Mechanisms shaping the gypsum stromatolite-like structures in the Llamara Salar

doi:10.21203/rs.3.rs-2092563/v1

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Mechanisms shaping the gypsum stromatolite-like structures in the Llamara Salar

https://doi.org/10.21203/rs.3.rs-2092563/v1

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published 12 Jan, 2023

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The explanation of the origin of microbialites and specifically stromatolitic structures is a problem of large relevance for decoding past sedimentary environments and deciphering the biogenicity of the oldest plausible remnants of life. We have investigated the morphogenesis of gypsum mushroom-shaped stromatolitic-like structures currently growing into shallow ponds (puquíos) in the Salar de Llamara (Atacama Desert, North Chile). The crystal size, aspect ratio, and orientation distributions of gypsum crystals within the structures have been quantified showing the existence of episodic nucleation and competitive growth of millimetric to centimetric selenite crystals into a radial, branched, loosely cemented aggregate. The morphogenetical process is explained by the existence of a vertical salinity stable gradient in the ponds. Due to the nonlinear dependency of gypsum solubility as a function of sodium chloride concentration, the density gradient produces undersaturated solutions dissolving gypsum crystals depth, narrowing the lower part of the structures, and producing their mushroom morphology. This novel mechanism is tested experimentally, showing the effective dissolution of gypsum crystals in stratified ponds, thus providing a purely abiotic mechanism for these stromatolitic-like structures.

Stromatolites are layered organo-sedimentary structures formed by sediment trapping, binding, and mineral precipitation within prostrate microbial communities termed algal mats^1–5. These structures, more frequent in the past than nowadays, are commonly used as evidence of ancient microbial life and as environmental markers for the study of ancient shallow-water environments, especially in Archean and Proterozoic deposits, where they show as one of the first forms of life on Earth⁶. The formation of stromatolites has been the subject of intense debate^5,7−9 that is still open because of a) the diversity of potentially organo-sedimentary structures (microbialites, stromatolites, thrombolites, etc.), b) the plausibility of producing them by abiotic mechanisms claimed both in field studies^10,11 and by numerical simulations^12,13, and c) because, despite the predominance of calcium carbonate stromatolites^14–16, similar structures made of silica^17–22 or gypsum^23–31 have also been reported. The proper use of these structures as a proxy to past environments depends critically on our knowledge of the processes shaping stromatolites or structures looking like stromatolites.

Gypsum microbialites, including stromatolites and thrombolites, have been reported in Messinian-age sediments outcropping around the Mediterranean Sea³² and are present in stratigraphic records of different ages in Ukraine, Australia, or Guatemala^33–35. Different types of gypsum microbialites are contemporaneously forming in Egypt²⁷, Saudi Arabia³⁶, Venezuela²⁵, and noticeably, because of their astrobiological relevance, in several locations of the Atacama Desert in the north of Chile^29,37. We have focused our investigation on the most conspicuous of these locations, termed the Salar of Llamara, which is located in the Tarapacá region, 141 km SE of Iquique, in the Atacama Desert (northern Chile), being the southern limit of the Pampa del Tamarugal region. This basin hosts some large evaporite deposits (“salares”), produced by lacustrine-evaporitic sedimentation since the Miocene^38,39, which represent good paleoclimatic and paleohydrologic indicators. “Salar” is the term used in South America and in this report, area salt-encrusted playas. Similar landforms in the United States have been termed salt flats, or, less frequently, salt pans. The surface of the Salar de Llamara is a hard-saline crust made of reddish sulfates and chlorides that become white where they are saturated in water. Most of them correspond to recycled materials from the Soledad Formation, which contains Pliocene halite and anhydrite deposits from ephemeral saline lakes, revealing a hydrological evolution from a saline pan to a salt-encrusted playa and, finally, to a gypsum-dominated playa lake^40,41. In the Salar de Llamara, there are currently forming mushroom-shaped stromatolite-like structures are currently forming in Huatacondo hypersaline shallow ponds, which are locally named “puquíos” (see Fig. 1). The explanation of the morphogenesis of these fascinating structures is a challenge in mineral pattern formation with important applications in the detection of primitive life and in the identification of past sedimentary environments. Using a combination of the previously studied complex hydrochemistry of the Huatacondo’s puquíos³⁷, textural study techniques, and ad-hoc crystal growth/dissolution experiments, we propose an abiotic mechanism for the formation of the mushroom shape of gypsum stromatolite-like structures.

