as the degree of regulation of the river network or the human footprint in the catchment, we base our selection of quasi-natural catchments on a combination of biome-specific thresholds and patterns of acceptable semi-natural vegetation, land cover classification, and potential natural vegetation maps. We then compare a global denudation rate model (trained on >4,000 CRN samples) against the contemporary SY observations and examine possible global and regional patterns of correlation. We further explore the potential of integrating both types of data into a combined global natural SY model, with the goal of further improving our understanding of nature- vs human-driven sediment dynamics worldwide.