Environmental controls on sedimentary deposits in saline lake environments

Saline lakes are known to be sensitive to changes in environmental conditions on a broad temporal scale. Therefore, variations in the mineralogical, geochemical, and sedimentological characteristics of these settings have often been interpreted to reflect oscillations in climatic conditions. However, recent work has shown that microbial communities can also influence the formation of carbonate and evaporite minerals in saline lake environments, especially in the salars of South America. Here, both abiotic and organomineralization pathways can be found to exist within the same salar environments, indicating a high degree of spatial heterogeneity of mineralization processes in such settings. Thus, the drivers of the resulting mineral assemblage can be complicated to disentangle through space and time. A process-level understanding of first-order controls on mineral assemblages can provide new insights into sedimentological dynamics of salar environments.

Babel (2004) published a conceptual model based on marine-fed systems that established links between salinity and the style of gypsum mineral deposition. Based on field and laboratory analyses conducted on sediments in the Salar de Llamara, we adapted this model for a continental saline lake setting (Reid et al., 2021). In the present study, we aimed to test whether our salar-scale conceptual model was applicable more generally to continental saline lake environments. To accomplish this goal, we investigated a 15-year time series of electrical conductivity, a proxy for salinity, collected in five saline lake/wetland systems situated along the margin of the Salar de Atacama. Based on this dataset, we predicted the style and mineralogy of mineral deposition in each setting using our salar-scale conceptual model. Next, we compared our predictions with published field descriptions of the occurrences of biofilms, microbial mats, microbialites, and evaporite deposits in these lakes. Through a principal component analysis, we evaluated environmental characteristics such as electrical conductivity, pH, and dissolved oxygen as controls on mineral morphology and mineralogy.

Results indicate that salinity is a first-order control on sedimentological expression in the lakes of the Salar de Atacama, although the transition between organomineralization pathways and physicochemical precipitation may occur at different salinity values than observed in other saline lake settings. Broadly in agreement with our model from the Salar de Llamara, granular precipitates of carbonate minerals formed within microbial mats were associated with environments characterized by low salinity, while microbial mats with laminated precipitates were found in settings with moderate salinity in the Salar de Atacama. High salinity environments contained crystalline bottom types characterized by selenitic morphology. Because some South American salars have been cited as living laboratories analogous to the ancient conditions that fostered the evolution of terrestrial and Martian life, these insights into mineralization are important. Improved constraints on the controls of carbonate and evaporite mineral deposition in saline lake environments will elucidate the definition of habitable environments, and provide a testing ground for the production and preservation of chemical and morphological biosignatures through time.