Characterization of mushroom-like stromatolitic structures

Gypsum stromatolite-like structures forming into the Puquíos de Huatacondo (21°16’06.5“S 69°37’02.1”W) were investigated in-situ during two field campaigns in October 2011 and March 2012. Figures 1a shows the location of the Salar de Llamara and the location of the different puquíos. A view of the mushroom-like gypsum structures forming inside the further East of these puquíos is shown in Fig. 1b. Figure 1c -d illustrates in further detail these “mushrooms” and shows the accumulation of detrital gypsum particles under the structure (Fig. 1c) as well as the alignment of structures in the same pond, with narrowing at a common, definite depth (Fig. 1d). Finally, Fig. 1e shows an ex-situ picture of one of these mushroom-shaped structures (Fig. 1c) that was collected in the last field campaign. That “mushroom” was supported by a thin stem that had recently broken, leaving the structure resting on one of its sides (see figures S1 and S2 of supplementary materials). The structure was cut radially in two halves. One of the halves was kept intact to observe the inner crystal distribution at the cutting surface (Fig. 2a). The second half was used to produce thin slices for textural studies using petrographic microscopy and for mineralogical characterization by X-ray diffraction studies (table S1). X-diffraction and optical microscopy shows that the structure is made of gypsum while evaporitic minerals such as eugsterite, halite, thenardite appear together along with gypsum, near the surface of the emerged part of the stromatolite. The submerged part is exclusively made of gypsum.

Figure 2a shows a high-resolution scanned image (11000x7300 pixels) of the cut surface used for crystal texture studies. The orientation, size, and shape of the crystals were measured by defining a pair of orthogonal segments along the longest and the shorter directions of the crystal (Fig. 2b). This measurement was repeated for ca. 1000 different crystals distributed over the section and representing the total population of crystals larger than 0.5 mm, limited by the image resolution of 34 microns per pixel (Fig. 2c). From these pairs of segments, a set of "projected" morphological variables were computed (Fig. 2b); the position of the crystal is defined as the intersection of the long and short axes, length and width correspond to the length of the long and short segments respectively, the aspect ratio was computed as the ratio between length and width, size as the product of them and orientation was computed as the angle made by the longest direction with the horizontal. It must be stressed that these are "projected" or "apparent" values. This is unavoidable for the non-destructive characterization of a large number of crystals. This was not an important limitation because the gypsum crystals are not randomly oriented with respect to the radial section: most of the crystals have c-axes oriented close to the section itself.

Crystals in the structure show a radial, branching distribution, already apparent in picture 2a, suggesting competitive growth limited by space availability. Crystal size (Fig. 2e) shows a continuous distribution (close to negative exponential), cut at the smallest sizes because the measurement was limited to features larger than 10 pixels. Crystals of similar sizes do cluster, with large crystals aligning along the "stems" of the radial fans and around a horizontal plane about two-thirds from the base of the structure. This level likely corresponds to the water level during the later growth stages. The largest crystals are longer than 1.5cm. The mean aspect ratio (Fig. 2d) of the crystals is between 2 and 3 and is relatively homogeneous within the structure.

Figure 2f confirms the radial, fan-like distribution of crystals: blue lines represent crystals oriented into the 2nd and 4th quadrants, red lines represent crystals oriented into 1st and 3rd quadrants, and white lines represent vertically growing crystals. Fans and branching are highlighted by the blue-white-red gradients in the underlying map. The orientation distribution (histogram in 2f) is bimodal, with maxima at ± 20º corresponding to the left and right sides of each fan. The overall structure consists of two main fans with an opening of around 80 degrees corresponding to twice the 40º degree separation between maxima. The skewed tails of the distribution correspond to the further bifurcation of the sub-fans. Notice that the left and right sides, in contact with the brine, show wider blue and red regions, respectively, more developed than the corresponding inner half fans. This feature is due to the availability of space to grow in that direction and confirm the competitive growth of the aggregate.

