THE INFLUENCE OF MESOAMERICAN OBSIDIAN MINING ON THE SOIL FORMATION OF TECHNOSOLS: A CASE STUDY IN LAS NAVAJAS, HIDALGO, MEXICO

Pre-Hispanic obsidian mining in Mesoamerica was one of the earliest and most widespread anthropogenic landscape modifications in the Mexican Central Highlands, yet its long-term impact on pedogenesis and Technosol formation remains poorly understood. The green-gold obsidian deposit at Cerro de Las Navajas (Hidalgo), exploited successively by the Teotihuacan, Toltec, and Aztec cultures over >1600 years, offers a unique opportunity to investigate anthropogenic soil evolution along a hillslope. We evaluated five soil profiles with different exploitation histories and two natural reference soils using morphological, physical, chemical, and micromorphological analyses. Soils on upper and middle slopes affected by older, less intensive Teotihuacan-period mining exhibited A-B-C horizons with well-developed aggregates, umbric and folic diagnostic horizons, and abundant biogenic features, and were classified as Spolic Technosols (Loamic, Hyperartefactic). In contrast, lower-slope soils impacted by intensive Aztec-period deep mining showed incipient development (O-C or CA-C horizons), apedal microstructures, and abundant fresh obsidian fragments. One profile with 40 cm of organic layers interbedded with obsidian flakes was classified as Coarsic Skeletic Sapric Hemic Fibric Folic Histosol (Hypertechnic); however, we propose reclassification as Folic Technosol to better reflect its technogenic origin and aerobic pedogenesis. Natural reference soils were classified as Umbrisol and Regosol. Micromorphological evidence demonstrates that soil biota (roots, mesofauna, fungi) play a crucial role in transforming technogenic deposits into functional soils in temperate forest landscapes. We conclude that pre-Hispanic obsidian mining generated Technosols in which the degree of soil development directly indicates the time elapsed since mining cessation and disturbance intensity. This study demonstrates that Technosols can serve as pedological archives of human history, and that soil biota plays a crucial role in transforming technogenic deposits into functional soils in temperate forest landscapes.