Soil production and transport on postorogenic desert hillslopes quantified with (super 10) Be and (super 26) Al
Soil production and transport on postorogenic desert hillslopes quantified with (super 10) Be and (super 26) Al
Geological Society of America Bulletin (January 2018) 130 (5-6): 1017-1040
- Al-26
- alkaline earth metals
- aluminum
- arid environment
- Australasia
- Australia
- Bayesian analysis
- Be-10
- bedrock
- beryllium
- burial
- erosion rates
- exhumation
- geomorphology
- grain size
- isotopes
- Lake Eyre Basin
- landform evolution
- metals
- Monte Carlo analysis
- pedogenesis
- radioactive isotopes
- sediment transport
- sediments
- slopes
- soils
- statistical analysis
- terrestrial environment
- transport
- weathering
- wind transport
- central Australia
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 (super 10) Be and (super 26) Al 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 approximately 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 approximately 0.2-2 m.y. or more within hillslope soils. Such long periods near the surface result in surface particles acquiring inherently low (super 26) Al/ (super 10) Be 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.