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

• Published in: Geochimica et cosmochimica acta 2001-04, Vol.65 (7), p.1087-1099
• Main Authors: Stewart, Brian W, Capo, Rosemary C, Chadwick, Oliver A
• Format: Article
• Language: English
• Subjects: USA, Hawaii
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/S0016-7037(00)00614-1

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 ∼150 ka Hawi alkali basalt aa flow, and median annual precipitation (MAP) changes from ∼16 cm along the west coast to ∼450 cm in the rain forest near the crest of the peninsula. We measured labile (plant-available) base cation concentrations and 87Sr/ 86Sr 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 87Sr/ 86Sr 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 87Sr/ 86Sr 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 87Sr/ 86Sr 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 (