Abstract EANA2025-52

Lake Salda (SW Turkey), a highly alkaline (pH 9.0 – 9.3), Mg-rich lake, hosts modern hydromagnesite stromatolites and represents one of the most compelling terrestrial analogues for Jezero Crater on Mars. Understanding the biotic versus abiotic pathways of hydromagnesite formation in this environment offers valuable insights into carbonate precipitation processes under extreme conditions, both on Earth and potentially beyond. This study integrates mineralogical, geochemical, and microstructural analyses to unravel the mechanisms behind hydromagnesite formation in living and fossil stromatolites, with a particular focus on microbial contributions. Field sampling across seasonal intervals was coupled with in situ hydrochemical and mineralogical measurements, saturation index modelling, and advanced imaging techniques, including SEM and TEM. Results reveal that hydromagnesite exists in distinct morphological forms such as nanospheres, platy crystals, and rosettes, frequently associated with organic matter. Extracellular polymeric substances, produced by bacteria and diatoms, serve as key nucleation sites for hydromagnesite nanospheres and platy aggregates. These biogenic features contrast with abiotic crystal morphologies such as acicular aragonite, indicating parallel but distinguishable precipitation pathways. Mineralogical XRD data suggest progressive dehydration of hydrated precursors into well-ordered hydromagnesite during stromatolite maturation. In living stromatolites, the structure of hydromagnesite is expanded due to hydration, which is progressively lost in more developed living mats and stromatolite pebbles. No X-ray reflections of nesquehonite or dypingite were observed. These two phases appear to have been misidentified in other studies, e.g. by Gunes et al. (2024). The dual biotic–abiotic mineralisation model highlights a complex geobiological system governed by high Mg/Ca ratios, elevated pH, and microbial metabolic activity. The presence of organic-rich, mineralised diatom frustules suggests a broader ecological role for non-cyanobacterial microorganisms in carbonate precipitation. Given the compositional and textural similarities to carbonates detected in Jezero Crater, these findings contribute to the development of biosignature criteria relevant to astrobiology. More broadly, this work provides a general framework for interpreting microbially influenced carbonates in other alkaline lacustrine settings and recommends future investigation into the specific microbial taxa and biochemical pathways involved.