Radial gypsum crystals aggregates (Fig. 3a) are identified, corresponding to events of high nucleation density that could be triggered by high supersaturation events or by the activity of cyanobacteria^6,18. Our observations cannot confirm this biological effect but certainly, support the contribution of these radial aggregates to the branching and layering properties of the structure. The crystals forming the stromatolite were also studied by optical microscopy after embedding pieces cut from the sample in resin to stabilize them mechanically. This resin was dyed (blue) to clearly see the pore space (fig. S3). Crystals are twined following the twin law (100) (Fig. 3c) and often show recurrent banded growth surfaces (Fig. 3b and 3c), revealing episodic growth, with periods of relatively fast and slow growth. These growth bands show similar spacing, which may be due to seasonal changes in brine composition.

All in all, our textural study suggests that the formation of these structures starts with the local nucleation of a few single crystals at the bottom of the pond. Competition for space during the growth of these crystals forced the development of a fan-like structure. The seasonal variations in brine composition and water level of the pound produce episodic nucleation of new fan-like structures and further growth of existing crystals. As a consequence, a semispherical dome-shaped structure with a radial distribution of crystals is formed, a thrombolytic structure that can reach almost one meter in diameter. Then, how the mushroom-like structure is obtained? We found the answer to that question in the complex hydrochemistry of the puquíos.

Gypsum Growth/dissolution In Stratified Brines

The hydrochemistry of the puquíos of Huatacondo in the Salar de Llamara is rather complex and very relevant for explaining the morphogenesis of mushroom-like stromatolitic structures³⁷. Three compositional changes were identified, mainly related to the salinity of the brines, i.e. to the concentration in calcium sulfate and sodium chloride: 1) a "lateral" gradient from West to East due to the progressive evaporation of the brines as they flow through the puquíos, 2) a vertical (depth) gradient related to a density stratification of the brines, and 3) a temporal "seasonal" variation of brine composition. The lateral gradient is obviously relevant for the formation of the stromatolitic structures since they only develop in the East end of the group of ponds, where salinity is the highest and algal mats are less developed. To test the relevance of the vertical stratification of the puquíos brines it is necessary to consider the change in gypsum solubility as a function of NaCl concentration. As shown in Fig. 4, calculated using the hydrogeochemical software PHREEQC using the Pitzer database⁴², solubility is maximum in solutions with NaCl concentration of around 3 moles/L and decreases either if the concentration is reduced by dilution or increased by evaporation. The counter-intuitive consequence of this maximum solubility is that by mixing two gypsum solutions saturated (for instance, dots in Fig. 4), it is obtained an undersaturated solution with respect to gypsum (any point in the blue line joining the two blue dots). The mechanism of precipitation or dissolution of a phase by mixing solutions with a non-linear solubility as a function of salinity has been theoretically proposed⁴³. Dissolution by brine mixing has been observed in carbonates⁴⁴. However, the mechanism of gypsum dissolution by brine mixing has never been reported.

Brines within the Huatacondo's puquíos are density-stratified; both the top and bottom brines in a given pond are almost in equilibrium with gypsum³⁷, corresponding to points in Fig. 4. At intermediate depths, the composition of the brines will be that of points in the segment joining the two limiting compositions and, therefore, will have an undersaturated composition. Within the stromatolitic structure, crystals located close to this halocline would dissolve.

