Influence of bacterial sulphate reduction on carbonate-sulphate mineral precipitation in ephemeral saline lakes

A long-term study of mineralogical and hydrochemical data analyses in playa lakes that host microbial mats, gives clues to the influence of sulphate reduction activity on the water chemistry and the mineral precipitation in... [ view full abstract ]

A long-term study of mineralogical and hydrochemical data analyses in playa lakes that host microbial mats, gives clues to the influence of sulphate reduction activity on the water chemistry and the mineral precipitation in ephemeral saline lakes. The study lakes, located in La Mancha (Central Spain), are shallow water bodies where the authigenic mineral assemblages are dominated by a variety of sulphates. Gypsum is the more abundant as it is relatively more insoluble and, thus, it may be preserved in the rock record. Discoidal gypsum grows episodically within the microbial mats, subjected to episodic stages of growing and dissolution. In addition, up to 30 % of Ca-carbonates precipitate, irrespectively of the low Ca and HCO3- concentration and negative saturation indices. The authigenic carbonates were found associated to microbial mats although the moment and mechanism of precipitation remained elusive.
The mineralogical, textural and statistical analyses applied to hydrochemical data suggested that as evaporation proceeds, a physicochemically driven mineral precipitation occurs following the predictable paths modeled by Eugster and Hardie (1978). Nevertheless, the precipitation of the Ca-carbonates and Ca-sulphates are inconsistent with this trend. The lowest concentration measured of Ca was coincident with the lowest pH value (8.06) and a low concentration of O2 in pools that were isolated as evaporation increases. By contrast, the water had the highest concentrations of HCO3-, CO32-, PO43- and NOX-, which is indicative of bacterial sulphate reduction processes. This mechanism also explain the presence of CaCO3 crystals found in the black layer of sediment.
Depite the negative saturation index for the Mg-sulphate (hexahydrite) , this mineral was crystallizing in the water-air interface of these remnant pools.
Thus, against what was previously modeled, these results provide evidence that carbonates precipitate simultaneously with Mg- sulphates and other more soluble minerals in ephemeral lakes. Bacterial sulphate reduction processes provide a mechanism for the precipitation of carbonate that commonly occur associated to gypsum in ancient lacustrine rocks. The more soluble minerals are not preserved due to their high solubilities.
Ca-carbonate precipitation by bacterial sulphate reduction processes prevents the microbial mat from mineralization and preservation in gypsum rocks.

Acknowledgment: Projects CGL2011-26781 and CGL2015-66455-R (Spanish Ministry for Economy and Competitivity