Water Dynamics on Landscapes and Soils of the Atacama Absolute Desert

The driest section of the Atacama Desert in northern Chile/nhas experienced nearly lifeless conditions for much of the past/nseveral million years. The extreme aridity of the region has/ndrawn the attention of scientists since Charles Darwin visited/nthe area in 1835. The unique conditions that have prevailed/nthrough this long time span have created a landscape that is/ndissimilar to all other areas on earth. However, what makes the/nAtacama particularly interesting is not just the low quantity of/nwater, but the infrequency and irregularity of rainfall. This/ndissertation examines the impact of water on the desert/nlandscapes at different space and time scales. Chapter 1/npresents the results of a stratigraphic and chronological study/non the desert lowlands. These areas are currently covered by/nhalite-encrusted salt pans (Salars) that have been thought to be/nfossil remnants of lakes that existed at the Plio-Pleistocene/ntransition. However, in this study I show that these are much/nyounger features from the late Quaternary. Numerous/nstratigraphic sections were observed and sampled in two subbasins/nlocated in the Central Depression of the Atacama Desert./nThe fossils and sedimentology of the stratigraphic sections show/nthat these environments supported a diverse hygrophyte/nvegetation, as well as an array of diatoms, ostracods and/ngastropods that indicate the presence of shallow lakes and/nwetlands periodically between ~46.9 ka and 7.7 ka. The formation/nof wetlands and lakes occurred due to an increase in groundwater/n2 levels as a result of increased Andean runoff during regional/nwetter intervals, particularly the Central Andean Pluvial Event/n(CAPE) that occurred between 17.5-14.2 ka and 13.8-9.7 ka./nChapter 2 examines the hydrological effect of a record/nhistorical rainfall that occurred on March 24-26 2015. From/nscattered weather station data, the storm was among, and in some/ncases the largest, recorded in the desert. The effect of this/nunusual storm was analyzed by observations made a few months/nafter the event in a N to S transect through the plant-free/nexpanse of the Atacama Desert, between 22 and 26° S. The main/nobjective of the field work was to characterize landscape/nchanges following the storm. The findings show that the storm/ninitiated some minor fluvial responses on the upland landscapes,/nbut overall those were not sufficient to reactivate many/nhydrological features that are prominent on the landscape, and/nthat must therefore be driven by larger, even less frequent/nstorms. The field evidence suggests that larger rainfalls (or/nperiods of rainfall) have occurred throughout the Quaternary,/nand that there are fossilized (or infrequently active) features/nin various stages of “repair” that provide evidence of rainfall/nre-occurrence. Radiocarbon dating of carbonate bearing soils at/nthe southern periphery of the desert reveals that more rainfall,/nand more biotic conditions, existed in the region up to the end/nof the Pleistocene. Additionally, the soils in the lifeless/nportion of the Atacama Desert have unique hydraulic properties./nIn most arid regions, a rainfall of this magnitude and intensity/nwould cause flash flooding, but the Atacama’s salt-rich soils/nhave very high infiltration rates, and the landscape is thus/nmore resilient to intensive rainfall events than most desert/nlandscapes. However, based on the fossilized geomorphic and/nhydrological features on the landscape, there is a rainfall/nthreshold, whose magnitude remains uncertain, above which this/nlandscape undergoes alteration and fluvial reshaping./nIn Chapter 3, the observations and laboratory analyses of a/nstudy of a rare calcium chloride rich soil in the Salar de/nLlamara are presented. The uniqueness of this project resides in/nthe fact that calcium chloride enrichments are extremely rare on/nthe earth surface, and that the hygroscopic properties of this/nsalt allow the soil to remain wet (8-16 % gravimetric water/ncontent) nearly continuously under modern – and essentially/nrainless - climatic conditions. The substrate for the/naccumulation are small aeolian dunes, of fine sand and silt,/nthat contain 60% of soluble salts by weight, of which ~15% is/nCaCl2. Based on an analysis of the regional geomorphology and/n3 hydrogeology, it is suggested that the source of the salt is/nfrom terminal recharge through fractures associated with the/nlocal fault system. Due to climate change, these deposits and/nthe salts began occurring ~14 ka ago. These deliquescent salts,/nin a rainless region, are unique habitats for life within the/nclimatic limits of life on Earth, and are potential analogs for/ntransient darkened linear features on Mars.

Advisor: Amundson, Ronald G