The development of laterite weathering profiles as a function of rainfall and time: A geophysical approach

Laterite weathering profile (LWP) thicknesses are functions of precipitation rate and time, but their exact dependence on them is uncertain. We investigate LWP development on ground surfaces in Hawai'i that are neither aggrading nor eroding across substrates from 0.01 to 4 Ma and rainfall rates of <250 to >3000 mm/a. The Hawaiian Islands provide an excellent opportunity for LWP studies across climates and over millions of years on a single rock type, basalt. LWP weathering rates are usually determined by geochemical approaches. We present a geophysical method once bedrock ages and precipitation rates are known. We employed multichannel analysis of surface waves and horizontal-to-vertical spectral ratio methods. Results indicate that >70% of the variability in LWP thickness is due to precipitation and bedrock age. The remainder is attributed to measurement uncertainty and heterogeneity in the permeability of basalt. LWPs develop by two paths. Dry (<1000 mm/a) areas have a negative water balance with evapotranspiration exceeding rainfall. LWPs thicken until they reach a steady state where the storage capacity of the saprolite precludes the percolation of water into subjacent lava. In wetter areas, downslope interflow produces thick laterite wedges near coastlines that migrate upslope over similar to 1 Ma. Subsequently, they thicken and reach a steady state where precipitation cannot deliver water through the vadose zone. LWPs' thickness (T) increases as a function of time (t) and precipitation (P) according to the expression (95% confidence): T =log(10)(t+1)(0.934(+/- 0.126)) x log(10)(P+1)(3.890(+/- 0.085)). The first partial derivative, or weathering rate, is partial derivative T/partial derivative tP = (0.406/(t+1)) x log(10)(P+1)(3.890). Weathering rates decrease with time and are strong functions of t and P. These expressions add considerable insight into the rates and processes driving weathering at large scales and can also be solved for t, giving a rough estimate of the time of landscape formation of eroded surfaces.