Soil production and transport on postorogenic desert hillslopes quantified with 10Be and 26Al

Abstract

Hillslopes stand at the top of the geomorphic conveyor belt that produces and transports mass throughout landscapes. Quantification of the tempo of hillslope evolution is key to identifying primary sediment production and understanding how surface processes shape topography. We measured cosmogenic 10Be and 26Al on three desert hillslopes in postorogenic central Australia and quantified their soil dynamics and evolution. We found that hillslope morphology is governed by lithological factors, and differing nuclide abundances reflect the main sediment transport processes. Slope wash is widespread, and shrink-swell soil processes drive downslope creep and upward migration of gravels detached from underlying bedrock. We applied Monte Carlo–based inversion modeling to reconstruct soil production and the exhumation histories of stony mantle gravels. Underlying silty soils derive from eolian dust inputs dating to at least 0.2 Ma and possibly more than 1 Ma, in line with intensified aridity. Exposed bedrock erodes at ∼0.2–7 m/m.y., and under soil, it erodes at maximum rates of <0.1 m/m.y. up to 10 m/m.y. Accordingly, particles spend 2–6 m.y. or more in the upper 0.6 m of the bedrock column and an additional ∼0.2–2 m.y. or more within hillslope soils. Such long periods near the surface result in surface particles acquiring inherently low 26Al/10Be ratios. Bedrock erodibility underpins regional variations in erosion rate, and the slow tempo of hillslope evolution is largely independent of base level. This suggests a distinctive top-down evolution among postorogenic hillslopes set by authigenic rates of sediment production, rather than by fluvial incision as in tectonically active settings. © 2021 Geological Society of America