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