To test this hypothesis, we have designed and performed ad-hoc experiments using a crystal growth cell that generates a permanent halocline (Fig. 5) that mimic the stratified brines of the Llamara ponds. This setup features a slow flow chamber with two inlets at different heights and a single outlet (waste) in the center of the opposite side. Two solutions, saturated with respect to gypsum but having different salinity, were slowly pumped from the left side to produce laminar flow towards the right side and will mix by diffusion at the interface, generating a steady halocline with a definite width depending on the residence time. Two NaCl solutions (1M and 5M) were equilibrated with gypsum for two weeks to obtain the composition of the blue dots in Fig. 4. The denser one was pumped through the lower inlet and the lightest one through the upper to keep the flow and mixing stable. One elongated gypsum crystal was fixed at the level of the halocline and perpendicular to it. During the flow experiment, the central part of the crystal was observed under the microscope through a 45º mirror to keep the flow chamber vertical (Figure S4). Time-lapse images collected during the flow experiment show a local dissolution of crystals at the level of the halocline (Fig. 5c). After 40 hours, the central part of most crystals was dissolved entirely, and the lower part of the crystals fell down. These falling gypsum fragments are interpreted as the origin of the detritical gypsum accumulation under the structures shown in Figs. 1c and 1d.

This experiment was performed using synthetic solutions with a high salinity contrast to test, in a reasonable time, the plausibility of locally dissolving gypsum crystals in contact with a mix of two saturated solutions having different salinities. But the salinity contrast between the top and bottom levels of the stratified puquíos is not that large. In a second run of experiments, we used natural brines sampled from Huatacondo's ponds. Two brines from the same pond were selected, one from the bottom of the pond (P12-9b), which is the most saline, and the other (less saline) from the top (P12-9)³⁷. The brines were pumped through the chamber using the same setup and flow conditions of previous experiments (see "Methods") and images were collected using the same protocol. Figure 6 shows the result of this experiment. The center picture shows one of the crystals after 60 hours. The narrowing of the central part is evident but, due to the lower contrast in salinity, and therefore lower undersaturation at the halocline, the local growth/dissolution kinetics is much slower, and quantitative measurement (right, top panel) and time-slicing images (right, bottom panels) are required to properly assess the changes in crystal width at levels A, B and C during the experiment. Different behaviors were found at these levels. At the top level (A), in contact with the lower salinity solution, the crystal grew slowly, while at the bottom level (C), in contact with the more saline solution, the width of the crystal keeps constant during the experiment. At the level of the mixing region (B), crystal dissolution was observed, as expected, due to the undersaturation at the interface between both brines. A similar behavior, neither grow nor dissolution, could be expected for levels A and C, but in this experiment, the solutions used were sampled from the top and bottom of the puquío and full equilibrium with gypsum cannot be assumed for the top solution, were active evaporation was taking place at the time of sampling. This produces a small supersaturation with respect to gypsum in this solution explaining the observed slow growth rate.

The above experimental results simply entail that density stratified pond brines, close to equilibrium with respect to gypsum, but with higher salinity at the bottom will be slightly undersaturated below a given depth. This will produce a dissolution and consequently a narrowing of the lower part of the stromatolitic structures explaining its mushroom shape.

There is clear evidence that biological activity can influence gypsum precipitation, either at the stage of nucleation or growth^31,45. Gypsum stromatolites in other Atacama Salares have been proposed to result from biological/abiotic processes²⁹. However, these structures are rather gas-triggered and develop by successive mat growth, upholstery of the mat, formation of the gypsum cover, and crystal growth. Still, in this model, the development of the mats is not related to gypsum precipitation and, the morphology of the structures (bubble-like, gas-filled domes covered with a single palisade of elongated gypsum crystals) is entirely different from the mushroom-like morphology of the stromatolitic structures found in the Huatacondo's puquíos. We have noticed the presence of organic matter remains around the stem and in the holes of the gypsum structures in the Salar of Llamara (Fig. 1e), but not on the surface. This scarcity of mats along with the isotopic data previously published³⁷ suggests a little relevance of biologically driven mechanisms in the control of the textural arrangements of the crystals. And certainly, there are no signs of the role of biology in the formation of the shape of the stromatolitic structures.

