Morphospace, composition and texture of Lake Salda microbialites

Abstract

<p>The discovery of hydrated magnesium carbonates in a paleolake in Jezero Crater, Mars, has inspired the study of carbonates associated with lacustrine systems on Earth. Lake Salda, Turkey, contains hydromagnesite-dominated microbialites in a mafic-rock hosted terrain. This alkaline lacustrine system provides an excellent natural laboratory in which to characterize various depositional environments and biosignature preserving potential of microbialites and other magnesium carbonate deposits. Here we aim to describe the textures, mineral composition, and macroscopic morphology of hydromagnesite structures in Lake Salda. All microbialites are covered by a thick, sticky organic-rich layer dominated by diatoms and cyanobacteria. A close association of contemporary hydromagnesite precipitation with organic-rich layers in microbialites indicates biologically mediated precipitation. Microbialites around the lake exhibit a range of morphologies, including finger-like, mini columnar, domical, and structureless. The recently exposed microbialites commonly contain colloform and cerebroid structures with fine, crude lamination, vuggy and globular textures. Layered microscopic textures preserve evidence of partially mineralized filamentous bacteria that trapped detrital grains and also contain globules embedded in extracellular polymeric substances, and dense hydromagnesite. The differences in mineralogy, density, and organic preservation potential within these textures likely correspond to the depositional environment. For example, the globular textures are closely associated with the purple layers in microbialites, including the layers in the recently sampled microbialites from the deeper part of the lake (ca. 10 m). These layers may form in the presence of anoxygenic purple sulfur bacteria. Our preliminary results suggest that the textures and macroscopic structures of hydromagnesite microbialites may record and reflect relationships among carbonate minerals, environmental factors, and microbial community composition.</p>