Rapid regolith formation over volcanic bedrock and implications for landscape evolution

Abstract

The ability to quantify how fast weathering profiles develop is crucial to assessing soil resource depletion and quantifying how landscapes evolve over millennia. Uranium-series isotopes can be used to determine the age of the weathering front throughout a profile and to infer estimates of regolith production rates, because the abundance of U-series isotopes in a weathering profile is a function of chemical weathering and time. This technique is applied to a weathering profile in Puerto Rico developed over a volcaniclastic bedrock. U-series isotope compositions are modelled, revealing that it takes 40–60kyr to develop an 18m-thick profile. This is used to estimate an average regolith production rate of 334±46mm/kyr. This value is higher by a factor of up to 30 when compared to production rates estimated for weathering profiles developed over granitic or shale lithologies. This quantitatively underpins the lithological control on rates of regolith production (in a neighbouring watershed but over a granitic bedrock, production rates are only ∼30–40mm/kyr). Moreover, by comparing these results to a compilation of soil erosion rates, it is clear that landscapes are controlled by the balance (or imbalance) between regolith production and erosion: soil-mantled landscapes are the result of a relative balance between production and erosion, whereas in cratonic areas, thicker weathering profiles are generated because erosion fails to match regolith production rates.