Fluvial responses to external and internal forcing: Upper Holocene dynamics in a low latitude semi-arid region in South America

Abstract

River streams are sensitive to environmental changes in drainage basins in response to external and internal forcing. These changes lead to drainage channel adjustments and may alter erosion-sedimentation cycles along valleys, as well as short and long term geomorphological evolution. Concerning the low latitude semi-arid region in South America, fluvial responses to environmental changes during the Upper Holocene have still not been adequately assessed, contrasting with evaluations in river drylands located in middle latitudes. To collaborate with assessments on Upper Holocene climatic fluctuations, alluvial deposits at the Itapicuru River, located in the Brazilian semi-arid, were analyzed to understand this river's dynamics in the last 5 ka and its relationship with both intrinsic and extrinsic factors. The following analyses were performed: [1] spatial terrace distribution throughout the river's longitudinal profile, [2] stratigraphic section assessments, [3] OSL dating, to estimate surface time elaboration, and [4] age correlations with regional paleoclimatic models. In the last 2.2 ka, downcutting and lateral migration occurred at the same time due to fluvial discharge changes over centuries and decades. Semi-arid conditions, such as those currently observed, prevailed throughout the drainage basin, with wet intervals identified upstream. Humidity variations were responsible for middle valley incision and deposition, forming terrace, bars, and natural levees. These findings indicate that, at least in our case study, intrinsic factors had big importance to control the fluvial dynamics since the establishment of the semi-arid low latitude climatic conditions.