Weathering of Mt. St. Helens Tephra under a Cryic‐Udic Climatic Regime

Abstract

AbstractThe initial stages of chemical weathering in tephra were examined during a 10‐yr period under field conditions in a cryic‐udic climatic regime. Unleached tephra from the 1980 eruption of Mt. St. Helens was applied to the surface of soils in 5‐ and 15‐cm depths to simulate natural tephra deposition on subalpine forest ecosystems. Solid‐phase tephra samples were collected 10 yr after addition and compared chemically and mineralogically to the original, unweathered tephra. The solid‐phase pH of the tephra decreased by approximately one unit, with the 5‐cm treatment being ≈0.2 units lower than the 15‐cm treatment. The accumulation of Al and Fe as alteration products in the weathered tephra was detected by selective dissolution analyses. Aluminum accumulated as Al‐humus complexes, hydroxy‐Al polymers in the interlayer of 2:1 layer silicates, and possibly as hydroxy‐Al precipitates external to the interlayer space; Fe accumulated primarily as a noncrystalline oxyhydroxide (e.g., ferrihydrite) and Fe‐humus complexes. The increase in the active Fe and Al fraction (i.e., oxalate extractable Fe and Al) contributes to a sevenfold increase in phosphate retention. Hydroxy‐Al interlayering of detrital 2:1 layer silicates present in the tephra was suggested from x‐ray diffraction and selective dissolution analyses. There was no evidence of opaline silica, allophane, or imogolite forming in the tephra. Preferential incorporation of Al into Al‐humus complexes and hydroxy‐Al interlayers of 2:1 layer silicates appears to inhibit formation of allophane and imogolite during the initial stages of weathering.