Effects of rainfall on weathering rate, base cation provenance, and Sr isotope composition of Hawaiian soils

A climate transect across the Kohala Peninsula, Hawaii provides an ideal opportunity to study soil processes and evolution as a function of rainfall. The parent material is the similar to 150 ka Hawi alkali basalt aa flow, and median annual precipitation (MAP) changes from similar to 16 cm along the west coast to similar to 450 cm in the rain forest near the crest of the peninsula. We measured labile (plant-available) base cation concentrations and Sr-87/Sr-86 ratios of labile strontium and silicate residue from soil profiles across the transect from 18 to 300 cm MAP. Depletion of labile cations and a shift in labile Sr-87/Sr-86 ratios toward rainwater values with increasing rainfall clearly show the transition from a mineral-supported to a rainwater-supported cation nutrient budget. In contrast, increases in soil silicate residue Sr-87/Sr-86 values with increasing MAP result primarily from input of exogenous eolian material (dust derived from Asian loess), with a greater dust fraction at the high MAP sites due to aerosol washout. Most of the soil silicate strontium in high-MAP sites is still derived from the original parent material, but the shallower portions of profiles can be dust-dominated. The variations in labile Sr-87/Sr-86 With rainfall allow us to calculate weathering rates as a function of MAP. The primary uncertainty is the degree to which Sr in rainwater actually interacts with the labile cation reservoir before being flushed from the system; mass balance calculations for the 150 ka evolution of the profile suggest that only on the order of 5 to 50% of rainwater strontium exchanges with the labile reservoir. Our models suggest that the present-day supply of strontium by weathering increases steadily with rainfall in the low-MAP (<140 cm) sites, then decreases dramatically as the soils become depleted in weatherable parent material. This implies that the initial weathering rate of the high-MAP sites was very high, and that there may be some change in soil weathering behavior in the 100 to 160 cm MAP range. Weathering rates calculated from the labile Sr-87/Sr-86, on the same order as other estimates for chemical denudation rates of basaltic terrains. Copyright <(c)> 2001 Elsevier Science Ltd.