Water Dynamics on Landscapes and Soils of the Atacama Absolute Desert

The driest section of the Atacama Desert in northern Chilehas experienced nearly lifeless conditions for much of the pastseveral million years. The extreme aridity of the region hasdrawn the attention of scientists since Charles Darwin visitedthe area in 1835. The unique conditions that have prevailedthrough this long time span have created a landscape that isdissimilar to all other areas on earth. However, what makes theAtacama particularly interesting is not just the low quantity ofwater, but the infrequency and irregularity of rainfall. Thisdissertation examines the impact of water on the desertlandscapes at different space and time scales. Chapter 1presents the results of a stratigraphic and chronological studyon the desert lowlands. These areas are currently covered byhalite-encrusted salt pans (Salars) that have been thought to befossil remnants of lakes that existed at the Plio-Pleistocenetransition. However, in this study I show that these are muchyounger features from the late Quaternary. Numerousstratigraphic sections were observed and sampled in two subbasinslocated in the Central Depression of the Atacama Desert.The fossils and sedimentology of the stratigraphic sections showthat these environments supported a diverse hygrophytevegetation, as well as an array of diatoms, ostracods andgastropods that indicate the presence of shallow lakes andwetlands periodically between ~46.9 ka and 7.7 ka. The formationof wetlands and lakes occurred due to an increase in groundwater2 levels as a result of increased Andean runoff during regionalwetter intervals, particularly the Central Andean Pluvial Event(CAPE) that occurred between 17.5-14.2 ka and 13.8-9.7 ka.Chapter 2 examines the hydrological effect of a recordhistorical rainfall that occurred on March 24-26 2015. Fromscattered weather station data, the storm was among, and in somecases the largest, recorded in the desert. The effect of thisunusual storm was analyzed by observations made a few monthsafter the event in a N to S transect through the plant-freeexpanse of the Atacama Desert, between 22 and 26° S. The mainobjective of the field work was to characterize landscapechanges following the storm. The findings show that the storminitiated some minor fluvial responses on the upland landscapes,but overall those were not sufficient to reactivate manyhydrological features that are prominent on the landscape, andthat must therefore be driven by larger, even less frequentstorms. The field evidence suggests that larger rainfalls (orperiods of rainfall) have occurred throughout the Quaternary,and that there are fossilized (or infrequently active) featuresin various stages of “repair” that provide evidence of rainfallre-occurrence. Radiocarbon dating of carbonate bearing soils atthe southern periphery of the desert reveals that more rainfall,and more biotic conditions, existed in the region up to the endof the Pleistocene. Additionally, the soils in the lifelessportion of the Atacama Desert have unique hydraulic properties.In most arid regions, a rainfall of this magnitude and intensitywould cause flash flooding, but the Atacama’s salt-rich soilshave very high infiltration rates, and the landscape is thusmore resilient to intensive rainfall events than most desertlandscapes. However, based on the fossilized geomorphic andhydrological features on the landscape, there is a rainfallthreshold, whose magnitude remains uncertain, above which thislandscape undergoes alteration and fluvial reshaping.In Chapter 3, the observations and laboratory analyses of astudy of a rare calcium chloride rich soil in the Salar deLlamara are presented. The uniqueness of this project resides inthe fact that calcium chloride enrichments are extremely rare onthe earth surface, and that the hygroscopic properties of thissalt allow the soil to remain wet (8-16 % gravimetric watercontent) nearly continuously under modern – and essentiallyrainless - climatic conditions. The substrate for theaccumulation are small aeolian dunes, of fine sand and silt,that contain 60% of soluble salts by weight, of which ~15% isCaCl2. Based on an analysis of the regional geomorphology and3 hydrogeology, it is suggested that the source of the salt isfrom terminal recharge through fractures associated with thelocal fault system. Due to climate change, these deposits andthe salts began occurring ~14 ka ago. These deliquescent salts,in a rainless region, are unique habitats for life within theclimatic limits of life on Earth, and are potential analogs fortransient darkened linear features on Mars