ASPECT-DEPENDENT ESCARPMENT RETREAT, SEDIMENT MOBILIZATION, AND SLOPE EVOLUTION UNDER HYPERARID CONDITIONS

Hillslope aspect has a prominent global influence on slope morphology. However, the relative role of micro-climatic factors in escarpment evolution and retreat pattern is still unknown. Here, we quantify aspect-dependent cliff retreat, clast transport rates, and slope and rill dynamics based on high-resolution (0.5 m²) LiDAR-derived topographic dataset, field observations, clast size measurements, and cosmogenic ¹⁰Be concentrations. Utilizing these datasets, we address the influence of aspect on cliff retreat and sediment transport and examine its long-term imprint on escarpment evolution.

Topographic analysis of escarpment and sub-cliff slopes in the hyperarid Negev (Israel) reveals a systematic decrease in cliff gradient between north- and south-facing slopes (mean gradient of 40 and 35, respectively) along with decreasing grain-size of the sediment derived from the cliffs. Coarser sediments characterize the north aspect and can explain its steeper rills. Escarpment retreat rates were evaluated using ¹⁰Be and extrapolated profiles of dated talus flatirons which are now disconnected from the active cliff. Our results highlight a systematic decrease in the long-term escarpment retreat from south-facing (80-90 m) to north-facing slopes (30-60 m). Furthermore, the transport rate of the clasts on the pediment is also aspect-dependent with southern clast transport rate ~5 times faster (0.05 - 0.25 cm year^-1). The clasts along these pediments are mobilized by sheetwash during rare transport events. Our results demonstrate that bedrock weathering of the cliff-face, and the related grain size distribution of the sediment delivered to the slopes, is a first-order factor controlling escarpment retreat rate and escarpment topography. The morphology of the escarpment covary with the solar radiation flux and the latter has a direct effect on the bedrock weathering pattern.