Field weathering rates of Mt. St. Helens tephra

Abstract The initial stages of chemical weathering in tephra were examined under field conditions in a cool and humid forest ecosystem in the Cascade Mountains of Washington. Unleached tephra from the 1980 eruption of Mt. St. Helens was applied in 5 cm and 15 cm depths to simulate natural tephra deposition. Leachate solutions from the tephra were then collected and analyzed over a 4 year period. Concentrations of dissolved elements were combined with the water fluxes to determine elemental fluxes from tephra and to estimate chemical weathering rates. Solutions leached from the tephra layer indicate incongruent dissolution resulting in formation of a cation-depleted, silica-rich leached layer on glass and mineral surfaces. Measured weathering rates were 1–3 orders of magnitude less than comparable rates reported in the literature for laboratory dissolution studies, but considerably greater than those measured for entire watersheds in field studies. Dissociation of carbonic acid, originating primarily from upward transport of carbon dioxide from the buried soil, was the dominant source of protons for weathering reactions. Weathering rates in the 5 cm treatment were approximately twice those of the 15 cm treatment. A greater flux of CO 2 per unit volume of tephra in the 5 cm treatment is believed to be responsible for the differential weathering rates.