Surface evolution of salt‐encrusted playas under extreme and continued dryness

Abstract

Miocene continental saltpans are scattered in the Central Valley of the Atacama Desert, one of the driest regions on Earth. These evaporitic deposits are hydrologically inactive, and are detached from groundwater brines or aquifers. The surface of the saltpans, also known as salars, comprises desiccation polygons, commonly with nodular salt structures along their sides. The morphology and bulk mineralogy of salt polygons differs between and within salars, and the shape and internal structure of salt nodules varies between different polygon types. Based on field observation, and mineralogy and crystallography data, we generated a conceptual model for the genesis and evolution of these surface features, whereby rare rainfall events are responsible for the transformation of desiccation salt polygons and the initial formation of salt nodules along polygon borders. In addition, frequent, but less intense, deliquescence events further drive the evolution of salt nodules, resulting in a characteristic internal structure that includes laminations, and changes in porosity and crystal morphologies. As a result, and despite the extreme dryness, the surfaces of fossil salars are dynamic on timescales of several years to decades, in response to daily cycles in atmospheric moisture, and also to rare and meager rainfall events. We propose that fossil salars in the Atacama Desert represent an end stage in the evolution of evaporitic deposits under extreme and prolonged dryness.

In situ metabolism in halite endolithic microbial communities of the hyperarid Atacama Desert

“…The present-day dry, fossil salar consists of a massive salt body approximately 70–90 m thick, with thick beds of almost pure halite formed approximately 1.8–5.3 million years ago, when the basin became hydrologically inactive. The surface of the salar is shaped in the form of polygons and nodular structures characteristic of this type of evaporitic settings (Figures 1A – C ; Artieda et al, 2015 ). A Relative humidity/temperature (RH/T) sensor (HOBO ® S-THB-M002) was inserted inside a salt nodule within the colonized zone (Figure 1D ) and set to record data every 10 min.…”

The Polygonal Surface Structures in the Dalangtan Playa, Qaidam Basin, NW China: Controlling Factors for Their Formation and Implications for Analogous Martian Landforms

“…In fact, polygonal ridges are formed through a combination of thermodynamic and geochemical mechanisms (e.g., Lowenstein & Hardie, ; Nield et al, , and references therein). The durability of salt polygons is in most cases limited by seasonal rainfall or flooding events, which often destroy the surface morphology and then start a new cycle of the formation and evolution of salt polygons (Artieda et al, ).…”

“…Salt polygons are usually formed in continental evaporitic environments when brine concentrates near the surface (Neal, 1975). These polygons are characterized by their uplifted rims (or pressure ridges) that formed from a subsurface process of lateral salt displacement (e.g., Artieda et al, 2015;Christiansen, 1963;Lowenstein & Hardie, 1985, and references therein) due to crystallization pressures (Handford, 1991) or to thermal contraction and expansion (Dellwig, 1968;Handford, 1991;Tucker, 1981). In fact, polygonal ridges are formed through a combination of thermodynamic and geochemical mechanisms (e.g., Lowenstein & Hardie, 1985;Nield et al, 2016, and references therein).…”

Analyses of Namibian Seasonal Salt Pan Crust Dynamics and Climatic Drivers Using Landsat 8 Time-Series and Ground Data

“…Carbonates are potentially deposited first, followed by sulfates, and finally halides that form a fresh efflorescent crust on top the more mixed sediment layers [96]. A similar process is described for halite efflorescence in salt pans of the hyperarid Atacama Desert [93,[95][96][97]. Once patches of purer NaCl form, they are preferentially wetted during subsequent nights, which leads to further enrichment and the development of an apparent bright salt crust over multiple diurnal cycles [96].…”

“…At that point, the amount of moisture absorbed from the air is enough for the dissolution of the hygroscopic salt and the formation of local brines. As the temperature on the salt pan surface increase after sunrise (up to 30 • C in winter) the brine that has been accumulated during the night will tend to move upwards driven by capillarity and evaporation processes [95]. As water migrates towards the surface, the brine becomes successively concentrated and salts are deposited when their solubility coefficients are exceeded.…”