The roles of salt (sodium nitrate) and fog in weathering: a laboratory simulation of conditions in the northern Atacama Desert, Chile

A laboratory experiment was used as a model to investigate interactions within a hyper-arid and hyper-saline weathering environment based on conditions observed in the Atacama Desert, northern Chile. The experimental design produced a matrix of 18 different weathering environments with six levels of salt (sodium nitrate) availability and three levels of fog precipitation. The experiment took place over an 11-week period. Over this time, 10 weekly fog applications modified the experimental conditions. From a theoretical perspective, the weekly fog applications generate an open rather than a closed system, which may be more representative of conditions in the field. The breakdown of the chalk blocks used was assessed in two ways. Firstly, qualitative observations were made at weekly intervals of the nature of morphological changes. Secondly, at the end of the experiment, breakdown was assessed by counting the number of discrete pieces greater than 1 g liberated by each block. The electrical conductivity at the base of each block was also measured. Collectively, the results indicate that fog appears to control the rate of breakdown and that salt availability controls the timing of the response. The style of breakdown was similar irrespective of the fog and salt levels. The morphological observations show the results of a sequence of events, with the blocks evolving as the experiment progresses; as such, they provide an experimental example of the role of inheritance in weathering processes.