Gypsum stromatolite-like structures currently forming at the Salar de Llamara are mushroom-shaped structures made of millimeter to centimeter-size crystals of selenitic gypsum showing episodic growth and arranged in radially distributed branching fans. The crystal texture and morphology of the structures are controlled by the three chemical gradients identified at the Huatacondo's puquíos: the lateral gradient is responsible for the fast, competitive growth of gypsum that produce these structures only in the ponds containing the most evolved, more concentrated brines. The seasonal variations in brine composition/level produce further spherulitic nucleation and competitive crystal growth leading to the formation of semispherical dome-shaped thrombolytic structures with a radial distribution of crystals. The mushroom shape structure is formed because of selective dissolution of gypsum controlled by the third gradient, namely, the stratification of highly concentrated brines, that produces a level of undersaturated solution at the halocline between the lighter (top) brine and the denser (bottom) brine. The gypsum solubility dependence on brine salinity (Fig. 5), along with the density stratification, creates gypsum undersaturation at the bottom of the brines, progressively dissolving the lower part of the thrombolytic structure and thus explaining the current mushroom shape. The rate of the resultant dissolution is proportional to the salinity contrast between the upper and lower brines and is faster in the more evaporated puquíos to the East, where the stromatolitic-like structures are found.

Our abiotic morphogenetic mechanism should be considered when using gypsum stromatolitic structures as a proxy for paleo-environmental conditions or as an indicator of biological processes either in the Earth stratigraphic record or in planetary research. Large gypsum deposits have been reported on Mars, and permanent hypersaline water bodies have been discussed in the context of life-search initiatives. Since our genetic mechanism is based on simple physicochemical premises, it applies to many different geological environments, current or past, as well as to non-terrestrial environments, and it can work either in the presence or in the absence of life.

The mineralogical composition of the samples was obtained by powder X-ray diffraction using samples from different parts of the stromatolitic structure. The composition was assessed by quantitative phase analysis (QPA) using Rietveld methods implemented in the Topas Academic V.5 software. The diffraction experiments were carried out Bruker D8 Advance Vario diffractometer (Cu Kα1) equipped with a Lynxeye detector and a primary germanium monochromator. Measurements span a 2θ angular range of 10º-100º with a 2θ step width of 0.015º.

Morphometric analysis of crystal distribution was done using custom Python 3 scripts to extract the position of all segments from a SVG overlay on top of the digitized image. Segments on this overlay were defined using the Inkscape software.

Hydrochemical calculations of solubility and supersaturation were performed using the Phreeqc 3.4 code and the accompanying Pitzer database⁴².

For the first gypsum dissolution experiments, the solution injected through inlet A (Fig. 6) was NaCl 1 M and the solution injected through inlet B was NaCl 5 M (both prepared from Sigma Aldrich 99% NaCl and MiliQ water type I). 2 g of CaSO₄·2H₂O powder (Sigma-Aldrich 98%) was set in contact with the solutions during 1 week under stirring to equilibrate them with respect to gypsum. After this period, solutions were filtered and stored into sealed bottles.

The halocline flow setup was implemented using two 10x5cm glass plates separated by a rubber spacer 1mm in thickness (Fig. 5). Two liquid inlets and one liquid outlet (waste) were implemented inserting syringe needles in the two opposite short sides. Pumping through the inlets creates an interphase layer between two solutions with different salinities. To avoid turbulent mixing, the flow rate was set to 1 mL/min and the setup was kept vertical and steady with the denser solution at bottom. Natural, elongated gypsum crystals obtained from the evaporation of Huatacondo's brines were glued to one of the glass plates so that their center lay at the halocline level.

Time-Lapse microscopic observations were performed with Nikon AZ100 microscope equipped with a Nikon AZ Plan Flour 2x objective and a Nikon DS-Fi1 photographic camera. Pictures were acquired and analyzed using the NIS-Elements BR software.

Data Availability

All data generated or analyzed during this study are included in this published article [and its supplementary information files]

Acknowledgments

This work was funded by the Spanish Ministerio de Economía y Competitividad project and Grant CGL2010-16882/BTE, CGL2010-12099-E and Grant BES-2014-069790, and Junta de Andalucía Project RNM5384. We thank Ismael Aracena from SQM for kindly sharing data and information on the geology and hydrochemistry of the Salar de Llamara, Alexander Van Driessche and Magí Baselga Bacardit for help during one of the field trips.

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No competing interests reported.

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Mechanisms shaping the gypsum stromatolite-like structures in the Llamara Salar

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Characterization of mushroom-like stromatolitic structures

Gypsum Growth/dissolution In Stratified Brines

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