Hydrochemical and Mineralogical Evolution through Evaporitic Processes in Salar de Llamara Brines (Atacama, Chile)

Abstract

The study of extreme evaporitic environments is a subject of increasing interest in sedimentary petrology and planetary geology. We report here the evaporitic precipitation in the shallow, small lakes (puquı ́os) at the Salar de Llamara (Atacama Desert, northern Chile). We have used a combination of in situ/laboratory/modeling methodologies allowing for the definition of a detailed, in-depth model for the sequence of evaporitic precipitation in the puquı ́os. The in situ measurements in these ponds reveal that brines are density-stratified with important gradients in salinity, temperature, and other properties of the solution (pH, oxidation/reduction potential, and dissolved oxygen). These vertical gradients, along with the lateral and seasonal variations, are the main factors of chemical variability controlling the precipitation processes. The evaporation of these brines drives the chemical evolution of the different ions and produces two distinct mineral assemblages: (i) large structures made of millimetric to centrimetric-size crystals of gypsum often forming thrombolite-like or stromatolite-like morphologies in the ponds and (ii) salt crusts made of mainly gypsum, eugsterite, halite, and thenardite surrounding the ponds. Evaporation experiments in the laboratory monitored by in situ X-ray diffraction and optical videomicroscopy reveal a precipitation sequence in accordance with field observations but distinct from the thermodynamically predicted precipitation. The actual formation of eugsterite, rather than glauberite, and the observed delay in gypsum dissolution are explained using kinetic evaporation models.

A Multidisciplinary Evaluation of Spatial Heterogeneity in the Puquios of the Salar de Llamara, Atacama Desert, Northern Chile

“…Gradients in environmental parameters have previously been described in the Puquios 32,59,60 , as well as in a variety of Andean microbial ecosystems, such as Lake Tebenquiche in Chile 25 , Lake La Brava in Chile, and Lake Socompa in Argentina 19,23,27 . These environmental settings are associated with biological communities that are tolerant to extreme conditions 17 , including a reduced group of invertebrates, such as brine shrimps, insects and other planktonic organisms, as well as macrophytes adjacent to the ponds [61][62][63][64][65] .…”

“…The ability of a community to shift between lifestyles, as observed in the endoevaporitic communities near the margins of the Puquios, should be considered a source of diversity and resilience. However, for a better understanding of the communities' response to typical environmental changes experienced on a seasonal time scale (sensu 32,54,55 ), a multi-disciplinary surveying effort across broader spatio-temporal scales is needed. Bottom types and coupled mineral precipitation revealed a wide depositional spectrum ranging from ne-grained granular gypsum precipitation with microbial in uences to large sparry accumulations of selenite with little or no microbial in uences.…”

“…The combination of these hydrological processes associated with the alluvial fan systems and the geomorphological characteristics at the boundary between the Coastal Cordillera and the Pampa del Tamarugal allows the shallow ground water levels to create the small lagoons called puquios with their biological communities. Brines in the Puquios are managed by SQM Ltd.; environmental protection measures were enacted in 2012 to preserve brine chemistry and water levels in the salar environment 32,40,54,55 .…”

“…One such example of a gypsum-depositing system is the series of lagoons locally known as Puquios, in the Salar de Llamara of the Atacama Desert. Recent studies of the Puquios suggest that multidisciplinary studies of the strong environmental gradients and a variety of microbial communities 17,24,31,32 , observed in this polyextreme environment will provide new insight into conditions that promote habitability at the dry limits of life.…”

“…Previous studies in the Puquios have focused primarily on microscopy and molecular analyses of microbial communities in subaerial crusts and domes 15,18,20,24,27,29,30 but detailed investigations of the relationship between physical, chemical, geological, ecological, and microbiological processes that may in uence mineral deposition in these lagoons are limited. Initial studies in the Puquios of brines 32,40 have begun to quantify strati cation and lateral gradients of brine chemistry within a portion of the Puquios system in the Salar de Llamara. In this paper, we expand on this foundation by incorporating two additional areas within the Puquios system that have yet to be described.…”