Climatically-Driven Changes in Bedrock Erosion Rate and Process on Semiarid to Hyperarid Hillslopes in the Atacama Desert, Chile

How does bedrock become soil on hillslopes? How does climate affect the rate of that conversion? How does the soil mantle change if there is almost no water? We are approaching these questions using cosmogenic radionuclide analyses and field observations of granitic hillslopes along a precipitation gradient in the Atacama Desert in northern Chile. At our study sites, annual precipitation decreases from ~100 mm in the south to <2 mm in the north. We have measured a large decrease in the rate of bedrock erosion (using ^{26}Al and 10Be) and observed a dramatic shift in the processes driving that erosion (from biotic to salt- dominated) with this change in climate. At the wettest site, the hillslopes are mantled by a relatively thick (~20 cm) soil which is underlain by 10-20 cm of saprolite. The saprolite is being incorporated into the soil via animal burrowing and root disruption at a rate of 20 m/Myr. The middle site is profoundly different: aridity limits plant cover to a few fog-adapted species, and the soil mantle is a few cm of bedrock and dust-derived material. Inspection of the soil-bedrock interface suggests that rock conversion to soil is driven by slow chemical/salt weathering of fractured bedrock within the upper few cm of the soil mantle where fog is the key moisture source. Animal burrowing and trampling is still an important mechanism for soil transport but not bedrock erosion. The calculated erosion rates in this nearly abiotic setting average 2.7 m/Myr. The driest hillslopes are completely lifeless due to hyperaridity, yet they are soil-mantled. The soils are dominated by sulfates and silicate dust which infiltrate the fractured bedrock by unsaturated flow. Physical expansion of the salts pries apart the bedrock and lofts it into the salt- and dust-rich soil mantle. The resulting bedrock lowering rate in this salt-driven environment is 0.83 m/Myr, among the lowest bedrock erosion rates measured on earth. Ample evidence indicates that the Atacama Desert achieved its present hyperarid state a few million years ago, and the slow rate of geomorphic processes we have measured indicates that the region may yet be adjusting to the present hyperarid, abiotic conditions.