Abstract

Along Tertiary times, aridity and volcanism defined in northern Chile a “saline domain”. Chlorides, sulfates, andborates are sourced from magmatism through volcanism, recycled by hydrothermal circulation or hydrologicalactivity. Gypsum frequently occurs in evaporitic environments and is also known as a habitable architecture formicroorganisms. One example is gypsum structures (GS). Recently, Chilean salt flats, like Salar de Pajonales (SP)and Salar de Gorbea, have received global attention as terrestrial analog environments to Mars because of the po-tential long-term microbial viability within gypsum crystal. Evidence of endolithic communities has been reportedin GS associated with volcanic rocks, calcite, quartz, and halite. However, nothing is known about the microbialparticipation in the formation and preservation of GS.Our main objective is to characterize the microbial communities in microbial mats with evident gypsum precipita-tion and in gypsum structures with a distinctive grade of formation and preservation.In order to distinguish abiotically and biotically gypsum precipitation in the formation process, hydrochemistryand brines evolution were registered. A geological map of SP exhibiting the localization of GS in-formation andin-preservation has been carried out. GS were classified according to morphology, colonization type, relative hu-midity, texture, macro and fine-scale mineralogy, fluorescence pattern, lipids biomarkers, microfossil composition,microbial abundance, and diversity indexes.SP is classified as arid region due to its low precipitation (80-150mm/year), low transpiration (1350 mm/year) andother data (397,3 W/m2, 3-10◦C, and 0,2%RH). Lagoons with <10% of salinity do not have abiotically gypsum pre-cipitation. Microbial mats with gypsum precipitation and in-formation GS are observed submerged in lagoons with14 to 44% of salinity in the center of SP, and fossil outcrops are preserved in the southeast part of SP. Endolithiccolonization is found in the last two cases. Thermal springs, carbonate terraces, and ulexite horizons associatedwith silica and halite are also present in SP. SEM and micro-XRF analysis revealed the occurrence of a millimeter-scale laminated structure at the bottom of the preserved GS and centimeter-scale vertical selenitic crystals at thetop of formed and preserved GS. Diatom frustules and biofilm remains were observed mainly in the laminatedstructure where Fe-SiO4-rich interplayed between gypsum-layers. Microbial pigments were observed in the up-per part of the selenitic gypsum in formed and preserved GS as well as microbial mats, and their fluorescencefingerprints indicated that phototrophic cells were intact and healthy. The microbial community was dominatedby Proteobacteria, Bacteroidetes, and Cyanobacteria. LDChip-3000 confirms that Cyanobacteria prefers seleniticcrystals stead of gypsum crusts and humid sediments. Selenitic crystals showed a predominance of Thermi andCyanobacteria, like Chroococcidiopsis which has been reported in hyper-arid habitats. Conversely, gypsum crustis dominated by Proteobacteria and Bacteroidetes.The different salinity gradient in lagoons, gypsum textures (like grains, layers, and crusts), and development ofdomical mounds exhibiting selenitic crystals growing syntaxially at the top of the structure suggest the microbialparticipation in the deposition process. Thus, searching for evidence of microbial induction or influence on gypsumprecipitation and on the GS’s formation is relevant for the exploration of biosignatures.