Evaporite microbial films, mats, microbialites and stromatolites

Abstract

Evaporitic environments are found in a variety of depositional settings as early as the Archean. Depositional conditions, microbial communities and mineralogical compositions vary significantly as no two settings are identical. The common thread linking all of the settings is that evaporation exceeds precipitation, resulting in elevated concentrations of cations and anions that are higher than in oceanic systems. The Dead Sea and Storrs Lake are terrestrial examples of two diverse-modern evaporitic settings, as the former is below sea level and the latter is a coastal lake on an island in the Caribbean. Each system varies in water chemistry; the Dead Sea-dissolved ions originate from surface weathered materials, springs, and aquifers while the dissolved ion concentration in Storrs Lake is primarily derived from sea water. Consequently, some of the ions, e.g., Sr, Ba are found at significantly lower concentrations in Storrs Lake than in the Dead Sea. The origin of the dissolved ions are ultimately responsible for the pH of each system, the alkaline versus mildly the acidic. Each system exhibits unique biogeochemical properties as the extreme environments select certain microorganisms. Storrs Lake possesses significant biofilms and stromatolitic deposits; the alkalinity varies, depending on rainfall and storm, activity. The microbial community in Storrs Lake is much more diverse and active than those observed in the Dead Sea. The Dead Sea waters are mildly acidic, lack stromatolites, and possess a lower density of microbial populations. The general absence of microbial and biofilm fossilization is due to the depletion of HCO3 and its slightly acidic pH.