Quantifying Catchment Scale Soil Variability in Marshall Gulch, Santa Catalina Mountains Critical Zone Observatory.

Quantifying regolith variation, both chemical and physical yields insights to the evolution of the subsurface. In this study we aim to quantify soil variability within a forested catchment, Marshall Gulch, AZ. Marshall Gulch (MG) lies within the Coronado National Forest, part of the Jemez River Basin-Santa Catalina Mountains Critical Zone Observatory (CZO).  MG is 5-hectare, mixed-conifer forested catchment situated on granitic parent material, with a mean elevation of 2400m, mean annual temperature of 8°C and mean annual precipitation of 75 cm. To ensure samples sites capture landscape variability, principal component analysis (PCA) were run on NAIP imagery and additional ancillary data from the study area. The PCA determined input layers of soil depth, slope, soil wetness index, NDVI and NAIP bands 3/2 as the variables needed to capture the landscape variability of MG. A conditioned Latin Hyper Cube (cLHC) model was then utilized to randomly determine 20 sample locations within the catchment to equally represent the six input layers, as determined from the PCA. Regolith profiles were described and sampled at all 20 locations. At each sample site a soil pit was dug to refusal (paralithic contact) and sampled according to genetic horizon. Each sample was then analyzed using methods of X-Ray Fluorescence (XRF), X-Ray Diffraction (XRD), particle size, color, pH, EC, C/N isotopes, and loss on ignition (LOI) to characterize chemical and physical soil properties. By quantifying chemical denudation and mineralogical variability of the collected soils, we establish a proxy for regolith weathering both on the profile scale (1m²) as well as the catchment scale (50k m²). GIS spatial techniques enable us to produce maps depicting the variability of soil properties. We confidently extrapolate our findings (pH, depth to paralithic contact, color, mineralogy etc.) throughout the entirety of the MG field site, generating a high-resolution understanding of the processes shaping the MG critical zone.