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

Abstract

AbstractThe 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 theSalar 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.