GEORADAR CHARACTERIZATION OF BIOTURBATION STRUCTURES IN CONTRASTING SOIL-GEOMORPHIC BIOTOPES, SOUTHERN ARIZONA

The Santa Catalina CZO of southern Arizona encompasses an ideal range of biotopes for investigating the structures of soil-dwelling organisms, from the semiarid Sonoran Desert to mountain slope regolith of the Santa Catalina Mountains. A new MALÅ GroundExplorer ground-penetrating radar (GPR) system with a 750 MHz monostatic antenna was used to visualize the subsurface extensions of bioturbation structures, soil thickness, and distribution of point-source reflections (gravel clasts, buried vegetation, etc.). Improved signal-to-noise ratio and real-time GPS allowed rapid and effective collection of subsurface data over relatively remote and rough terrain. Radargrams allowed the assessment of burrow dimensions and gross geometry in real time, with subsequent post-processing offering quantitative attribute analysis of key features (e.g., velocity structure of the fill and matrix, burrow dip angle, and relative volume of reworked material). At two adjacent desert sites (elev. ~1,000 m), continuous imaging along 4x4 m grids (2D profiles) allowed 4-5 cm vertical resolution of mammal, insect, and arachnid burrows. At two mountain localities (elev. ~2,100-2,500 m), with large burrow complexes excavated primarily by pocket gophers, GPR profiles were collected over open entrances, spoil mounds, and subsurface tunnels. On non-surface-normalized B-scans, traverses over spoil mounds are recognized by a characteristic inverted signature and yield information about mound material (relative soil compaction, presence of infilled burrow entrance). Electromagnetic signal polarity structure was used as a means of distinguishing between air-filled tunnels and morphologically similar roots or infilled burrow segments. At each site, a dense survey grid (area: 1.5x1.5 m to 4.0x2.6 m; line spacing: 10-20 cm) was established for 3D reconstruction of a representative structure (e.g., ant nest, gopher burrow complex). Burrow casts and sedimentological analyses aid in groundtruthing geophysical data, with the ultimate goal of using GPR for in situ burrow examination and identification of additional abandoned and buried structures. Our findings provide the first step in near-surface characterization of semi-fossorial organisms in semiarid soils, with potential applications to the ichnology of paleosols.