Relief inversion and denudation dynamics in a semi-arid landscape (Araripe Plateau, NE Brazil): Insights from cosmogenic nuclides and geomorphic surfaces

Abstract

The Araripe Plateau, a relict landscape in the semi-arid northeastern region of Brazil, provides a unique setting to investigate sediment dynamics and long-term denudation rates using in situ cosmogenic nuclides (

and

). In this study, we analyze bedrock and modern river sediment samples to determine denudation rates, evaluate sediment mixing, and assess the landscape’s response to erosional processes. Our results indicate that, while bedrock samples largely reflect a state of cosmogenic secular equilibrium, river-borne sediments exhibit significantly lower

(Al-Be) ratios, pointing to a mixing process between freshly eroded material and recycled sediment from buried sources in the alluvial plain. Denudation rates in the Araripe region are low ($\le$ 20 m Myr⁻¹) and show limited variability across different slopes and precipitation levels, consistent with other slow-tectonic settings like the Anti-Atlas in Morocco. However, differential denudation between the sedimentary rocks of the Araripe Basin and the crystalline basement rocks primarily controls relief evolution, driving the process of topographic inversion along the Ceará and Pernambuco slopes. In the Parnaíba basins, while this pattern holds, watersheds developed in Palaeozoic sedimentary rocks exhibit notably higher erosion rates, indicating faster landscape evolution in these areas. A speculative analysis of the geomorphic surfaces around the Araripe Plateau suggests long-term denudation rates of 3–6 m Myr⁻¹, consistent with our cosmogenic nuclide-derived rates. These findings indicate a stable landscape with low relief, where tectonic activity has minimal influence, and lithological factors play a dominant role in controlling erosional patterns. The differential denudation rates across the plateau, supported by geomorphic metrics and the spatial distribution of laterite covers, provide insights into the ongoing evolution of this relict landscape and potential future drainage rearrangements. Our study underscores the complexity of sedimentary processes and the importance of combining cosmogenic nuclide analysis with geomorphic context to understand the interplay between erosion, sediment mixing, and landscape stability. Using a two-source mixing model, we estimate that up to 84% of river sediment in some watersheds originates from previously buried material. This trend correlates with the extent of the cumulative drainage network, suggesting that longer transport pathways enhance sediment mixing. The apparent burial times inferred from Al-Be ratios also show a negative correlation with the fraction of freshly eroded material, highlighting the role of deep-seated sediment contributions in building the cosmogenic signal. This integrated approach provides a refined perspective on sediment dynamics and denudation in slowly eroding landscapes, with broader implications for interpreting cosmogenic nuclide data in similar settings worldwide.