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 op portunity 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, an d 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 p eninsula. We measured labile (plant-available) base cation concentrations a nd Sr-87/Sr-86 ratios of labile strontium and silicate residue from soil pr ofiles across the transect from 18 to 300 cm MAP. Depletion of labile catio ns and a shift in labile Sr-87/Sr-86 ratios toward rainwater values with in creasing 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 f rom input of exogenous eolian material (dust derived from Asian loess), wit h a greater dust fraction at the high MAP sites due to aerosol washout. Mos t of the soil silicate strontium in high-MAP sites is still derived from th e original parent material, but the shallower portions of profiles can be d ust-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 calcula tions for the 150 ka evolution of the profile suggest that only on the orde r of 5 to 50% of rainwater strontium exchanges with the labile reservoir. O ur models suggest that the present-day supply of strontium by weathering in creases steadily with rainfall in the low-MAP (<140 cm) sites, then decreas es dramatically as the soils become depleted in weatherable parent material . This implies that the initial weathering rate of the high-MAP sites was v ery 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 rat es of basaltic terrains. Copyright <(c)> 2001 Elsevier Science Ltd.