NEOICHNOLOGY OF SEMIARID ENVIRONMENTS: SOILS AND BURROWING ORGANISMS OF THE SONORAN DESERT

Whereas soils in semiarid environments have received considerable attention, their biota and the traces they produce are poorly understood. Soils in a field site in the Santa Catalina Critical Zone Observatory in the semiarid Sonoran Desert (MAT 18.2° C, MAP 420 mm) of southern Arizona were investigated to develop a model to interpret analogous paleosols and associated ecosystems in the geologic record. A flat plateau of desert scrubland composed of various cacti, small trees, shrubs, and grass (~70% vegetation cover) was divided into 30 four-meter-square grids for description and study. Field methods included mapping of soil surfaces and burrow openings, imaging with high-frequency ground-penetrating radar, casting of open burrows, description of soil trenches, and soil coring. Laboratory methods included analysis of bulk geochemistry and clay mineralogy of soil samples, study of soil thin sections, and detailed descriptions of burrows. The soils consisted of Entisols and Inceptisols with 10-40 cm thick profiles composed of light brown sandy silt with blocky peds and pervasive, coarse (3-5 mm) to very fine roots (<1 mm) roots. The soils lacked well-developed clay or carbonate accumulation horizons. The soils were dominated by the burrows of ground squirrels and ants as well as burrows of various lizards, snakes, scorpions, spiders, centipedes, termites, and insect larvae. Burrow openings were circular to elliptical and varied from 1-10 cm in diameter, with an average diameter of ~5 cm. Burrow morphologies included simple vertical shafts, isolated ovoid chambers, subhorizontal tunnels, subvertical to subhorizontal networks of branching tunnels, and complex branching galleries. Small diameter burrows were highly concentrated in the upper 10 cm of the soil profile whereas larger burrows often extended to the base of the profile. Additional evidence of bioturbation included 5-20 cm high mounds of excavated sediment with several burrow openings beneath cacti and other large vegetation. Results from this study will be invaluable to the interpretation of the paleosols from semiarid environments in the rock record and advance our understanding of the traces produced by different soil organisms and their impact on soil development in modern and ancient settings.