The Alteration Of Volcanic Ash and Soil Development Across a High Elevation Climate Gradient In Mauna Kea, Hawaii.

Hydrothermal alteration processes are known to influence short-range order mineral abundance as well as the presence of allophane in volcanic ash, even in the absence of weathering.   However, the extent to which in-situ weathering processes versus hydrothermal alteration mechanisms influence volcanic ash properties in high elevation environments remains unclear.  We examined soil organic matter dynamics and volcanic ash weathering across a high altitude (3400 - 3000 m) 20 ky climate gradient on Mauna Kea in Hawaii.   Four sites along the climate gradient were selected, which range from arid-periglacial at the top two 3400, 3350m sites, with sparse C4 grass vegetation to inversion-layer influenced at the bottom two (3200,3000m) sites, where rainfall increased noticeably and the influence of shrub vegetation is more pronounced. At each site, three pits were dug and major diagnostic horizons down to bedrock (in-tact lava).  These fractions, along with bulk soil were analyzed for particle size, organic C and N, δ13C, δ15 stable isotope composition mineral assemblage and metal composition using oxalate and citrate-dithionite extraction. Soil pH (2:1 water), exchangeable cations, base saturation as well as NaF pH and phosphorous sorption was measured at each of the pits. Bulk elemental composition of soils were determined using HF digestion and quantification of major elements. Mass balance of major elements as well as pedogenic metal accumulation (oxalate and citrate-dithionite) was used to study changes in mineralogy, extent of weathering and accumulation of soil organic matter across the climate sequence.  We found evidence for pronounced soil development at the two lower elevation sites (3200, 3000m), where  short-range ordered minerals were appreciable and soil organic matter stocks increased significantly.  Higher elevation sites contained very limited evidence for soil development, or major elemental losses in the subsoil and were low in soil carbon content. However NaF pH and phosphorous sorption patterns suggest cryogenic (freeze-thaw) and arid evaporative mechanisms have altered the reactive properties of the volcanic ash throughout the depth profile, with evidence for little alteration at greatest depth.  The results suggest that pedogenic processes have influenced the nature and composition of volcanic ash even in this weathering environment where organic acid influences from plant material and rainfall is severely limited.  A strong and discernible impact of vegetation at the lower two sites (which was greatest at the lower elevation site) resulted in increased short-range order mineral abundance, the formation of pronounced Bw soil horizons as well the substantial increase in soil C content observed.