Supplement to: A global rate of denudation from cosmogenic nuclides in Earth’s largest rivers

Sampling large river´s sediment at outlets for cosmogenic nuclide analysis yields mean denudation rates of the sediment producing areas that average local variations in denudation commonly found in small rivers. Using this approach, we measured in situ cosmogenic 26Al and 10Be concentrations in sands of >50 large rivers over a range of climatic and tectonic regimes covering 32% of Earth’s terrestrial surface.
River samples were processed in the Helmholtz Laboratory for the Geochemistry of the Earth Surface (HELGES) (von Blanckenburg et al., 2016). 10Be/9Be ratios were measured by Accelerator Mass Spectrometry (AMS) at the University of Cologne and normalized to the KN01-6-2 and KN01-5-3 standards. Denudation rates were calculated using a time-dependent scaling scheme according to Lal/Stone ”Lm” scaling (see Balco et al., 2008) together with a sea level high latitude (SLHL) production rate of 4.13 at/(gxyr) as reported by Martin et al. (2017).
Measured in the mineral quartz, the cosmogenic nuclides 26Al and 10Be provide information on how fast Earth´s surface is lowering through denudation. If sediment is however stored in catchments over time spans similar to the nuclides half-lives (being 0.7 Myr and 1.4 Myr for 26Al and 10Be, respectively), the nuclide´s budget is disturbed, and meaningful denudation rates cannot be calculated. The ratio of 26Al/10Be informs us about these disturbances. In 35% of analyzed rivers, we find 26Al/10Be ratios significantly lower than these nuclides´ surface production rate ratio of 6.75 in quartz, indicating sediment storage and burial exceeding 0.5 Myr. We invoke mainly a combination of slow erosion, long transport, and low runoff for these low ratios.
In the other 65% of rivers we find 26Al/10Be ratios within uncertainty of their surface production-rate ratio, indicating cosmogenic steady state, and hence meaningful denudation rates can be calculated. For these rivers, we derive a global source-area denudation rate of 140 t/km^2/yr that translates to a flux of 3.10 Gt/yr. By assuming that this sub-dataset is geomorphically representative of the global land surface, we upscale this value to the total surface area for exorheic basins, thereby obtaining a global denudation flux from cosmogenic nuclides of 15.1 Gt/yr that integrates over the past 5 kyr.
In Table S1, we provide detailed 10Be nuclide production rates and their correction due to ice shielding and carbonates that are necessary to calculate denudation rates. We provide International GeoSample Numbers (ISGN) for samples used in the analysis, except values that were compiled from published sources. We then compare these denudation rates, converted to sediment fluxes, to published values of sediment fluxes from river load gauging. We find that our cosmogenic nuclide-derived sediment flux value is similar, within uncertainty, to published values from cosmogenic nuclides from small river basins (23 Gt/yr) upscaled using a global slope model, and modern sediment and dissolved loads exported to the oceans (23.6 Gt/yr). In Table S3, we compiled these modern sediment loads and give their references. We also compiled runoff values (mm/yr) from published sources (Table S2) that are used to infer what controls denudation rates.
For more details on the sampling and analytical methods, please consult the data description part of this publication.