Non-Steady-State ¹⁴C-¹⁰Be and Transient Hillslope Dynamics in Steep High Mountain Catchments

Paired in situ cosmogenic nuclides ¹⁴C and ¹⁰Be present an opportunity to explore erosion rate disequilibria over Holocene to latest Pleistocene timescales and are a new avenue in surface processes research. ¹⁴C and ¹⁰Be concentrations in quartz from river sand collected at the outlets of five mountainous catchments in the Argentine Andes are compared in this study. River gauge and ¹⁰Be-derived erosion rates are in good agreement; however, ¹⁴C concentrations are approximately 2.7–4 times lower than expected relative to ¹⁰Be under steady-state erosion. Low ¹⁴C to ¹⁰Be ratios imply that sediment eroded from the high mountains was shielded for at least 7–15 ky. Neoglacial advances and storage in terraces may account for some of the reduced ¹⁴C concentrations but are insufficient alone. Transient storage in dynamic talus slopes in the steep topography of the High Andes provides the best explanation for the observed ¹⁴C concentrations.