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 analogues for Jezero Crater on Mars due to its highly alkaline, Mg-rich waters and abundant hydromagnesite-forming microbialites. This study integrates hydrochemical measurements, saturation index modelling, SEM–TEM imaging, and 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 EPS, demonstrating a dominant biomediated mineralisation pathway. TEM 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 have precipitated, indicating a previously under-recognised nucleation mechanism. XRD data show that living microbialites contain predominantly expanded, poorly ordered hydromagnesite (95%), whereas reworked 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 any 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 mineralisation system shaped by both 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-related deposits in Jezero Crater, the biosignature-relevant textures identified here — nanosphere aggregates, organic-associated platy hydromagnesite, and dehydration trends detectable by XRD — provide a robust interpretive framework for analysing returned Martian samples. More broadly, this work advances understanding of microbially influenced carbonate formation in alkaline lakes and strengthens the use of Lake Salda as a high-fidelity analogue for Mars.