CHARACTERIZING THE PROCESS AND RATE OF SUBAERIAL ROCK WEATHERING IN DESERT ENVIRONMENTS WITH GROUND-BASED LIDAR

Subaerial weathering of rocks is one of the primary erosion processes governing post-deposition and/or post-exposure geomorphic evolution of rocky desert surfaces. And yet, quantitative aspects of this fundamental geomorphic process, such as the contribution of the different weathering mechanisms that drive it (e.g., rock shattering, chemical weathering, aeolian abrasion and/or exfoliation) as well as the change in its rate over time remain loosely determined. Here, we present a new and widely applicable approach for systematically constraining these aspects of subaerial rock weathering by examining the evolution of surface roughness on co-genetic surfaces of different ages.

This study focuses on 2 alluvial chronosequences from the hyper-arid Negev desert in southern Israel as these effectively record discrete stages in the in-situ subaerial weathering of young (5±1 ka) boulder-strewn deposits into mature (87±2 ka) and smooth desert pavements. High resolution (mm-scale) LiDAR scans of representative 30-50 m² areas were acquired for 6 terraces with distinct OSL abandonment ages within this time range. Analysis of the LiDAR data in power spectral density (PSD) space allowed us to characterize the geometric manifestation of the weathering process at 0.03 to 1.50 m length-scales. The OSL abandonment ages provided independent constraints for the time elapsed since weathering began on the respective surfaces. As commonly observed on natural surfaces, we found that PSD values, which correlate with roughness, increase with measured length-scale within each given terrace. Our results revealed a consistent pattern where roughness at any given length-scale decreases monotonously with time. The rate of this smoothing process follows a positive power-law function of length-scale and a negative power-law function of time. The observed PSD evolution also distinguishes rock shattering as the primary rock weathering mechanism driving roughness evolution on these surfaces. In addition, the older PSD curves display time-progressive slope moderation at longer length-scales, which reflects the weathering and effective removal of larger roughness elements (boulders) from the surfaces. This allowed us determine a characteristic weathering rate for the system and to constrain its non-linear decay with time.