Denudation rates from meteoric 10Be/9Be in lithologically heterogeneous catchments

Accurate determination of denudation rates is crucial for the quantification of Earth surface processes like soil formation and the consumption of CO2 during silicate weathering. In situ 10Be is the most widely used proxy to quantify denudation rates over millennial timescales, but this system is severely limited in settings where quartz is absent. In contrast, the meteoric cosmogenic 10Be/9Be ratio is independent of quartz presence, but one important parameter that must be known is the 9Be concentration ([9Be]) of the underlying parental bedrock. In lithologically homogenous catchments, an average upper continental crust 9Be value is commonly assumed.

However, this approach may not be appropriate in the lithologically heterogeneous catchments that we encountered along a N-S (26°S to 38°S latitude) gradient in Chile. By sampling ~100 hand-sized pebbles per catchment from the riverbed near the catchment mouth, we investigated the variability in [9Be] of different rock types from each catchment, and calculated area-weighted mean parental [9Be] via a combination with the distribution of lithologies in the catchments. We assessed the consistency of denudation rates calculated from the meteoric 10Be/9Be (Dmet) against denudation rates derived from the in situ technique (Dinsitu).

Current data from 7 catchments shows that Dmet are in very good agreement with previously published Dinsitu along Chile: a peak in D of around 0.25 mm/yr at 34°S latitude, and consistent decreases to <0.1 mm/yr in N and S directions was reproduced. This supports the versatility of the meteoric 10Be/9Be proxy and encourages its application in catchments containing multiple lithologies.