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Water Dynamics on Landscapes and Soils of the Atacama Absolute Desert

  • Author(s): Pfeiffer Jakob, Marco Matias
  • Advisor(s): Amundson, Ronald G
  • et al.

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.

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