GEOCHEMICAL DIFFERENCES IN PETROCALCIC AND CALCIC HORIZONS DUE TO SOIL PARENT MATERIAL IN SOUTHEASTERN ARIZONA

Petrocalcic and calcic horizons are quintessential features of arid and semiarid regions and store significant amounts of soil carbon. Past studies identified the source of calcic and petrocalcic horizons as calcium-rich dust, linking calcic horizon formation to dust input. To date, there are few previous studies that explicitly test this hypothesis. Further, throughout Southeastern Arizona (SEAZ), a number of locations contain little calcium, despite significant soil development throughout the Quaternary period. We tested this widely accepted hypothesis by comparing the geochemical properties of petrocalcic and calcic horizons originating on different parent materials in SEAZ. We collected petrocalcic and calcic horizons sourced from basalt, rhyolite/andesite, limestone, and mixed alluvium and measured color, pH, electrical conductivity (EC), and loss on ignition (LOI) for each soil sample. We determined the bulk elemental content of each sample using portable x-ray fluorescence. We hypothesized that the petrocalcic and calcic soils will differentiate by parent material. Using a principal component analysis (PCA), we found geochemical differences between the calcic horizons forming on different parent materials, with limestone-derived calcic horizons separating from volcanic-derived horizons. We observed notable differences in pH, EC, and LOI between the samples, likely stemming from their different parent materials. We calculated dust input for each sampled horizon using titanium and zirconium concentrations and constant Ti:Zr values for dust and parent material. We found that many horizons contained a significant dust fraction, but generally limestone and rhyolite derived samples had the lowest contributions from dust. Thus, while dust is a significant factor in calcic and petrocalcic horizon formation, other factors such as parent material partially explain the formation of petrocalcic and calcic horizons.