NEOICHNOLOGY AND PEDOLOGY OF MONTANE SOILS IN THE SANTA CATALINA MOUNTAINS, SOUTHERN ARIZONA

The long-term goal of this study is to examine a wide range of modern soils and associated biota to develop a model to interpret analogous paleosols and trace fossils in the geologic record. Few studies have examined the biogenic modification of mountain soils. While such soils may be geologically fleeting, several factors make them worth studying. Soil organisms likely influence strongly the rate and magnitude of regolith erosion, soil development, and slope stability. In addition, montane environments act as refugia for many species and frequently develop their own “island” biogeographies. Two field sites on the slopes of Mount Lemmon in the Santa Catalina Critical Zone Observatory in southern Arizona were investigated; Oracle Ridge at mid-elevation (~2150 m elev., MAT 11.9° C, MAP 840 mm), and the higher elevation site at Marshall Gulch (~2500 m elev., MAT 10.4° C, MAP 940 mm). Field methods at both sites included mapping soil surfaces and burrow openings, imaging with high-frequency ground-penetrating radar, soil trenches, and soil coring. Soil were sampled for thin section analyses, bulk geochemistry and clay mineralogy, while open burrows were cast in plaster. Flat lying areas at Oracle Ridge and Marshall Gulch were divided into 32- and 40-m² grids, respectively, for description and study. Both sites are pine-oak forests with ground cover consisting of pine needles, grass, shrubs, ferns, and sapling and adult pine and oak trees. Soils are organic-rich, sandy loam Inceptisols up to 103 cm thick with large roots (1-5 cm diameter) and pervasive fine roots (<1 cm) in a granular soil structure composed primarily of earthworm casts. The upper 5-12 cm of the soils are completely bioturbated by earthworms, ants, beetle larvae, and mole crickets, but most extensively by pocket gopher burrows. Pocket gopher burrow openings were circular to elliptical, varied from 5-10 cm in diameter (average ~6.5 cm), and were often surrounded by a 5-20 cm high mound of excavated soil. Tunnel morphologies included simple subhorizontal tunnels 50 or more cm long, but also networks of branching tunnels, galleries, and helical subvertical shafts. Our study results will help link the extant biotic structures to their specific pedogenic environments, thus assisting in our interpretations of similar trace fossil assemblages in the geologic record.