CL DATING OF SOIL FORMATION IN THE HYPER-ARID ATACAMA DESERT, CHILE

Meteoric ³⁶Cl has been used for the first time to date a hyper-arid soil in the Atacama, Desert and used to understand the timescale of soil formation processes in hyper-arid regions. A 225 cm deep trench in the Baquedano region of the Atacama was sampled at five centimeter vertical resolution. The observed ³⁶Cl/Cl ratios show a clear systematic decline with depth with the highest values (354~394 × 10^-15) occurring near the surface, and the lowest ratios (70~74 × 10^-15) at the bottom of the profile. A simple model of a soil matrix, including chloride, that builds up as layers over time via atmospheric inputs and ³⁶Cl that radioactively decays in situ reproduces the data remarkably well. The model suggests an age of 689 (±36) ka for the deepest soil and the soil accumulation rate of 0.33 cm/ky. This soil accumulation is conceptually counter to the net loss of soil mass in humid regions or static soil surfaces in arid regions. The age of the soil at 225 cm is radically different from soils in other deserts, indicating hyper-aridity has prevailed at least since the middle Pleistocene in the Atacama. The soil accumulation rate is comparable to the dust deposition rate measured in previous research, demonstrating an efficient retention of atmospheric deposition in the Atacama. Despite the good fit of the model, observed deviations between the model and observations probably arise from some of the inherent assumptions in the model (constant ³⁶Cl production, constant chloride input, constant accumulation, and no chloride migration). De-convolving these deviations using ¹⁰Be may yield additional insight into soil formation mechanisms in the Atacama.