ROUGHNESS EVOLUTION THROUGH TIME AS A MEASURE FOR SUBAERIAL ROCK WEATHERING ON DESERT SURFACES

Emerging 3D surveying technologies, such as LiDAR and/or automated photogrammetry, are significantly advancing our ability to readily acquire spatially extensive and/or repeated high-resolution roughness data for natural terrains. Here, we examine the spectral (i.e., scale-dependent) evolution of surface roughness through time as a new measurable that can offer unique and quantitative insights into geomorphic surface processes. We focus on the subaerial weathering process of surface rocks in the hyperarid Negev desert of Israel and utilize ground-based LiDAR data to characterize weathering mechanisms and rates during the late Quaternary.

High-resolution, mm-scale, LiDAR scans were used to capture surface roughness evolution on eight abandoned alluvial chronosequence terraces that record sequential stages in the subaerial weathering of boulder-strewn alluvial deposits into smooth, gravel-armored desert pavements. Roughness was examined using power spectral density (PSD) calculated for each terrace at length-scales (λ) ranging from ~0.04 to 1.50 m. All terraces displayed PSD values that increase as power-law functions of λ with a ~constant roughness exponent of 2.0±0.2 and scaling factor that decreases as a logarithmic function of surface age. Interpreted as a measure for scale-dependence in weathering efficiency on the examined surfaces, this PSD evolution indicates a fragmentation weathering process driven by rock cracking and shattering. Luminescence terrace abandonment ages, which range from 5±1 to 87±2 ka, allowed us to also quantify rock-weathering rates as time-variant functions rather than as time-averaged constants, which are typically determined for rock weathering with other techniques such as cosmogenic radionuclides. Our analysis revealed an intrinsic power-law decay in rock weathering rates as a function of time from >20 mm/kyr at the initial stages down to < 1 mm/kyr within ~60 kyr. These measurements of time-variant rock weathering rates highlight the due caution in the commonly utilized assumption of subaerial rock weathering as a constant-rate process over geologic time-scales.