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

Abstract Paired in situ cosmogenic nuclides 14 C and 10 Be present an opportunity to explore erosion rate disequilibria over Holocene to latest Pleistocene timescales and are a new avenue in surface processes research. 14 C and 10 Be 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 10 Be‐derived erosion rates are in good agreement; however, 14 C concentrations are approximately 2.7–4 times lower than expected relative to 10 Be under steady‐state erosion. Low 14 C to 10 Be 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 14 C 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 14 C concentrations.