The Tool-size Effect: old weathered soils limit the erosive power of bedrock streams

The conversion of rock to soil through weathering tends to reduce grain size [1] and prepares Earth’s surface for erosion processes to take hold. Warm, wet climatic conditions that lead to the formation of clays and weathering of bedrock clasts to sand might increase the transportability of the particles. For this reason, and potentially others, high weathering rates and high rates of erosion often go hand in hand. Here, we identify a potential process that mechanistically links highly weathered soil profiles with fine-grained particles at the surface – often formed during pre-Quaternary climate regimes – to decreased rates of fluvial incision. When bedrock type allows it, increased weathering in the hinterlands reduces the number and sizes of clasts to efficiently abrade the bed. What impact, then, does this previous climatic state have on the fluvial response times to uplift?

We test our ‘Tool-size Effect’ hypothesis in similar-sized tributaries in the Luquillo Critical Zone Observatory. Here, a series of well-defined knickpoints separate a relict, weathered landscape from a rejuvenated landscape downstream of the knickpoints. Knickpoint initiation results from Early Pliocene tilting of the platform that changed the base level ca. 600 m in <10⁶y. Above the knickpoint, the soils are depleted of easily-weatherable minerals and the sand-bedded streams are graded to the former sea level. We tested the ability of the tool effect to account for different knickpoint propagation rates by ascribing different values of retreat efficiency to each knickpoint based on sediment grain size at each knickpoint lip and cosmogenic nuclide-derived mass fluxes in different grain fractions. We find that grain-size does impact the retreat velocity; the retreat rate with the smallest grain-size at the lip travels ca. half the speed of the stream with the coarsest particles. Knickpoint propagation has been slowing down with time at a rate faster than that predicted by the decrease in upstream drainage area. This mechanism explains the persistence of other high-elevation tropical landscapes with highly weathered soils. It also bears on the response of rivers to climate change and geomorphic hysteresis resulting from time-dependence of weathering processes. [1] Ruxton, B.P.; Berry, L. 1957. GSA Bull. 68(10): 1263-1292.