Water Dynamics on Landscapes and Soils of the Atacama Absolute Desert

摘要

The driest section of the Atacama Desert in northern Chile has experienced nearly lifeless conditions for much of the past several million years. The extreme aridity of the region has drawn the attention of scientists since Charles Darwin visited the area in 1835. The unique conditions that have prevailed through this long time span have created a landscape that is dissimilar to all other areas on earth. However, what makes the Atacama particularly interesting is not just the low quantity of water, but the infrequency and irregularity of rainfall. This dissertation examines the impact of water on the desert landscapes at different space and time scales. Chapter 1 presents the results of a stratigraphic and chronological study on the desert lowlands. These areas are currently covered by halite-encrusted salt pans (Salars) that have been thought to be fossil remnants of lakes that existed at the Plio-Pleistocene transition. However, in this study I show that these are much younger features from the late Quaternary. Numerous stratigraphic sections were observed and sampled in two subbasins located in the Central Depression of the Atacama Desert. The fossils and sedimentology of the stratigraphic sections show that these environments supported a diverse hygrophyte vegetation, as well as an array of diatoms, ostracods and gastropods that indicate the presence of shallow lakes and wetlands periodically between ~46.9 ka and 7.7 ka. The formation of wetlands and lakes occurred due to an increase in groundwater 2 levels as a result of increased Andean runoff during regional wetter 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 record historical rainfall that occurred on March 24--26 2015. From scattered weather station data, the storm was among, and in some cases the largest, recorded in the desert. The effect of this unusual storm was analyzed by observations made a few months after the event in a N to S transect through the plant-free expanse of the Atacama Desert, between 22 and 26° S. The main objective of the field work was to characterize landscape changes following the storm. The findings show that the storm initiated some minor fluvial responses on the upland landscapes, but overall those were not sufficient to reactivate many hydrological features that are prominent on the landscape, and that must therefore be driven by larger, even less frequent storms. The field evidence suggests that larger rainfalls (or periods of rainfall) have occurred throughout the Quaternary, and that there are fossilized (or infrequently active) features in various stages of "repair" that provide evidence of rainfall re-occurrence. Radiocarbon dating of carbonate bearing soils at the southern periphery of the desert reveals that more rainfall, and more biotic conditions, existed in the region up to the end of the Pleistocene. Additionally, the soils in the lifeless portion of the Atacama Desert have unique hydraulic properties. In most arid regions, a rainfall of this magnitude and intensity would cause flash flooding, but the Atacama's salt-rich soils have very high infiltration rates, and the landscape is thus more resilient to intensive rainfall events than most desert landscapes. However, based on the fossilized geomorphic and hydrological features on the landscape, there is a rainfall threshold, whose magnitude remains uncertain, above which this landscape undergoes alteration and fluvial reshaping.;In Chapter 3, the observations and laboratory analyses of a study of a rare calcium chloride rich soil in the Salar de Llamara are presented. The uniqueness of this project resides in the fact that calcium chloride enrichments are extremely rare on the earth surface, and that the hygroscopic properties of this salt allow the soil to remain wet (8--16 % gravimetric water content) nearly continuously under modern---and essentially rainless---climatic conditions. The substrate for the accumulation are small aeolian dunes, of fine sand and silt, that contain 60% of soluble salts by weight, of which ~15% is CaCl2. Based on an analysis of the regional geomorphology and 3 hydrogeology, it is suggested that the source of the salt is from terminal recharge through fractures associated with the local fault system. Due to climate change, these deposits and the salts began occurring ~14 ka ago. These deliquescent salts, in a rainless region, are unique habitats for life within the climatic limits of life on Earth, and are potential analogs for transient darkened linear features on Mars.