Weathering under coastal hyperaridity – Late Quaternary development of spectral, textural, and gravelometric alluvial fan surface characteristics

Abstract

Hyperarid coasts develop under relatively high air humidity and abundant sea salt aerosols, resulting in physical and chemical weathering processes that distinctly differ from those present in inland deserts. However, neither the geomorphic effects of the weathering processes nor the timescales on which they alter depositional surfaces are sufficiently understood. In this study we therefore integrate spectral, textural, and gravelometric analyses based on high to very high resolution satellite and aerial remote sensing to assess the degree of post-depositional alteration of a multi-stage alluvial fan at the coast of the Atacama Desert. To further deduce the present-day as well as past intensities of responsible weathering processes we establish a morphochronology of the fan's evolution using cosmogenic 10Be exposure dating.Spectral, textural, and gravelometric parameters relatively date a succession of fan generations, assuming the weathering of fan surface sediments prior to deposition can be largely excluded. Under coastal hyperaridity, effects on the spectral surface signatures are primarily explained by the development of weathering rinds and related formation of secondary iron oxides. Spectral iron oxide ratios show little to no change during the Holocene but strongly increase during the Late Glacial, while over longer timescales an asymptotic increase is suggested. Clast breakdown can largely be attributed to salt weathering causing sublinear to roughly linear decreases in mean size, coarseness, and size variation of coarse clasts. Governed by clast breakdown as well as lateral remobilization of clasts, nano-scale surface roughness exhibits a non-monotonic, peak-shaped relationship with age, whereas the micro-relief experiences continuous smoothing due to denudation. Findings are summarized in a conceptual model showing that in hyperarid coastal environments fan surface weathering results over timescales of 104 to 105 a in a fully developed desert pavement. Two hypothetical scenarios are introduced which differ based on whether desert pavements remain stable over timescales exceeding the Late Pleistocene or not. Late Quaternary weathering intensity along the coast of the Atacama Desert is found to be related to sea surface temperature variability and sea level jointly driving the oceanic moisture supply.