CHEMICAL WEATHERING VS. PHYSICAL EROSION IN HYPERARID, ARID, AND SEMIARID HILLSLOPE SOILS IN THE ATACAMA DESERT – DISCOVERING A PROCESS THRESHOLD

Soil transport on hillslopes occurs through physical and chemical processes and their relative importance appears to change with climate.  However, previous studies have focused on humid to semiarid sites.  The Atacama Desert, Chile, offers a unique opportunity to study hillslope soil processes at the dry end of Earth's climate spectrum. We use an inverse modeling approach, based on a steady state soil mass balance equation and immobile element concentrations, to calculate the rate of physical erosion and chemical weathering along a downslope transect of soil pits on our hillslopes (semiarid, arid, and hyperarid).  Two key inputs to this model are the rates of soil production from bedrock (calculated using the cosmogenic radionuclides ¹⁰Be and ²⁶Al) and atmospheric input (measured using passive dust traps).

Field observations indicate changes in the relative importance of different soil processes with decreasing precipitation.  At our semiarid site (MAP~80mm) there is a thick soil mantle, no soil carbonate or sulfate, thriving desert vegetation, and evidence of bioturbation.  The arid hillslopes (MAP~10mm) are weathering-limited such that they are almost free of soil and support only a few fog-adapted plants.  Lastly, our hyperarid site (MAP~1 mm) has a fairly thick soil mantle but the soil is dominated by sulfates and carbonates of atmospheric origin and there is almost no life. 

Our preliminary calculations show a shift in process from the semiarid to arid and hyperarid sites.  Three trends emerge: (1) physical erosion is an order of magnitude larger at the semiarid site than at the arid and hyperarid sites (which are ~equal), (2) chemical weathering results in mass loss in the semiarid site, slight mass gain in the arid site, and substantial mass gain in the hyperarid site, and (3) physical erosion is 100 times greater than chemical weathering at the semiarid site but only one order of magnitude greater at the arid and hyperarid sites (and of the opposite sign). 

Our semiarid site calculations are similar to other semiarid sites studied in Australia by Yoo but the arid and hyperarid sites show that precipitation plays a critical role in hillslope soil process and offer a glimpse into the dry side of this geochemical and geomorphic threshold.