Processes and “rates” of pedogenesis across a Late Quaternary chronosequence dated by in situ cosmogenic 10Be – a case study from the coastal Atacama Desert (N Chile)

Studies on soil formation under hyperaridity have so far focussed on the hyperarid core of the desert. Although the coastal sector of the Atacama Desert receives much larger amounts of moisture due to the orographic blocking of fog by the Coastal Cordillera, neither the pedogenic processes nor the timescales on which the coastal soils formed have as yet been studied. We therefore assessed physical and chemical soil parameters across a chronosequence of a coastal alluvial fan at Paposo, which is composed of four morphostratigraphic units. From each surface generation, we sampled four shallow soil profiles and multiple boulders for exposure dating using in situ cosmogenic 10Be. The combined establishment of a pedostratigraphy and morphochronology allows us to indirectly assess rates of soil formation and to make use of a soil catena as an unprecedented palaeoenvironmental archive for the Atacama Desert.

Physicochemical soil properties mostly exhibit monotonic relationships with increasing time since abandonment of the first fan surface in the early Late Pleistocene. Pedogenic processes and weathering result in a loamification reflected in the soil texture by continuous fining with age. In contrast to typical Aridisols, trends towards desalinization as well as complete decalcification become apparent. Moreover, spectrophotometric soil colour measurements as well as the content of pedogenic iron oxides indicate that rubification plays a major role in the pedogenesis under coastal hyperaridity. Significant effects of biotic activity become apparent in organic carbon and phosphorus contents. Eventually, the results provide strong indications for in situ formation of clay-sized particles and colloids.