El Niño forcing on ¹⁰Be-based surface denudation rates in the northwestern Peruvian Andes?

High magnitude precipitation events provide large contributions to landscape formation and surface denudation in arid environments. Here, we quantify the precipitation-dependent geomorphic processes within the Rio Piura drainage basin located on the Western Escarpment of the northern Peruvian Andes at 5°S latitude. In this region, monsoonal easterly winds bring precipitation to the >3000m asl high headwaters, from where the annual amount of precipitation decreases downstream toward the Pacific coast. Denudation rates are highest in the knickzones near the headwaters, ~200-300mm ky^-1, and sediment discharge is limited by the transport capacity of the channel network. Every few years, this situation is perturbed by westerly, wind-driven heavy precipitation during El Niño events and results in supply-limited sediment discharge as indicated by bedrock channels.The detailed analysis of the stream-long profiles of two river basins within the Rio Piura catchment reveals a distinct knickzone in the transition zone between the easterly and westerly climatic influences, suggesting an En Niño forcing on the longitudinal channel profiles over at least Holocene timescales. Measured trunk stream catchment-wide denudation rates are up to ca. 300mm ky^-1 and decrease successively downstream along the river profiles. Denudation rates of tributary rivers are ca. 200mm ky^-1 near the plateau and show a stronger downstream decreasing trend than trunk stream rates. This suggests that the landscape is in a transient stage of local relief growth, which is driven by fluvial incision. This corroborates the results of paleoclimate studies that point towards higher El Nino frequencies during the past ca. 3000years, leading to higher runoff and more erosion in the trunk channel compared to the hillslopes and thus growth of local relief. Downstream increases in channel gradient spatially coincide with the reaches of highest precipitation rates during El Niño events, we therefore interpret that Holocene landscape evolution has largely been controlled by climate. The ky-timescale of the ¹⁰Be data together with the transience of the landscape implies that El Niño events in northwestern Peru have occurred since at least the Holocene, and that adjustment to channel incision is still taking place.