Hydromagnesite Precipitation in an Ultramafic‐Hosted Alkaline Lake: Insights From Lake Salda for Jezero Crater, Mars

Lake Salda (SW Turkey) is one of the most compelling terrestrial analogs for Jezero Crater on Mars due to its highly alkaline, Mg‐rich waters and hydromagnesite‐forming microbialites. This study integrates hydrochemical measurements, saturation‐index modeling, SEM–TEM imaging, and X‐ray diffraction (XRD) analysis to disentangle the biotic and abiotic pathways responsible for carbonate precipitation in this ultramafic‐hosted lacustrine system. Hydromagnesite occurs as nanospheres, platy crystals, and rosettes, frequently replacing filamentous microorganisms and exopolymeric substances, demonstrating a dominant biomediated mineralization pathway. Transmission electron microscopy observations reveal that diatoms also contribute to carbonate formation: their frustules are coated by an external organic layer on which platy and anhedral aggregates of hydromagnesite precipitate, indicating a previously under‐recognized nucleation mechanism. XRD data show that living microbialites contain predominantly expanded, poorly ordered hydromagnesite (95%), whereas shoreline pebbles exhibit more crystalline, dehydrated forms (13% expanded), reflecting progressive structural maturation linked to episodic desiccation. In contrast, acicular aragonite crystals infilling voids lack association with organic substrates and represent an abiotic precipitation pathway driven by extreme Mg/Ca ratios and high pH. The coexistence of biologically influenced hydromagnesite and abiotic aragonite highlights a dual‐mode mineralization system shaped by microbial activity and physicochemical controls. These findings refine the mineralogical and microtextural criteria needed to distinguish biogenic from abiotic Mg‐carbonates in ancient lacustrine environments. Given the detection of hydrated Mg‐carbonates and possible shoreline deposits in Jezero Crater, the biosignature‐relevant textures identified here—nanosphere aggregates, organic‐associated platy hydromagnesite, and dehydration trends detectable by XRD—provide a framework for analyzing returned Martian samples.