Biosedimentary and geochemical constraints on the precipitation of mineral crusts in shallow sulphate lakes

Abstract

Seasonal desiccation of Mg2+–(Na+)–(Ca2+)–SO42−–(Cl−) saline lakes in La Mancha (Central Spain) that host microbial mats led to the precipitation of hydrated Na-Mg sulphates and gypsum. Sulphates precipitated in the submerged conditions form extensive biolaminites, whilst in marginal areas they produce thin crusts. Sedimentological, mineralogical, petrographic and high resolution textural studies reveal that the crusts were formed within the benthic microbial mats that thrive at salinities ranging from 160 to 340 g·L−1. The minerals of the crusts are primary bloedite (Na2Mg(SO4)2·4H2O), epsomite (MgSO4·7H2O), gypsum (CaSO4·2H2O) and mirabilite (Na2SO4·10H2O), as well as secondary hexahydrite (MgSO4·6H2O) and thenardite (Na2SO4). Primary bloedite crystals, which form the framework of surficial and submerged crusts are seen to nucleate subaqueously and grow incorporatively within the matgrounds. Displacive and incorporative epsomite grows on previous bloedite crystals and also on the ground. Mirabilite is precipitated rapidly at the brine-air interface over bloedite and epsomite. Hexahydrite and thenardite are formed due to dehydration of epsomite and mirabilite, respectively.Hydrochemical modeling with PHREEQC indicated that evaporitic biolaminites are forming from brines undersaturated with respect to bloedite, epsomite and mirabilite, which suggests that the microorganisms contribute to the heterogeneous nucleation of the sulphates in the microbial mats.Unlike carbonates, the influence of microbes on the growth and morphology of complicated double salts such as bloedite has not been documented previously and provides a new perspective on the formation of hydrated sulphate minerals that are common on Earth as well as other planets.