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.