THE ARID TO HYPERARID HYDROLOGICAL INTERFACE ON EARTH: A BIOGEOCHEMICAL THRESHOLD

Rainfall declines with decreasing latitude in northern Chile, reaching levels as low as ~1 mm yr^-1 in the Atacama Desert. The persistence of these patterns since at least the Pliocene makes the region an ideal natural rainfall experiment. We have focused on soil and landscape processes from semiarid (~80mm) to hyperarid (~1mm) conditions, and have found that for many biogeochemical processes, the arid/hyperarid transition represents a threshold across which many soil properties vary discontinuously with rainfall, and certain landscape processes change in both rate and form as rainfall declines and as life disappears. Our initial work shows that: (1) On level, Tertiary, landforms, semiarid to arid soils exhibit net elemental losses and volumetric collapse via weathering while hyperarid soils are nearly passive collectors of atmospheric deposition, and exhibit large mass gains and volumetric expansion. Soils at the arid/hyperarid transition exhibit little net change. (2) In semiarid to arid soils, total soil N is relatively low, predominantly in an organic form, and at steady state. In hyperarid soils, total N is relatively high, most soil N is biologically unaltered tropospheric NO₃ , and the N pool is not at steady state. (3) Bedrock erosion rates (boulders and bedrock hillslopes) decline with decreasing rainfall, and are among the lowest yet measured on Earth. Semiarid hillslopes are vegetated and mantled with soil derived from underlying rock. In the hyperarid extreme, slopes are mantled with soil dominated by atmospheric dust and salt. At the broad arid/hyperarid transition, hillslopes are dominantly soil-free due to an absence of plants to stabilize bedrock-derived sediment, but sufficient rainfall to rapidly remove atmospheric deposition and sediment. In summary, the hyperarid Atacama Desert, when placed into a global context, reveals patterns and processes that would be unanticipated based on extrapolations from more humid regions. In general, profound changes in key processes occur at the arid/hyperarid transition, the most fundamental being the near cessation of life and the processes it mediates.